Press Tool-1.pdf

DIE AND PUNCH A typical die and punch set used for blanking operation is shown in Fig 8.1. The sheet metal used is called strip or stock. The punch which is held in the punch holder is bolted to the press ram while die is bolted on the press table. During the working stroke, the punch penetrates the strip, and on the return stroke of the press ram the strip is lifted with the punch, but it is removed from the punch by the stripper plate. The stop pin is a gage and it sets the advance of the strip stock within the punch and die. The strip stock is butted against the back stop acting as a datum location for the centre of the blank.

Fig 8.1. The die opening is given angular clearance to permit escape of good part (blank). The waste skelton of stock strip, from which blanks have been cut, is recovered as salvaged material. The clearance angle provided on the die (Fig 8.1) depends on the material of stock, as well as its thickness. For thicker and softer materials generally higher angular clearance is given. In most cases, 2 degree of angular clearance is sufficient. The height of cutting land of about 3 mm is generally

sufficient. TYPES OF DIES The components generally incorporated in a piercing or blanking die are shown in Fig 8.3. This Figure shown the die in the conventional closed position. The die set is made up of the punch holder which is fastened to the ram of the punch press and the die shoe which is fastened to the bolster plate of the punch press. Generally, the punch is fastened to the punch holder and aligned with the opening in the die block. Fig 8.2 shows one type of stripper plate and push – off pins. The stripper holds the scrap strip so that the punch may pull out of the hole. The push – off pins are needed to free the blank in instances where the material strip clings to the bottom of the punch. This may be necessary for thin material, or where lubricants are used on the material.

Fig 8.2 Sometimes the die and the punch positions may be interchanged. This may become necessary when the opening in the bolster plate is too small to permit the finished product to pass through the bolster opening. Fig 8.3 shows such a die.

Fig 8.3 Inverted die (Fig 8.3) It is designed with the die block fastened to the punch holder and the punch fastened to the die shoe. During the downward stroke of ram, the blank is sheared from the strip. The blank and shedder are forced back into the die opening, which loads a compression spring in the die opening . At the same time the punch is forced through the scrap strip and a spring attached to the stripper is compressed and loaded. On the upstroke of the ram, the shedder pushes the blank out of the die opening and the stripper forces the scrap strip off the punch. The finished part (blank) falls, or is blown, out the rear of the press. Compound die (Fig 8.4) It combines the principles of the conventional and inverted dies in one station. This type of die may produce a workpiece which is pierced and blanked at one station and in one operation. The piercing punch is fastened in the conventional position to the punch holder. Its matching die opening for piercing is machined into the blanking punch. The blanking punch and blanking die opening are mounted in an inverted position. The blanking punch is fastened to the die shoe and the blanking die opening is fastened to the punch holder.

Fig 8.4 Progressive dies are made with two or more stations arranged in a sequence. Each station performs an operation on the workpiece, or provides an idler station, so that the workpiece is completed when the last operation has been accomplished. Thereafter each stroke of the ram produces a finished part. Thus after the fourth stroke of a four – station die, each successive stroke will produce a finished part. Operations which may be carried out in a progressive die are piercing, blanking, forming, drawing, cut – off, etc. The list of possible operations is long. The number and types of operations which may be performed in a progressive die depends upon the ingenuity of the designer. Fig 8.5 shows a four – station progressive die. The die block is made up of four pieces and fastened to the die shoe. This permits easy replacement of broken or worn die blocks. The stock is fed from the right and registers against a finger strop (not shown). The first stroke of the press Fig 8.5(a) produces a square hole and two notches. These notches form the left end of the first piece. During the upstroke of ram, the stock is moved to the next station against a finger stop (not shown). The stock is positioned for the second stroke. The second station is an idler, Fig 8.5(b). The right end of the first piece, the left end of the second piece, and a second square hole are pierced.

Fig 8.5 The ram retracts and the scrap strip is moved to the third station against an automatic stop, Fig 8.5(c). This stop picks up the notched V and positions the scrap strip. The third stroke of the ram pierces the four holes as shown in Fig 8.5(c). The fourth stroke, Fig 8.5(d), cuts off and forms the radii at the ends of the finished piece. Thereafter every stroke produces a finished part, Fig 8.5(e). Progressive dies generally have the cut – off or blanking operation as the last operation. It is preferred to have piercing operation as the first operation so that the pierced hole can be advantageously used as a pilot hole. Alternatively, special pilot holes are pierced in the scrapped part of the stock. In certain special cases, blanking is done at the first station, and the blank returned to the die by using spring plates and then moved to the subsequent station by mechanical means or manually. Progressive dies are used where higher production rates are desired and the material is neither too thick nor too thin. Their use helps in cutting down the material handling costs.

SHEARING THEORY Shearing Theory : Shearing is the method of cutting sheets or strips without forming chip.The material is stressed i a section which lies parallel to the forces applied.The forces are applied by means of shearing blades or punch

and die. Critical stages of shearing Stage 1: Plastic deformation The force applied by the punch on the stock  Material tends to deform it into the die opening .When the elastic limit is exceeded by further application of force.  The material is forced into the die opening in the form of an embossed pad on the lower face of the material .  A corresponding depression is formed on the upper face. This stage impacts a radius on the upper edge of the penning in the strip and on the lower edge of the punched cut material (which may e blank or slug) Stage 2: Penetration As the load is further increased ,the punch will penetrate the aterial to a certain depth.An equally thick portion of the metal is forced into the dies.This impacts a bright polished finish (cut band) on both the strip and the blank or the slug.on optimum cutting conditions,the cut band will be 1/3rd the sheet thickness. Stage 3: Fracture I n this stage,fracture starts from both upper and lower cutting edgesAs the punch travels further,these fractures will extend towards each other and meet to cause complete separation.This stage impacts a dull fractured edge.

Cutting Clearance In press tool Cutting Clearance:  Cutting clearance is the gap between a side of the punch and the corresponding side of the die opening when the punch is entered into the die opening.  Cutting clearance should always be expressed as the amount of clearance per side.

 Proper cutting clearance is necessary for the longer life of the tool.Quality of the piece part also depends on proper cutting clearance.A visual examination of the punched components will indicate the amount of clearance and whether the punch and die have - optimum cutting clearance or - excessive clearance or - misalignment Optimum Cutting Clearance: When optimum cutting clearance condition exists a small edge radius is formed. The edge radius is the result of the plastic deformation (first stage of shearing).A highly burnished cut band is approximately one third the thickness of the stock material.The balance of the cut is the break resulting from fracture(third Stage). Excessive cutting clearance: The gap between the punch and the die is comparatively more in this case.The stock material reacts to the initial pressure on a manner approaching that of forming rather than cutting.Therefore the edge radius becomes larger.It does not blend smoothly with the cut band.The cut band becomes smaller.The break shows greater irregularities. Heavy burrs are noticeable all along the cut contour.The burr results from the dragging of the material. Insufficient cutting clearance: When cutting clearance is slightly less,the width of the cut band will be more.If cutting clearance is too less two ormore cut bands will be formed.Because of the steeper angle between the punch and the die cut edges the resistance of the stock material to fracture is increased.The resulting pressure will cause the initial fracture to originate at clearance rather than at the cut edges.Burr may be caused by compressive forces. Misalignment between punch and Die: The cutting characteristics also indicate whether the punch and die openings are in accurate alignment.Because of misalignment,clearance on one side increases and the other side decreases.The component will show

the corresponding difference on cut band. Burr side The burr side is adjacent to the break.Burr shouldbe practically nonexistent - if the cutting clearance between the punch and die is optimum. - if the cutting edges are sharp. The burr side of a blank or slug is always towards the punch(die stars shearing)The burr side of a punched opening is always towards the die opening (punch starts shearing).The characteristics of the blank or slug and the punched opening are inversely identical.

Relationship of piece part size to punch and die size : when pierced or blanked piece parts are measured,the measurement is made at the cut band. Blanking The actual cutting of the blank or slug is done by the cutting edge of the die opening .Therefore the die opening determines the size of the blank or slug. BLANKING PUNCH SIZE=BLANK SIZE-TOTALCLEARANCE BLANKING DIE SIZE=BLANK SIZE Piercing The actual cutting of the opening in the stock material is done by punch.Therefore the size of a punched opening is determined by the punch. PIERCING PUNCH SIZE=PIERCED HOLE SIZE PIERCING DIE SIZE=PIERCED HOLE SIZE+TOTAL CLEARANCE Clearance calculation: The ideal clearance can be caluculated by the following formula Clearance=C X s X square root of (T max/10) where C is a constant=0.005 for very accuratecomponent =0.01 for normal components s=sheet thickness in mm T max=shear strength of the stock material in N/mm square T max for some materials: St.steel=400N/mm square

MS=400N/mm square Brass=200N/mm srqare aluminium=100N/mm square Copper=250N/mm square

Blanking Tool A blanking tool consists of: 1. Top plate 2. Bottom plate 3. Punch 4. Die 5. Stripper 6. Punch holder 7. Thrust plate( Punch Back Plate) 8. Guide pillar 9. Guide bush 10. Ball Cages ( Al., Brass , Steel) 11. Shank

Top Plate This is the plate on which the punch holder is fixed.The top plate also holds the shank and the bushes.Top plate absorbs the stock resulting due to the cutting action. This plate is made from C-45 or EN-8 Steel material. Bottom Plate It is the base of the tool.The die and pillars are fitted to this plate.The opening for the blank is made in this plate.The opening should be simple in profile and at the same time provide enough support to the die. This plate is made from C45 or EN-8 Steel material. Punch and Die They are the cutting elements of a blanking tool.They are made of( HCHCr) high carbon high chromium steel and are hardened and tempered to 58-62 HRC. Stripper Its main function is to strip the stock scrap off the punch and to guide the

punch into the die. This plate is also made from HCHCr material or if stripper insert provided in that case the stripper plate material is C-45 or EN-8 material to be selected. Punch Holder Punch is fitted to the punch holder with a light press fit(H7/m6).It is made of mild steel means C-45 or EN-8 steel material. Thrust Plate( Punch Back Plate) While performing the cutting the punch will exert an upward thrust.So the punch should be backed by a hardened(45 - 50 HRC) plate to avoid digging on the top plate.For this purpose the thrust plate is used. Guide Pillars & Guide Bushes They are used to achieve a well guided movement of the moving part with respect to the fixed part. These are made from SAE-8620 or EN-31 Case hardened steel material. Ball Cages : It is used on the guide piller for smooth running operation. It is made from Brass, Aluminum ( cage body material) and the ball made from steel. Shank Shank connects the top half of the tool to the press ram.It is screwed on to top plate.

(1) Blanking Force Knowing the forming force necessary for carrying out a press blanking operation (shearing operation) is indispensable for selecting the press machine and for carrying out the die and punch design. The blanking force (P) is obtained using the following equation.

■Equation 1 P=L*t*S P: Blanking force (kgf) L: Forming circumference (mm) t: Plate thickness (mm) S: Shear Strength (resistance)

(kgf/mm2）

However, when it is difficult to know the shearstrength(resistance) (S), it is substituted by a value equal to 80% of the tensile strength (Ts) of the material. The equation in this case will be as follows.

■Equation 2 P = K * L * t * Ts P: Blanking force (kgf) K: Coefficient = 0.8 L: Forming circumference (mm) t: Plate thickness (mm) Ts: Tensile strength (kgf/mm2） Taking the example of Fig. 1, the blanking force will be as follows. In this case the tensile strength of SPCC is taken as 30 kgf/mm2. P = 0.8 * n * 40 * 1 * 30 = 3014.4 (kgf)

As a method of reducing the blanking force, there is a method of providing a shearing angle as shown in Fig. 2. The shearing angle is provided in the die in the case of blanking operations and in the punch in the case of hole punching operations. Most often the shearing angle is provided so that the dimension H is roughly equal to or more than the plate thickness. By the way, the blanking force can be reduced by about 30% when the dimension H of the shearing angle is made equal to the plate thickness.

BUCKLING OF PUNCHES The punches in a press tool are subjected to compressive stresses during the cutting operation.If these stresses are overlooked thin punches in the tool may fail by buckling.The maximum force that a punch can withstand with out buckling can be calculated from the following formula. FB=pie square EI/Lp square Fb=Maximum force beyond which buckling occurs E=Modulus of Elasticity of punch material (in GN/mmsquare) I=Moment of inertia in mm 4 Lp=length of punch in mm The cutting force coming on the punch should be less than or equal to the buckling force. Worked out examples: Is it possible to punch 1mm brass sheet with a5mm square punch. Length of punch=60mm, br>T max=320N/mm square Pierce the hole=L*s*T max L=cut length in mm s= sheet thickness in mm T max=shear force in N/mm square Shear force=20*1*320=6400N=6.4kN Buckling force=pie square E I/Lp square E=210 GN/mm square I=a^4/12=5^4/12=52.08mm^4 Lp=60mm Buckling force=(3.14^2*210*52.o8*10^-12/0.06^2)=29.9835kN The punch can withstand a force of 29.9835kn.The force coming on the punch is only 6.4kN.Therefore it is possible to use the punch.

Cutting Force Cutting force is the force which has to act on the stock material in order to cut out the blank or slug.This determines the capacity of the press to be used for the particular tool. Caluculation of cutting force Cutting force = l*s*T max where, l = length of periphery to be cut in mm s = sheet thickness in mm T max = shear strength of stock material in N/mm square

Relationship between shearing action and cutting force the three critical stages of shearing action are related to cutting force.Resistance begins when the punch contacts the stock material.The load builds up rapidly during the plastic deformation stage.It continues to increase while penetration takes place.The accumulated load is suddenly released when fracture occurs.The curve levels off near the bottom.the last portion of the load curve represents frictional resistance developed. - as the punch travels through the stock material - as the blank or slug passing through the die. If proper cutting clearance condition exists between the punch and the die fracture will occur when cutting force equals the shear strength of the material.

Methods of reducing press force

In some cases it will be necessary to reduce cutting force to prevent press overloading. A method to reduce press force is to grind the face of the horizontal plane.This reduces the area of contact during shear at any one time. Providing shear angle also reduces shock to the press and smoothens out the cutting operation. The shear angle should provide a change in punch length from 1 to 1.5 times the sheet thickness. Double shear angle is preferred over single shear angle because it does not create lateral forces. double shear angled punches should be concave to prevent stretching the material before it is cut. to prevent distortion on the stock material

- for blanking operation the shear angle will be on the die member. - for piercing operation the shear angle will be on the punch member. Another method to reduce cutting force is to step punch lengths.Punches or groups of punches are made progressively shorter by about one sheet thickness

FASTNERS Screws and Dowels Dowels Dowels hold parts in perfect related alignment by absorbing side pressures and lateral thrusts. They facilitate quick disassembly and reassembly of tool elements in their exact former relationship. Dowels are precise in their accuracy and in their surface finish. They are made of alloy steels and case hardened to 58HRC. Standard dowels are available in the range of sizes from 1.5mm to 20mm dia, in various lengths. The dowels are made to IT6 grade and are finish ground. The fit between the dowel hole and dowel pin is H7/m6.The surface finish is maintained at N6.The extremely smooth finish reduces the possibility of seizing of the dowel holes. The dowels are driven inside the reamed holes or in jig ground holes. The hole sizes are maintained to H7 tolerance. Fasteners The main function of the fastener(screws)is to hold or clamp two or more tool elements together in position. The fasteners are divided into two types. Threaded fasteners Non threaded fasteners. Threaded fasteners These type of fasteners include different types of screws and bolts. Socket head screws

These are used to fasten the plate elements of a press tool. eg: Punch holder assembly to the top plate. Stripper and die assembly to

bottom plate. counter sunk screws

These are used to fasten elements like nest gauges, spacers, plate stoppers etc. Cheesse head screws These are used to fasten sheet metal elements like spring holders,leaf springs etc. Set screw(grub screw)

These are used to fasten parts which are to be confined with in a hole.eg:spring. Eye bolts

Eye bolts are used for lifting heavy die sets.It is also called carrier bolts. Non threaded fasteners This group includes the elements like rivets and cotter pins. Rivets Rivets are used to fasten the support plates of an extension table in press tool. They are made of mild steel,aluminum,copper or brass. Cotter pins These are used to prevent loose parts from coming out of holes. Screw and dowel position The components of the tools are held together by screws and are located in position by dowels.A minimum of one screw and two dowels are necessary to position and hold a tool component in place accurately.More screws may be used depending upon the size of the tool but only two dowels should be used for positioning.The size of the screws and dowels are determined by the size the tool.Dowels should be located diagonally across from each other and as far

apart as possible to increase location accuracy. All screws and dowels should be located from 1.5 to 2 times their diameter away from the die edge.Whereever possible screw and dowel holes should be located nearer to the outer edge of the die block. Dowel holes are made through holes.So that they can be easy removed. The effective depth for screws should be - 1.5 times the screw diameter for general application. - two times the diameter when the element is subjected to shock loads.The dowel engagement length in each plate should be of minimum two times the diameter of dowel.

PILOTS Purpose of Pilots The pilot positions the stock strip in relation with the die opening. This tripped is normally overfed more than the pitch length. When the press is tripped the pilot comes down and engages the pre pierced hole. The strip is dragged back into the registry position. When mechanical feeding is employed the strip is underfed. The pilot pulls the strip into registry position.

Pilot Size diameter of pilot for average work, Diameter of hole to be piloted 0.05 to 0.1mm For close work, Diameter of hole to be piloted 0.03 to 0.05mm. For accurate work, Diameter of hole to be piloted 0.01 to 0.02mm. Thick stock materials and materials like aluminium and copper require larger tolerance. Pilot length Registering of the strip should be done before the punches come and engage the strip. The pilot should extend beyond the punch face equal to one sheet thickness. Pilot opening in the die If the pilot opening in the die is larger, instead of registering the material will be drawn into the opening. Diameter of the opening = diameter of pilot + double clearance. Pilot opening in the bottom plate through hole is provided in the bottom plate for the following reasons: 1.Slugs produced due to mis -feeding can be cleared. - if by chance the strip jumps the stopper the pierced hole will be out of alignment with the pilot. The pilot will hit the strip and punch out a deformed slug.

2.Accumulated burrs dislodged from the pierced from the pierced hole is cleared. - During piercing operation burr is formed on the pierced hole. The pilots, while entering the prepiercedhole,will dislodge the burr. Pilot nose profile Bullet nose The most common pilot nose profile is bullet nose. The bullet shape is formed by radius 'R' can be increased to reduce the lateral force is strong, simple to make and smooth in action. 45 degree conical stub nose - shorter nose profile. - used for piloting thick materials. 30 degree conical stub nose This is a compromise between bullet nose pilot and 45 degree stub nose pilot. 15 degree conical stub nose Used for small pilots and for soft thin materials. Pilot in punches Pilots mounted in punches are called punch pilots. The pilot extends beyond the punch face by a distance of at least one sheet thickness(minimum 1.5mm) and secondary operation tools. Types of pilots pilots are held in the punch holder. In special cases the following pilots are used. Retractable pilots Mis-feeding may occur due to the overshooting of stock strip over the stoppers. In such cases pilots may buckle or break. To overcome this difficulty retractable pilots are used. They are spring loaded pilots, they will be pushed up when they come into contact with un pierced area during operation. Removable type pilots Pilots break often due to mis-feeding of stock strip. Changing of broken pilots consume considerable time and leads to loss in production. Removable type pilots are used to overcome this difficulty. The pilots are

inserted through the top plate into the punch holder and fastened with screw. Methods of piloting Direct piloting Piloting in holes pierced in that area of the strip which will become the blank is called direct piloting. Indirect piloting Indirect piloting consists of piercing holes in the scrap area of the strip and locating by these holes at subsequent operations. Direct piloting is the preferred method, but certain blank conditions require indirect piloting. Indirect piloting is preferred under the following conditions: close tolerance on holes - pilots can enlarge holes by pulling a heavy strip in position. Holes too small - fragile pilots can break or deflect in operation. Holes too close to the edge of the blank - distortion can occur on the blank because of enlargement of holes. Holes in weak areas - piloting in projecting tabs is impractical because they may deflect before the strip is pulled in position. Holes spaced too closely - piloting in closely spaced holes does not provide an accurate relationship between holes and relative edges of the blank. Blanks without holes - piloting has to be done in the scrap area whenever the blank does not contain holes.

STOPPER Function Of Stopper :

The function of a stopper is to arrest the movement of the strip when it is fed

forward to one pitch length. After each of the press stroke, the stock strip is stopped at definite position in order to perform cutting/non cutting operation on the strip to obtain the component correctly. Classification of Stopper Stoppers are classified into three categories - Primary - Secondary and - final. The first stop in the tool is called primary stop. The last stop in the tool is called final stop. The stops in between the primary stop and final stop are called secondary stops. Stop Position This is the location at which the stock strip is stopped. Registry position This is the exact location in which the stock strip must be established in order to obtain dimensionally correct component. The work is located by the stop and is registered by the pilots. The registry position may or may not be the same as the stop position. When a stop functions as an approximation gauge the stop position does not coincide with the registry position. If the stop acts as a true gauge the stop position and the registry position are same. If a stock strip is piloted the stop need act only as an approximation gauge. The strip is overfed against the gauge and is registered by the pilots. If the stock strip is not piloted the stop has to function as a true gauge. Primary stops act as true gauges, registering the stock strip. Secondary stops normally serve as approximation gauges. Therefore overfeeding is allowed when installing them. Types of stop Solid stops A hardened steel block is mounted at the required location Plain pin stop It is a plain cylindrical pin mounted in the die block. A clearance hole for the pin stop in the bottom plate is provided for the following reasons. To permit adjustment of the height of the pin stop without removing the die plate from the assembly.

While re-sharpening the die the stop pin can be removed. The pin can be driven down in the event of misfeed.This reduces the chance of damage to the tool. Headed pin stop When the stop is to be very nearer (very small scrap bridge)to the die opening a headed pin stop is used. A plain pin stop cannot be used in such cases because the opening made for locating thee stop will weaken the die. The mounting hole of the headed pin stop will be away from the die opening. Spring loaded pin stop It is a spring pin located at the required stopping position. These stoppers do not require clearance in the opposing tool member. The pin is pressed down by the opposing tool member during operation. Finger stops In progressive tools designed for manual feeding finger stops locate the strip for each station except for the final station. This is actuated manually. It is mounted in the stripper plate. Provision is made for moving the stopper through a predetermined distance. It is pushed inwards to enable the stock material to halt against it. After the press stroke the stopper is released. When a new stock strip is fed the stopper has to be actuated again. Trigger stops For faster manual feeding, trigger stops are preferred. There are two types of trigger stops. - Front acting. - Side acting. The working mechanism for front acting and side acting trigger stops are same. The front acting trigger stop is mounted in the front end of the tool and the side acting trigger stop is mounted in the side of the tool. The lever shaped trigger stop fits freely in the slot milled in the guide plate. One side of the wall of the slot is machined with a taper angle. It gives the necessary movement to the trigger. An inclined set spring at the other end of the trigger holds the trigger holds the trigger in position. When the strip is pressed against the tip face of a trigger, the trigger moves back wards and stops against the non tapered wall of the slot. This allows the strip to advance. This advancement is equal to one margin width. When the tool is tripped a knocker bar fixed to the top assembly of the top assembly of the tool comes down and knocks the free end of the trigger. This action lifts to clears the thickness and then jumps back to its old

position to fall on the strip(blanked portion).The strip can be fed forward.

Gauges In most press tools the stock material is fed in the form of long strips. For the efficient functioning of the tool the strip should be guided longitudinally during its travel through the tool. This is achieved by employing gauges. When unit stocks are used, pin gauges nest them in the required position. Gauges used in secondary operation tools locate the pre-blanked or preformed component in relation to the operation to be carried out. Back gauge and front gauge " Whenever the stock material is fed in the form of a strip it is fed in between the back gauge and the front gauge. The back gauge is the one which is on the far side of the press operator or located on rear side of press tool. The front gauge is on the near side of the operator or located in front portion of press tool. While feeding, the operator should always keep the strip pressed against the back gauge. Back gauge is the actual gauging member and the function of the front gauge is only to provide approximate gauging. The required dimensional coordinates are maintained from the back gauge to the die opening. Bulge clearance Thick and soft materials tend to bulge sidewise as soon as the blanking operation is performed. This makes it difficult to feed as well as to gauge the strip further. A bulge clearance is provided usually in the back gauge only. Size of back gauge and front gauge Extended back gage for easier gauging the back gauge is extended beyond the die on the feeding side. Its length is equal to the strip width for roll feeding and two and a half times the strip width for manual feeding. Strip support During manual feeding, to reduce fatigue to the operator a strip support should be provided while feeding pliable(flexible)strips. The strip support should be made wider and brought closer to die block to provide better support and guidance. Roll feed does not require strip support. Pushers

Pushers are provided to keep the strip firm against the back gauge during its travel through the tool. Spring loaded pushers are used for this purpose. Nesting Gauges Nest gauges are used whenever - secondary operation tools are used - unit stock is fed into the tool. The function of the nest gauge is to align the unit stock or the component for the secondary operation in correct relation to the punch and die. The nest gauges should meet the following three conditions to achieve the best result. Accuracy The fit between the piece part and the gauge should be perfect and consistent throughout the life of the tool. It is not necessary for the nest to locate the entire contour of the piece part. Only sufficient number of locating points are needed. But they should be strategically located in relation to the piece part contour. The number of locating pins depends upon the life and the shape of the piece part. The minimum requirement are - Three points for circular and triangular shapes. - four points for other shapes. Easy and quick unloading Nest gauges should facilitate fast and easy loading and unloading of components. to achieve this good visibility and accessibility are required. Adequate lead angle should be provided around the nesting profile for easy loading, unloading is difficult than loading. So through consideration should be given to this point. For low production tools simple pick off slots are machined in the nest. the operator can manually pick the piece part out of the nest. Piece parts can be ejected out by means of lever operated ejectors. Thin piece parts can be expelled from the nest by means of compressed air jets. Fool proofing Possibility of the piece part being loaded in incorrect manner by the operator should be prevented by the nest. This can be easily achieved by fool proof pins. Types of nest gauges Pin type nest gauges This is the simplest form of nest gauge .It consists of plain or headed

cylindrical pins. They are arranged in such a way as to provide enough number of locating points for the piece part. The pins are hardened and ground and are press fitted in the die block. The upper end of the pins must be beveled for easy loading and unloading. The opposing tool member should have relief holes to receive these pins. In inverted tools the nest pins are fitted into the travelling stripper. The relief holes are drilled in the die block. If these holes are to be provided near the die opening the die will be weakened. In such cases the pins are spring loaded. They are pushed below the face of the stripper upon contact with the die block. Retracting nest pins are less accurate and should be used only if inevitable. Plate type nest gauges This type of nest gauge is a plate into which an opening is machined to receive the piece part. This opening need not fit the entire contour of the piece part. The plate nest can be of split construction for easiness in machining and hardening. They should be perfectly screwed and dowelled into position. All gauging elements should be made out of tool steels and hardened to 48 to 53 HRC.

LAND & ANGULAR CLEARANCE Land: The inner walls of the die opening are not usually made straight through. If they are straight,the blanks or slugs tend to get jammed inside the die opening.This may lead to the breakage of punch or die. To avoid this,the die walls are kept straight only to a certain dimension from the cutting edge.The straight wall is called as land. LAND=3mm for sheet thickness upto 3mm and for thicker material equal to the sheet thickness. Angular clearance The die walls below the land are relieved at an angle for the purpose of enabling blanks of slugs to clear the die. Soft material require greater angular clearance than hard materials. The normal value of angular clearance is 1.5 Degree per side. Dies for materials like silicon steel and stainless steel are provided with angular clearance from the cutting edge.(no land is provided).These materials are abrasive in nature and tend to bell-mouth the die opening rapidly if land is provided.

Stripper Plate : This plate is also called as stripper plate. In guide plate tool this element is known as guide plate. This plate helps in stripping operation. It not only strips the strip from the punch but the main function of this plate is to guide the punch accurately and maintain the alignment of this plate is to guide the punch accurately and maintains the alignment between punch and die. Hence the plate is made with the same care as die. It is mounted on die plate. It is made out of mild steel. In some cases this guide plate is also made out of tool steel. A channel is milled in the plate, which will guide the stock strip.

SHANK  Shank is an element of a press tool.The shank acts as a connecting link between the press tool and the press.  The diameter of the shank should fit into the bore in the press ram.  The shank diameter is standardized in relation with the size of the press ram bore.  The size of the bore varies from press to press depending on the capacity of the press.  The shank can be fixed to the tool top plate by - Rivetting - Screw thread - Making as integral part of the top plate.  The threaded types are used commonly. The shank has two flats milled on diameter at top to facilitate its fastening to the tool by using a spanner.

Self aligning type shank The shank permits quick loading and unloading of the press tool on a press.The design of the shank is different from the other types.A tee coupling mechanism is made in two sections.The half mounted on the tool is the male member.The half fixed to the press ram is the female member.They are case hardened.

Location of shank on a tool The balancing of cutting punches is very important for press tool operation. Unbalanced force distribution on the tool top will cause undue wear on the punch,die and also on the pillars. The resultant force of all the cutting forces acting on different punches should pass through the shank centre. The resultant force of all the partial cutting forces can be fund by applying the following two methods. - by calculation - by graphical method(polygon of forces).Centre point of shank location can be found by calculating the x and y coordinates for the point.The formula to be used for this calculation is x=(l1x1)+(l2x2)+(l3x3)+.../l1+l2+l3+.... y=(l1y1)+(l2y2)+(l3y3)+.../l1+l2+l3+... Polygon System The centre point of shank location can be determined by graphical method which is also known as polygon system.To construct the polygon force diagram the sequential steps given here have to be followed. 1. Draw the cutting forces to a scale in a straight line. 2. Draw the arrow heads at the ending points of each force as shown. 3. Draw two more lines at 45degree angle from the starting and finishing points of the total length of the forces so as to form an isosceles and call the intersecting point as pole. 4. Draw the lines from each arrow head to join the pole point and call them as

pole beams. 5. Draw the forces to scale at the given distance. 6. Draw the lines parallel to the pole beams,cutting force line graphically. 7. The line of action of the resultant goes through that point where those two pole beams intersect.

Basic Rules To Decide The Strip Layout 

Strip development refers to the choice of operations to be performed in each of the stations in a progressive die. It is essential that the strip is properly developed since it will ultimately result in the best design of the die. Some of the principles that are to be followed are as follows:

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Always pierce the piloting holes in the first station, this helps in proper registering of the strip for the subsequent stations.

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If numbers of punched holes are very close, distribute them in more than one station so that the die block remains strong. Similarly, holes nearer to the edge should be done in separate stations.

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A complex contour be normally split into a combination of simple shapes and punched out a number of stations. This eliminates the expensive way of making a complex punch. Use ideal stations in the strip development stations refer to the stations in a progressive die where no piercing or blanking is done. Some of them are used for simple piloting.

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Side Cutters The side cutter is installed in the first position of the toolthis eliminates extra stops and simplifies both construction and operation of the tool.Usually side cutter is located along the front edge of the stockstrip.Two side cutters on each side is used when the number of stages are more or if the pitch is less. Side cutter is an accurate method of stopping arrangement.It is mainly used for thinner strips where it is difficult to accommodate other types of stoppers.Side cutter is a trimming punch which trims the side of the stock material.A shoulder is formed on the strip.The shoulder is stopped against a

hardened insert provided in the spacer.The width of the side cutter is equal to the pitch. The allowance for side cutting depends on the type and thickness of the stock material.The size of the side cutter will be more than the pitch by 0.05 to 0.01mm when pilots are used in the tool.The pilots will register the strip at the correct location.But in tools without pilots the side cutter is made equal to the pitch. Unbalanced forces act on side cutters during the cutting operation.Therefore the side cutters are provided with heels. Thorns(small projections) occur on the side of the strips due to side cutter wear out. The undercuts provided on side cutter eliminates the difficulties of feeding due to formation of thorns.The slugs may go up with the punch.Slug pushers are used to avoid this. Advantages of using side cutters 1. It is a safer method than stop pins. 2. Avoids deformation of thinner strips by stop pins. 3. Preferred for small punching where it could be difficult to employ other types of stops. 4. It is economical and avoids complications in tools where number of stages are more. 5. Pilots can be avoided for punching components with moderate accuracy.

PUNCHES Punches can be classified into three categories. Punch Classification:  Cutting punches Cutting punches do operations like blanking ,piercing,notching,trimming etc...  Non cutting punches They do operations like bending,forming, drawing,etc...  Hybrid punches Hybrid punches do both cutting and non cutting operations like shear and form,pinch,trim,etc... Punch Groups :  Segregated punches These punches are positioned and retained by means of self contained screws and dowels.

 Integrated Punches This group of punches are located and positioned by punch holders. Types of punches  Plain Punches: The side walls of the plain punches follow the cutting contour originating at the cutting edge and extending straight through the base surface.Plain punches are self mounting-straight through punches.  Pedestal punches: The base area of the punch is larger .The cutting force is distributed toa larger area.These punches are recommended for heavy duty work.In case of narrow pedestal punches angular fillets are used.These punches are also called as broad based punches.  Off Set Pedestal Punches: These pedestal punches have their base off set.The reason for off setting the base are - Space consideration for other components in the assembly. - Machining and grinding accessibility.The distribution of cutting forces is non-uniform in these punches.  Keyed Punch: A key is provided for non circular punches to prevent their rotation. Punches Mounted In Punch Holders  Headless punches: This is a plain punch except that it does not require dowels.The positioning of the punch is done by the punch plate.The punch is fastened to the top plate by means of screws.  Step Head Punches (Shouldered Punches): These types of punches are fitted in the punch plate without screws and dowels.  Beveled Head Punch: When the punch is made with an angular seating it is called bevel head punch.The bevel angle is usually between 30Degree to 45 Degree.Thebeveled portion may be either machined or peened.  Floating Punches: They are made loose in the punch holder and are guided in the stripper plate.  Perforators: A punch of diameter 2.5mm or below is called a perforator. Bevel Head Perforators: On these type of perforators a beveled seating is machined or peened. Headless Perforators: These punches do not have shoulders.A whistle notch is milled on the

shank of the perforator.A screw from the side will fasten the perforator in position. StepHead Perforator: These are the commonly used perforators.They have a stepped head shank and a point diameter. Step Head Perforator –ShankLess These are similar to step head perforators except that the shank diameter is more by 0.025mm than the point diameter. Pyramid Perforators: This type of perforator is considerable disparity between the point diameter and the shank. QuilledPerforators: Slender punches are to be protected from buckling. Quills are provided to prevent buckling. Slug Ejector Perforator To prevent slug pulling,air pressure or spring pins are used.These are known as slug ejector perforators.

STRIP LAYOUT Strip layout plays an important role especially in the case of the design of the press tool. Strip decides the economic utilization of the work piece & helps in the decrease of cost of the job & reduction in the production time by increasing the number of components. Some Images For Strip Layout

Economy factor Stock material conservation is a decisive factor in means should be tried to attain this without sacrificing the piece part. Economy of any strip layout in percentage is find out by the following formula. Economy factor(E) in %=(Area of the blank X Number of rows X 100) (Width of the strip X Pitch) A minimum economy of 60% to 75 % should be aimed at.The position of the blank in the strip decides the economy factor. Note:Terms used in a strip layout. 1) Scrap bridge a) This is the portion of the material remaining after blanks operation between one edge of the strip and the cutout portion. b) The portion of material remaining between the two adjacent openings after blanking is also called as the scrap bridge. 2) frontScrap: This is the scrap bridge on that edge of the strip which is towards the operator.

3) Back Scrap:This is the scrap bridge on that edge of the strip which is away from the operator Example : Calculate the economy factor to punch the mild steel washer in single row feeding. Outside diameter is 30mm, inside diameter 18mm and thickness 2mm. Scrap bridge width == 1.2 X S ( Sheet Thickness)== 1.2 X 2 mm == 2.4 mm Pitch=Product Out Side Dimension + Scrap bridge width = 30+ 2.4=32.4 mm Strip width=Product Out Side Dimension + 2 X Scrap bridge width =30+4.8 =34.8 mm Number of rows =one Area of blank= (pie /4) X 30 2= 706.50 mm E=Area of blank X 100 X No of rows (Pitch X Strip width) E=706.50 X 100 X 1/(32.4 X34.8)=62.66% Calculate the economy factor to punch the same washer in double row feeding. E=Area of blank X100 X Number of rows/(pitch X strip width) Pitch=32.4mm Area o blank=706.95 mm square Number of rows =2,br>Center distance between two washers are required to determine the strip width. center distance=Cos 30 degree X pitch=0.866 X 32.4=28.05mm strip width=2.4+2.4+30+28.05=62.85 mm E =706.95 X 100 X 2/(32.4 X 62.85)=69.42%

Strip Layout For Blanking Tools Choice of strip layout method Blanking tools produce blanks out of the strip or unit stock.None of the edges of the strip or unit stock forms an edge of the blank.Blanking is the most efficient and popular way of producing intricate and closely tolerated blanks.The profile and accuracy built into the toolwill be reproduced on the blank. In the strip layout blanks can be positioned in different ways in the strip .Choice of the method depends on the following factors. 1.Shape of blank

The contour of the blank is the main factor which decides the way in which it is to be positioned. - Production requirement If lesser production is anticipated more emphasis should be given for material conservation without increasing the tool cost. - Grain direction When sheets are produced by rolling, the rolling direction orients the grains. Standard sizes of rolled sheets or strips will have the grains along its length. Bending the strip along the grain direction may result in cracks or fractures. If the blank is to be bent at a later stage the strip should take care of the grain direction. The grain direction should be at right angles or at an angle more than 45 degree to the direction of the bend when harder verities of strips are used. - Burr side In a blanking operation burr is formed on the face of the blank which comes in direct contact with the punch.In piercing it appears on the face which comes in direct contact with the die.In some piece parts the burr resulting from either blanking or piercing would be required to appear on a particular face of the blank in relation to details of the blank contour.Whiledeciding the strip layout care must be taken to see that such requirements are met. - Stock material A comparative study of stock material conservation,tool cost and labor cost is necessary while the strip layout is made.If the stock material is precious every means to conserve the stock material should be employed.If double pass is a one complete pass of the strip,it is reserved fed again for maximum utilization of stock strip. This is decided by the shape of the component(two pass or double pass both mean the same)Double pass method is employed,labor cost increases.So a double pass layout should justify the cost of stock material conserved. Based on the above factors, different strip layouts are explained in following paragraphs.  Single Row One Pass Layout This is the most popular way of laying out the strip. The blanks are arranged in a single row.The strip is passed through the tool only once to punch out the blanks from it. There are two possible ways of laying out this strip. - Narrow run - wide run Wide run is more desirable due to the following reasons. Shorter advance distance of the strip promotes easy feeding.

More blanks could be produced from a given length of strip compared to narrow run.Therefore a fewer number of strips are to be handled to produce a given number of blanks. Narrow run is used when the grain direction of the piece part has importance.  Blanks Having AlleastTwo Straight Parallel Edges In such cases the strip width should be equal to the distance between the parallel sides.Theblanks are produced by a cut off or parting operation.If the blank has got two sets of parallel sides, a cut off operation is sufficient to produce the blanks.But if the blank has got only one set of parallel sides.These sides become the sides of the stock strip and the other non-parallel sides are produced by a parting operation.  Strip Layout For Different Operation Cut off & Parting Cut off punch cuts with only one edge.No scrap is produced.A parting punch cuts with two opposite edges there by producing a scrap. Notching Notching is a cutting operation for cutting off small portions from the edge of a strip or a pre blanked component. Trimming Trimming is an operation of cutting off material to alter the shape of the strip or blank. In notching only a small area of the blank is cut off.In trimming a larger area of material is removed.Blank can be produced by combining notching ,trimming and piercing operations with cut off parting operations.  Blanks Having Irregular Contour The following factors must be considered before determining the best method of positioning a blank in the strip. Contour If the blank has two parallel sides,it can be produced by cut off operation.The advantages of cut off or parting operation are - minimum material wastage. - less tool cost. - no scrap strip to handle. - speeds up production. Accuracy InStrip Width Sheared strips cannot be held to an accuracy closer than +/-0.2mm.If the blank must be held to closed limits on its width sides cutting off or parting cannot be employed.When the blank dimensions are to be controlled to close limits it should be produced with a blanking

tool,regardless of the parallel sides it may contain. Flatness If the blank has to be flat,a blanking tool is preferred ,a blanking tool produce considerably flatter components than other tools. Single Row Two Pass Method This strip lay out demands the strip to be fed twice through the tool.This is to achieve greatereconomy in stock material utilization. A two pass tool requires two stops.The stop used for the first pass should be removed or made to retract(spring loaded stoppers)from the working surface so as not to interfere with the second pass.The front and back scrap as well as the scrap bridge should be wider than those for the single pass (about 50 to 100%) Two pass lay outs are justified only when the wastage is considerable and the stock material is costly. Double Row Lay Out Higher economy can be attained by positioning the blanks in double rows. Gang dies A gang die consists of two or more similar sets of tool members so as to produce two or more number of components during a single stroke of the press ram.A gang die eliminates the cumbersome process of double pass. The higher tool cost will be off set by higher rate of production.Gang dies are not recommended for very complex work. Angular layout Some piece parts require to be laid out in an angular position to make the lay out more economical.

Die Sets The die set consists of a bottom plate and top plate together with guide pillars and bushes.The guide pillars and bushes align the top and bottom plates. The advantages of die sets are - Accuracy of set up. - improved piece part quality. - Increased die life. - Minimum set up time. - Easy maintenance. - Alignment of punch and die. - Easiness of storing. Die sets can be classified as

- Precision - Commercial The difference between them are the accuracy of the fits between the bushes and the pillars. Precision die sets are used for cutting operation tools. Commercial die sets are used for operations like bending,forming and other non cutting operations. Die set materials Die sets are manufactured by using the following materials: - Cast iron containing 10 to25% steel. - Hot rolled steel. - Semi hardened or hardened tool steels. Die set components - Top plate - Guide bushes - Guide pillars - Bottom plate. Top plate The upper working member of the die set is called the top plate.The punch holder is clamped to the top plate. Bottom plate The bottom plate is the lower working member of the die set.It's shape corresponds with that of the top plate except that it is provided with clamping flanges.The flanges have provision for fastening the die holder to the bolster plate of the press.Usually the bottom plate is made thicker than the top plate.This is to compensate for the weakening effect of slug and blank holes. Guide pillar Guide pillars are presicion ground pins which are press fitted into accurately bored holes in the bottom plate. Guide pillars are assembled into corresponding guide bushes to align punch and die components with a high degree of accuracy. The commonly used types of pillars are - small diameter guide pillars which are usually hardened and centreless ground. - large diameter pillar which are ground between centres after hardening or case hardening. Removable guide pillars can be easily removed from the die set for resharpening the cutting elements.They are employed for large dies and for dies having more than two pillars. Guide Bushes Guide bushes are precision ground bushes which are press fitted into accurately bored holes in the top plate. Ball cage die set some die sets are provided with ball cages instead of guide bushes. Guide pillars are pressed into the bottom plate.They are assembled into linear ball cages whichin turn are guided in hardened sleeves resting in the top plate. Ball cage die sets are used where high rate of production is required and accurate alignment is necessary. Types of die set 1. Standard die set.

2. Non standard die set 1.Standard die sets - used for bending tools. - secondary operation tools. Center pillar die set - used for round working area. Diagonal pillar die set - used for progressive tools with rectangular working area. Four pillar die sets - used for heavier press working operation. Non standard die sets These die sets are made for a particular design when standard die sets are not suitable or not available.These are usually made of mild steel with case hardened pillars and bushes. Shut height Shut height is the distance from the bottom plate to the top of the top plate when the tool is in closed position.The height of the pillars must be less than the shut height in order to ensure that the press ram will not strike against the ends of the pillars.If possible, the pillars should be so designed to accommodate the reduction in the shut height which will be due to resharpening of die.

Die Set

The die set consists of a bottom plate and top plate together with guide pillars and bushes.The guide pillars and bushes align the top and bottom plates. The advantages of die sets are - Accuracy of set up. - improved piece part quality. - Increased die life. - Minimum set up time. - Easy maintenance. - Alignment of punch and die. - Easiness of storing. Die sets can be classified as - Precision - Commercial The difference between them are the accuracy of the fits between the bushes and the pillars. Precision die sets are used for cutting operation tools. Commercial die sets are used for operations like bending,forming and other non cutting operations. Die set materials Die sets are manufactured by using the following materials: - Cast iron containing 10 to25% steel. - Hot rolled steel. - Semi hardened or hardened tool steels. Die set components

- Top plate - Guide bushes - Guide pillars - Bottom plate. Top plate The upper working member of the die set is called the top plate.The punch holder is clamped to the top plate. Bottom plate The bottom plate is the lower working member of the die set.It's shape corresponds with that of the top plate except that it is provided with clamping flanges.The flanges have provision for fastening the die holder to the bolster plate of the press.Usually the bottom plate is made thicker than the top plate.This is to compensate for the weakening effect of slug and blank holes. Guide pillar Guide pillars are presicion ground pins which are press fitted into accurately bored holes in the bottom plate. Guide pillars are assembled into corresponding guide bushes to align punch and die components with a high degree of accuracy. The commonly used types of pillars are - small diameter guide pillars which are usually hardened and centreless ground. - large diameter pillar which are ground between centres after hardening or case hardening. Removable guide pillars can be easily removed from the die set for resharpening the cutting elements.They are employed for large dies and for dies having more than two pillars. Guide Bushes Guide bushes are precision ground bushes which are press fitted into accurately bored holes in the top plate. Ball cage die set some die sets are provided with ball cages instead of guide bushes. Guide pillars are pressed into the bottom plate.They are assembled into linear ball cages whichin turn are guided in hardened sleeves resting in the top plate. Ball cage die sets are used where high rate of production is required and accurate alignment is necessary. Types of die set 1. Standard die set. 2. Non standard die set 1.Standard die sets - used for bending tools. - secondary operation tools. Center pillar die set - used for round working area. Diagonal pillar die set - used for progressive tools with rectangular working area. Four pillar die sets - used for heavier press working operation. Non standard die sets These die sets are made for a particular design when standard die sets are not suitable or not available.These are usually made of mild steel with case hardened pillars and bushes. Shut height Shut height is the distance from the bottom plate to the top of the top plate when the tool is in closed position.The height of the pillars must be less than the shut height in order to ensure that

the press ram will not strike against the ends of the pillars.If possible, the pillars should be so designed to accommodate the reduction in the shut height which will be due to resharpening of die.

PROGRESSIVE DIE STRIP EVELUATION The most important step in designing a progressive stamping die is developing the strip layout. Strip layouts directly impact die size and initial die cost, as well as costs for die maintenance and repair. The strip layout affects press selection and the costs for press maintenance and repair, as well as initial stamping cost, in-process reliability, dimensional accuracy of the finished part and the cost of poor quality. The strip layout serves as a master plan that determines—and restricts—nearly every decision made during die design. The main steps to design a strip layout: 1) Input data, typically consisting of a 3D model of the finished product. 2) Flatten the blank through use of unfolding software or metalforming-simulation codes. 3) Nest the blank, to optimize material consumption and establish carrier location(s), coil width, pitch distance (progression) and part orientation in the strip. 4) Plan the sequence to determine the operations conducted at each station, including idle stations. For any given part design, numerous variations of strip layouts can be proposed. The final layout largely depends on the designer’s personal experience or opinion, and the customary practices of the press shop. Often a team of experienced and skilled individuals from various engineering and manufacturing disciplines will work together to make the determination. D o n ’ t G u e s s ; G a th e r D a t a A friend of mine frequently says, “Solutions without data are nothing more than guesses.” That same philosophy can be applied to strip layouts: Selecting a progressive-die process based on a consensus of opinions is nothing more than a group’s best guess. Financial business decisions often rely on multiple sources of data, such as financial statements, financial ratios, and forecasting and investment analysis. But mission-critical technical decisions such as the selection of a strip layout, which can affect a business’s bottom line for years, often are based on a collection of opinions. At a recent PMA METALFORM symposium on advanced high-strength steels (AHHS), I presented a talk titled, Performance-Based Die-Engineering Strategies. These strategies rely on knowledgebased systems rooted in science and mathematics, rather than on traditional experience-based systems that rely on individual and group experiences. A knowledge-based system is particularly important for processing AHSS stampings, since experience-based systems either are nonexistent or woefully insufficient. Another shortcoming of experience-based systems: They are dominated by fears, especially the fear of repeating past mistakes and the fear of the unknown. These fears work their way into our strip layouts and compromise the die design.

Let’s consider how performance-based engineering strategies can be applied to progressive-die strip layouts. In order to select the best strip layout from the several available options, the designer should compare and rank each layout using a relevant scoring system. Among the factors that influence die cost and quality, four have been proposed as most critical (by Lin and Sheu, authors of Knowledge-based Sequence Planning of Shearing Operations in Progressive Dies, International Journal of Production Research, 2010). They are: • Station number factor, Fn • Moment balancing factor, Fb • Strip stability factor, Fs • Feed height factor, Fh An evaluation score (Ev) then can be computed using these four factors and their corresponding weighting factors—wn, wb, ws, wl: Ev = (wn x Fn) + (wb x Fb) + (ws x Fs) + (wh x Fh) The designer or process engineer selects the four weighting factors (from 10 to 100) based on how much each factor contributes to the strip evaluation. A higher score indicates better efficiency in cost and production. For example, presses for progressive dies usually require large beds to accommodate multiple workstations. More stations require a wider press bed, and a longer die results in higher construction costs and increased tolerance accumulations. Station-number factor Fn determines a strip layout’s effectiveness in terms of the number of stations it includes. An Fn value of 100 (best possible) represents a minimum number of stations—two. In contrast, an Fn of 10 represents the maximum number of stations, typically the total number of punches used for cutting and bending. N – Nmin Fn = 100 – 90 x Nmax – Nmin

N = total number of stations in the strip layout; Mmax = total number of punches (cutting and bending); and Nmin = the possible minimum

number of stations, Nmin = 2 T i p p in g M o m e n t Also required is a moment-balancing factor, Fb. When two or more die stations perform their task on the die strip, the forces simultaneously act on the strip at different points. If the reaction forces are unbalanced relative to the press centerline, ram tipping occurs. The moment-balancing factor indicates how near to center the equivalent reaction forces are to the axis of the press ram. Since the center of the die usually aligns under the center of the ram, this factor considers tipping-moment severity—seldom considered in strip layouts. All stamping presses have a maximum tipping moment, established by the press builder, that represents the maximum off-center loading condition that the press can safely handle without suffering long-term damage. Designers use this rating to establish a maximum off-center loading parameter, Dmax. To calculate the moment balancing factor:

d Fb = 100 – 90 x Dmax When d = 0, the center of the ram and of the stamping loads are completely matched, so Fb = 100 (best condition). When d >Dmax, the deviation is so serious that Fb = 10 (worst condition). Similar factors can be established for strip stability and strip lift. Then the designer assigns corresponding weighting factors based on their importance to the strip layout. The resulting evaluation score (Ev) provides die and process engineers with a performance-based numerical rating to evaluate each strip layout. The evaluation score has relative meaning for different layouts producing the same part. It therefore can be used to find the best solution for that particular part.

Progressive Die design Progressive dies perform fundamental cutting and forming operations simultaneously at various stations within a die during each press stroke. This Progressive Die Design program explores the essential design variables used in part/strip development that contribute to part quality, progressive die tool maintenance, tool life, and tool cost. The Progressive Die Design Essentials segment details part orientation, part lift, the use of cams, part transport, stretch webs, ribbon strips, stock positioning using direct and indirect pilots, first station operations, piercing distribution, die punches, forming operations, idle stations, part & scrap ejection, die clearances, and more. The Lubrication Factors in Progressive Die Design segment examines the types and effective use of lubricants in progressive dies to control friction, extend tool life and improve part surface quality.

Sheet Metal Bending Bending of sheet metal is a common and vital process in manufacturing industry. Bending is the plastic deformation of the work over an axis, creating a change in the part's geometry. Similar to other metal forming processes, bending changes the shape of the work piece, while the volume of material will remain the same. In some cases bending may produce a small change in sheet thickness. For most operations, however, sheet metal bending will produce essentially no change in the thickness of the sheet metal. In addition to creating a desired geometric form, bending is also used to impart strength and stiffness to sheet metal, to change a part's moment of inertia, for cosmetic appearance and to eliminate sharp edges.

Figure:264

Bending enacts both tension and compression within the material. Mechanical principles of metals particularly with regard to elastic and plastic deformation are important to understanding bending, and are discussed in the fundamentals of metal forming section. The effect that material properties will have in response to the conditions of manufacture will be a factor in sheet metal process design. Usually sheet metal bending is performed cold but sometimes the work may be heated, to either warm or hot working temperature. Most bending operations involve a punch die type setup, although not always. There are many different punch die geometries, setups, and fixtures. Tooling can be specific to a bending process and a desired angle of bend. Bending die materials are typically gray iron, or carbon steel, but depending on the work piece, the range of punch-die materials varies from hardwood to carbides. Force for the punch and die action will usually be provided by a press. A work piece may undergo several bending processes. Sometimes it will take a series of different punch and die operations to create a single bend. Or many progressive bending operations to form a certain geometry. Sheet metal is referenced with regard to the work piece when bending processes are discussed in this section. However, many of the processes covered can also be applied to plate metal as well. References to sheet metal work pieces may often include plate. Some bending operations are specifically designed for the bending of differently shaped metal pieces, such as for cabinet handles. Tube and rod bending is also widely performed in modern manufacturing.

Bending Processes Bending processes differ in the methods they use to plastically deform the sheet or plate. Work piece material, size and thickness are an important factor when deciding on a type of bending process. Also important is the size of the bend, bend radius, angle of bend, curvature of bend, and location of bend in the work piece. Sheet metal process design should select the most effective type of bending process based on the nature of the desired bend and the work material. Many bends can be effectively formed by a variety of different processes, and available machinery will often determine the bending method. One of the most common types of sheet metal processes is V bending. The V shaped punch forces the

work into the V shaped die and hence bends it. This type of process can bend both very acute and very obtuse angles, also anything in between, including 90 degrees.

Figure:265

Edge bending is another very common process, and is performed with a wiping die. Edge bending gives a good mechanical advantage when forming a bend. However, angles greater than 90 degrees will require more complex equipment, capable of some horizontal force delivery. Also wiping die employed in edge bending must have a pressure pad. The action of the pressure pad may be controlled separately than that of the punch. Basically the pressure pad holds a section of the work in place on the die, the area for the bend is located on the edge of the die, and the rest of the work is held over space like a cantilever beam. The punch then applies force to the cantilever beam section causing the work to bend over the edge of the die.

Figure:266

Rotary bending forms the work by a similar mechanism as edge bending. However, rotary bending uses a different design than the wiping die. A cylinder with the desired angle cut out serves as the punch. The cylinder can rotate about one axis, and is securely constrained in all other degrees of motion by its attachment to the saddle. The sheet metal is placed cantilevered over the edge of the lower die similar to the setup in edge bending. Unlike in edge bending, with rotary bending there is no pressure pad. Force is transmitted to the punch causing it to close with the work. The groove on the cylinder is dimensioned to create the correctly angled bend. The groove can be less than or greater than 90 degrees allowing for a range of acute and obtuse bends. The cylinders V groove has two surfaces. One surface contacts the work transmitting pressure and holding the sheet metal in place on the lower die. As force is transmitted through the cylinder it rotates, causing the other surface to bend the work over the edge of the die, while the first surface continues to hold the work in place. Rotary bending provides a good mechanical advantage. This process provides benefits over a standard edge bending operation, in that it eliminates the need for a pressure pad and it is capable of bending over 90 degrees without any horizontally acting equipment. Rotary bending is relatively new and is gaining popularity in manufacturing industry.

Figure:267

Air bending is a simple method of creating a bend without the need for lower die geometry. The sheet metal is supported by two surfaces a certain distance apart. A punch exerts force at the correct spot, bending the sheet metal between the two surfaces.

Figure:268

Punch and die are manufactured with certain geometries in order to perform specific bends. Channel bending uses a shaped punch and die to form a sheet metal channel. A U bend is made with a U shaped punch of the correct curvature.

Figure:269

Many bending operations have been developed to produce offsets and form the sheet metal for a variety of different functions.

Figure:270

Some sheet metal bending operations involve the use of more than 2 die. Round tubes, for example, can be bent from sheet metal using a multiple action machine. The hollow tube can be seamed or welded for joining.

Figure:271

Corrugating is a type of bending process in which a symmetrical bend is produced across the width of sheet metal and at a regular interval along its entire length. A variety of shapes are used for corrugating, but they all have the same purpose, to increase the rigidity of the sheet metal and increase its resistance to bending moments. This is accomplished by a work hardening of the metal and a change in the sheet's moment of inertia, caused by the bend's geometry. Corrugated sheet metal is very useful in structural applications and is widely used in the construction industry.

Figure:272

Edge Bending Processes Sheet metal of different sizes can be bent and innumerable amount of ways at different locations, to achieve desired part geometries. One of the most important considerations in sheet metal manufacture is the condition of the sheet metal's edges, particularly with regard to the part after manufacture. Edge bending operations are commonly used in industrial sheet metal processing, and involve bending a section of the metal that is small relative to the part. These sections are located at the edges. Edge bending is used to eliminate sharp edges, to provide geometric surfaces for purposes such as joining, to protect the part, to increase stiffness, and for cosmetic appearance. Flanging is a process that bends and edge, usually to a 90 degree angle.

Figure:273

Sometimes the material is purposely subjected to tensions or compressions, in the processes of stretch flanging and shrink flanging respectively. In addition to bending the edge these operations also give it a curve.

Figure:274

Beading is common in the edge treatment of sheet metal parts, and can also be used to form the working structure of parts such as hinges. Beading forms a curl over part's edge. This bead can be formed over a straight or curved axis. There are many different techniques for forming a bead. Some methods form the bead progressively with multiple stages using several different die arrangements. Other sheet metal beading processes produce a bead with a single die. In a process called wiring the edge is bent over a wire. How the bead is formed will depend on the specific requirements of the manufacturing process, and sheet metal part.

Figure:275

Hemming is an edge bending process in which the edge of the sheet is bent completely over on itself.

Figure:276

Seaming is a sheet metal joining process. Seaming involves bending the edges of two parts over on each other. The strength of the metal resists breaking the joint, because the material is plastically deformed into position. As the bends are locked together each bend also helps resist the deformation of the other bend providing a well fortified joint structure. Double seaming has been employed to create watertight or airtight joints between sheet metal parts.

Figure:277

Roll Bending Roll bending provides a technique that is useful for relatively thick work. Although sheets of various sizes and thicknesses may be used, this is a major process for the bending of large pieces of plate. Roll bending uses three rolls to feed and bend the plate to the desired curvature. The arrangement of the rolls determines the exact bend of the work. Different curves are obtained by controlling the distance and angle between the rolls. A moveable roll provides the ability to control the curve. The work may already have some curve to it, often it will be straight. Beams, bars, and other stock metal is also bent using this process.

Figure:278

Roll Forming Roll forming is a continuous manufacturing process that uses rolls to bend a sheet metal cross section into a certain geometry. Often several rolls may be employed in series to continuously bend stock. Similar to shape rolling, but roll forming does not involve material redistribution of the work only bending. Like shape rolling, roll forming usually involves bending of the work in sequential steps. Each roll will form the sheet metal to a certain degree, in preparation for the next roll. The final roll completes the geometry. Channels of different types, gutters, siding, and panels for structural purposes are common items manufactured in mass production by roll forming. Rolls are usually fed from a sheet metal coil. The entry roll is supplied as the coil unwinds during the process. Once formed, continuous products can be cut to

desired lengths to create discrete parts. Closed sections such as squares and rectangles can be continuously bent from sheet metal coil. Frames for doors and windows are manufactured by this method. Sheet metal coil is often roll bent into thin walled pipe that is welded together at its seam. The welding of the continuous product is incorporated into the rolling process. Roll forming of channels is a continuous alternative to a discrete channel bending process, such as the one illustrated in figure 269. Figure 279 shows a simple sequence used to produce a channel.

Figure:279

This channel could be produced with a punch and die. However in that case, the length of the channel would be limited by the length of the punch and die. Roll forming allows for a continuous part, limited practically to the length of the coil, that can be cut to whatever size needed. Productivity is also increased, with the elimination of loading and unloading of die. Rolls for roll forming are typically made of grey cast iron or carbon steel. Lubrication is important and effects forces and surface finish. Sometimes rolls will be chromium plated to improve surface quality.

Mechanics Of Sheet Metal Bending To understand the mechanics of sheet bending, an understanding of the material properties, characteristics, and behaviors of metal, is necessary. Particularly important is the topic of elastic and plastic deformation of metal. Information on the properties of metals with relation to manufacturing can be found in an earlier section, fundamentals of metal forming. It should be understood also that bending produces localized plastic deformation and essentially no change in sheet thickness for most operations. It does not create metal flow that effects regions away from the bend. The force required to perform a bend is largely dependent upon the bend and the specific process, because the mechanics of each process can vary considerably. Proper lubrication is essential to controlling forces and has and effect on the process. In punch and die operations, the size of the die opening is a major factor in the force necessary to perform the bending. Increasing the size of the die

opening will decrease the necessary bending force. As the sheet metal is bent the force needed will change. Usually it is important to determine the maximum necessary bending force to access machine capacity requirements. The important factors influencing the mechanics of bending are material, sheet thickness, width over which bend occurs, radius of bend, bend angle, machinery, tooling, and specific bending process. Bending a sheet will create forces that act in the bend region and through the thickness of the sheet. The material towards the outside of the bend is in tension and the material towards the inside is in compression. Tension and compression are opposite, therefore when moving from one to the other a zero region must exist. At this zero region no forces are exerted on the material. When sheet bending, this zero region occurs along a continuous plane within the part's thickness, called the neutral axis. The location of this axis will depend on the different bending and sheet metal factors. However a generic approximation for the location of the axis could be 40 percent of sheet thickness measured from the inside of the bend. Another characteristic of the neutral axis is that because of the lack of forces the length of the neutral axis remains the same. To one side of the neutral axis the material is in tension, to the other side the material is in compression. The magnitude of the tension or compression increases with increasing distance from the axis.

Figure:280

If a relatively small amount of force is exerted on a metal part, it will deform elastically and recover its shape when the force is removed. In order for plastic deformation of metal to occur a minimum threshold of force must be reached. The force acting on the neutral axis is zero and increases with distance from this region. The minimum threshold of force required for plastic deformation is not reached until a certain distance from the neutral axis in either direction. The material between these regions is only plastically deformed due to the low magnitude of forces. These regions run parallel to, and form an elastic core around, the neutral axis.

Figure:281

When the force used to create the bend is removed the recovery of the elastic region results in the occurrence of springback. Springback is the partial recovery of the work from the bend to its geometry before the bending force was applied. The magnitude of springback depends largely on the modulus of elasticity and the yield strength of the material. Typically the results of springback will only act to increase the bend angle by a few degrees, however, all sheet metal bending processes must consider the factor of springback.

Figure:282

Methods Of Eliminating Springback Techniques have been developed in manufacturing industry that can eliminate the effects of springback. One common technique is over bending. The amount of springback is calculated and the sheet metal is over bent to a smaller bend angle than needed. Recovery of the material from springback results in a calculated increase in bend angle. This increase makes the recovered bend angle exactly what was originally planned.

Figure:283

Another method for eliminating springback is by plastically deforming the material in the bend region. Localized compressive forces between the punch and die in that area will plastically deform the elastic core preventing springback. This can be done by applying additional force through the tip of the punch after completion of bending. A technique known as bottoming or bottoming the punch.

Figure:284

Stretch forming is a technique that eliminates most of the springback in a bend. Subjecting the work to tensile stress while bending will force the elastic region to be plastically deformed. Stretch forming can not be performed for some complex bends and for very sharp angles. The amount of tension must be controlled to avoid cracking of the sheet metal. Stretch forming is a process often used in the aircraft building industry.

Figure:285

Sheet Metal Bendability Bendability of sheet metal is the characteristic degree to which a particular sheet metal part can be bent without failure. Bendability is related to the more general term of formability discussed in sheet metal basics. The bendability will change for different materials and sheet thicknesses. Also the mechanics of the process will effect bendability, since different tooling and sheet geometries will cause different force distributions. Bending tends to be a less complicated process than deep drawing in the analysis of forces acting during the operation. One simple method to quantify bendability is to bend a rectangular sheet metal specimen until it cracks on the outer surface. The radius of bend at which cracking first occurs is called the minimum bend radius. Minimum bend radius is often expressed in terms of sheet thickness, (ie. 2T, 4T). The higher the minimum bend radius, the lower the bendability. A minimum bend radius of 0 indicates that the sheet can be folded over on itself. Anisotropy of the sheet metal is an important factor in bending. If the sheet is anisotropic the bending should be performed in the preferred direction. A test to determine anisotropy is discussed in the sheet metal basics section. The condition of a sheet metal's edges will influence bendability. Often cracks may propagate from the edges. Rough edges can decrease the bendability of a sheet metal part. Cold working at the edges, or within a part can also reduce bendability. Vacancies within sheet metal can be another source of material failure while bending. The presence of vacancies will reduce bendability. Impurities in the material, particularly in the form of inclusions, can also propagate cracks and will decrease bendability. Pointed or sharply shaped inclusions are more detrimental to bendability than round inclusions. Surface quality of the sheet metal can make a difference in bending manufacture. Rough surfaces can increase the likelihood of the sheet cracking under force.

To mitigate these problems and optimize the bendability of sheet metal care should be taken all the way through the manufacturing process. High quality sheet metal comes from high quality metal. Effective refining techniques, along with a sound sheet metal rolling process should close up vacancies, break up or eliminate inclusions, and provide a sheet metal product with a smooth surface. Edge treatment such as trimming or fine blanking can improve edge quality. Sometimes cold worked areas can be machined out. Annealing the part to eliminate regions of cold working and increase ductility also improves bendability. Bending operations are sometimes performed on heated parts because heating will cause the metal's bendability to go up. Sheet metal may also on occasion be formed in a high pressure environment, which is another way to make it more bendable.

Cutting And Bending Processes Some processes involve both cutting and bending of the sheet metal. Lancing is a process that cuts and bends the sheet to create a raised geometry. Lancing may be used to increase the heat dissipation capacity of sheet metal parts, for example. Another common process that employs both cutting and bending is piercing. Not to be confused with the forging process of piercing. Piercing is used to create a hole in a sheet metal part. Unlike blanking, which creates a slug, piercing does not remove material. The punch is pointed and can pierce the sheet. As the punch widens the hole the material is bent into an internal flange for the hole. This flange may be useful for some applications.

Figure:286

Tube Bulging Tube bulging is a process in which some part of the internal geometry of a hollow tube is subjected to pressure causing the tube to bulge outward. The area being bulged is usually constrained within a die that can control its geometry. Total length of the tube will be decreased because of the widening of the bulging area. There are different bulging techniques employed in manufacturing industry. One main group of processes uses an elastomer plug, usually polyurethane. This plug is placed within the tube. Pressure is applied to the elastomer causing it to bulge. Expanding outward the plug bends the sheet metal tube. Upon removal of the force the elastomer plug returns to its original shape and can be easily removed. Polyurethane plugs are durable and will create a good pressure distribution over the surface during bending. Hydraulic pressure may also be used to produce the same bulging effect. However elastomer plugs are cleaner, easy to remove, and require less complicated tooling. Split dies are used to facilitate the removal of the part.

Figure:287

Tube Bending Tubes, rods, bars and other cross sections are also subject to bending operations. It should be remembered that when bending a metal part, springback is always a factor. Several special processes have been developed for the bending of hollow tubes. These operations can also be used on solid rods. Hollow tubes have the characteristic that they may collapse when bent. Tubes may also crack or tear, the material's ductility is important when considering tube failure. As the bend radius goes down, the tendency to collapse increases. Bend radius in tube bending is measured from the tubes centerline. The other major factor determining collapse is the wall thickness of the tube. Tubes with a greater wall thickness are less likely to collapse. Bending a thick walled tube to a large radius is usually not a problem as far as collapse is concerned. However, as wall thickness decreases and/or bend radius goes down, solutions must be found to prevent tube collapse. One solution is to fill the tube with sand before bending. Another method would be to place a plastic plug of some sort in the tube then bend it. Both the sand and the plastic plug act to provide internal structural support, greatly increasing the ability to bend the tube without collapse. Stretch bending is a process in which a tube is formed by a stretching force parallel to the tubes axis, and a simultaneous bending force acting to pull the tube over a form block. The block is fixed and the forces are applied to the ends of the tube.

Figure:288

Draw bending involves clamping the tube near its end to a rotating form block. A pressure pad is also used to hold the tube stock. As the form block rotates the tube is bent.

Figure:289

Compression bending is a tube bending process that has some similarities to edge bending of sheet metal with a wiping die. The tube stock is held by force to a fixed form block. A wiper like die applies force bending the tube over the form block.

Figure:290

Posted by Bhavani Singh Choudharyat12:05 Email ThisBlogThis!Share to TwitterShare to FacebookShare to Pinterest Labels: Sheet metal Design Guidelines

Sheet metal Design Guidelines

Bends Bends are the most typical feature of sheet metal parts and can be formed by a variety of methods and machines which negate the absolute need for some of the below tips. However for typical parts meant to be cost effective and easily produced the following tips should be useful. o o

o

o

The minumum flange length is based on the die used to bend. Consult and Air Bend Force Chart to determine typical minimum flange lengths. When multiple bends are on the same plane try and design the part so the bends all face the same direction. This will prevent the need for the operator to flip the part. This also benefits man leaf and panel benders which can only bend one direction per setup. Avoid large parts when possible, and especially large parts with small or detailed flanges. Chasing a large part through each bend can be dangerous and exhausting for an operator. This also makes you vulnerable to reduced part accuracy. Always consult a tooling profile chart when developing your part. Know the tools available in your shop or the standards if you are outsourcing production. Specialized tooling cen be very expensive.

Counterbores& Countersinks While thinner gauge sheets won’t often be countersunk there are a few guidelines to try and follow on thicker sheets to preserve the strength of the material and prevent deformation fo the features during forming. o o o o

The distance between two countersinks should be kept to at least 8 times the material thickness. To ensure strength the distance between a countersink’s edge and the edge of the material should be 4 times the material thickness. There should be at least %50 contact between the fastener and the surface of the countersink. To prevent any deformation of the hole the edge of the countersink should be at least 3 times the material thickness from the tangent point of the bend.

Curls When adding a Curl to the edge of a sheet the following guidelines will ensure that no special tooling is required. o o o

The outside radius of a curl can be no smaller than 2 times the material thickness. This will create an opening with a 1 material thickness radius. A hole should be at least the radius of the curl plus material thickness from the curl feature. A bend should be at least the radius of the curl plus 6 times the material thickness from the curl feature.

Dimples o o o o

The diameter of a dimple should be no more than 6 times the material thickness. The inside depth of a dimple should be no more than the inside radius. A hole should be at least three times material thickness away from the edge of the dimple. Or the inside radius of the dimple plus 3 times material thickness. From the part’s edge, dimples should be at least 4 times material thickness plus the radius of the dimple.

o o

From a bend, dimples should be at least 2 times material thickness plus the dimple radius plus the bend radius. From another dimple, dimples should be 4 times material thickness plus the inside radius of each dimple.

Embossments & Ribbing o o o o o o

Embossments and offsets should be measured to the same side of material unless it is necessary to hold an outside dimension. For round embossments or ribs, maximum depth is equal to the internal radius of the embossment. For flat embossments, the maximum depth is equal to the inside radius plus the outside radius. For V embossments the maximum depth is equal to 3 times material thickness. Embossments should be at least 3 times material thickness from a hole’s edge. Between two parallel ribs, minimum distance is 10 times material thickness plus the radius of the ribs.

Extruded Holes o o o

Between two extruded holes, distance should be at least 6 times material thickness. From edge to extruded hole, distance should be at least 3 times material thickness. From bend to extruded hole, distance should be 3 times material thickness plus bend radius.

Gussets Gussets are used to strengthen a flange without the need for secondary processes such as welding. While gussets will almost always require custom tooling some basic guidlines should help. Be sure to consult with your factory’s Brake Press department to learn what they are equipped to bend. o o

45° gussets shouldn’t be designed to be more than 4 times material thickness on their flat edge For holes, the distance between the gusset and the hole’s edge should be at least 8 times material thickness.

Hems Hems are used to create folds in sheet metal in order to stiffen edges and create an edge safe to touch. o o o o

For tear drop hems, the inside diameter should be equal to the material thickness. For open hems, the bend will lose its roundness when the inside diameter is greater than the material thickness. For holes, the minimum distance between the hole’s edge is 2 times the material thickness plus the hem’s radius. For bends, the minimum distance between the inside edge of the bend and the outside of the hem should be 5 times material thickness plus bend radius plus hem radius.

Holes / Slots o

Distance from outside mold line to the bottom of the cutout should be equal to the minimum flange length prescribed by the air bend force chart. o Rule Of Thumb: 2.5* Material Thickness + Bend Radius. o When using a punch press the diameter of a hole should always be equal to that of your tooling and you should never use a tool who’s diameter is less than that of the material’s thickness. o Rule Of Thumb: Never design a hole smaller than .040” Diameter unless laser cutting. o When using a punch press holes should be at least 1 material thickness from any edge. This prevents bulging along the edge.

Lances & Louvers Formed lances and louvers will almost always require specialized tooling so be sure to understand what is available to you before designing the feature. o o o o

The minimum depth of a lance should be twice the material thickness and at least .125” If the lance if formed with standard tooling be sure that the length of the bend is dividable by a standard set of Sectionalized Tooling. From a bend, lances should be at least 3 times material thickness plus bend radius, however the actual minimum is often much greater than this and driven by the tooling profile. From a hole, lances should be at least 3 time material thickness from the edge of the hole.

Notches & Reliefs o o

o o

The minimum width of a notch is equal to the material thickness and at least .04”. This is negated if the blank is being cut by a Laser System in which case the minimum is only the kerf of the laser. When determining the length of a notch it is very important to understand the tooling used to cut the notch. When possible the notch should be equal to a multiple of the punch’s length in order to prevent nibbling from occurring. From a bend, the minimum distance is 3 times material length plus the bend radius. When fabricating with a Punch Press the minimum space between two notches should be at least 2 time material thickness and at least .125”

Welding o o

Welding by hand should be restricted to gauges thicker than 20 gauge. Spot welding should be used for joining equally thick co-planar surfaces. The arm geometry and throat depth of the spot welder will be a limiting factor.

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Welded joints should be designed with as tight of tolerances as possible to remove the need for a welder to add wire. Wire material should always be the same as the material being welded.

Plating o o o o

o o

Sharp edges and corners will typically receive about twice as much as the plating material because of the current density in these areas. If possible tap and thread after plating, else assume that the material will grow up to 4 times the typical platting thickness, compensate pitch and depth accordingly. Avoid recessed areas which are difficult to reach. Because the parts are going to be hung from hooks and dipped it is beneficial to design hanging holes into your part rather than leaving the decision to the plater. These holes can be small, just enough to get a wire hook through. These holes will also give you control over how the part is positioned when it is dipped. In addition to hanging holes design drainage holes. Knowing the orientation of the part from your hanging holes make sure the part can be easily cleaned after plating. Assume all areas of the part will be plated, masking is not recommended.

SOLID DIES The required shape of opening for blanking or piercing made on a single metal block is solid die.

Split die /the die contour built up from two or more pieces. the following factors influence the design of the die. 1.Pierce part size. 2.stock thickness 3.Intricacy of piece part contour. 4.Type of tool. 5.Machinery available for manufacturing tool. Material:die blocks are made out of non shrinking tool steels. Heat treatment Hardened and tempered to 58 to 62 HRC.Nonshinking steel is selected to avoid dimensional instability.To withstand the high cutting pressure and to have wear resistance,the die block is hardened and tempered.

Design requirement: The die block thickness is influenced by the following factors: - Severity of the specific operation. - Expected tool life. - Properties of the material used in the manufacture.

General Guidelines for die block thickness for die block length upto 125 mm/125 to 200 mm/200 to 400 mm Stock material thickness in mm upto 1mm 16/20/24 1 to 2 mm 20/24/28 2 to 3 mm 24/28/32 3 to 4 mm 28/32/36 4 to 6 mm 32/36/50 6 mm and above 36/40/60 Wall Thickness For tools(approximately up to a working size of 100*100 mm)1.5 times die block thickness. For tool having larger working area than 100*100 mm - 2 times die block thickness. Screw and dowel holes Screw & dowel holes should not weaken the die. Screw and dowel hole centers should be away from the edge and opening of die.at least by 1.5d. Where d=diameter of the screw or the dowel hole. Deciding factors for solid dies/split dies 1.Size of the die block for higher size sectional construction. - Reduces cost of material. - Easiness of machining. - Reduces hardening failures. when die opening is too small for internal working sectional construction is preferred. - when the component profile is complex,split die construction makes manufacturing easy. - When die opening consists of many sharp corners split construction avoids cracking of die during hardening. Perishability Manufacture and replacement of perishable portions is easier when the die is split. Profile grinding When conventional internal grinding is not possible due to the profile feature,split construction facilitates grinding. Results of spliting Tilting of splits due to the downward thrust of cutting force. Lateral displacement due to lateral thrust. Hence effective fastening of splits is a must. Methods of fastening - Only screws and dowels are used when press force is less (this stock materials). - Nesting is a must hen lateral thrusts are more. - Nesting does not eliminate the need for use of screws and dowels. Nesting Dies sections are nested in two ways: -dies sections are nested into a pocket milled in the die set. - Seperate nest blocks.

Pocket milled in the die set The die sections should fit tightly into the pocket.Too much assembly pressure will distort the die set. Liners To facilitate the accurate and easy assembly and dismantling of sectional dies-hardened lines are used. Liners eliminate the shearing of the walls of pocket. Discrepancies in the size and position of pockets can be adjusted as the liners are the last to be fitted. Knoc out are to be provided in the die pocket right under the liners to facilitate removal of liners. Nest blocks Separate nest blocks are preferred to direct pocketing for the following resons: -Pockets weaken the die sets,nest blocks do not. - Easy to handle. - Regrinding is possible without dismantling the die. Die bushes Hardeneddie bushes in mild steel retainer plates are used in large piercing dies. Carbide diesTungsten carbide is used as die material for blanking,piercing,trimming,forming and drawing operations. Carbide dies are used when the production rates are high and the parts have close tolerances.The principles of design are similar to those of steel dies. The carbide die insert is subjected to high impact load.The inserts should be supported externally by pressing or shrinking the carbide ring into a hardened steel case. STOCK MATERIAL ( STRIP MATERIAL)

The material out of which stampings are made is known as stock materials stampings can be made from metallic or non metallic materials. Metallic materials include ferrous and non ferrous metals. Ferrous metals - hot rolled steels - cold rolled steels - stainless steels - spring steels Non ferrous metal - copper - Brass - bronze - aluminium - tin - zinc etc. Non metallic materials - plastic - rubber - wood - cloth - paper etc.

Stock strip Stock strips are fed into the tool.They are advanced through the required advance distance at each press stroke for a series of repetitive operations. Unit stock Stock materials which are fed individually into the tool for processing are called unit stock. Piece part A piece part is a product of a tool.It may be - a complete product in itself. - one component of a product. Ferrous metals Hot rolled steel sheets They are used for manufacturing where scaling and discolouration are not objectionable.The surfaces are painted after operation(if required). Pickled and oiled sheets Immersing hot rolled in acid solutions results in smooth clean scale free surface.oiling protects the surface against rusting.Pickled and oiled sheets are used in the manufacture of parts for household appliances,automobileparts,toysetc.The sheets can take long lasting painting due to the absence of scales. medium carbon steels These are hot rolled steels having 0.4% to 0.5 % carbon.They are hard,tough and resistant to abrasion. Cold rolled sheets They have a smooth deoxidisedfinish.This provides excellent base for paint lacqur or enamel coating.The thickness of the sheet is uniform.They are available in six gades of hardness. Hard Hard sheets and strips cannot bend in either direction of the grain without cracks or fracture.Such sheets are used for producing flat blanks that require resistance to bending and wear. Three quarter hard Three quarter hard strips can be bend to an angle 60 degree from flat only across the grain. Half hard Half hard steel strips can be bent to 90 degree across the grain. Quarter hard This can be bent over flat (180) across right angle along the grain. Soft Soft grades of steel can be bent over flat(180 degree) both across and along the grain direction. Dead soft This grade of steel is used for severe forming and drawing operations. Deep drawing steel sheets They are cold rolled low carbon steels.They are throughlyannealed,deoxidised and oiled.Deep drawing sheets are used for difficult drawing,spinning and forming operations. Silicon steel Silicon steel is used for electrical laminations. Stainless steel They are used where corrosion resistance is a requirement. Non ferrous metals Copper and its alloys are widely used as a stock material.They are good conductors of heat electicity and also are highly non-corrosive. Copper alloys include Beryllium copper

Red brass Low brass Cartridge brass Yellow brass Muntz metal Phospher bronze Othernon ferrous metals used are - aluminium and aluminium alloys. - magnesium and magnesium alloys. Rare metals Rare metals like zirconium,tantalum,vanadium,tungstun and molybdenum and their alloys are used press working. Precious metals Precious alloys like that gold silver,platinumetc are used for manufacturing laboratory equipment and electrical industry. Clad metals It consists of a core of one metal and a covering layer of dissimilar metal. Preparation of stock In steel mills the metal is formed into large sheets by rolling.The sheets are cut into strips in a sheraingmachine.Slitting machines are also used to cut the sheets.