Ver2 Forming

IE 217 Manufacturing Processes

Dr. J. Cecil FORMING PROCESSES

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Characteristics of sheet metal forming processes

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Sheet Metal Operations - Stamping • One of several sheet metal operations • Includes many operations such as punching, blanking, bending, coining, etc • Simple or complex shapes formed at high production rates • Tooling and equipment costs can be high • Labor cost is low

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Overview of Contents in this module • Shearing, Stamping and Bending

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Shearing • Before a sheet metal part is made, a blank is first removed from a large sheet by shearing • Viz, the sheet is cut by subjecting it to shear stresses between a punch and a die (see figure 16.2 a) • What is sheared is normally called a slug

Figure 16.2 a

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Shearing (c)

FIGURE 16.2: (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) slug. Note that the scales of the two figures are different. 6

Shearing Process • Shearing starts with the formation of cracks on both the top & bottom edges of the workpiece (Fig 16.2 --A&B, C&D regions) • When these cracks meet each other, separation occurs • Major parameters in shearing are: – – – –

Shape and materials for punch and die Punching speed Lubrication Clearance c between punch and die

• Clearance is major factor in determining the shape and quality of sheared edge • As c increases, material tends to be pulled rather sheared – Also, sheared edge becomes rougher and zone of deformation becomes larger 7

Shearing process

• The width of deformation zone depends on punch speed (figure 16.3 above) • With increasing speed, the heat generated by the plastic deformation is confined to a smaller and smaller zone • A Burr is a thin edge or ridge (see fig 16.2 b, c) • Burr height increases with increase in clearance and ductility 8

Burrs • Dull tool edges contribute greatly to burr formation • Height, shape and size of burrs can significantly affect subsequent forming operations • Edge quality can improve with increasing punch speed – Speeds high as 10-12 m /s (33-39 ft/s)

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Punch Force • The force required to punch is the product of the shear strength of the sheet metal area and the area being sheared • Friction between the punch and workpiece can however increase this force substantially. • The maximum punch force F can be estimated from the equation F = 0.7 T L *(UTS) Where T – sheet thickness, L – total length sheared (such as perimeter of hole) UTS – ultimate tensile strength of material 10

Force calculation • As clearance increases, the punch force decreases and the wear on the dies and punches is also reduced • A second force is also required to strip the punch from the sheet during its return stroke • Example: – Estimate the force required to punch a 1 inch (25 mm) dia hole through a 1/8 inch thick (3.2 mm) annealed titanium alloy Ti-6t 6A1-4V sheet at room temperature. – Using eqn 16.1 from book. UTS (table 6.10) for this alloy is 1000 Mpa or 140,000 psi. – F = 0.7 * (1/8) ? * (1)* (140,000) = 38,500 lbs 11

Operations • Punching: The sheared slug is discarded

• Blanking: The slug is the part and rest is scrap • Die Cutting • Shearing process consisting of – Perforating: punching a number of holes in a sheet – Parting: shearing the sheet into two or more pieces – Notching: removing pieces from the edges – Lancing: leaving a tab without removing any material 12

Operations • Die cutting: – Parts produced from these operations are used usually in assembly with other components – Eg: perforated sheet metals with hole diameters ranging from around 1mm to 75 mm can be used as • Filters, screen, machinery guards, for weight reduction etc.

– Punched using crank presses at rates as high as 300,000 holes per minute (using special dies and equipment)

• Fine Blanking: – Very smooth and square edges can be produced (fig 16.5a) – A V-shaped stinger (or impingement) locks the sheet tightly in place and prevents distortion (such as in fig 16.2b and 16.3) 13

One set-up for fine blanking

FIGURE 16.5: (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. Source: Feintool U.S.Operations. 14

Operations • Fine blanking involves clearances on the order of 1% of sheet thickness – – – –

Ranges from 0.5 mm to 13 mm (0.02 to 0.5 in) Carried out on triple action hydraulic presses Motion of punch, pressure pad and die are separately controlled Usually for parts having holes punched simultaneously with its blanking

• Shearing Die Features: – – – –

Punch and Die shapes (beveling) Compound Dies Progressive Dies Transfer Dies 15

Types of Shearing Dies • Punch and Die Shapes: • In fig 16.2 a, punch and die surface are both flat – Punch force builds up rapidly during shearing as the entire thickness is sheared at the same time – Location of the regions being sheared at any moment can be controlled by beveling the punch and die surfaces

FIGURE 16.10 Example of the use of shear angles on punches and dies 16

Punch and Die Shapes …contd • See Fig 16.10 • Geometry of bevel is similar to that of the paper punch • Beveling is suitable for shearing of thick blanks as it – reduces the force at the beginning of the stroke – Reduces the operation’s noise level

• Fig 16.10 c and d, the punch tip is symmetric – No lateral force acting on punch

• 16.10 b has a single taper, so there is a lateral force acting on punch – So these need rigid punch and press to avoid producing a hole in wrong position and avoid hitting the edge of lower die (B & D in fig 16.2 a)

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Punching applications • See figure below of satellite shell • Using punching, holes are created to reduce the weight of the satellite

Punched holes 18

Compound Dies • Using compound dies, Several operations on the same strip of metal can be performed in one stroke at one station (fig 16.11 a and b)

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Compound Dies FIGURE 16.11: Schematic illustrations: (a) before and (b) after blanking a common washer in a compound die. Note the separate movements of the die (for blanki ng) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed.

(d)

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Compound Dies • Combined operations are usually limited to relatively simple shapes – Relatively slower and more expensive than those used for individual operations

• See fig 16.11 a and b for before and after blanking a common washer in a compound die • Note separate movements of die (for blanking) and the punch (for punching hole in the washer)

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Progressive Dies • Using these dies, Parts requiring multiple operations, such as punching, blanking, notching at high production rates can be achieved • Sheet metal is fed through as a coil strip and a different operation is performed at the same station with each stroke of a series of punches (see fig 16.11 c) • Example of part made using such a die is shown in figure 16.11d

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Transfer Dies • Here, the sheet metal undergoes different operations at different stations • These stations can be linear or circular path • Part moves from one station to another

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BENDING • • • • •

One of the most common forming operations Eg: parts in a car, plane; paper clips,etc Bending also imparts stiffness to a part In bending, the outer region of the material is in tension, While inner region is in compression

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BENDING • Common Bending Operations – – – – – – – – –

Press Brake forming Roll Bending Beading Flanging Dimpling Roll Forming Deep Drawing Rubber Forming Explosive Forming

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Press Brake Forming • Sheet metal can be bent easily with simple fixtures using a press • Sheets 7m (20 ft) or longer and other narrow pieces can be bent using a press brake (fig 16.23) • This m/c uses long dies in a mechanical or hydraulic press and is suitable for small production runs • Tooling is simple and adaptable to a wide variety of shapes • Process can be automated • Die materials for these m/c’s can be hardwood to carbides; generally, carbon steel or gray iron is used 26

Press brake

FIGURE 16.23: (a) through (e) Schematic illustrations of various bending operations in a press break. (f) Schematic illustration of a press brake. Source: Verson Allsteel company 27

Roll Bending

• Here the plates are bent using a set of rolls • By adjusting the distance between the 3 rolls, various curvatures are obtained 28

Bending in a 4 slide machine • Bending of relatively short pieces can also be done on machines such as in fig 16.22 c below

• Machines are available in a variety of designs • To form part to desired shapes, – Lateral movement of the dies can be controlled and synchronized with the vertical movement of the die 29

Beading • Here, the periphery of the sheet metal is bent into the cavity of a die (fig 16.24 a, b)

• The bead imparts stiffness to the part by increasing the moment of inertia of that section • This improves the appearance of the part and eliminates exposed sharp edges 30

Flanging and Dimpling • Flanging: Process of bending the edges of sheet metals usually to 90 degrees (fig 16.25 a) • Dimpling: fig 16.25 b – First a hole is punched – Then hole is expanded into a flange – In fig 16.25 c, a shaped punch is shown which can be used for piercing

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Flanging (Fig 16.25)

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Hemming • Hemming (also called flattening) involves folding the edge over itself – This improves stiffness, improves appearance and eliminates sharp edges

• Seaming involves joining two edges of sheet metal by hemming

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Roll Forming • This process is used for forming continuous lengths of sheet metal and for large production runs (fig 16.26) • Also called contour roll forming or cold roll forming • Products made include channels, gutters, panels, etc • Sheet thickness around 0.005 inch to 0.75 inch • Design and sequencing of rolls requires significant experience – Tolerance, tearing, buckling of strip have to be considered – Rolls made of carbon steel or gray iron and chromium plated (to improve surface finish of product and wear resistance of rolls) – Lubricants used (for roll life, surface finish and to cool rolls and product) 34

Roll forming (fig 16.26)

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Deep Drawing • Here, a round sheet metal is placed over a circular die opening and is held in place with a blankholder or hold down ring (fig 16.32 b) • Punch travels downward and forces the blank into the die cavity, forming a cup

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Deep drawing (fig 16.32)

FIGURE 16.32: (a) Schematic illustration of the deep-drawing on a circular sheet-metal blank. The stripper ring facilitates the removal of the formed cup from the punch. (b) Process variable in deep drawing. Except for the punch force, F, all the parameters indicated in the figure are independent variables.

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Deep drawing …contd • During drawing, when the blank moves into the die, compressive circumferential stresses are induced in the flange – This causes flange to wrinkle – Eg: try forcing a circular sheet of paper into a drinking glass

• Failure results from thinning of the cup wall under longitudinal stresses

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Deep drawing Practice and guidelines • Guidelines for deep drawing – Blank holder pressure is generally chosen as 0.7 to 1% of the sum of the yield strength and the ultimate tensile strength of the sheet metal – Too high a blankholder force increases punch force and causes the cup wall to tear – If blankholder force is too low, wrinkling will occur

• Draw Beads: – Are used often control the flow of blank into the die cavity – They increase the force required to pull the sheet into the die – Reduce the required blank holder forces as a beaded sheet has a lower tendency to wrinkle 39

Deep drawing Practice and guidelines • Drawbead diameters range from 0.5 to 0.75 inch – Large ones for automotive panel stampings

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Embossing • Moderate draws made with female and male matching dies

• Enables stiffening and for decoration • Eg: Car license plates

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Equipment for drawing • Most common materials for tools and dies are cast irons and tool steels (carbides and plastics can be used) • A double action hydraulic or mechanical press is widely used • Punch speed range is 20 ft/min and 60 ft/min • Modern production facilities are highly automated – Single plant can produce upto 100,000 automotive oil filter cans per day – Robots part handlers, lubricant sprayers and inspection systems work together

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Rubber Forming • In processes discussed so far, dies are generally made of solid materials • In rubber forming, one of the dies in a set can be made of a flexible material (such as polyurethane membrane) • Polyurethane can be used due to: – Their resistance to abrasion – Resistance to cutting by burrs or by sharp edges on sheet metal – Their long fatigue life

• In bending and embossing of sheet metal, female dies are replaced by a rubber pad • The outer surface of sheet is protected from damage or scratches as it is not in contact with a hard metal surface 43

Rubber forming • Pressures in rubber forming around 1500 psi (10 MPa)

FIGURE 16.38: Examples of the bending and the embossing of the sheet metal with a metal punch and with a flexible pad serving as the female die. Source: Polyurethene products Corperation. 44

Rubber forming ..contd • When selected properly, rubber forming has the following advantages: – – – – –

Low tooling cost Flexibility and ease of operation Low die wear Surface damage of sheet can be avoided Capability to form complex shapes

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Explosive and other forming processes • Explosive forming: • By controlling the quantity and shape of explosives, they can be used as a source of energy for metal forming • First used in early 1900’s – – – – – –

Typically, the sheet metal blank is clamped over a die the entire assembly lowered into a water tank (fig 16.44 a) Air in die cavity is evacuated Explosive charge place at certain height and detonated Rapid conversion of charge into gas creates a shock wave The pressure of this wave is enough to form sheet metals

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Explosive Forming

FIGURE 16.44: (a) Schematic illustration of the explosive forming process. (b) Illustration of the confined method of explosive bulging of the tubes. 47

Explosive forming • Peak pressure P (due to explosion) is – P = K * W (a/3) * R (1/a) – – – –

P is in PSI K is a constant based on explosive type (for TNT, 21,600) W is weight of explosive in pounds R is distance of explosive from workpiece surface (or Standoff) in feet – a is a constant taken to be usually 1.15

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Explosive forming ..contd • Variety of shapes can be formed by this process • Provided material is ductile at the high rates of deformation • Explosive forming is versatile – – – –

No size limit of workpiece Suited for low quantity production runs of large parts Eg: aerospace applications Steel tubes 1inch thick and 12 ft in dia have been formed

• Dies may be made of steel, aluminum alloys, zinc alloys, plastics or composite materials 49

Equipment for Sheet Metal Forming • Be prepared for quiz and exam questions • Sample questions: – What are the various components in a punching operation? – Give examples of parts you use in daily life which can be made using operations discussed in this module – How do you estimate the punch force? – Mention the Benefits of rubber forming

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