Chapter 02 - Cutting Processes (essential).pdf

CHAPTER 02 CUTTING PROCESSES 1

INTRODUCTION TO TESTING

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CLASSIFICATION OF TESTING

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CLASSIFICATION OF TESTING

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CLASSIFICATION OF TESTING

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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CLASSIFICATION OF TEST EQUIPMENT

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MEASURING INSTRUMENTS

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28.7 mm

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Accuracy = 0.1 mm or 100 μm

Accuracy = 0.05 mm or 50 μm

Accuracy = 0.02 mm or 20 μm

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PRACTICAL TEST • Ask the students to bring one workpiece (their bench work exercise), one ruler and one vernier caliper each then practice measuring their workpiece and write down on the paper comparing between ruler and vernier caliper the measurement result. • Then give his workpiece to classmate to measure.

• Finally exchange the result with their classmate and discuss. 25

DISCUSSION 01 1. How test is classified? 2. What is subjective testing and what is objective testing? 3. What is the difference between measuring and gauging?

4. What are the cause of measurement error? 5. Explain the method of indirect measurement by caliper?

6. What are the units of length? 7. Reading the following vernier illustration on the next page:

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MARKING WORKPIECES

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MARKING WORKPIECES

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MARKING WORKPIECES

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MARKING WORKPIECES

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MARKING WORKPIECES

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MARKING WORKPIECES

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DISCUSSION 02 1. What are the unit of angle?

2. Bring one workpiece from workshop and let the student measuring by micrometer and spell out its dimensions? 3. Bring one workpiece from workshop and let the student measuring the squareness by steel square (90O angle) and spell out? 4. What is a divider?

5. What can we do with center punch? 6. Reading the following illustration on the next page: 58

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CUTTING • Cutting is a production process, in which the shape of a solid body is changed by overcoming the cohesive force of the material at a specific point (point of cutting). • 2 types of cutting:

1. Cutting by dividing: consists of serving individual parts off a workpiece without chip formation, e.g., knife-edge cutting with pliers, chisels in the vertical position and cutting with shears. 2. Cutting by chip forming: consists of removing material particles (chips) mechanically (planning, filing) with tools whose cutters are precisely shaped, or with tools whose cutters do not have a precise geometrical form (grinding). 60

CUTTING

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WEDGE SHAPE • Wedge shape: What is common to all cutters is the wedge shape. 1. Rake angle (wedge angle)

2. Cutting edge 3. Wedge surface

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WEDGE SHAPE • Cutting effect of the wedge shape: • The force exerted on the cutting tool first acts at the cutting edge. When the force is large enough, the cohesion of the material is overcome, and the workpiece in indented. • The material of the workpiece distributes some of this force in the direction of least resistance and this results in the formation of a bulge on both sides of cutting edge. • When the wedge penetrates further, large lateral cutting forces develop, which widen the notch and lead to crack formation. The lateral forces finally lead to a sudden splitting of material. 63

WEDGE SHAPE • Resolution of forces acting through the cutting wedge shape: • The Force F exerted on the wedge resolves into two lateral forces, F1 and F2, when the wedge penetrates the material. • These cutting forces depend upon the wedge angle Beta. The narrower the wedge is, the greater the cutting forces. 64

CUTTING BY DIVIDING • Cutting by dividing: consists of serving individual parts off a workpiece without chip formation. • We divide cutting by dividing into 3 main categories: 1. Cutting with wedge such as chisel, tube cutter, end cutting pliers, knife-edge pliers, double action pliers and bolt clipper. 2. Cutting with shear such as straight shears ,through-cut shear, bent shears (piercing shears), figuring shears, portable electric plate shears and cutting machines. 3. Form cutting such as punching tools without guidance, punching tools with plate guides, sequential (follow die) punching tool. 65

CUTTING BY DIVIDING 1. Cutting with wedge: • Wedge cutting consists of cutting the workpiece with one or two wedgeshaped cutters, in which process the workpiece in disrupted. • Hard and strong materials necessitate the use of large wedge angles and large forces, soft materials of lower strength allow the use of small wedge angles and small forces. • The material of the cutting wedge must always be harder than that of the piece being cut.

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CUTTING BY DIVIDING • Single-edged and two-edged wedge cutting tools: • All types of chisels belong to the single-edged cutting tools as well as punches and tube cutter. • All long-shaped cutting tools, such as end cutting pliers, knife-edge pliers, double action pliers and bolt clipper, belong to the group of twoedged wedge cutting tools.

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CUTTING BY DIVIDING 2. Cutting with shear: • Shear cutting consists of dividing a workpiece by means of a two-edged wedge cutting tool.

• The shears-in which the two cutting edges cross each other in their movement.

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CUTTING BY DIVIDING • Wedge angle, clearance angle and play of the blades:

• The upper and lower blades of the shears have cutting wedges. • Wedge angle β: is between 750 to 850. so that the cutting wedges do not rub against the shear surface of the workpiece. • Clearance angle α: is about 20. • A blade play: of 1/10 to 1/20 of the material thickness ensures that the blades do not damage each other. If the blade play is too large, the material is squeezed between the cutting wedges, damaging the shears and giving rise to a burr on the workpiece. 69

CUTTING BY DIVIDING • Lever effect and cutting force: • The rotary moment of M2 = F2 x l2 at the cutting edge (cutting force times the distance to the fulcrum).

• Clockwise moment = anticlockwise moment (F1 x l1 = F2 x l2)

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CUTTING BY DIVIDING • Example: Calculate the cutting force. • An aluminum sheet poses a resistance of 300N when cut with hand shears. When a force F1 = 60N is applied the sheet is cut at point l2=3cm but can not cut at l2=6cm.

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CUTTING BY DIVIDING • Rake angle of the shear blade: • An opening angle of 140 is the most favorable; if the rake angle is greater, the workpiece slides out of the shears. If it is smaller, the cutting cross-section and, therefore, the cutting force required, become too large.

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CUTTING BY DIVIDING • Working with hand operated, plate shears: • Straight shears are used for straight short cuts when the sheet is not too curved. • Through-cut shear are used for cutting long straight pieces. • Bent shears (piercing shears) have pointed cutting blades to cut out inside shapes. • Figuring shears have narrow blades, which make it easy to cut out complicated shapes.

• Portable electric plate shears. • All hand shears are available in left-hand and right-hand models. 73

CUTTING BY DIVIDING

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CUTTING BY DIVIDING • Cutting machines:

• Cutting machines are built for hand operation as well as with mechanical drives. • Lever shears possess a fixed lower blade and a moving upper blade. The sheet being cut is prevented from tilting by a clamping device which can be adjusted to the thickness of the sheet. • Lever board shears are used to cut long thin strips of sheet metall. The upper shear blade is curved in order to maintain a constant shear angle of 140. • Circle cutting and curve-cutting machines are used to cut circles and curves of the desired shapes. 75

CUTTING BY DIVIDING

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CUTTING BY DIVIDING • Evaluating the Quality of Shearing:

• Precaution:

– Marked ridge formation at the cutting point.

– Always handle sharp-edged sheets with gloves.

– Too much force has been applied.

– Lift up lever arm and hook in after using the lever shears.

– Blades are notched. – Cut deviates from marked line.

– Clamp down the sheet firmly with the suppressor. – Do not increase the turning moment of the lever by lengthening the lever arm. – Do note overload the shears.

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CUTTING BY DIVIDING 3. Form cutting: • Any desired shape can be cut out of sheet metal by using cutting tools consisting of punches (top dies) and cutting plates (bottom dies, negative dies).

• This process is particularly suited to production, since the cutting tool must be specifics for each shape.

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CUTTING BY DIVIDING • Punching tools without guidance: • Used to cut round discs and simple shapes from thin sheets. • Punching tools with plate guides: • A guide plate guides the punch precisely. It pulls the material off the punch after cutting.

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CUTTING BY DIVIDING • Sequential (follow punching tool:

die)

• This combines several punches in succession in one tool.

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DISCUSSION 03 1. How cutting is classified? 2. Why should the play between the blades of shears not be too great? 3. Mention the characteristics of wedge shape? 4. What is cutting by dividing? How many types of it? 5. Pipe cutter is classified into which category?

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CUTTING BY HAND CHIPPING • Cutting by hand chipping: • Consists of removing material particles (chips) mechanically with tools. A chip forms as a result of the forward movement of the cutter parallel to the surface of the workpiece. • We divide cutting by hand chipping into 6 main categories: 1. Chiselling 2. Sawing

3. Filling 4. Scraping 5. Reaming

6. Threading 82

CUTTING BY HAND CHIPPING • Structure and cutting effect of the cutter: • Wedge angle β: Angle between the cutting face and the flank. The harder the material to be cut the larger the wedge angle. • Clearance angle α: Angle between the flank of the tool and the surface being cut. Soft materials require a larger clearance angle because they generate more heat and friction. • Rake angle γ: Influences the chip formation. It is between the cutting face and the plane perpendicular to the cut surface. 83

CUTTING BY HAND CHIPPING • Small rake angle (γ=0o…8o): The material is first compressed strongly in front of the chip surface. When the material is hard, a so called leading crack forms in front of the cutter. the particles of the material are then torn off and form a brittle tear chip. • Large rake angle: The material is compressed only slightly. The chip does not break up but is continuous. Because the change of load on the cutter is slight, the worked surface is smooth. 84

CUTTING BY HAND CHIPPING • Effects of positive and negative rake angle: • Positive rake angles: – Make the tool more sharp and pointed. This reduces the strength of the tool, as the small included angle in the tip may cause it to chip away. – Reduce cutting forces and power requirements. – Helps in the formation of continuous chips in ductile materials. – Can help avoid the formation of a built-up edge. 85

CUTTING BY HAND CHIPPING • Negative rake angles: – Make the tool more blunt, increasing the strength of the cutting edge. – Increase the cutting forces. – Can increase friction, resulting in higher temperatures. – Can improve surface finish.

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CUTTING BY HAND CHIPPING • Relation between β, α and γ: • The clearance and wedge angles always have positive values. • The rake angle may have a negative value. • Clearance angle + wedge angle + rake angle = 90o. – When γ is positive: α + β + γ = 90o

– When γ in negative: α + β - γ = 90o

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CUTTING BY HAND CHIPPING 1. Chiselling: • In chiselling, the cutting edge of the tool is driven into material by impact. • The parts of the chisel are known as the head, the shank and the cutting edge. • The chisel must be harder than the piece being worked. Most chisels are made of alloyed tool steels. • Chisels are used for dividing and chipping. 88

CUTTING BY HAND CHIPPING • Suitable wedge angles of the chisel cutter are: – Aluminum and soft Al-alloys β = 30o to 40o, β = 50o to 60o – Unhardened carbon steel, grey cast iron, cast steel β = 65o to 70o

– Alloyed steels, chilled cast iron β = 75o to 85o

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CUTTING BY HAND CHIPPING • Types of chisels: 1.

Flat chisel: Commonest sort of chisel. For surface finishing, cutting and deburring.

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Drift punch: To drive out webs between bore holes.

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Hewing chisel: The cutting edge is curved. Used to carve out curves and circles from the sheet metal.

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Cross head chisel: The cutting edge and the breadth of the shank form a cross. Used to carve out grooves.

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Groove chisel: To cut out lubricating grooves in bearing shells.

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CUTTING BY HAND CHIPPING • The hammer as a striking tool: • The energy required is acquired by the hammer from its mass (m) and the velocity (v) imported to it by the hand or the arm.

• A doubled mass yields twice the energy and a doubled velocity four times the energy. 92

CUTTING BY HAND CHIPPING • Chiselling work: • The type of chisel is chosen in accordance with the work to be done. • The chisel must be struck on the center of the head, in the direction of the axis of the chisel.

• The workpiece must be clamped when chiselling. • Precaution: • A bead, which is hard and brittle, is formed on the head of the chisel as a result of cold hardening. It must be removed by grinding. • Flying chips can cause injury to you and your workmates. Use protective goggles when chiselling. • Replace damaged hammer shafts promptly. • Observe directions regarding accident prevention. 93

CUTTING BY HAND CHIPPING 2. Sawing: • Several cutting wedges work successively in the saw. Saws are used for cutting and scoring slits and grooves.

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CUTTING BY HAND CHIPPING • Relationship between tooth division and material:

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• Video : How to use handsaw in a proper way? • https://www.youtube.com/watch?v=G0NGoPF3Q6A

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CUTTING BY HAND CHIPPING • Sawing machines for metals: • Power hacksaw: used for many purposes. It works by drawing. The saw blade is lifted up by an eccentric cam on its return stroke.

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CUTTING BY HAND CHIPPING • Circular saw: Used to cut thick or broad workpieces. It uses segmental saw blades or solid steel saw blades. – Solid steel saw blades are used to cut thin-walled steel pipes and nonferrous metals. The saw blade is set or hollow ground.

– A segment saw blade used to cut steel and casting materials.

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CUTTING BY HAND CHIPPING • Bandsaw machine: An endless saw blade runs over two rollers.

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CUTTING BY HAND CHIPPING • Cutting machine: • Uses steel blade, some machine uses abrasive blade to cut materials.

• Abrasive blades can cut materials such as brick and concrete. Some abrasive blades are suitable for metalcutting applications. Like the diamond blades, they don't have teeth. They cut with an abrasive material such as aluminum oxide or silicon carbide. 102

CUTTING BY HAND CHIPPING • Precaution: • The risk of accidents with sawing machines is very great. • Bandsaw blades must be enclosed and circular saw blades must be covered. • Injuries often occur when sawing through a workpiece.

• Blades which have been clamped too tightly may break. Examine the saw blade for cracks. 103

DISCUSSION 04 1. What is cutting by hand chipping? How many types of it?

2. How are the following terms related: rake angle, clearance angle, wedge angle? 3. Compare the chipping effects of a positive and a negative rake angle. 4. Compare the favorable wedge angle for working a) cast steel, b) copper. 5. What can we do to make sure that the saw can cut freely? 104

CUTTING BY HAND CHIPPING 3. Filling: • Files are toothed tools.

chip-forming

• When using a file, several cutting wedges always act at the same time. • The resistance of the material to cutting must be overcome by motive power of the hand. 105

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CUTTING BY HAND CHIPPING • Cut of files: – Single cut files: Used for working soft materials such as lead, aluminum, zinc, tin and copper, chips do not clog these so easily. – Rasps: have separate conical teeth and 7 to 28 notches per cm2 of cut surface. These are used for working wood, leather and hard board.

– Cross-cut files: Are used to work with harder materials such as steel, castings and non-ferrous alloys. 107

CUTTING BY HAND CHIPPING • Cut count and cut number of crosscut files: • The number of cuts (notches) per cm of file length in the direction of the axis is known as the cut count. • A cut file of any particular length will be available in four different cut counts, designated by cut numbers 1 to 4.

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CUTTING BY HAND CHIPPING • Classification and nomenclature: • The shapes of cross-sections are designated by alphabets from A to H. Codes indicate which side of the file has been cut.

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CUTTING BY HAND CHIPPING • Special files: • Include circular files and rotating shank files also known as milling files or rotary files. • In choosing the correct file, the following rule is maintained: Hard material-fine tooth spacing and low r.p.m. Soft material-coarse tooth division and higher r.p.m. 110

CUTTING BY HAND CHIPPING • Working with files: • The speed of work and surface finish depend upon the correct choice of file for the work at hand.

• File with low cut counts remove more material and thus reduce the working time, but produce marked filling grooves.

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CUTTING BY HAND CHIPPING • Clamping workpieces: • Clamp all workpieces as close as possible to the centers of the jaws of the vise. • Video: How to use file correctly? • https://www.youtube.com/watch?v =32FUysj-Lu4 • https://www.youtube.com/watch?v =WoOrSna7QBg 112

CUTTING BY HAND CHIPPING • Precaution: • Never work with a file which does not have a grip. • Ensure that the grip is firmly seated, that it is of the right dimensions and that it is not split. • Ensure that the lighting is correct and sufficient.

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CUTTING BY HAND CHIPPING 4. Scraping: • Scraping are production processes for micro-finishing workpieces manually, in order to improve the surface quality and dimensional accuracy. • Grooves always remain on a surface which has been filled, planed or milled. When grooved workpiece surfaced slide against each other, the friction between them is high. This causes the surfaces to wear rapidly. Groove peaks are removed by scraping. Scraped surface can be sealed perfectly. 114

CUTTING BY HAND CHIPPING • Scrapers: • The scraper removes extremely fine chips. In order to accomplish this, the tool is inclined against the workpiece when shaving. This result in a negative rake angle.

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CUTTING BY HAND CHIPPING • Video: (How to scrap the workpiece?) • https://www.youtube.com/watch?v =TFCVO0fXW1k • https://www.youtube.com/watch?v =esAqz6bCVyQ

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CUTTING BY HAND CHIPPING 5. Reaming:

• Cylindrical or conical drill-holes are to improve their inner surface quality and the accuracy of their shape and size. • Cylindrical and conical pins can then be sealed exactly and screws or bolts fitted easily with very little play. • The hole is drilled undersize (0.2 … 0.3mm) smaller than the nominal reaming dimension.

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CUTTING BY HAND CHIPPING • Reamers: • Straight fluted reamers are used for through holes and blind holes for short depth. • Spiral-fluted reamers are used for hole of greater depth. • Reamers with clockwise fluting are used only for deep blind holes. • Reaming can be carried out with manual reamers or with machine reamers. • Visually, you can distinguish the manual reamers by the square end of the tool. • Reamers have an irregular tooth pitch. This prevents vibration and rattling of the tool during reaming. 118

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CUTTING BY HAND CHIPPING • Operation of reamers:

• Reamers take off material at the circumference of the cylindrical bore hole. This happens when they are turned with the help of a tap wrench with little pressure from above in clockwise direction in the bore hole. • Straight and spiral-fluted reamers cannot carry off any chips during the process, the chips remain in the chip grooves filling them after a short time.

• Especially with longer bores, reamers must be taken out and cleaned during the reaming process, otherwise they will become jammed and break.

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CUTTING BY HAND CHIPPING • Note: • Do not ream dry but use cutting oil to increase the service life of the reamer and to increase the surface finish of reaming. • Never turn the reamer backwards, not even to pull out of the hole after reaming.

• Video: (How to hand ream a workpiece?) • https://www.youtube.com/watch?v=0nZvhsggJBA • https://www.youtube.com/watch?v=Yi2YSkVzb7s 122

DISCUSSION 05 1. How many types of cuts of file? What are they used for?

2. What is scraping process? 3. What is reaming process?

4. How many type of reamers? 5. For reaming a hole, the hole sized should be: 1. Oversize of the ream diameter 2. Undersize of the ream diameter 3. The same size 123

CUTTING BY HAND CHIPPING 6. Threading: The thread is a groove that runs along the helix around a cylinder. Threading is the process of producing threads on cylinder internally (1) or externally (II).

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• Metric ISO thread profiles:

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• Whitworth thread profiles:

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• Other threads profiles:

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CUTTING BY HAND CHIPPING I.

Cutting internal threads (tapping):

• The process of cutting internal threads using taps is called tapping. • A set of hand taps (taper tap, plug tap, bottoming tap with different thread profiles and starting taper lengths) is used to cut internal thread in blind holes or open (through) holes.

• The quantity of material to be chipped away is thus distributed over 3 taps, which results in a clean thread and saves the individual tool from being overworked.

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CUTTING BY HAND CHIPPING • Taps: • Taps are tools for the production of internal threads. By itself, the tap is nothing more than a screw, which is turned into an existing hole, the so-called core hole. The cutting edges are formed by the grooves.

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CUTTING BY HAND CHIPPING • Hand nut taps & single cut hand taps: in order to save time and tool cost, hand nut taps (long lead taper) and single cut hand taps are used for threaded through holes. • Because the starting taper of a single cut hand tap is spiral-fluted, better chip removal become possible, so that a shorter starting taper suffices.

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CUTTING BY HAND CHIPPING • Tapping process:

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CUTTING BY HAND CHIPPING • Measuring internal thread by Thread gauges:

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CUTTING BY HAND CHIPPING II. Cutting external threads (threading): • The outer thread is cut with the threading die, often called as 'Filiere' in the workshop. The threading die is like a nut. Again here, the cutting edges are formed by the grooves. • Threading dies are made of HSS. They cut a dimensionally stable thread in one operation. Bolt threads are cut with threading dies.

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CUTTING BY HAND CHIPPING • Threading dies:

• There are closed and open threading dies. The diameter of the open type can be adjusted. Thus thread, which are surfacetreated subsequently, can be cut slightly smaller in diameter. • Threading dies are supplied by manufacturers always in closed state and are opened when needed in the workshop or tool store.

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CUTTING BY HAND CHIPPING • Threading process:

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CUTTING BY HAND CHIPPING • Measuring external thread by Thread templates:

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DISCUSSION 06 1. Name 3 types of tap? Which one is used first for cutting internal thread? 2. What is drill bit diameter for tap hole with M10?

3. How to make sure that the tap doesn’t get clogged during tapping? 4. What should it be the diameter of the bolt d1 with which we use to make external thread by hand die cutting M16? 5. Bring the bolts and let the student practices measuring the thread pitches? 146

HOMEWORK

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Q &A 148