6-Bench work.pdf

Production Technology

Chapter 6

Dr. M. Atta

Processes Using Hand Tools (Bench Work) Using hand tools in the workshop still has a place in modern industry, although the large range of machine tools available, and the high rates of material removal that are possible with these machine tools and cutters. In spit of, bench work is too slow and costly for batch and flow line production, but it has a place in the making of prototypes and development projects, and in jig and tool making. So, Hand tools are used to remove small amounts of material, usually from small areas of workpiece. This may be done because no machine is available, the workpiece is too large to go on a machine, the shape is too complex or simply that it would be too expensive to set up a machine to do the work. Since the use of hand tools is tiring, it is important that the amount of material to be removed by hand is kept to min. and the correct tool is chosen for the task. Powered hand tools is used, not only to reduce fatigue but also to increase the speed of the operation and so reduce the cost. Feb. 2011

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Advantages of bench work • Hand tools are relatively cheap and versatile for making small components of complex shapes that would be difficult to hold on machines. • Small and weak components may not be strong enough to withstand the clamping and machining forces, hand processes will then be the only choice. choice • Skilled crafts persons can work to relatively high levels of accuracy and finish using hand tools. • No capital investment in costly plant is required. • Hand tools are more easily maintained compared with machine t l cutters. tool tt

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Disadvantages g of bench work • Compared with machining, the rate of material removal by hand tools is limited and production using hand tools is relatively slow. slow • Compared with machining processes such as surface and cylindrical y ggrinding, g, the accuracy y and finish achieved even byy a skilled craftsperson are limited. • The unit cost of production by using hand tools is high because off th the limited li it d rate t off material t i l removall andd the th relatively l ti l high hi h wages that can be commanded by skilled fitters and toolmakers.

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The bench work can be classified into three basic processes. 1 Cutting 1. C tti processes (shears, (h chisels, hi l andd saws). ) 2. Drilling processes ( Punches, and threading). 3. Finish work (files and scrapers).

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1- Cutting processes 11-11 Hand shears shears. Used to cut sheets with thickness not exceed 1mm. There are different Th diff t types t off shears h with ith different diff t shapes h andd size according to the cause of use. 1 The normal shear, 1shear used to cut straight and outer curved edges.

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2- Inclined blade shear, its edge has inclined angle up to 45° to the right or to the left. Used to cut inclined edges. 3- Shear for cut long straight edges.

4- Shear for cut circular and curved edges.

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5- Shears for cut internal holes.

6- Shears for cut tube walls.

7- Shears for cut wires.

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8- Bench shear, used for cut thicker sheets faster and more effective.

Shear handle

Pivot of the shear handle Shear body

Pivot of the moving edge Bolts for fixed the shear body

Moving edge

Fixed edge Bolts for fixed the fixed edge

Bench

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1-2 Chipping Chipping is the removal of metal by the use of cold chisels. This process is used for rapidly breaking down a surface. It is the quickest way of removing metal by hand but the accuracy is low and the finish is poor. However, in some instances there are no alternatives. Cold chisels are made from high-carbon steel, hardened and tempered at the cutting end. The opposite end, which is struck by the hammer, is not hardened but is left to withstand the hammer blows without chipping.

The chisel should be inclined to the cut surface, surface as shown in the figure to make the cut is easy and the surface smooth and horizontal.

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20°

Chapter 6

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60° 10° 10

Correct angles of chisel

If the cutting angle of the chisel is not correct the machined surface will not be flat, as shown in the figure.

Correct angle leads to flat surface.

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Low inclination angle

high inclination angle

The chisel clamping up on the workpiece.

The chisel goes into the workpiece.

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Types of chisels

Marking out chisel

Cutting an oil groove with a half-round chisel;

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Squaring S i out a corner with a diamond point chisel;

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Chipping a flat surface

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1-3 Sawing The hacksaw is used to cut metal. The hacksaw blade fits into a hacksaw frame on two holding pins, one of which is adjustable in order to tension the blade. The hacksaw frame should be rigid, hold the blade in correct alignment, tension the blade easily and have a comfortable grip. The blade is fitted to the frame with the teeth pointing away from the handle, as shown in the figure, and is correctly tensioned by turning the pp y g a further three turns only. yA wingg nut to take upp the slack and then applying loose blade will twist or buckle and not cut straight, while an over tightened blade could pull out the ends of the blade.

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The blade is made of carbon tool steel or high speed steel with thickness 0.6 mm and width 10-14 mm. for the blade that has teeth in the two sides off it the th blade bl d width idth will ill be b increased i d to t 25 mm.

Cutting angles of saw tooth.

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The blade specifications are; 1. Dimensions 2. Materials 3. Pitch = (1/ no. of teeth in cm.) Hints when sawing • Thee coarser co se thee pitch p c of o thee teeth ee thee greater g e e will w be thee ratee of o metal e removal and the quicker the metal will be cut. However, there must always be a minimum of three teeth in contact with the metal as shown in Figure. g

correct

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•To prevent the blade jamming in the slot that it is cutting, side clearance must be provided by giving the teeth of the blade a set as shown in Figure. There are three ways in which set may be applied; •For coarse pitch blades for general workshop use, the teeth are bent alternatively to the left and right with each intermediate tooth left straight to clear the slot of swarf. Some blades leave every third tooth straight. •For fine tooth blades used for cutting sheet metal and thin walled tubes, b the h edge d off the h bl blade d is i given i a ‘wave’ ‘ ’ set. •When cut copper the blade is sloped to its back edge.

Cutting edge

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2- Drilling processes 2-1- Punching or blanking Punch is used to make holes in sheets either by hand or by fixed it to a bench press machine. The sheet is held to a lead, lead wood or steel die with a hole for the punch. Punch is made of hard steel with a solid or hollow cutting end. The solid punch used to make small holes. The hollow punch used to make holes not exceed 12 mm dia. The bench punching machine used to make holes in the thick sheets and with large diameters.

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hollow punch for making a disc with dia. (D)

Solid punch

Dr. M. Atta

hollow p punch for making a hole with dia. (D)

Cutting edge

Workpiece

Workpiece Stand of lead or wood

Stand

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punch holding

Workpiece holding Workpiece Base

The pressing force (P) P=σ*A where; h σ is i the th shear h stress t off workpiece k i material. t i l A is side area of workpiece

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2-2 Threading 2-2-1 Internal threads • When tapping by hand, hand straight-flute straight flute hand taps are used. These are made from high-speed steel or carbon tool steel and are supplied in sets of three three. • The taps are heat treatment to increase the hardness • The three taps differ in the length of chamfer at the point, known as the lead. The one with the longest lead is e e ed to as the t e taper tape or o first st tap, the t e referred next as the second or intermediate tap and the third, which has a very short p g tap, p, lead,, as the bottomingg or plug Figure.

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Tap set

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• When taping through hole the tapper tap is used only. • When taping blind holes the three taps are used. • A square is provided at one end so that the tap can be easily rotated by holding it in a tap wrench. The chuck type of wrench is used for the smaller tapp sizes. • The taps are classified according to the diameter and the thread pitch. • The hole diameter that required q for threading is equal to the diameter of the thread bottom. Small dia. breaks the tap while large one make incorrect thread.

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2-2-2 External thread • Screw thread dies are used to cut external threads on engineering p components. • The split button die is the type most widely used by a fitter. • The diestock has three adjusting screws. The centre screw engages the slot in the die and spreads the die to reduce the depth of cut. The other two screws close the die and increase the depth of cut. • As for a tap the die must be started square with the work axis as shown. • It is difficult to control a screw-cutting die and any attempt to cut a full thread in one pass will result in a ‘drunken’ thread. • It is better to open up the die to its fullest extent for the first cut. (This will also produce a better finish on the thread.) Then close the die down i stages for in f the h subsequent b cuts until il the h thread h d is i the h required i d size. i • The thread size can be checked with a nut. • The diestock is rotated in a clockwise direction and should be reversed after ft every one or two t revolutions l ti to t break b k up the th swarf. f • A thread-cutting lubricant must be used.

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Production Technology

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Split button dies: ((a)) cuttingg angles g applied pp to a thread-cutting die; (b) split die; (c) diestock; (d) positioning of die in stock – the engraved face of the die is visible, ensuring that the lead of the die is in the correct position iti

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Production Technology

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3- finishing processes 3-1 Filing • Filing operations can range from roughing down to fine and accurate finishing operations. • There is a wide variety of files, and to specify any given file you must state the length, length shape and grade of cut. cut • The main parts of the file is shown in the figure.

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Classification of file; files could be classified according to, 1 File nominal length. 1. length 2. File cross section shape. 3. Teeth type 4 No. 4. N off tteeth. th 1- File length between 80 mm. to 450mm. 2- File cross section shape

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3- Teeth type; •Single Si l • double teeth. •Dreadnought file •Needle dl fil file In double cut the inclination angle is differ for the two set of teeth. 45-60° f the for th first fi t andd 70-80° 70 80° for f the th second. d this to increase the smoothness of the surface and to prevent the formation of slots in then filling surface surface.

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Dreadnought file

Needle file

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4- No. of teeth When the number of teeth/ unit area is increased the smoothness of the surface is increase and the volume of the teeth is decreased. coarse 5-11, bastard 7-16, second cut 7-17, smooth 13-24, dead smooth 22-35 rake angle

File cutting angles The rake angle is negative to prevent the file from go into the work (to remove a very small layer form the work).

wedge angle

clearance angle

File manufacturing; •The file is made of high carbon steel. • Primary shaping the file and annealed •Final shaping then hardened.

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Correct grip for different file applications

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3-2 Hand Scrapers •Scraping, unlike filing or chiselling, is not done to remove a great deal of material. The material is removed selectively in small amounts, usually to give a flat or a good bearing surface. •The purpose of scraping is therefore to remove high spots to make the surface flat or circular, and at the same time to create small pockets in which lubricant can be held between the two surfaces. •The point is slightly curved, and the cutting edges are kept sharp by means of an oilstone. The scraper cuts on the forward stroke, the high spots being b i removedd one at a time i by b short h forward f d strokes. k

Scraper types

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•The flatness is checked with reference to a surface plate. A light film of engineer’s g blue is smeared evenlyy on the surface plate, p and the surface being scraped is placed on top and moved slightly from side to side. Any high spots show up as blue spots, and these are reduced by scraping. •The surface is again checked, rescraped and the process is repeated until the desired flatness is obtained. Flatness of the surface is indicated when the whole area being scraped is evenly covered by blue from the surface plate. The scraper cutting angles are rake angle = -27.5°, wedge angle = 95° and clearance angle = 22.5 22 5° the scraper is made of tool steel with forging, grinding and hardening then re-grinding. re grinding. For very hard workpice metals an edge of hard metals can be used

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ASSIGNMENT (6) Bench work 1. What are the advantages of the bench work? 2. What are the disadvantages of bench work? 3. State seven different types of shears? 4. Describe the bench shear, with neat sketch. 5. What is the effect incorrect cutting angle of the chisel? 6. Describe with neat sketch four types of chisel. 7. Describe the saw blade? 8. Describe how to prevent jamming of the saw. 9. What are the types of the punch, with neat sketch? 10. What are the tap set, and what is the function of each? 11. Describe how to make an external thread for a tube? 12. How the file could be classified? 13. What are the classifications of files according to its cross section? 14. What are the classifications of files according to its teeth types? 15. In the double teeth file why the inclination angles of each row are different? 16. Give reason for the negative rake angle for the file’s teeth. 17. Describe the steps for producing the files.