Unit5 Power Press Machine
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- 1. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/5/1 hehek k liclic POWER PRESS MACHINE CC womwom ww w. w.A B B Y Y.cA B B Y Y.cUNIT 5 POWER PRESS MACHINEOBJECTIVES General Objective:To understand the use of presses and press tools in cold metal working. Specific Objectives : At the end of the unit you will be able to; Know the types and function of presses and presstools. Sketch and know the parts of presses and press tools. Elaborate on the methods of cold working in sheetmetal i.e. shearing, bending and drawing.
2. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/5/2 he hekk lic lic POWER PRESS MACHINE C C wom wom w w w.w.A B B Y Y.c A B B Y Y.cINPUT 5.0. INTRODUCTION In the manufacturing section of the engineering industry, metal articles can be worked to shape either by metal cutting, or by metal forming. Metal cutting can often be wasteful because, on the average, 40% of the original component material is removed by expensive machining operations to become scrap. This scrap material will then on the average be worth 5% of its original value as raw material. In many cases machining operations can be more economically carried out by metal forming. It is interesting to carry out a break-even cost analysis upon a manufacturing operation where metal cutting or metal forming is possible alternative processes. Often the forming process requires expensive dies and fixed costs are higher; material wastage is negligible, labour costs are low and hence variable costs are lower. The metal cutting process is often lower on fixed costs, but higher on variable costs. Hence, the forming process will be more economical when quantities required are large. In this chapter we shall be concerned solely with the use of presses and press tools ( Fig. 5.1 (a), (b) and (c) ) as a means of cold working metal objects into shape. Most of this type of work is carried out upon ductile metal in sheet or strip form of relatively thin section. It represents an important part of the 3. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/5/3 hehek k liclic POWER PRESS MACHINE CC womwom ww w. w.A B B Y Y.cA B B Y Y.c manufacturing industry, being used for the cheap production for large quantities of components, such as motor car bodies, electric motor parts, domestic electrical articles, etc. (a) (b) (c) Figure 5.1. Press machine 4. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re reJ3103/5/4 he hekk lic lic POWER PRESS MACHINE C C wom wom w w w.w.A B B Y Y.c A B B Y Y.c The metal if in coiled strip form, may be fed automatically into the press tool by power rolls or power slide, or may be hand fed by operator. If the metal is in some other form, such as a sheet or partially formed shape, it may be located in the tool by mechanical hands which have gripping fingers or locating pans which drop the metal drop part into the correct position.Again, an operator may hand feed the part. The mechanical feed or location devices must of course be synchronized to operate every time the press ram lifts the top tool clear of the bottom tool.Where the press is hand fed, stringent safety precautions must be taken to ensure that the operators' hands cannot be trapped in the press tool. Efficient guards must be provided which are completely fool proof, and it should always be remembered that a press is potentially a very dangerous machine. The presses mi which press-tool work is done may be divided into (a) hand-operated presses, and (b) power-operated presses. Hand operated presses are very simple in construction and are generally used, only for small batches of simple work; an example is shown in Fig. 5.1 (a) Power presses may be subdivided into (1) Single-acting presses, (2) Double-acting presses, and (3) Triple-acting presses. Single-acting presses are essentially similar to the simple hand-operated press inn that they have only one ram; the means of operating the ram are various. These means may be a crank, an eccentric or a toggle-lever mechanism In Fig. 109 is shown a typical crank-operated single-acting press. The ram is guided in the frame and is actuated by a crankshaft through the medium of a connecting rod. The press is provided with a fly-wheel and may be driven either by a belt or by an electric motor. A clutch enables the flywheel to be coupled to the crankshaft when the press is required to work; this clutch may be arranged to disengage automatically when the crankshaft has made one complete revolution and the 5. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re reJ3103/5/5 he hekk lic lic POWER PRESS MACHINE C C wom wom w w w.w.A B B Y Y.c A B B Y Y.c ram has reached the top of its stroke, a brake being used to prevent overrunning or, alternatively, the crankshaft may be allowed to keep on revolving as long as may be desired. In the latter case the feed to the stock must be automatic and the press must have additional mechanism to provide this automatic feed during the time that the ram and punch are clear of the work. Blanking, piercing, and trimming operations on strip material will generally be done with a continuously running press and automatic feed, but second operation work must generally be put into the die by hand and then the clutch must be operated every time the ram is required to make a stroke. Because of the great danger to an operator's hands which arises from the starting of a press cycle before the operator's hands are clear of the dies all presses must, by law, be fitted with guards. These are arranged to push the operator's hands away (if they are not already clear) as the press ram begins to descend; the guard is clearly visible in Fig. 5.1 (c). In addition some presses are arranged so that two levers have to be moved simultaneously before the press will start and the levers are situated so that the operator must use both hands to move them. This,, however, generally slows down production. The use of an eccentric instead of a crank enables greater forces to be applied to the ram for a given diameter of shaft and is consequently found in presses for very heavy work. Very large presses, such as are used for the production of motor-car body panels, sometimes have four eccentrics, one at each corner of the "ram," so as to ensure an even pressure and to eliminate tilting. When very heavy forces must be used as, for example, in coin embossing, toggle mechanism is sometimes used, but this form of operation generally necessitates a comparatively short working stroke. 6. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/5/6 he hekk lic lic POWER PRESS MACHINE C C wom wom w w w.w.A B B Y Y.c A B B Y Y.c Single-acting presses are made either open-fronted as Shown in Fig. 5.1 (b), the frame being C-shaped, or double sided as in Fig. 5.1 (c). They are often inclinable so that gravity may be used to discharge the job clear of the dies. The double-acting press has two reciprocating parts, an inner member actuated usually by a crankshaft and connecting rod and an outer member actuated usually by cams carried by the crankshaft. Double-acting presses are used chiefly for drawing operations and the outer member is used to actuate the holder or pressure-plate, while the inner member carries the drawing punch. The use of cams makes it easy to arrange that the holder descends ahead of the punch so that the blank is gripped before the drawing starts and also to keep the holder at rest during the drawing. Double-acting presses do not usually run continuously. A triple-acting press is similar to a double-acting press but has, in addition, a third reciprocating member carried in guides in the base of the machine so that a second punch can be made to draw the blank upwards into a suitably shaped recess formed in the top punch. They are used only for large work such as motor-car body panels. In order to appreciate press tool design, it may help to briefly reconsider the elementary principles of metal plasticity. Standard tensile or compression tests which cold work the specimen being used are an ideal means of obtaining data about the plastic range of metals. 7. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rere J3103/5/7 hehek k liclic POWER PRESS MACHINE CC womwom ww w. w.A B B Y Y.cA B B Y Y.c Consider Fig. 5.2, which shows the results of a tensile test upon a relatively ductile material, such a low carbon steel. Figure 5.2 Tensile test of a Ductile Metal The metal is elastic up to point A and will return to its original size if the force is withdrawn. However, if the force is increased to point B, before being withdrawn, the force extension graph follows the line BC, parallel to line AO, as the force is removed.The test piece will then be permanently extended by amount OC, and will not return to its former size. CD represents the elastic contraction ( recovery ) which occurs as the force is removed. Area OABD represents the work required to cause deformation OC. If the overstrained material is again subjected to a tensile force upon a testing machine, we shall plot an entirely different force-extension graph than we first derived. This second graph will now have its origin at C ( instead of O), its yield point approx. at B ( instead of A ), and its breaking point at approx. the same point as would have occurred if the first test had been completed to failure. In effect the original piece of metal in being cold-worked to point B well above 8. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rere J3103/5/8 hehek k liclic POWER PRESS MACHINE CC womwom ww w. w.A B B Y Y.cA B B Y Y.c the yield point acquires a new set of properties. These new properties result in a different force-extension graph being derived if the metal is