Unit5 Power Press Machine

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Unit5 Power Press Machine

  1. 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/1 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 5 POWER PRESS MACHINE OBJECTIVES 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; Ø Know the types and function of presses and press tools. Ø Sketch and know the parts of presses and press tools. Ø Elaborate on the methods of cold working in sheet metal i.e. shearing, bending and drawing.
  2. 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/2 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 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. 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/3 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A 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. 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/4 he he k k lic lic POWER PRESS MACHINE C C w om w om 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. 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/5 he he k k lic lic POWER PRESS MACHINE C C w om w om 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. 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/6 he he k k lic lic POWER PRESS MACHINE C C w om w om 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. 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/7 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A 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. 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/8 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A 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 reworked. This is the most important first effect of cold working, which means that a cheaper material can be specified for a cold forming operation, the component finishing with new superior properties comparable to a more costly material. The results of cold working a metal well within the plastic range of the metal can be summarized as follows: a) The yield point, and hence the stress at yield point is raised, where Force at yield point stress at the yield point = sy = Cross sectional area b) The ductility is lowered, and hence the elongation % is reduced, extension where elongation % = x 100 original length Should high ductility be the important property, such as in a deep drawing operation which must be carried out in several stages, then the metal must be annealed after cold working to restore it to its original state. From figure 5.2. it can be seen that no annealing takes place, then between points A and B, each successive increment of elongation will require an increasing increment of work. In other words, as cold working proceeds, the resistance to deformation rises steadily. In cold working there will be minor changes in dimensions of the work piece when the work is removed from the tool. This is the elastic -recovery of the material shown in the graph due to the release oil stresses causing the deformation. The work after removal from the press tool will spring back from
  9. 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/9 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c the die shape to take up a different shape. This must be allowed for in the tool design. One other factor to consider in press tool work is the direction of rolling of the strip to be worked in the press tool. Cold rolled sheet or strip is used which has been rolled in a certain direction thus giving the strip directional properties. It will be found that bending can be carried out more successfully across the grain (direction of rolling), than along the grain. Components which are produced from tools which effect bending along the direction of rolling will almost certainly crack during the bending operation. There are three different ways of cold working sheet metal in press tools tools. These are: 1) Shearing. In this case the required shape of work is sheared from the metal strip, the metal being deformed to shear failure. There are three variations of shearing, viz., a) Blanking, in which a blank is punched from the strip, the blank removed by the punch being the required article. The metal left is waste. The die is made to the required shape and size, i.e., the punch is made smaller than the die by the amount of clearance required. b) Piercing, in which the blank punched or pierced from the metal strip is waste, the hole left in the strip being required. The piercing punch made to the required shape and size, the clearance being added to the die. c) Cropping, in which the piece blanked from the strip is waste, left with the cropped ends being the required part. A cropping tool is in principle, a blanking tool.
  10. 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/10 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Each of these operations is shown at Fig 5.3(a), (b) and (c) respectively Piercing Waste Waste Required part Strip feed Required part Strip feed b) Piercing a) Blanking Waste Strip feed Required part c) Cropping Figure 5.3. Press tool shearing operations In Fig 5.3 (b) it can be seen that the operation is one of piercing three holes followed by the strip moving forward one pitch. Then a blanking operation follows to give a blanked and pierced component. Piercing is most commonly carried out in conjunction with blanking. Figure 5.3 (c) shows a cropping operation which is used when the component is relatively long, and the width is sufficiently accurate without blanking. This is economically good sense where it can be done, as the tool is cheaper than a full blanking tool. 2) Bending. This is carried out on blanks, strip, sheet, rod or wire and consists of local deformation, as opposed to a change of shape of the complete article. Forces must be high enough to cold work the material within the plastic range.
  11. 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/11 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 3) Drawing. This is carried out on blanks, and involves considerable deformation or a complete change of shape of part. Again the deformation must be carried out in the plastic range. As stated earlier, deep drawing may require several drawing stages with interstate annealing. In the next section we will consider each of these methods of press work, and the principles of design of a typical tool of each type. 5.1. SHEARING Figure 5.4 shows a blanking and piercing press tool. This is a simple shearing tool, and the punches and die can be of any required profile. The tool shown has the main features of any press tool which we will examine in more detail. Shank Press ram Top set Pressure plate Punch plate Blanking punch Piercing punch Pilot Stripper Die Strip feed Bottom set Press bed Figure 5.4. Blank and pierce tool
  12. 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/12 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 5.1.1. Punch Made from a non-shrinking, non-distorting alloy tool steel, such as a high carbon, high chromium alloy steel. Punches are hardened and ground, and are held in a punch plate. The shanks may be a drive fit in the punch plate as shown. Alternatively, where there are many piercing punches, say, which require precise location in the punch plate to match the die, the punches may be held in place in the punch plate by Cerro matrix or some other low melting point alloy. This method greatly eases the problems of manufacture of the press tool. 5.1.2. Die Made from the same metal as the punch, and like the punch is retargeted by grinding the top face. Complex die shapes may necessitate the die being made in more than one piece. The die profile is ‘backed off’ with taper as shown to allow the blanks and piercing slugs to easily fall clear into tote boxes positioned under the press bed. 5.1.3. Stripper This may be made out of mild steel and is a clearance fit for the punches at the top, and the strip at the side which it guides into the correct position under the punches. The stripper prevents the metal strip lifting up with the punch as the ram returns. It also provides a means of housing the stops.
  13. 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/13 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 5.1.4. Stop There are many forms of stops, the one shown being a simple spring loaded type. As the strip is pushed under the stop it rides up over the strip to drop into the space left by the blank. If the strip is then pulled back against the stop (against the direction of feed), the strip is correctly located for the next shearing operation, leaving the minimum of waste metal between the blanked holes. This feeding operation up to a stop can be carried out at high speed by the operator, and a well designed stop must be simple and efficient in use. More complex tools may require more than one stop, and arrangements can be made to operate stops by the movement of the press ram if desired. Automatic feed devices do not require soon as the strip is automatically moved along the correct pitch length, between each stroke of the press. 5.1.5. Pilot These are held in the blanking punch and are a clearance fit for the pierced hole. As the ram descends, the pilot locates the previously pierced hole and positions the strip more precisely under the blanking punch. Pilots are used where the relationship of pierced holes to a blank profile must be precise; therefore a stop initially positions the strip, but the final positioning depends upon the pilot. It can be seen then that this technique is ideal for a roll feed press which has no stops upon the too].
  14. 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/14 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 5.1.6. Sets These may be made from mild steel as they are bolsters to which the die and punch assemblies are screwed. The top set holds the shank by means of which the punch assembly is located and held in the press ram. The bottom set is bolted or clamped to the press bed in the correct relationship to the top set so that the axes of the punches tied die holes are in line. Cast iron die sets are commercially available which are made complete with guide puts and bushes. Hence the top set can always be located in exactly the same position relative to the bottom set. The press tool maker then ensures that the punch and die are always a perfect fit when the tool is closed. This makes the setting operation on the press quick and easy, because the complete tool is simply mounted and fastened on the press, no locating between punch and die being necessary. 5.1.7. Pressure Plate This is a hardened and ground steel plate which is inserted between punch and top set, in order to take the impact on the head of the punch as it shears through the strip. Let us now consider some other important features of sheet metal shearing, which affect the design of the tool. 5.2. CLEARANCE BETWEEN PUNCH AND DIE When the punch hits the work metal strip, it penetrates a certain percentage of the strip thickness before the metal shears and the whole blank ruptures. The depth of penetration depends upon the hardness of the work
  15. 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/15 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c material, being greater for a soft material, and can be seen as a polished rim around the edge of the blank. The amount of clearance allowed between the punch and die also affects the appearance of the blanked edge and the accuracy of the finished blank. The clearance varies with the thickness and the hardness of the metal being sheared and will be some value up to 10% of the strip thickness. The edge surface appearance and quality produced upon a standard tool will not be as good as a machined surface, but can be adequate if the correct clearance is chosen. Figure 5.5 shows the effect on the blank edge of allowing insufficient clearance. Note that the clearance is the gap between the adjacent walls of punch and due, i.e., radial clearance for circular punches. Polished rim Ruptured surface Blank a) Correct clearance b) Insufficient clearance Fig. 5.5. Effect of Insufficient Clearance 5.3. FORCE AND WORK DONE REQUIRED FOR SHEARING It is necessary to know the force for a particular shearing operation in order to choose a press of adequate capacity for the press tool. The shearing force varies during blanking (piercing or cropping) because of the nature of the process as just described. This is illustrated in fig. 5.6 which shows a force penetration graph for a blanking operation in which the correct amount of punch and die clearance is allowed.
  16. 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/16 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Punch force (F) MN % Penetration (c) Fig. 5.6. Force penetration It can be seen from the graph curve that the force reaches the maximum (Fmax) as the punch penetrates the metal, then falls away rapidly as the metal ruptures. The graph shown is for a fairly hard steel where, say, c = 15%. For soft steel, c might equal 40% but Fmax would be much lower. This maximum punch force depends upon the edge area to be sheared and the shear strength of the metal, therefore: Fmax = (ultimate shear stress of material) x (shearing area) = t x material thickness (t) x work profile perimeter. =t xtxx The area under the force-penetration curve is equal to the work done during the shearing operation, therefore: Work done = ( maximum punch force ) x (% penetration) x (material thickness) = Fmax x c x t.
  17. 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/17 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c This is an estimate of course, and depends upon the assumption that maximum punch force is sustained during complete punch penetration of the work metal. Example 5.1 Calculate the maximum punch force necessary to blank a steel washer 44-45 mm OD x 22.23 mm ID x 1.59 mm thick, if t = 432 N/mm2. Estimate the work done if % penetration is 25 Solution Work profile perimeter, x = p (44-45 + 22-23) = 209-5 mm. Fmax = t. t . x = 0-432 x 1-59 x 209.5 = 0-144 MN Work done =Fmax . c . t =144 x 0-25 x 1-59 = 57 J. 5.4. SHEAR It can be seen from the above expression for work done, that if the work is spread over a greater stroke movement of the punch, then Fmax will be reduced. Hence the tool could be used upon a smaller capacity press (assuming the press shut height and stroke are satisfactory) which could be both convenient and
  18. 18. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/5/18 he he k k lic lic POWER PRESS MACHINE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c more economical. This effect can be achieved by grinding either single or double shear upon either the punch or die. Shear is usually applied when thick metal is being blanked, or if the work has an extensive contour. Examples are shown at Fig. 5.7. Shear is applied to the punch { Fig.5.7 (a) } when piercing or cropping, because the slug which is punched out will be deformed. Shear is applied to the die { Fig 5.7 (b) } when blanking, because the flat face of the punch produces a flat blank without distortion. b) Die with double a) Punch with shear single shear Punch Punch Depth of shear (s) Die Die Figure 5.7. Shear applied toa press tool The amount of shear (s) to be ground upon the tool depends upon the reduction in punch force required. By a consideration of the amounts of work done with, or without shear, the following expression can be deduced: Fmax - F x c x t s= F This is true for single or double shear.

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