Machinery's Handbook, 31st Edition
1430 Drawing Sheet Metal When laying out the blank it is usually advisable to plan for a form that will produce cor ners a little higher than the sides. The wear of the die is at the corners, and when it occurs, the material will thicken and the drawn part will be low at the corners if no allowance for this wear has been made on the blank. Blank for Rectangular Flanged Shells: The shape of the blank for a rectangular flanged shell may be determined in practically the same way as described in the foregoing, except that the width of the flange must be considered. Referring to Fig. 20, the dimension h in the flat blank is made equal to the height of the drawn part plus the width of the flange; however, the blank diameter D for a cylindrical shell having a flange can be determined by the formula (47) where D = blank diameter (inch or mm) d = diameter of drawn shell (inch or mm) d 1 = diameter measured across the flange (inch or mm) h = height of shell (inch or mm) After determining diameter D and the corresponding radius R , the outline of the blank is drawn the same as for a rectangular shell without the flange. Ironing Process.— The ironing process is the reduction in thickness of drawn shell walls by pulling them through tight dies. Ironing is a very useful process when employed in combination with deep drawing to produce a uniform wall thickness and to increase the wall height. It is done to obtain a wall that is thin compared with the shell bottom or merely to correct natural wall thickening toward the top edge of a drawn shell. Basically, in the ironing processes, a previously deep-drawn shell is placed on a punch and pushed through one or more ironing die rings that have a smaller inside diameter than the outside diameter of the shell (Fig. 21). Hence, the clearance between the ironing rings and the punch is less than the shell’s wall thickness, so the shell after ironing has a constant wall thickness equal to the clearance. The theoretical maximum reduction in wall thick ness per operation due to ironing is approximately 60 percent. D d dh 4 2 = +
d 0 F,v T Drawn cup Ironed cup
Punch
Workpiece
c
Ironing ring
a) b) Fig. 21. Ironing of Drawn Cup: a) Cup Before Ironing; b) Cup After Ironing.
Ironing Force : Ironing involves the compression and additional work hardening of the metal. The stress on the sidewall can be quite severe, creating the additional possibility of cup failure. The force required to iron a cylindrical workpiece can be calculated as (48) where F ir = ironing force (lb); d 0 = outside diameter of cylindrical cup (in.); S c = compres sive strength of metal (lb/in 2 ); T = thickness of material (in.); and, c = clearance between punch and die (in.). F S d T c ir c 0 = π − ^ h
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