After all pieces have been ground to exact dimensions, they are attached to the die shoe and placed in position by soft test pieces that have been machined the right thickness. These test pieces are placed in the opening of the die, and the die pieces are brought to bear against the test piece, in which position the hardened pieces are securely fastened in place by screws. After tightening the screws the die should be gone over again to see if some piece had moved a trifle when tightening the screws; then drill and ream for dowels.
While the die pieces are attached to the die shoe, the die shoe is in turn attached to the punch holder, as the die in all sub-press construction where piercing punches are employed in conjunction with blanking operates as the upper member. It will be noted that the piercing punches for the holes shown in Fig. 26 have not been inserted in the die as yet, for the reason that it is easier to put the piercing punches in place after the die is complete. This is accomplished by drilling holes in the die shoe approximately where the punches will come but by making the holes a trifle larger than the larger diameter of the punch and then inserting the piercing punches in small individual punch holders, these punch holders in turn being attached to the back of the die shoe, the punches can be properly located from the holes in the blanking punch by shifting the individual holders one way or another and may be securely screwed to the die shoe when the punches are in proper location. The individual punch holders can be inserted in recesses in the die shoe, or can be screwed on top of the die shoe, and the punch holder recessed to clear them. The locating of punches in this manner eliminates considerable boring of a very accurate nature. Another method of inserting the piercing punches in the upper half is to complete the blanking punch, then the die, and, by entering the punch in the die, the holes in the blanking punch which are the piercing dies can be transferred to the die shoe. This is not as accurate, however, as the adjustable punch-holder method. Making Blanking Punch. The blanking punch is also made up of pieces and is more difficult to make than the die, for the holes in the blanking punch which are the piercing dies must be very accurately located and bored, and, after hardening the punch pieces, the punch pieces are ground to shape and dimensions from the holes. Measuring from Piercing Holes. The holes are put in the soft pieces of the punch after the pieces have been roughed out to within the grinding allowance. After hardening, the holes are thoroughly cleaned, and taper plugs are turned of soft steel to fit the tapered holes. That portion of the plugs, however, that extends beyond the face of the punch, Fig. 34, is straight and all diameters must be exactly alike. When grinding the punch sections, the plugs rest on the hollow square or on the top of the angle iron to insure the edge being ground parallel with the hole, and the method of measuring for proper thickness is as shown in Fig. 34. The small projecting plugs must be of exactly the same diameter, but the actual diameter is immaterial. The punch thickness is ¾ inch, and, assuming that the projecting ends of the plugs in the holes are .250 inch in diameter, it means in this instance that the distance A must be one-half the thickness of the punch plus one-half the diameter of a plug or .375+.125 = .500 inch.
Fig. 34. Measuring Thickness of Piece with Depth Gage.
When a portion of a punch is at an angle, the grinding of the punch sections requires considerable care and skill due to the fact that discrepancies on abutting ends of punch sections will greatly multiply themselves at the extreme ends of the punch as in Fig. 35. A slight opening as at a, Fig. 35, is permissible.
The object in making the punch thin, then attaching it by screws to the soft-steel punch block a, Fig. 34, is for rigidity. It would be easier to make the punch as in Fig. 36, leaving enough stock to grind to size, then grinding out the holes to proper location, but the face of the punch that bears against the holder is too narrow for a working seat, and any miscut by the press operator, or a piece of scrap punching adhering to the sheet being punched, would cause the punch to spring to one side. This causes the edge of the punch to strike the edge of the die, and a broken member or a sheared punch is the result.
Fig. 35. Sectional Blanking Punch, Showing Exaggerated Opening for Making Adjustment* and Completed Sections.
Fig. 36. Punch Easier to Build than Fig. 35 but Not So Satisfactory.