One reason is that the individual pieces of alarge die can be machined to shape easier. Another reason is that a long die or a large die would distort appreciably when being hardened, which would mean that the die could not be finished to exact size, but that a considerable amount of stock must be left on the large solid die in order to grind to shape after hardening. In the case of weak projections in a die, if the die is left solid and one point breaks, the entire die is ruined, whereas, with a sectional die, that part containing only the broken portion can be removed and a new one made at a small cost.
The procedure in making such a sectional die as in Fig. 325, Tool-Making, Part III, would be to machine the two strips as in Fig. 22, herewith, and, clamping them together, to lay out the outline from a templet or drawing in the same manner as the die in Fig. 4 was laid out.
Fig. 22. Clamping Sectional Die for Outlining.
The parting line of the two halves should come in the center of the scribed outline. The operation of drilling along the line as in the case of the die in Fig. 4, would be useless, for the end hole can be drilled, and reamed tapering, then each half in turn may be gripped in a shaper vise, Fig. 23, and, by placing a small wire or a strip of folded paper of the right thickness to tilt the strip at the desired angle for clearance, then placing, say, a ½-iuch rod at the mid height of the jaws, the opening in the strip can be easily machined to size and the clearance can be machined at the same time.
Fig. 23. Shaping Die, Shoring Method of Obtaining Clearance.
Several dowel holes are drilled in each strip, if possible, in addition to the screw holes for fastening to the die shoe, and, after hardening and grinding the die strips, the die shoe is machined to receive the two strips. A snug fit is necessary. The dowel holes are now transferred from the die strips to the die shoe, and the dowels are inserted. There is always more or less spring to a sectional die when shearing the punch. Care must be exercised in not attempting to shear too much stock, as the dies will spread.
Fig. 24 shows the method employed in making a small sectional die of simple design having weak projections. It is obvious that weak points, such as shown on this die, would not withstand the pressure necessary to shear much stock. Therefore, when shearing a punch having such points, always remove the stock by scraping or filing and do not allow the points in the die to shear the punch. Great care also is necessary in withdrawing a punch from a die of this character when shearing the punch.
Fig. 24. Making Sectional Die with Weak Projections.
The cores for coils and armatures are made up of variously shaped laminated soft-iron punchings, which, as a rule, must be extremely accurate. The iron sheets are rolled hot, producing a hard scale or oxide which causes severe wear on punches and dies. The dies must be frequently ground, as burrs on punchings are prohibited, and if the dies were given clearance each grinding would produce a larger punching. Therefore, due to intricate shapes that must be exact and the fact that the wear on the dies is severe, the dies are invariably made of the sub-press construction and also made up of pieces, the latter being ground all over to size after hardening. There is no clearance given these dies and they are known as sectional or built-up dies.
When the die is of extremely intricate shape, having a great number of pierced slots, Fig. 25, the usual method is to make a single punch and die, and, by the use of an accurate indexing fixture which holds the blank, the notches are pierced one at a time and the indexing done automatically by the stroke of the press. One operator can attend to several presses. The object in indexing and using a single piercing die is to eliminate the high cost of making a die to produce the blank in one stroke and also to eliminate the cost of repairs, for if one small point on such a die should break it would render the entire die useless until repaired.
Fig. 25. Die with Pierced Slots.
Fig. 23. Typical Punching with Dimensions.
The punching shown in Fig. 26, while extremely accurate, is of a comparatively simple design, but the method of making is the same as for dies for more intricate shapes.. The punchings are built up as in Fig. 27, and every other one is reversed, so that the error must be slight, for in reversing the blanks the error is doubled.
When laying out a built-up die that is to be ground to shape and size after hardening, the sections should be as free from internal corners as possible, as it is a difficult job to grind a good sharp inside corner. For instance, referring to Fig. 28, it would be easier to make and grind the four pieces, as at a, than it would be the two pieces with inside corners, as at 6. Therefore, the division of the die at hand for sections requires considerable study.
Fig. 27. Method of Mounting Punchings shown in Fig. 26.
Fig. 23. Two Methods of Making Sectional Die.
Referring to Fig. 29, it will be seen that the division lines are so placed that there are no corners in any one piece and that each piece is comparatively easy to make. Instead of laying out from a templet, or roughing out the pieces and placing them in position, then laying out the outline on the pieces, a better way is to make each piece by measurement. The pieces a and i are simple end pieces, perfectly straight, and can be eliminated from the description of making. Referring to piece 6, Fig. 29, we find by totaling the dimensions on the drawing, Fig. 26, that the piece is 2 7/16 inches wide and that the angle is 45 degrees. This piece is carefully planed or milled to exact shape, but, say, .010 inch is left on each surface for grinding. Piece c, Fig. 29, is important, as the width governs the length of the end of the punching. Referring to the drawing, Fig. 26, we find this width to be 1¼ inches, and .010 inch is left on each surface for grinding; and so on, until all pieces are roughed out to within grinding allowance. The edges and bottom of all pieces must be at right angles even when in rough size.
Fig. 29. Building Up Die for Punching Shown in Fig. 26.