The die block is fastened to lathe the faceplate, and the prickpunch mark is indicated true. The recess for the hub of the forging is turned in the block the proper depth and tapering. Usually 5 degrees clearance is given on drop-die work of ordinary depth, and if the forging is somewhat deep, say 2 inches, 10 degrees is better; it is obvious that straight sides in the die would cause the metal to stick so that the red-hot bar would not be stiff enough to pry the forging from the die. The center lug a, Fig. 70, for forming the hole in the hub may be made solid or inserted, as suits shop practice. At the same setting the rim b and the flash clearance c are turned in the die. When turning the rim it is well to use a formed turning tool that gives the proper angle to the side of the rim and at same time shapes the bottom of the rim.
Fig. 70. Elevation of Die Block, Showing Shape of Lugs. Etc.
The spokes are milled or cut in the shaper. The spokes and teeth can best be done on a die-sinking machine, which is a vertical milling machine having the dividing head on a table in the horizontal plane.
Dies for forging sprockets and similar work are extremely simple to make as the work is mostly lathe work. The die for producing the monkey wrench, Fig. 63, and for the crank, Fig. 65, are also easy to make, as the die can be cut out in a shaper or a milling machine, except for the sharp corners which have to be chipped out by hand using a cold chisel and hammer.
As steel in the molten state shrinks 1/8 inch per foot when cold, every dimension given on the blue print must be increased at the rate of 1/8 inch per foot to allow for shrinkage. For instance, if the finished diameter of the sprocket forging, in Fig. 66, called for 12 inches, the diameter of the corresponding recess on the die in Fig. 70 should be 12$ inches, or if the diameter is 6 inches, then allowing for shrinkage the die must be 6 1/16 inches, and if the forging is to be 3 inches the die must be 3 1/32 inches, etc. There are shrink rules or scales on the market that instead of being 12 inches long are 12 1/8 inches long, but- the graduations are the same style as on an ordinary scale - in inches, from 1 to 12. In other words, the 1/8-inch shrinkage allowance is taken care of by being evenly distributed throughout the 121/8 inches.
All that is necessary, with this scale is to set the dividers to, say, 6 inches on it, and they are really set at 6 1/16 standard inches.
When making drop dies having round or tapered parts, as in Fig. 65, it is a good plan to turn up a piece of steel at the proper taper, or to the proper diameter if the work is straight, and to use this piece as a templet, with which, by placing an even thin coating of Prussian blue on the templet, the trueness of the recess in the die can be tested frequently. When both halves of the die are finished and the impressions are smoothed nicely, the blocks may be placed face to face, and, after slightly warming the blocks, the impression may be filled with molten babbitt. Babbitt does not shrink very much and the babbitt test piece may be examined to make sure that the impressions in each die match. Also the babbitt may be measured to check diameters and length.
The dies are hardened by being heated face down on a charcoal fire, and, when heated to a depth of several inches, by having a heavy stream of water played on the face of the die. Cast-iron dies may be used when only a few hundred forgings are required.
Trimming dies are usually made in halves to facilitate making, and are also frequently made of machine steel and casehardened as the forging is red hot when trimmed. If the forging is thin or light, the flash would cool so quickly that soft dies would be out of the question. The punch, however, can be case-hardened.
The clearance on trimming dies varies from 5 degrees to 10 degrees, according to the nature of the forging. In any case the forging must be able to pass through the die without being distorted. Trimming dies are open at one end to allow the sprue and rod to pass through, for the forging should stay on the rod as it often happens that the forging becomes distorted during trimming and it is necessary to strike it between the drop dies for final shaping of the forging.