When cutting threads in screw machines and turret lathes, dies are held in die holders, which are constructed in two parts, as shown in Fig. 140. The shank A fits the hole in the turret, while the die holder B has a stem that fits the hole in the shank. While the die is cutting, the pins D and C are engaged, and prevent the holder B from turning. When the turret slide of the screw machine has traveled to its limit, the holder is drawn out of the shank until the machine is reversed, when the pins engage on their opposite sides. A pin is put through the stem of the holder at E; this strikes the end of the shank just at the time the pins D and C become disengaged.
Both shank and body may be made of machinery steel; the shank may be finished to size, except the portion marked A, which should be left .010 inch large for grinding. The front end of the hole should be rounded, as shown, to allow the fillet in the shoulder of the stem to enter. This fillet is left for strength. The pinhole should be drilled and reamed. When the holders are to take dies not over 1/8 inch in size, this pinhole may be 3/16 inch in diameter; for dies from 1/8 to 5/16 in size, the hole should be 1/4 inch in diameter. As the dies increase in size, the pin must increase proportionately. The shank may be casehardened in a mixture of granulated charred leather and charcoal; it should run about two hours, and then be dipped in a bath of oil. The hole should be lapped straight and true, and the outside ground to fit the hole in the turret. The pin C should be of tool steel, hardened and drawn to a blue color, and forced into place.
Fig. 140. Die Holder.
The holder B may be made from a forging, or turned from a solid piece. After roughing to size somewhat larger than finish, the stem may be turned and fitted to the hole in the shank, in which it should turn freely. The larger portion, or body, is next turned to size. This should be run in the steady rest, and the end drilled and bored for the die and for clearance back of the die, as shown. Three or four large holes drilled into the clearance hole provide the chips and oil with a way of escape, thus preventing injury to the threads of a screw long enough to reach through the die when being threaded.
Screw holes should be drilled and tapped as shown. The screws are to hold the die in position in the holder, and also to adjust to size dies that are split. The stem may be placed in the shank, and the pinhole transferred through the pinhole in the shank into the body; this should be done before the pin C is pressed into place. The pin D should be hardened the same as C. The pinhole for the pin E should be drilled in a location that allows C and D to become disengaged, and yet have no play between them.