While round dies for screw-machine work may be made solid for roughing out a thread that is to be finished by another die, the finish die should be made adjustable. When making adjustable dies, the general instructions given for solid dies may be followed, except that some provision must be made for adjustment. This is done by splitting the dies at one side as shown at A, Fig. 133. In order that the die may not spring out of shape in hardening, it is advisable to cut the slot from the center of the die, leaving a thin margin as shown at A, Fig. 133; after the die is hardened, this may be cut away with a beveled emery wheel.
If the thickness at B is too great to allow the die to close readily when adjusted to size, the hole may be drilled and connected with the clearance hole by means of a saw cut.
If many round dies of the same diameter are to be made, it is economical to have a holder with a shank which fits the hole in the spindle of the lathe; the opposite end should be made to receive the die blanks, which should be turned to fit the die hholer in the screw machine. Fig. 134 shows the holder to be used in the lathe. A represents a die blank in the holder; B is the shank which fits in the spindle of the lathe; C is a recess in the holder to provide for the projection left on the blank when it is cut from the bar, and also to provide an opening to receive the drill and tap after they run through the die. After the blank is placed in the holder and secured in position by the screw DD, the outer surface may be faced smooth and true with the circumference, after which the blank should be reversed and the opposite side finished to the proper thickness. The die is now ready to be drilled and tapped.
Fig. 133. Die Blank Showing Margins Left for Hardening.
Fig. 134. Die Holder for Lathe.
Before drilling, the die should be carefully centered in the lathe. To insure a full thread in the die, a drill a few thousandths of an inch smaller than tap size should be used, after which a reamer of the proper size may be run through. When tapping the thread, it is advisable to use two or three taps of different sizes; the finish tap should be the size of the desired hole in the. die, and should be of the form known as screw die hob. Where several taps are used for a die, there should be some difference in the diameter so that any inequality in the shape or pitch of the thread may be removed by the larger tap; otherwise imperfect threads will result. For instance, if three taps are to be used for a 1/4-inch die, the first one may be .230 inch in diameter; the second .240 inch in diameter, and the finish tap, if the die is to be solid, .250 inch in diameter. If it is to be an adjustable die, the finish tap should be .253 inch in diameter, in order to furnish clearance to the lands when it is closed to .250 inch.
Dies should be heated very slowly for hardening, either in an oven furnace, or in some receptacle that protects them from the action of the fire. When heated to a uniform low red, they may be immersed in a bath of lukewarm brine and worked back and forth to insure hardening the threads. The temper should be drawn to a full straw color. If it is an adjustable die, the portion marked B, Fig. 133, should be drawn to a blue color in order that it may spring without breaking. This is done by placing this portion of the die on a red-hot iron plate; or the jaws of a heavy pair of tongs maybe heated red hot, and the die grasped in the tongs and held until the desired color appears. The blue color must not be allowed to extend to the threads, or they will be too soft. When the desired color has been obtained, the die may be dropped into oil to prevent drawing the temper more than is desired.
A great many threading dies are made from high-speed steel. In order to secure the best results it is necessary to harden them properly. Tools having projecting portions that must retain their exact shape and size cannot be heated to so high a temperature as lathe and planer tools that are to be ground to shape after hardening.
Threading dies should never be hardened in a blast of air, as the oxygen in the air might attack the metal, oxidize the threads, and so spoil the die. A furnace specially designed for such tools is shown in Fig. 22, Part I. The die may be suspended by means of a hook, or a specially designed holder in the center of the furnace. The flame circulating around the outside of the opening in the furnace leaves the center portion unaffected by the blast. When the tool has reached a temperature of 2150° F., it should be removed and immediately plunged into a bath of cottonseed oil and worked back and forth to force the oil through the opening. Threading dies should have their temper drawn to 490° F. in order to reduce the brittleness to a point where the cutting edges will stand up when in use.
Better results are achieved if the dies are pack hardened. Heat them to a yellow heat and allow them to remain at this temperature for from one-half hour to one hour; then quench them in cottonseed oil. When cold, the temper may be drawn to 480° F.