When making taps 1/4 inch in diameter and smaller, the threads are often cut with screw dies, of which there are two styles. The form of screw plate shown in Fig, 93 is termed a jam. die plate. With this form the die is opened to allow the wire to pass through, until it is even with the outside edge of the die, which is now forced into the wire by means of the adjusting screw; the screw plate fa revolved until a thread of the desired length fa cut. This operation is continued, the die being closed a trifle each time, until the right size is obtained. The method taken for gaging the correct size varies in different shops; if only one tap fa made, the tops of the threads are measured with a micrometer caliper; but for many taps of the same size, such as for sewing machines, guns, and bicycles, a sizing die fa used to give the threads an exact size. The threads are cut to within a few thousandths of an inch with the die plate, and finished with the sizing die. One form of sizing die is shown in Fig. 94.
Fig. 92. Dctails of Nut for Milling-Machine Arbor.
Where a great many taps of one size are cut, it is customary to use several dies of different sizes, one of which, the finishing die, is always made adjustable. The roughing dies may be made solid or adjustable, but the finishing must be adjustable for wear and for the changing size of the taps. These dies are sometimes held in separate holders of the form shown in Fig. 94, but a more convenient form of holder is the one shown in Fig. 95. If all the dies are in one holder, they are not scattered around the shop. When many taps are made at a time, the work can be done better and more cheaply if the wire is held in a chuck in a lathe. The die plate should be placed against a drill pad held in the tail spindle of the lathe, in order to insure starting the threads true. The largest die should of course be run on first, the second largest next, and so on to the finish die.
Fig. 93. Typical Jam Die Plate Courtesy of Morse Twist Drill and Machine Company, New Bedford, Massachusells.
Fig. 94. Simple Form of Sising Die.
For taps up to and including those 1/4 inch in diameter, it is customary to use a drill rod. The taps should be chamfered for a distance of three or four threads, as shown at A, Fig. 90, in order that the point may enter the drilled hole.
Fig. 96. Drill Rod for Small Taps.
Taps larger than 1/4 inch are made from tool steel. Taps of 1/4- to 1/2-inch diameter should be made of stock at least 1/15 inch large, which should be centered quite accurately with a small drill, because a large center hole weakens the tap and increases the liability of its cracking when hardened. After taking a chip sufficiently deep to remove all the outer coating, the tap should be box annealed, if possible.
Taps for general use around the shop are often made in sets of three. The first tap to enter the hole is called the taper tap, because of the long chamfering or taper. The second is known as the plug tap; this tap has the first two or three end threads chamfered, and is used when the screw is to go nearly to the bottom of the tapped hole. The bottoming tap is used when the thread is to go to the bottom of the hole; the end of this tap is not chamfered, Fig. 97.
Hand taps are intended for tapping holes by hand, and are usually made in sets of three, as previously explained.
After being annealed, the shank should be turned to size and the square end milled for a wrench. The body should now be turned to size, and the thread cut. Before turning any of the parts to size or starting to cut the thread, be sure that the centers of the lathe are in good condition - the live center should run true, the dead center should fit the center gage and be in good shape.
It is advisable to cut the tap slightly tapering, the thread being from .0005 to .001 inch smaller at the end toward the shank. This prevents the tap from binding when slightly worn, yet does not taper enough to affect the accuracy of the thread. The thread tool should be an exact fit to the gage, and placed in the tool post so that the top of the shank stands about level. The top of the blade shown at A, Fig. 98, should be ground parallel with the top of the shank and the cutting point should be set at the exact height of the point of the head center. Many tool-makers consider it advisable to rough the thread nearly to size with a single-point tool, finishing it with a chaser held in the same holder. A chaser blade is shown in Fig. 99.