When cutting a thread of large lead or “steep pitch,” the top of the thread tool should be ground so that it is at right angles to the thread; then the thread groove will be cut to the same width as the tool.
Testing the Size of a Thread.—When the thread tool has been fed in far enough to form a complete thread, the screw is then tested for size. If we assume that a bolt is being threaded for a standard nut, it would be removed from the lathe and the test made by screwing a nut on the end. If the thread were too large, the nut might screw on very tightly or not at all; in either case, the work would again be placed in the lathe and a light cut taken over it to reduce the thread to the proper size. When replacing a threaded part between the centers, it should be put back in the original position, that is, with the “tail” of the driving dog in the same slot of the faceplate it previously occupied.
Fig. 25. Testing Diameter of Thread with Calipers and Micrometer
As it is difficult to tell just when a thread is cut to the exact size, special thread calipers having wedge-shaped ends are sometimes used for measuring the diameter of a V-thread or a U. S. standard thread, at the bottom of the grooves or the root diameter, as shown at A in [Fig. 25]. These calipers can be set from a tap corresponding to the size of the thread being cut, or from a previously threaded piece of the right size.
The Thread Micrometer.—Another form of caliper for testing threads is shown at B. This is one of the micrometer type and is intended for very accurate work. The spindle of this micrometer has a conical end and the “anvil” is V-shaped, and these ends bear on the sides of the thread or the surfaces which form the bearing when the screw is inserted in a nut or threaded hole. The cone-shaped point is slightly rounded so that it will not bear in the bottom of the thread. There is also sufficient clearance at the bottom of the V-shaped anvil to prevent it from bearing on top of the thread. The diameter as indicated by this micrometer is the “pitch diameter” of the thread and is equal to the outside diameter minus the depth of one thread. This depth may be determined as follows:
Depth of a V-thread = 0.866 ÷ No. of threads per inch;
Depth of a U. S. standard thread = 0.6495 ÷ No. of threads per inch;