While ordinary crucible tool steel is extensively used in making taps, many makers assert that the best steel for use in tapping cast iron and brass is one which has, in addition to the usual composition of high-carbon crucible tool steel, from two to three per cent tungsten. It is said that the amount of change in length due to hardening is the same for tungsten steel as for most tool steel.
Fig. 122. Typical Tap Wrench Courtesy of S. W. Card Manufacturing Company, Mansfield, Massachusetts.
Vanadium tool steel is used rather extensively in making taps for tapping steel and is especially satisfactory in making long stay-bolt taps. It is strong and is not so easily broken by shock and irregular strains as ordinary tool steel, nor is it so easily affected by slight variations of heat when hardening.
There are several oil-hardening steels on the market that have won the approval of the tap-makers. The taps made from some of these steels, it is asserted, will not change in pitch when hardened.
A solid tap wrench may be made for taps whose squares are all of a size. This wrench is forged nearly to shape, the handles turned to size in. the lathe, and the square hole in the center drilled and filed. For general shop work adjustable tap wrenches are commonly used, Fig. 122.
When holes are to be tapped to a uniform depth in a screw machine or a turret lathe, a tap holder is used which automatically releases the tap when it reaches the required depth. A very common form, which gives excellent results when properly made and adjusted, is shown in Fig. 123. Its essential parts are a sleeve A, which fits the tool holes in the turret of the screw machine, and a tap holder B, which fits the hole in the sleeve in such a manner as to slide longitudinally. The sleeve should be made of tool steel, if of a diameter that makes the wall around the hole thin; the hole should be drilled and reamed to size, and the outside turned to size. The portion of the sleeve which enters the hole in the turret must be a snug fit. The tap holder should be made of tool steel, or of a grade of machine steel possessing great stiffness and good wearing qualities. After roughing out to sizes somewhat larger than finish, the end which is to hold the tap may be turned to size, and the stem end, which is to run in the sleeve, fitted, after which the hole I, to receive the tap, may be made of a convenient size. In order that the hole may be perfectly concentric with the holder, it will be necessary to run the large end of the holder in the steady rest of the lathe; the opposite end should be fastened against the head center of the lathe in such a manner that the stem runs perfectly true. With work of this nature, the head center of the lathe must be in good condition and run true.
Fig. 123. Releasing Tap Holder.
After the hole has been drilled somewhat smaller than finish size, it is necessary to true the hole with a boring tool; the hole should be bored to within .010 inch of finish size, after which it may be reamed with a rose reamer. Before reaming, however, the outside edge of the hole should be chamfered to the shape of the point or cutting end of the reamer, to avoid any possibility of the reamer running. Some tool-makers never ream a hole of this nature if it can be avoided, always boring to size with a tool that makes a smooth cut. If extreme care is used and the holes are finished to size with a reamer, results good enough for a tool of this character may be obtained.