Machinery's Handbook, 31st Edition
TAPPING 2153 Copper Alloys: Most copper alloys are not difficult to tap, except beryllium copper and a few other hard alloys. Pure copper is difficult because of its ductility and the ductile contin uous chip formed, which can be hard to control. However, with reasonable care and the use of medium heavy duty mineral lard oil it can be tapped successfully. Red brass, yellow brass, and similar alloys containing not more than 35 percent zinc produce a continuous chip. While straight fluted taps can be used for hand tapping these alloys, machine tapping should be done with spiral pointed or spiral fluted taps for through and blind holes respec tively. Naval brass, leaded brass, and cast brasses produce a discontinuous chip and a straight fluted tap can be used for machine tapping. These alloys exhibit a tendency to close in on the tap and sometimes an interrupted thread tap is used to reduce the resulting jamming effect. Beryllium copper and the silicon bronzes are the strongest of the copper alloys. Their strength combined with their ability to work harden can cause difficulties in tapping. For these alloys plug type taps should be used and the taps should be kept as sharp as possible. A medium or heavy duty water soluble oil is recommended as a cutting fluid. Other Tapping Lubricants.— The power required in tapping varies considerably with dif - ferent lubricants. The following lubricants reduce the resistance to the cut when thread ing forged nuts and hexagon drawn material: stearine oil, lard oil, sperm oil, rape oil, and 10 percent graphite with 90 percent tallow. A mixture of cutting emulsion (soluble oil) with water reduces resistance to threading action well. A few emulsions are almost as good as animal and vegetable oils, but the emulsion used plays an important part; the majority of emulsions do not give good results. A large volume of lubricant gives somewhat better results than a small quantity, especially in the case of the thinner oils. Kerosene, turpentine, and graphite proved unsuitable for tapping steel. Mineral oils not mixed with animal and vegetable oils, and ordinary lubricating and machine oils, are wholly unsuitable. For aluminum, kerosene is recommended. For tapping cast iron use a strong solution of emulsion; oil has a tendency to make cast-iron chips clog in the flutes, preventing the lubricant from reaching the tap cutting teeth. For tapping copper, milk is a good lubricant. Diameter of Tap Drill Tapping troubles are sometimes caused by tap drills that are too small in diameter. The tap drill should not be smaller than is necessary to give the required strength to the thread as even a very small decrease in the diameter of the drill will increase the torque required and the possibility of broken taps. Tests have shown that any increase in the percentage of full thread over 60 percent does not significantly increase the strength of the thread. Often, a 55 to 60 percent thread is satisfactory, although 75 percent threads are com - monly used to provide an extra measure of safety. The present thread specifications do not always allow the use of the smaller thread depths. However, the specification given on a part drawing must be adhered to and may require smaller minor diameters than might otherwise be recommended. The depth of the thread in the tapped hole is dependent on the length of thread engage ment and on the material. In general, when the engagement length is more than one and one-half times the nominal diameter a 50 or 55 percent thread is satisfactory. Soft ductile materials permit a slightly larger tapping hole than brittle materials such as gray cast iron. It must be remembered that a twist drill is a roughing tool that may be expected to drill slightly oversize and that some variations in the size of the tapping holes are almost inevi table. When a closer control of the hole size is required it must be reamed. Reaming is recommended for the larger thread diameters and for some fine pitch threads. For threads of Unified form (see American National and Unified Screw Thread Forms on page 1940 ) the selection of tap drills is covered in Table 2, and the hole size limits are given in Table 3. See the section Factors Influencing Minor Diameter Tolerances of Tapped Holes on page 2169 . Tap drill sizes for American National Form threads based on 75 percent of full thread depth are given in Table 4 and Table 5. For smaller-size threads the use of slightly larger drills, if permissible, will reduce tap breakage. The selection
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