Machinery's Handbook, 31st Edition
2150 TAPPING ferrous and nonferrous materials including plastics, hard rubber, mild steel, and tool steel. Other surface treatments that have been used successfully to a limited extent are vapor blasting and liquid honing. Rake Angle: For the majority of applications in both ferrous and nonferrous materials the rake angle machined on the tap by the manufacturer is satisfactory. This angle is approxi mately 5 to 7 degrees. In some instances it may be desirable to alter the rake angle of the tap to obtain beneficial results and Table 1 provides a guide that can be used. In selecting a rake angle from this table, consideration must be given to the size of the tap and the strength of the land. Most standard taps are made with a curved face with the rake angle measured as a chord between the crest and root of the thread. The resulting shape is called a hook angle. Table 1. Tap Rake Angles for Tapping Different Materials Material Rake Angle, Degrees Material Rake Angle, Degrees Cast Iron 0-3 Aluminum 8-20 Malleable Iron 5-8 Brass 2-7 Steel Naval Brass 5-8 AISI 1100 Series 5-12 Phosphor Bronze 5-12 Low Carbon (up 5-12 Tobin Bronze 5-8 to .25 percent) Manganese Bronze 5-12 Medium Carbon, Annealed 5-10 Magnesium 10-20 (.30 to .60 percent) Monel 9-12 Heat-Treated, 225-283 Brinell 0-8 Copper 10-18 (.30 to .60 percent) Zinc Die Castings 10-15 High Carbon and 0-5 Plastic High Speed Thermoplastic 5-8 Stainless 8-15 Thermosetting 0-3 Titanium 5-10 Hard Rubber 0-3 Cutting Speed.— The cutting speed for machine tapping is treated in detail on page 1140. It suffices to say here that many variables must be considered in selecting this cutting speed and any tabulation may have to be modified greatly. Where cutting speeds are men - tioned in the following section, they are intended only to provide a guideline to show the possible range of speeds that could be used. Tapping Specific Materials.— The work material has a great influence on the ease with which a hole can be tapped. For production work, in many instances, modified taps are recommended; however, for toolroom or short batch work, standard hand taps can be used on most jobs, providing reasonable care is taken when tapping. The following concerns the tapping of metallic materials; information on the tapping of plastics is given on page 604. Low Carbon Steel (Less than 0.15% C): These steels are very soft and ductile resulting in a tendency for the work material to tear and to weld to the tap. They produce a continuous chip that is difficult to break and spiral pointed taps are recommended for tapping through holes; for blind holes a spiral fluted tap is recommended. To prevent galling and welding, a liberal application of a sulfur base or other suitable cutting fluid is essential and the selection of an oxide coated tap is very helpful. Low Carbon Steels (0.15 to 0.30% C): The additional carbon in these steels is beneficial as it reduces the tendency to tear and to weld; their machinability is further improved by cold drawing. These steels present no serious problems in tapping provided a suitable cutting fluid is used. An oxide coated tap is recommended, particularly in the lower carbon range.
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