Machinery's Handbook, 31st Edition
2152 TAPPING Free Cutting Steels: There are large numbers of free cutting steels, including free cutting stainless steels, which are also called free machining steels. Sulfur, lead, or phosphorus are added to these steels to improve their machinability. Free machining steels are always easier to tap than their counterparts that do not have the free machining additives. Tool life is usually increased and a somewhat higher cutting speed can be used. The type of tap recommended depends on the particular type of free machining steel and the nature of the tapping operation; usually a standard tap can be used. High-Temperature Alloys: These are cobalt or nickel base nonferrous alloys that cut like austenitic stainless steel, but are often even more difficult to machine. The recommenda tions given for austenitic stainless steel also apply to tapping these alloys but the rake angle should be 0 to 10 degrees to strengthen the cutting edge. For most applications a nitrided tap or one made from M41, M42, M43, or M44 steel is recommended. The tapping speed is usually in the range of 5–10 ft/min (1.5–3.0 m/min). Titanium and Titanium Alloys: Titanium and its alloys have a low specific heat and a pro nounced tendency to weld on to the tool material; therefore, oxide coated taps are recom mended to minimize galling and welding. The rake angle of the tap should be from 6 to 10 degrees. To minimize the contact between the work and the tap an eccentric or con- eccentric relief land should be used. Taps having interrupted threads are sometimes helpful. Pure titanium is comparatively easy to tap but the alloys are very difficult. The cutting speed depends on the composition of the alloy and may vary from 10–40 ft/min (3.0–12.2 m/min). Special cutting oils are recommended for tapping titanium. Gray Cast Iron: The microstructure of gray cast iron can vary, even within a single cast ing, and compositions are used that vary in tensile strength from about 20,000–60,000 psi (138–414 MPa) and 160 to 250 BHN. Thus, cast iron is not a single material, although in general it is not difficult to tap. The cutting speed may vary from 90 ft/min(27.4 m/ min) for the softer grades to 30 ft/min (9.1 m/min) for the harder grades. The chip is discontinuous and straight fluted taps should be used for all applications. Oxide coated taps are helpful and gray cast iron can usually be tapped dry, although water soluble oils and chemical emulsions are sometimes used. Malleable Cast Iron: Commercial malleable cast irons are also available having a rather wide range of properties, although within a single casting they tend to be quite uniform. They are relatively easy to tap and standard taps can be used. The cutting speed for ferritic cast irons is 60–90 ft/min (18.3–27.4 m/min), for pearlitic malleable irons 40–50 ft/ min (12.2–15.2 m/min), and for martensitic malleable irons 30–35 ft/min (9.1–10.7 m/min). A soluble oil cutting fluid is recommended except for martensitic malleable iron where a sulfur base oil may work better. Ductile or Nodular Cast Iron: Several classes of nodular iron are used having a tensile strength varying from 60,000–120,000 psi (414–827 MPa). Moreover, the microstructure in a single casting and in castings produced at different times vary rather widely. The chips are easily controlled but have some tendency to weld to the faces and flanks of cutting tools. For this reason oxide coated taps are recommended. The cutting speed may vary from 15 fpm (4.6 m/min) for the harder martensitic ductile irons to 60 fpm (18.3 m/min) for the softer ferritic grades. A suitable cutting fluid should be used. Aluminum: Aluminum and aluminum alloys are relatively soft materials that have little resistance to cutting. The danger in tapping these alloys is that the tap will ream the hole instead of cutting threads, or that it will cut a thread eccentric to the hole. For these reasons, extra care must be taken when aligning the tap and starting the thread. For production tapping a spiral pointed tap is recommended for through holes and a spiral fluted tap for blind holes; preferably these taps should have a 10 to 15 degree rake angle. A lead-screw tapping machine is helpful in cutting accurate threads. A heavy duty soluble oil or a light base mineral oil should be used as a cutting fluid.
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