7127 Grinding and Machining

When very close tolerances are required, grinding is the preferred method. Grinding wheels from reputable manufacturers such as Norton, Carborundum, and others have been successfully used. Recommended wheels and coolants for these alloys are available from the manufacturers of these wheels. Nickel alloys are readily machinable by the conventional methods. Tungsten carbide and recently ceramic-tipped tools have been successfully used for machining these alloys. High-speed steel tools have also been used. During machining some of the highnickel alloys work harden rapidly, generate high heat during cutting, may weld to the cutting tool surface, and offer high resistance to metals removal due to their high shear strength in comparison to stainless steels and/or carbon steels. The following points should be kept in mind when machining nickel alloys:

• Machine should have sufficient power and be rigid.

• Workpiece and cutting tool should be held rigid without any chatter. Vibration should not be present during machining operation.

• Tools should be sharp at all times and should be changed as the need arises. A 0.015-in. wear land is considered a "dull tool." Use positive rake angle tools for most machining operations. Negative rake angle tools can be used for intermittent cuts and heavy stock removal.

• Use heavy constant feed to maintain positive cutting action, that is, the tool should be constantly engaged with low cutting speed and higher feed rate. If feed slows and the tool dwells in the cut, work hardening will occur, tool life will lessen and close tolerances will be difficult to achieve.

• Lubrication is desirable. Water-based cutting oils are recommended to avoid overheating of the cutting tools. However, operations such as threading, tapping, or drilling require sulfonated or chlorinated lubricants. However, all traces of these fluids must be removed prior to any heat treatment or high-temperature service.

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