Tech Talk
What are the faults usually made in “Tap Sharpening”?
A tap is a tool used to produce internal (female) threads to receive screws, bolts of threaded parts in accordance with established classes of fit. Tape manufactured to slightly varying limits (cut thread, commercial ground and precision ground thread) are suggested for producing holes to meet the established classes of fit. No tap, however, is guaranteed to produce a thread of a given size.
Tape that become dull cut oversize: require more power to drive; cause rough threads or breakage, and generally slow the tapping operation. However, it is not always profitable to re-sharpen taps unless the proper tap grinding equipment is available.
Sharpening is usually restricted to the chamfer and must have great accuracy to minimize torque load, produce accurately sized hole, and promote longer life.
In sharpening spiral pointed taps, care must be exercised to maintain the original angle and shape of the spiral point. The spiral point flute should extend approximately 2 threads beyond the chamfer. When ends of lands become thin, grind end of tap straight back until lands are of normal thickness. It is good practice to refer to a new spiral pointed tap of the same size for guidance in sharpening.
In re-sharpening the cutting faces, or in refitting a regular tap with a different rake angle or hook for a special material, a cutter grinder should be used. However, this operation can be performed by hand on a bench grinder if precautions are taken to properly shape the wheel.
Excessive breakage and other tap failures can usually be attributed to: faulty tap holder, machine spindle, fixture, or work alignment; wrong tapping speed; wrong type or lack of lubricant; improperly designed tap for operation; peculiarities of the material being tapped; and condition of hole, finish and size.
