Taps, like all other cutting tools, become dull through use. Dull taps can produce either oversize or undersize holes. They may chip, produce poor threads, or break in extreme cases.
Wear is greatly accelerated by the dullness of the tap. For example, a tap that has produced 2,000 tapped holes may be resharpened to its original usefulness with little effort and with the possibility of obtaining several additional sharpenings. However, if the same tap were allowed to produce 4,000 holes, it may be worn to a point where it is impossible to resharpen it satisfactorily. A determination of the number of holes to be produced by a tap must always include the factors of machine down-time for changing taps and the economic feasibility of resharpening. In many cases, it is most economical to run small taps until they will no longer produce satisfactory threads after which they are discarded. Larger and more expensive taps should be resharpened before wear becomes excessive.
Smaller sizes of taps are generally reconditioned by grinding the chamfer portion only, while larger taps may also require flute grinding. Taper pipe taps must be sharpened by grinding the flutes and spiral-pointed taps must be reground in the angular portion of the flute as well as on the chamfer.
In any type of reconditioning, it is generally advisable to duplicate the previous grind as close as possible. The chamfer must be true and uniform, and the original amount of relief should be maintained. Hand grinding of chamfers is not recomended; however if it is done, it should be done only in the simple fixtures or machines sold for this purpose.
Flute grinding or spiral point grinding should also be done on a tap reconditioning machine or on a cutter grinder. Flute grinding wheels should be shaped with a diamond dresser of appropriate type to produce a rake face or hook face as required. Both chamfers and flutes should be ground carefully so as not to burn the cutting edges of the tap, careful grinding will also produce a smooth finish on the tap. Wheels of K or L hardness and 60-grain or finer should be used.