Three modes of tap failure have been considered: too high a cutting force, jamming of the swarf in worn corners of the cutting edges, and loss of thread form. In tests in which a high speed steel tap cut a 1.5 percent Ni-Cr-Mo steel with neat mineral oil as the lubricant, chip thickness and tap wear increased with tap use until the jamming limit was reached. Coating the tool with an inorganically-bonded molybdenum disulphide film did not alter the failure mode but increased tap life by reducing the rate of progress to failure. Adding an EP agent to the oil completely changed the route to failure and increased tap life even more. The tool coating reduced tapping forces only initially. It was soon mainly swept away. Thereafter, however, the residue still had an influence on tap performance through its action in reducing pick-up on the tap rake faces. Supplementary tests contrast the behavior of the coating in tapping and turning operations.

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