Tapping

Conventional tap drill charts should be followed to maintain 65 to 75% depth of thread. Two-flute taps are recommended for

5 5 I, diameters up to 8 mm (16 in.). Three-flute taps should be used for diameters of 8 to 12.5 mm (16 to 2 in.). Spiral-point taps are desirable because they throw the chip out instead of driving it into the pores of the workpiece. Some experimenting in tapping P/M parts may be required to determine which tap is best for a specific metal. As shown in the following example, chip clamping can degrade the performance, but it is improved by the use of cutting fluid. Evaluation of tapping with the use of cutting fluid for the first three holes and after tapping 50 holes is presented in Fig. 9. Tapping under dry condition reveals problems with chip clamping. Cutting fluid improves the performance. Carbon addition decreases the performance and additives like MnS decrease the torque. The distribution during the measuring length still indicates problems with chip clamping. Selection of tap geometry is considered to solve the problem.

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Fig. 9 Influence of carbon/MnS addition for the moment length. Bottom hole was used in the initial investigation.

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