On Tapping Torque and Tap Geometry

Summary Variations in tap geometry are reflected in the measurements of the tapping torque. The effects of cutting speed, thread relief, chamfer relief and rake angle on torque have been investigated in a statistically designed experiment when using 7/16 - 20. UNF bottoming taps and EP-oil on a pitch-gear controlled tapping machine. The analysis of the experimental data showed that the higher order interactions of the variables, in particular those of speed and chamfer relief, significantly affect the torque. The thread relief and the chamfer relief must be selected for the tapping speed used in production, if the torque is to be reduced. At fast speeds only a combined increase of thread relief and rake angle is expected to yield a substantial reduction in torque, which in turn will reduce the enlargement cut by the tap. The experiments also showed little change in tapping torque around a rake angle of γ = 7°, when tapping CS 1114 free-machining steel. Furthermore, the results suggest that tapping torque measurements should be carried out at three speed levels when the ratings of cutting fluids have to be established.