Investigation of thread tapping load characteristics through mechanistics modeling and experimentation

A mechanistic model for the prediction of tapping torque and axial force is developed. The model is capable of predicting tapping torque and axial force resulting from chip formation and tool flank/workpiece friction under various machining conditions, including dry tapping and tapping with different cutting fluids. Extensive tests on tapping torque and axial force measurements are conducted to verify the predictive model. Characteristics of the measured tapping loads are studied. It is found that the total tapping load consists of a base load and a chip packing load. The base load results from chip formation and tool/workpiece friction. The predicted tapping load is found to be in good agreement with measured base load. The chip packing load is the result of chip clogging in the flutes, and is random in nature. The chip packing load may be many times that of the base load, depending on tap geometries and the machining conditions. Factors causing severe chip clogging and excessive torque leading to tap breakage are also reported.