Mechanics of tapping process with emphasis on measurement of feed error and estimation of its induced indentation forces

Feed error induced by the mismatch between the spindle motor and Z-axis motor of the machine can significantly influence the cutting loads during thread tapping process. In this paper, a comprehensive study, which allows predicting the feed error-induced force together with measuring the actual tapping feed error, is presented by establishing simplified methods. Convenient experimental setup and procedure are first proposed to measure the actual feed error by using a laser tachometer along with a laser interferometer, followed by modeling the contact status between the tap flank and workpiece under feed error as an indentation effect, under which the corresponding indentation force is derived as a simplified proportional function of the indentation volume by using the basic principle of contact mechanics. Identification scheme is also developed to determine the proportional coefficient, i.e. the indentation force coefficient. Total tapping forces are obtained by summing cutting forces related to material removal mechanism and the indentation forces associated with feed error. Experimental verifications have been carried out to validate the proposed methods.