Analysis of Feed Errors in Tapping by Contact Stress Model

One of the most common tapping errors, mismatch of tap feed rate and the thread pitch, has a significant effect on the tapping thrust signal and the resultant thread profile. The underfeed of the tap leads to an upward pull on the workpiece and overfeed causes a downward push. Tapping experiments are conducted with intentionally magnified feed errors to study the phenomenon and aid in the model development. Using contact stress theory for elastic materials, a model for tap-workpiece interaction is developed and integrated with the mechanistic force model developed earlier to predict the tapping forces. In most cases there was a good agreement between the simulations of combined model (contact stress model coupled with mechanistic model) and the experimental results. Tap tooth design with relief on the flank has been shown to significantly reduce the tapping thrust sensitivity to the feed error, thereby affecting the tool life and improving the thread quality.