An Analysis of Conical Drill Point Grinding — the Generation Process and Effects of Setting Errors

Abstract

The final drill point geometry generation process for general purpose twist drills using the popular conical grinding method is described and modelled. Based on the “ideal” case analysis previously reported, the fundamental elements of this process are identified and analysed. For this “ideal” (design) case, the grinder described can produce all the recommended specified drill point feature values for a range of grinding cone angles. The introduction of grinder setting deviations from the “ideal” case (setting errors) severely complicates the analysis and the generated curved lip drill point shape. New definitions for the geometrical features and the “acceptable grinder criteria” had to be established. The effects of deviations in six grinder settings and two drill flute-features have been studied for 80 general purpose combinations of drill point feature values. It is shown that even with small deviations, unacceptable drill shapes occur for 26 combinations. For the remaining 54 combinations essentially straight drill lips are obtained although the ranges between the design and generated features can differ substantially. The dominant setting errors and the susceptible generated features are identified. This study highlights the difficulties in achieving and controlling the specified geometry in practice.