An Analytical Study of Three Point Grinding Methods for General Purpose Twist Drills

Abstract

This paper presents ‘acceptable grinder criteria’ for assessing the potential of drill point grinding methods for general purpose twist drills. The criteria are based on geometrical requirements involving the shape and the recommended values of the commonly specified drill point features of conventional general purpose drills as well as the ability of the grinder to independently achieve the specified features. Three plausible drill point grinding methods are considered, namely; the ‘plane flank’, the ‘cylindrical flank’ and the ‘conical flank’ methods. The relevant analyses relating the commonly specified drill point features to the grinding parameters are presented and used to assess the three grinding methods by means of the above criteria. It is shown that for the ‘plane flank’ grinding method, the drill point geometry is over-specified and the acceptable grinder cannot be satisfied. For the ‘cylindrical flank’ method the drill point geometry is uniquely specified although only very few combinations of the values of the specified drill point features meet the acceptable grinder criteria. In contrast, the ‘conical flank’ analysis indicates that the drill point geometry is under-specified but that the acceptable grinder criteria can be satisfied for most values of the specified drill point features. It is concluded that the first two methods are unsuitable for general purpose drill point sharpening while the ‘conical flank’ method is most promising. The criteria proved to be essential for assessing the sharpening methods.