Improvement of Hole Accuracy by Piloting Drills.

Abstract

This paper deals with the improvement of hole accuracy by piloting drills. The piloting drills have two short cutting edges and a piloting part on the projection of twist drill point. Experiments showed the following results. These drills improve the roundness at the hole entrance, because the wandering motion of drills can be suppressed by a short chisel edge and has already damped under the effect of a piloting part when the corners of primary cutting edges enter workpiece. In the rotating drill-stationary workpiece system piloting drills produce smaller hole oversize, hole deviation of axis and its scatter at the hole entrance than conically ground drills. Further improvement of them can be obtained by two-step feed, which consists of a very small feed rate and an ordinary feed rate before and after the primary cutting edges enter workpiece, respectively. The improvement of hole accuracy results from small oversize of pilot hole at a small feed rate. Longer piloting part is desirable from the viewpoint of the reduction in hole oversize.