A Study of Lip Clearance on Twist Drills

Abstract This paper discusses the form of the warped surface on twist drills called “lip clearance” as produced by five makes of drill-sharpening machines. Theoretically this lip-clearance angle should increase as measured from the circumference toward the center of the drill. So-called correct angles are assumed for various-diameter drills with a given feed, and illustrations show how much the drills actually vary from the correct angles. The contour of the lip-clearance surface is measured by a special measuring machine. The drills are rotated by 10-deg increments and the dial-indicator pointer on the measuring machine moved a definite distance from center for each measurement. Thus, deviations of the surface from a true cone are measured. The angle is easily computed from these data. Detailed description shows how the lip-clearance surface is developed and a comparison is made of the actual with the assumed correct angle of clearance. Figures show the developed surfaces of ⅜-in. and 1-in. drills as sharpened on these five drill-grinding machines. Some drills have beveled corners and a negative clearance may be noted on these because of lack of care in sharpening the drills. The developed surface of a drill with a thinned web shows how the possibility of rubbing back of the cutting edge in the web may be reduced. The figures show that too much clearance is ground near the circumference of the drill in most of the cases studied. A few, however, approach quite near to the so-called correct angles. The drills are tested on drill presses equipped with dynamometers in annealed cast iron at approximately 60 fpm surface speed. Torque and thrust curves are plotted to show the power required to operate these drills. Considerable variation will be noticed in the torque and thrust. The advantage of thinning the web is brought out quite positively in these curves. The paper suggests a future study to endeavor to ascertain if one machine sharpens drills that are better adapted for drilling one material than another, also that endurance tests should be run to test the life of drills between grinds. There is no intention to criticize the design of the various sharpening machines, but rather their operation.