A Brief Introduction to Drilling
[Excerpt from Drill Point Grinding, Toycen] Drilling is one of the most complex machining processes. The chief characteristic that distinguishes it from other machining operations is the combined cutting and extrusion of metal at the chisel edge in the center of the drill. The high thrust force caused by the feeding motion first extrudes metal under the chisel edge with a conventional helical tip profile.
Center Cutting tip profiles greatly increase the efficiency of the drilling operation by shearing the material at the center of the tool.
Drill wear starts as soon as cutting begins and instead of progressing at a constant rate, the wear accelerates continuously. Wear starts at the sharp corners of the cutting edges and, at the same time, works its way along the cutting edges to the chisel edge and up the drill margins. As wear progresses, clearance is reduced. The resulting rubbing causes more heat, which in turn causes faster wear.
Unless buying a drill designed for a specific purpose, the average drill as it comes from the manufacturer has a “best general purpose’ tip on it when it comes out of the package. What this means to the shop owner is that there is good chance the tip on the drill will be wrong for the intended application. It has been estimated that about 90 percent of drilling troubles are due to improper grinding of the drill point. Therefore, it is important that care be taken when re sharpening drills.
A good drill point will have: both lips at the same angle to the axis of the drill; both lips the same length; correct clearance angle; and correct thickness of web.
Drill Point Grinders for Standard Chisel Tips and Split Points
Our recommended drill point grinders for drills from 1/16″ to 1-1/4″ are timed-chuck grinders (once the drill is timed in the chuck the complete sharpening process happens by feeding the chuck into the sharpening port with no other set up required). They are the best of their class and are designed to be used on the shop floor, can be operated with minimal training, and produce NAS 907b tip profiles in less time than it takes to load a new drill.