Cuttermasters Handbook on Twist Drills and Drill Point Geometry, CM-BD

Cuttermasters Handbook on Twist Drills and Drill Point Geometry, CM-BD

This is the second in our series of shop reference books about cutting tools. This handbook provides general information about two flute drills. Most commonly associated with producing machined holes, drilling process is one of the fastest material removal opportunities in the machine shop. Many other processes contribute to the production of holes including boring, reaming, broaching, and internal grinding however drilling accounts for the majority of holes produced in most shops. This is because drilling is the most simple, quick and economical method of hole production. The other methods are used principally for more accurate, smoother, larger holes. They are often used after a drill has already made the pilot hole. 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. Then the Material tends to shear under the action of a negative rake angle tool. Center Cutting tip profiles discussed later in the book greatly increase the efficiency of the drilling operation by shearing the material at the center of the tool. The cutting action along the lips of the drill is not unlike that in other machining processes. Due to variable rake angle and inclination, however, there are differences in the cutting action at various radii on the cutting edges. This is complicated by the constraint of the whole chip on the chip flow at any single point along the lip. Still, the metal removing action is true cutting, and the problems of variable geometry and constraint are present, but because it is such a small portion of the total drilling operation, it is not a distinguishing characteristic of the process. 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. Wear lands behind the cutting edges are not the best indicators of wear, since they depend on the lip relief angle. The wear on the drill margins actually determines the degree of wear and is not nearly as obvious as wear lands. When the corners of the drill are rounded off, the drill has been damaged more than is readily apparent. Quite possibly the drill appeared to be working properly even while it was wearing. The margins could be worn in a taper as far back as an inch from the point. To restore the tool to new condition, the worn area must be removed. Because of the accelerating nature of wear, the number of holes per inch of drill can sometimes be doubled by reducing, by 25 percent number of holes drilled per grind. 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. In North America we have had easy access to inexpensive tools a few cost effective options in drill point grinding equipment. To a great extent we have become accustom to using drills “right out of the bag”. This approach is flawed and we may run the risk of losing our competitive edge to those who can run faster cycle times, less expensive set ups, and reduced down time because all of their shop staff can drill point grind.