Drill Grinding Improves Machine Performance
Form grinding or facet grinding of your drills? The question is an important one for metalworkers because as much as 50% of machining time is spent drilling holes. Obviously, machine capability is dependent on more than just the spindle. Properly sharpened and applied tools boost productivity; improve part quality, and accelerate payback on machine investment.
For example, properly sharpened drills can reduce center drilling and reaming operations, extend tool life, reduce breakthrough burrs, and improve part quality. But how do you go about improving drill performance? How do you increase part output while improving quality? Let's examine two basic methods used to put a point on a drill.
In conventional facet drill point grinding, the grinding wheel and drill point make contact along only one axis. The result is a drill point that has two large flat facets separated by a straight chisel edge. There is no cutting action in the area of this chisel due to a large negative rake cutting surface. Consequently, conventional drills create high thrust forces causing them to cut oversize holes with poor location. (See figure 1)
In formed tri-axial drill point grinding, the grinding wheel and drill make contact through three simultaneous axes of motion. The result is a drill point with a continuous formed relief separated by a formed chisel edge. Being formed, this S-shaped chisel includes a relieved leading edge that has a small negative rake cutting surface. For this reason, formed drill points reduce cutting forces and improve hole quality. (See figure 2)
The major benefit of formed grinding is found at the center of the drill or in the chisel area. Generated tri-axial ground drill points include an active cutting chisel that cuts chips instantly and cleanly upon part contact. In contrast, a conventionally ground drill point with the straight chisel edge can only push or extrude material not cut it.
Productivity increases result from higher efficiency at the drill point. For high performance drilling, it is important that a chip is formed immediately upon entering the material. This creates chip flow, takes the heat away from the cutting edge, and allows the drill to actively hold location. The drill cuts to size and drills a straighter hole.