I had to drill 16 blind holes that were 0.101 in. in diameter and 0.300-in. deep in parts made of 316 stainless steel. I tried high-speed steel, carbide and cobalt drills, but all of them broke before completing the 16 holes.
I did some asking around and here are some of the tips I received.
Coolant mixture should be rich, at least 12 percent. Also, a squirt of a 50/50 coolant concentration in a spray bottle or a squirt of Tapmatic fluid during the drilling helps to prevent drill breakage.
For drill speeds and feeds, use standard formulas. For my application, I run the drill at a cutting speed of 60 and chip load of 0.002 in.
If peck drilling, a good initial peck is equal to the drill diameter. Then, all preceding pecks are reduced to 25 percent or less of the drill diameter.
For the drill itself, make sure cutting edges have good edge strength to resist chipping. If the tool is edge prepped properly, it will not dig into the material and can be fed a bit more aggressively which will minimize work hardening the workpiece. Adding a T land or negative chamfers to the cutting edges will not help. The edge preparation must be smooth, properly sized and shaped to resist digging.
Online name:
caldude
Hollywood, Calif.
