Cobalt Drill
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difference between Rockwell hardness C63 and C67?
M35 & M42 Cobalt drills are specified at HC65-67 hardness and M1, 2, 7, 50 general purpose drills are specified as HC 63-65. The first question is - percentage wise what does that difference represent?
Ultimately, what difference does it make in the life of a drill or the ability to drill into something hard like stainless steel?
What does the extra 3% cobalt in M42 drills over M35 drills buy operation / life wise?
Stainless steel isn't necessarily particularly hard, though it can be. It does tend to be harder to machine, which is a different thing altogether.
(Some knives made of high-alloy steels are almost impossible to hand sharpen, but are no harder than low alloy but high carbon knife blades of the same Rockwell hardness.)
I have no experience with drills, but a web search turned up the following pages that might be of interest:
http://en.wikipedia.org/wiki/High_speed_steel
http://www.eaglemountstudios.com/Cobalt-vs-Carbide-End-Mills-article13953--7.htm
http://www.icscuttingtools.com/Tooldata.htm
One thing that is clear, is that M42 does better at red-heat temperatures than M35 and so allows faster cutting.
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