I have a 2 1/2" diameter fingernail bit. I only wanted to use the top part of it, which is only about 1" diameter. I figured that full speed would be okay, and it seemed to work out; but I wonder if the speed is for the bit, or the part of the bit I actually used?
There is no free lunch.
The centripetal forces are applied to the entire bit, with the max force at the largest radii, not just the part that is cutting.
Consider yourself lucky that the bit was probably conservatively rated.
I would not suggest going there again.
Lew
It's both. You shouldn't ever exceed the maximum recommended speed of the bit (which you may not actually be given my the manufacturer unless you contact them), but the best speed to run the bit within that range may not be the same whether you are using all or part of the bit.
When in doubt, spin it slower.
-Leuf
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