Brain's40,
Keep in mind the RPM of the drill is not dependent on the thickness of the material to be drilled, but on the diameter of the drill(generally speaking).
If 540 was the appropriate RPM for a 1/2" HS drill in A36 steel, the hole could be 6" deep...makey-no-diff!
The larger the drill diameter, the more SFPM the drill travels in one rotation, and this is the critical factor.
I have a real simple formula for figuring drill RPM based on drill diameter..it's down in the shop, and I ain't. I will bet someone else has that info and will be along shortly.
I wouldn't feel nervous about turning a 1/2" bit at 540 RPM in mild steel...use a liberal amount of Rapid-Tap, or any coolant or cutting fluid.
When the chips come out bright red and stick to your cheek, it's time to back off!
Stuart
