In Veeduber wrote:

AN3's need about 4000rpm, AN4's about 3000.

You can work all this out for yourself. The equation is: SFM / (.
2618 x drill diameter) = RPM.

SFM is 'Surface Feet per Minute;' a constant reflecting the type of
material being drilled. Aluminum is 100, cast iron 70, mild steel 60,
alloy steel 40... it's in all the manuals. (Or usta be :-)

I don't claim to have anything close to your experience, but in A&P
school I was taught to use up to 300fpm for aluminum with hss bits,
depending on the alloy used. Out in the garage I have a copy of a speed
chart which seems to justify this. With a plain carbon steel bit your
numbers make a lot more sense so that is probably their basis, but I
think most builders are probably using hss bits if not something better
like cobalt. I do know that my 6000rpm air drill works slicker than
snot for drilling #30 and #40 holes in aluminum. Drilled around 1500 #
40 holes in my wing ribs last weekend using it. The standard 2700-
2800rpm air drills work OK for aluminum but that is the slowest I would
want to use. I'm thinking of picking up a 3600rpm motor for
countersinking, but that isn't a huge priority just yet.

----------------------------------------------------
Del Rawlins-
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