# RPM calculations

• posted
Could anybody tell me the formula for calculating drill speeds, (
specifically mild steel ).
I normally just go by eye or from experience, but I need an accurate answer
for a college
assignement.
I have found a couple of items on the net, but they seem to result (in my
opinion), in
totally unrealistic RPM figures.
John
• posted
For mild steel about 100 ft / min should be O.K, for a 1/4" drill that is about 1500 rpm
What speeds have you worked out ?
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
• posted
I was looking to find the speeds for 3.3mm and 4.1mm drills, I was coming up with speeds of around 2500 RPM for a 4mm drill. Your 100ft a minute also brings up a similar figure. Is this the sort of speed you would use? I have been a toolmaker for 20 years and have always done things by sight (and sound) and would only use about 850-900rpm for a 4.1dia, and about 1000rpm for the 3.3mm.
John
• posted
Far be it for me to argue with twenty years experience, but I was quoting from the students shop reference handbook.
I generally find that with a machine ground ( new ) drill what they recommend works, but if I have resharpened a drill by hand I generally want to run a little slower.
Normally I'm to lazy to change from the pillar drills bottom speed of 500 which is defiantly as fast as I want to run a 1/2" drill in 316 stainless steel :-( ...... I'm looking for a drill that will run a lot slower, as I have to use the mill to work with hole saws at present.
Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
• posted
Your figures are right - 2366 rpm at 11.8ipm feed for 4.1mm - 2940 14.7ipm for 3.3mm
Triple those for Carbide drills.
Those figures are from Cimco's tables. They are for coolant and maximum speed, not taking into account tool wear.
There's a good article and tables here