@MLDJR,
My old LeBlond is running at 492
The circumference of the work is C = 2 * pi * radius = pi * dia.
Since we need the circumference in feet (not inches) to match with our surface FEET per minute:
C[ft] = pi * dia.[in] / 12
since pi ~= 3 (close enough for these calculations)
C[ft] ~= 3 * dia.[in] /12
C[ft] ~= 3/12 * dia[in]
C[ft] ~= 1/4 dia.[in] or dia.[in] / 4
The SFPM is the circumference(in feet) multiplied by the RPM:
SFPM = C[ft] * RPM
SFPM = dia.[in] /4 *RPM
Then using the above approximations;
492 RPM is about 31 SFPM at 1/4" dia. (1/8" radius)
492 RPM is about 492 SFPM at 4" dia. (2" radius)
(so I agree with
@Randall Marx within a bit of slop due to my simplifications)
For cutting mild steel with a HSS tool bit I aim about 80 SFPM.
80 SFPM is about 1280 RPM at 1/4"dia. (1/8" radius)
80 SFPM is about 80 RPM at 4" dia. (2" radius)
So your original 492 RPM was actually slower than required at the inner diameter, but way too fast at the 4" outer diameter. Too slow is usually okay. Too fast can be a big problem.
(Don't forget to convert the diameter in inches to feet in the calculation!)
-brino