Cutting Feed and Speed Calculator
A cutting feed and speed calculator converts a target surface cutting speed and chip load into the two values a CNC control reads: spindle RPM and feed rate. This one handles both milling and turning, both HSS and carbide tools, and ten common workpiece materials. Feed rate is returned in IPM for milling and IPR for turning, matching the units each operation expects.
Cutting speed equations
- RPM = (SFM x 12) / (pi x D) where SFM is surface feet per minute and D is diameter in inches. For milling, D is the cutter diameter. For turning, D is the workpiece diameter being cut.
- Milling feed rate (IPM) = RPM x chip load x flutes.
- Turning feed rate is expressed as IPR (inches per revolution), typically 0.005-0.020 IPR.
For a 1/2 inch carbide end mill in 4140 alloy steel at 250 SFM: RPM = (250 x 12) / (pi x 0.5) = 1,910 RPM. At 3 flutes and 0.0015 IPT, feed rate = 1,910 x 0.0015 x 3 = 8.6 IPM.
For a 2 inch diameter bar of mild steel turned at 350 SFM with carbide: RPM = (350 x 12) / (pi x 2) = 668 RPM. At 0.010 IPR, feed rate = 668 x 0.010 = 6.68 IPM.
Why tool material matters
The biggest driver of cutting speed after material is tool material. HSS (M2, M42) stays hard up to about 1000 degrees F; above that it softens and wears fast. Carbide stays hard past 1800 degrees F, which is why carbide inserts can run at 3-5x the SFM of HSS. The cutting feed and speed calculator picks SFM from a material table indexed by both workpiece and tool material; a carbide row is always significantly faster than the HSS row for the same material.
IPM vs IPR
Milling controls program feed in IPM (inches per minute) because the cutter moves through the work at a rate largely independent of RPM. Lathes program feed in IPR (inches per revolution) because the feed-per-rev is what determines chip thickness and surface finish. The calculator returns both so you can paste either one straight into the control.