CNC / machinist calculator
Turning (Lathe) Feeds and Speeds Calculator
On a lathe the part spins, not the tool, so the spindle RPM is set by the workpiece diameter and the cutting speed, and it changes every time the diameter changes. Enter the material and the diameter you are cutting and this calculator returns the spindle RPM, the table feed from your feed per revolution, and the material removal rate, so you can size a roughing pass against the cut you actually have on the lathe.
- Feed rate
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- Material removal rate
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- Required power
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- Spindle torque
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How it works
Turning uses the same surface-speed relationship as milling, but with the workpiece diameter in place of the tool diameter, because the part is what rotates against a fixed tool. The spindle RPM is twelve times the surface speed in SFM divided by pi times the diameter being cut. As you face in toward the center the diameter shrinks and the RPM for a constant surface speed climbs, which is why lathes offer a constant-surface-speed mode that adjusts RPM automatically.
Feed in turning is a feed per revolution, the distance the tool advances along the part each turn. The table feed in inches per minute is that feed per revolution times the RPM. A finer feed leaves a smoother finish but takes longer; the surface finish calculator turns a feed and a tool nose radius into a theoretical roughness.
Material removal rate on a lathe is twelve times the surface speed times the depth of cut times the feed per revolution, which is the productivity number to watch when roughing and the basis for the spindle power the cut demands.
Worked example
Turning a 1 in 1018 steel bar at 350 SFM: RPM = 12 x 350 / (pi x 1.0) = 1,337 RPM. Face toward the center and the RPM must climb to hold that surface speed.
Frequently asked questions
How do I calculate lathe RPM?
Multiply the surface speed in SFM by 12 and divide by pi times the workpiece diameter in inches. Because the part rotates, the diameter you use is the diameter being cut, not a tool diameter.
What is constant surface speed on a lathe?
Constant surface speed mode raises the spindle RPM as the cutting diameter shrinks so the surface speed at the tool stays the same. It keeps finish and tool life consistent when facing or turning a tapering profile.
What feed per revolution should I use for turning?
Roughing in steel often runs around 0.010 to 0.020 inch per revolution and finishing far less, but it depends on the insert nose radius and the finish you need. A larger nose radius tolerates a heavier feed for the same roughness.
How is turning MRR calculated?
Material removal rate equals twelve times the surface speed in SFM times the depth of cut times the feed per revolution, giving cubic inches per minute. It is the productivity figure that also sets the spindle power the roughing cut needs.
Why does my RPM keep changing as I face a part?
At a constant surface speed the RPM is inversely proportional to diameter, so as you face toward the center the diameter falls and the required RPM rises. Near the center it can spike, which is why many controls cap the maximum RPM.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.