CNC Speeds and Feeds Calculator
Speeds and feeds decide tool life, cycle time and finish. This calculator converts a catalog surface speed and chip load into the RPM and feed rate you type into your program, and tells you when your spindle limit changes the answer.
Your numbers
Surface feet per minute from your tool maker's catalog for this material and coating.
Use the actual cutting diameter, not the shank.
Feed per tooth from the catalog. Reduce it for slotting or small diameters.
The calculated RPM is capped at this value.
Your results
Starting values only. Confirm against your tool manufacturer's data and your machine, fixture and workpiece rigidity before cutting.
Feed rate (IPM) vs surface speed
Sweeps Surface speed from 100 to 1000, holding your other inputs fixed.
Review cutting standards against ERP routing data
Optional: send us your routing and run-time exports and we will point out where standards drift from what the shop actually runs.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Start with the surface speed (SFM) and chip load per tooth that your cutter manufacturer lists for your workpiece material. Enter the cutter diameter and flute count, then your machine's maximum RPM. The calculator returns the ideal RPM, the RPM actually programmed after the spindle cap, the feed in inches per minute and millimeters per minute, and the real surface speed you end up running. The chart shows how feed changes as you raise or lower SFM, which helps when you are tuning a job and want to see the cycle time cost of running cooler.
- Use catalog values as the starting point, not the finish line.
- Enter the cutting diameter, not the shank diameter.
- Check the capped RPM against the actual SFM before accepting the feed.
The formulas behind it
Surface speed is how fast the cutting edge travels past the material: SFM = pi x diameter x RPM / 12 when diameter is in inches. Rearranged, RPM = SFM x 12 / (pi x diameter), which is the familiar shortcut of about 3.82 x SFM / diameter. Feed rate then follows from chip load: IPM = RPM x number of flutes x chip load per tooth. Metric users can use Vc in meters per minute with RPM = 1000 x Vc / (pi x diameter in mm), which is the same relationship in different units.
Worked example
A half inch four flute carbide end mill in aluminum at 350 SFM gives an ideal RPM of about 2,674. At 0.003 inch per tooth the feed is 2,674 x 4 x 0.003, roughly 32 IPM. On a small high speed machine the same cutter could run at 800 SFM or more and the spindle cap would start to matter: if RPM is limited to 12,000 the actual surface speed is lower than the target and the feed must be recalculated from the capped RPM, which this tool does for you.
Common mistakes
The most common error is using a chip load that ignores the cut. In slotting the full diameter is engaged and heat has nowhere to go, so lower SFM and feed. In light radial cuts, chip thinning means the true chip is thinner than the programmed feed per tooth, so many shops raise the feed to compensate. Other mistakes include mixing metric and inch values, forgetting that long stickout reduces usable speed, and ignoring the machine's actual power at low RPM.
Frequently Asked Questions
How do I convert SFM to RPM?
Use RPM = SFM x 12 / (pi x diameter in inches), or roughly 3.82 x SFM divided by the diameter. For metric, RPM = 1000 x Vc in meters per minute / (pi x diameter in millimeters). This calculator does the conversion and applies your spindle limit.
What chip load should I use?
Use the value your tool manufacturer lists for the cutter diameter, material and type of cut. Smaller diameters need much smaller chip loads. The companion chip load worksheet on this site gives typical starting ranges for common materials.
What if my calculated RPM is higher than my spindle can run?
Cap the RPM at the spindle maximum and recalculate feed from the capped value. You lose some surface speed, so you also lose some productivity, but the cut remains stable. This tool shows both ideal and capped values.
Does this work for turning or drilling?
The RPM formula works for any rotating cutter or workpiece diameter. For turning, feed is per revolution rather than per tooth, so multiply RPM by feed per revolution. For drilling use feed per revolution with the drill diameter.
If your programs live in an ERP or MES, Netray can help surface the routing data, tooling and run-time standards behind them so cutting data is reviewed against real job history.
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