Milling speed and feed are connected calculations, not interchangeable settings. Surface speed and effective cutter diameter determine spindle RPM. RPM, chip load, and the number of effective cutting edges then determine table feed.
Metric RPM: n = (vc × 1,000) ÷ (π × D)
Imperial RPM: n ≈ (SFM × 3.82) ÷ D
Table feed: vf = fz × n × zc
The complete calculation chain
Begin with a cutting speed supplied for the exact cutter, cutting material, work material, and engagement. Convert that value to RPM using the effective cutting diameter. For a square end mill this is usually the nominal diameter; ball-nose and form tools can require a diameter at the actual contact point.
Next choose chip load per tooth from current tooling data and multiply it by RPM and the effective tooth count. Metric chip load produces mm/min; inch chip load produces IPM.
Metric worked example
A 10 mm, four-flute end mill is assigned 120 m/min and 0.05 mm/tooth.
RPM = (120 × 1,000) ÷ (π × 10) = 3,819.7.Feed = 0.05 × 3,819.7 × 4 = 763.9 mm/min.- A practical starting program could use 3,820 RPM and 764 mm/min, provided both remain inside the tool, holder, and machine limits.
| Input | Value |
|---|---|
| Cutting speed | 120 m/min |
| Diameter | 10 mm |
| Chip load | 0.05 mm/tooth |
| Effective edges | 4 |
| Result | 3,820 RPM · 764 mm/min |
How to select the inputs
The equations convert selected cutting data; they do not create safe cutting data. Start with the cutter manufacturer’s current recommendation for tool grade, coating, diameter, work-material group, hardness, radial engagement, axial depth, coolant condition, and toolpath. A value copied from a different cutter family is not equivalent evidence.
Count effective edges, not simply the number of visible inserts. Face mills, staggered cutters, and special indexable bodies may define an effective tooth count differently. For light radial engagement, calculate whether radial chip thinning changes the programmed feed per tooth.
Checks before running the cut
- Cap RPM to the lowest rating among spindle, holder, tool, and balance class.
- Check the machine feed limit and whether short moves can reach the programmed feed.
- Do not carry a chip-thinning feed increase into slots, tight corners, or plunges.
- Account for runout, overhang, workholding, interrupted cuts, coolant, and machine rigidity.
- Prove the operation conservatively and inspect chips, sound, spindle load, size, and finish.
Technical references
Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.
Worked chain: the machine cannot reach the calculated RPM
Assume a hypothetical 6 mm, three-flute cutter has been assigned 120 m/min and 0.025 mm/tooth for a validated application. The unrestricted arithmetic gives n = 120,000/(π × 6) = 6,366.2 RPM and feed = 6,366.2 × 3 × 0.025 = 477.5 mm/min. These cutting inputs are illustrative, not a recommendation for a material.
If the permitted spindle speed for this setup is 5,000 RPM, use that actual speed throughout the remaining chain. Achieved surface speed is π × 6 × 5,000/1,000 = 94.25 m/min. Feed for the same programmed chip load becomes 5,000 × 3 × 0.025 = 375 mm/min. Keeping the original 477.5 feed after reducing RPM would instead produce 0.03183 mm/tooth, about 27.3% above the assumed target.
| Quantity | Unrestricted calculation | After the stated RPM cap |
|---|---|---|
| RPM | 6,366.2 | 5,000 |
| Surface speed | 120 m/min | 94.25 m/min |
| Feed preserving 0.025 mm/tooth | 477.5 mm/min | 375 mm/min |
The lower surface speed must still be suitable for the tool and work material. Preserving chip load is only an arithmetic condition; it does not automatically validate the new operating point. A feed-axis cap or engagement change requires another check.
Use this page as the full calculation chain. For individual questions, see RPM and effective diameter, forward feed-rate calculation, or reverse-checking a program’s chip load. The formulas follow the unit conventions in Sandvik’s milling formula reference.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.