CNC formula reference
CNC RPM, Feed Rate and Chip Load Formulas
Keep the formula and unit basis visible before a starting parameter enters a CNC process plan. This reference page explains the relationships; it does not supply material, tool, or machine presets.
Source policy: record the source for every input and validate the result on the machine before production.
Core formulas
| Value | Formula | Input basis |
|---|---|---|
| RPM | RPM = (surface speed × 1000) ÷ (π × tool diameter) (metric) | Surface speed and diameter use matching units. |
| Feed rate | Feed = RPM × flute count × chip load | Chip load is per tooth; flute count comes from the installed tool. |
| Chip load | Chip load = feed rate ÷ (RPM × flute count) | Use when checking a recorded feed against spindle speed. |
| Surface speed | Surface speed = (π × diameter × RPM) ÷ 1000 (metric) | Convert diameter and speed to the desired reporting unit. |
Unit conversion
- Metric: diameter in mm, surface speed in m/min, feed in mm/min.
- Imperial: diameter in inches, surface speed in surface feet/min, feed in inches/min.
- Cross-check: convert the final feed to the controller unit before comparing it with a program or log.
- Limit: a unit conversion cannot validate a tool, material, machine, or workholding choice.
Input and validation checklist
Keep these fields together when transferring a value to CAM, a worksheet, or the calculator.
| Record | What to capture | Validation |
|---|---|---|
| Tool | Diameter, flute count, tool identity, and tool condition. | Matches the tool loaded for the operation. |
| Process | Operation, material record, surface-speed or chip-load source, and unit system. | Source applies to the selected operation and machine context. |
| Machine | Spindle limit, controller units, coolant/air condition, and workholding note. | RPM is below the recorded machine limit and the program units are explicit. |
| Verification | Test-cut result, sound/load observation, measured finish, and reviewer/date. | Release only for the documented scope; revise when conditions change. |
Use the formula, then validate the process
The formulas connect tool diameter, surface speed, spindle speed, flute count, and chip load. They do not decide whether the selected values are appropriate for a tool, material, machine, or workholding setup.
Metric RPM = (surface speed × 1000) ÷ (π × diameter). Feed rate = RPM × flute count × chip load. Imperial versions use the matching inch and surface-feet units.
Do not treat this page as a machine preset or material parameter table. Verify starting values with tool-maker data, machine records, CAM review, and a controlled test before production.