How it works
Every cutting edge removes a small chip on each turn. The chip load is the thickness of that chip, measured as the distance the tool advances per tooth. Feed rate is how fast the tool travels, so feed = RPM × flutes × chip load.
Surface speed is how fast the cutting edge moves past the material. When the sideways width of the cut is less than half the tool diameter, the chips come out thinner than the advance per tooth because the edge only nicks the surface. That is chip thinning, and you speed up the feed to compensate. The calculator shows the feed with and without that adjustment.
Worked example
A 1/4 inch two-flute bit at 18,000 RPM with a 0.005 inch chip load, cutting a full-width slot 1/4 inch deep:
- Feed = RPM × flutes × chip load = 18,000 × 2 × 0.005 = 180 in/min
- Surface speed = π × 0.25 in × 18,000 ÷ 12 = 1,178 SFM
- Chip thinning at a 0.25 in width of cut = 1×, so feed 180 in/min keeps the chip load
- Removal rate = 180 × 0.25 × 0.25 = 11.25 in³/min
| Input | Value |
|---|---|
| Tool diameter | 0.25 in |
| Number of flutes (cutting edges) | 2 |
| Spindle speed | 18000 RPM |
| Chip load per tooth | 0.005 in |
| Radial width of cut (stepover) | 0.25 in |
| Axial depth of cut | 0.25 in |
| Result | Value |
|---|---|
| Feed rate for the chip load entered | 180 in/min |
| Surface speed | 1,178 SFM |
| Chip thinning factor at this radial width | 1 × |
| Feed rate to keep that chip load with chip thinning | 180 in/min |
| Material removal rate at the entered feed | 11.25 in³/min |
Assumptions and limits
- Chip load is an input. This tool does not include material recommendations, because they depend on the tool, machine, and stock.
- Feed is the linear feed of the tool axis. Corner slowdowns, acceleration, and arc feed compensation are not included.
- The chip thinning factor applies to end milling with a straight-flute geometry at radial widths below half the diameter, and is a first-order correction.
- Metric units use the same formulas: mm per tooth, mm/min, and meters per minute.
Common questions
How do I calculate feed rate for a CNC router?
Multiply spindle RPM by the number of flutes and by the chip load. A 2-flute bit at 18,000 RPM and 0.005 in chip load feeds 180 in/min.
What chip load should I use for wood?
Use the tool manufacturer's range for your species and bit, which differs for hardwoods, softwoods, and sheet goods. This calculator does not guess a value for you.
What is chip thinning?
When the stepover is less than half the tool diameter, each tooth takes a thinner chip than the feed per tooth suggests. To keep the intended chip load you raise the feed. At 10% of the diameter the factor is 1.67, and at 25% it is 1.15.
Why is my bit burning the wood?
Usually the chip load is too low for the RPM, so the edge rubs instead of cutting, or the bit is dull. Raise the feed rate or lower the RPM.
Sources
- Standard machining relationships (feed = RPM × flutes × chip load; SFM = π × D × RPM ÷ 12), as given in Machinery's Handbook and tool-maker technical guides.
- Radial chip thinning factor from the geometry of the tool engagement: chip thickness scales as 2 √(ae(D − ae)) ÷ D for engagement under half the diameter.
Updated 2026-09-30