CNC feeds and speeds calculator
A bit that burns wood is spinning too fast for its feed; a bit that snaps is feeding too fast for its speed. This tool computes a starting speed + feed pair from the material’s surface speed and the chip load — the two quantities forum spreadsheets hide.
Machining throws chips and can snap tools: safety glasses are mandatory, tie back hair and clothing, and never leave a CNC cutting unattended. These cautious starting values do not replace your tool manufacturer’s recommendations.
Spindle speed
18,568RPM
- Feed rate
- 175in/min
- Chip load (fz)
- 0.0047in
Machining throws chips and can snap tools: safety glasses are mandatory, tie back hair and clothing, and never leave a CNC cutting unattended. These cautious starting values do not replace your tool manufacturer’s recommendations.
The method
Everything starts from the surface speed Vc: how fast the cutting edge must sweep through the material, specific to each material (450 m/min in pine, 40 in mild steel). The tool’s circumference converts that speed into revolutions per minute — a small bit must spin faster than a big one.
If the theoretical speed exceeds your spindle, it is capped at the machine’s maximum — almost always the case in wood with small bits, and harmless: the feed adapts accordingly.
Feed follows from the chip load fz: the thickness of material each tooth must bite per revolution. Too small, the tooth rubs instead of cutting and overheats; too big, it snaps. fz grows with diameter (a bigger bit takes more), hence fz = k × D.
In the formula: Vc the surface speed (m/min), D the diameter (mm), N the spindle speed (RPM), Z the flute count, fz the chip load (mm).
N = Vc × 1,000 / (π × D), capped at spindle max · feed = N × Z × fz · fz = k × D
Good to know
- These are cautious STARTING values: a rigid, well-tuned machine often takes 30–50% more; a flexy lightweight machine sometimes needs 30% less.
- Depth of cut is not computed here: start from half the diameter in wood, a quarter in aluminium, and listen to the machine.
- Sound tells the story: a high-pitched squeal = rubbing (raise the feed or lower the speed); a dull hammering = overload (the opposite).
- In plastics, the enemy is re-welding: single flute, a decisive feed, and compressed air to clear chips if you can.
- Steel on a hobby CNC assumes a slow, rigid spindle: at a 24,000 RPM minimum, a router simply cannot machine steel.
FAQ
- Why is my bit burning the wood?
-
Because each tooth removes too little material: it rubs, heats up and chars instead of cutting. Classic causes: feed too slow for the speed, a dull bit, or the machine dwelling in the material. The counter-intuitive fix: feed FASTER, or spin slower.
- One or two flutes for aluminium?
-
One, on hobby machines: aluminium makes big sticky chips that pack the flutes, and a fast spindle leaves no time to clear them between teeth. A single-flute bit gives the chip all the room. On a proper slow, coolant-fed mill, 2 or 3 flutes make sense again.
- My spindle cannot spin slowly enough: what then?
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That is the steel-on-a-router case (10,000 RPM minimum when 2,000 is needed): surface speed is exceeded fivefold, the edge overheats and dies in seconds. No setting compensates — it is a hardware limit. Stick to wood, plastics and cautious aluminium.
- What is the displayed chip load worth?
-
It is the target chip thickness per tooth per revolution — the figure machinists compare. Ballparks at 6 mm: 0.10–0.15 mm in wood, 0.03–0.05 in aluminium. If your tool manufacturer publishes its own values, they override this generic calculation.
- Why do my “working” settings differ from the calculation?
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Because the calculation ignores your actual rigidity: machine, workholding, tool stick-out. A slower setting that works is not wrong — it fits your machine. The point of the calculation is knowing which DIRECTION to correct when things go badly, instead of guessing.