CNC Feeds & Speeds Calculator

Frequently Asked Questions

What is chip load and why does it matter?

Chip load is the thickness of material each cutting tooth removes per revolution. Too low causes rubbing and heat; too high overloads the bit. Getting it right maximizes tool life and surface quality.

What SFM should I use for each material?

Common starting points for carbide tooling: softwood 500-600 SFM, hardwood 450-550, MDF/plywood 500-600, acrylic 400-600, and aluminum 200-400 SFM.

Why is my calculated RPM higher than my spindle can reach?

Small-diameter bits require very high RPM to achieve proper surface speeds. If your spindle cannot reach the target, lower the SFM value or use a larger cutter.

Are these numbers a final recipe?

No - they are starting points. Real-world results depend on machine rigidity, bit sharpness, hold-down, and cutting depth. Listen to the cut and adjust.

Does this work for drag knives or laser cutters?

No. This calculator is for rotary end mills and router bits only. Different tools follow entirely different parameters.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

CNC routing quality and tool life depend on hitting the right combination of spindle speed (RPM), feed rate (how fast the gantry moves), and chip load (how much material each flute removes per revolution). Enter your cutter diameter in inches, the number of flutes, and the surface speed (SFM) recommended for your material. The tool converts SFM to spindle RPM, estimates a chip load scaled to the cutter diameter, and multiplies the three together to give a feed rate in inches per minute.

The constant 3.82 in the RPM formula comes from 12 divided by pi, which converts surface feet per minute at the cutting edge into shaft revolutions per minute. A larger diameter cutter sweeps more surface area per revolution, so it reaches the same surface speed at a lower RPM.

Chip load is the critical quality variable. Too low and each tooth rubs instead of slicing, generating heat that dulls the cutter and scorches the wood. Too high and the flutes become overloaded, causing vibration, deflection, and broken bits. The calculator uses a starting chip load of 0.005 inch per tooth for a 1/4 inch bit and scales linearly with diameter as a reasonable starting point.

Use Cases

Feed and speed calculations are needed whenever you set up a CNC routing job:

Setting up a hobbyist CNC router (Shapeoko, X-Carve, Onefinity) for the first time with a new bit.

Optimizing passes on a production CNC for furniture parts in hardwood, MDF, or plywood.

Dialing in acrylic and soft-aluminum cuts where burning and melting are the failure modes.

Selecting the right RPM when moving from a small end mill to a larger surfacing bit on the same machine.

Troubleshooting poor surface finish, excessive fuzz, or premature bit wear by checking whether current settings are in range.

Tips

Start conservatively and dial in from there:

Always check your spindle's maximum rated RPM and cap the calculated value to that limit before cutting.

For the first run on a new material or bit, use 50-70% of the calculated feed rate and listen for chatter or burning, then step up.

Deep cuts (full-depth) demand a reduced feed rate even if the RPM is correct; take multiple shallower passes instead.

Chip color tells the story: light tan or clear chips are ideal; dark or scorched chips mean reduce feed or increase RPM slightly; fine dust instead of chips means chip load is too low.

Measure your real chip load by cutting a known length, counting flute marks in the material, and comparing to the target - it is the single best feedback on whether the recipe is right.

FAQ