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CNC Speeds and Feeds Calculator

Estimate spindle speed, programmed feed rate, and material removal rate for milling operations from cutter geometry and chip load.

Category: Manufacturing & Materials. Formulas cited: RPM = (SFM x 12) / (pi x diameter); metric RPM = (Vc x 1000) / (pi x D); feed rate = RPM x flutes x chip load.

Inputs

Use manufacturer data when available. Presets are practical starting hints, not fixed recommendations.

Choose a material to see conservative starting ranges for surface speed and chip load.
Cutter diameter at the cutting edge.
Use SFM for imperial or Vc in m/min for metric.
Number of effective cutting edges.
Feed per tooth from toolmaker or process data.
Axial depth of cut.
Radial engagement.

Results

Calculated from the current inputs.

Spindle speed

3,820

RPM

Feed rate

22.9

in/min

MRR

0.716

in³/min

With a 0.5 in tool at 500 SFM, spindle speed is 3,820 RPM and feed rate is 22.9 in/min.

RPM = (SFM x 12) / (pi x diameter) Feed rate = RPM x flutes x chip load MRR = feed rate x depth of cut x width of cut

Self-Tests

Golden cases verify the RPM, feed, and MRR formulas used by this page.

Self-tests: not run

About the CNC Speeds and Feeds Calculator

CNC speeds and feeds calculations help programmers and machinists turn cutter diameter, surface speed, flute count, and chip load into spindle RPM, feed rate, and material removal rate. The tool supports first-pass setup choices for milling or drilling, then leaves room for adjustment based on toolholder rigidity, coolant, machine power, and chip evacuation.

How it works

  1. Enter cutter diameter, flute count, and recommended surface speed.
  2. Add chip load per tooth or feed per revolution.
  3. Choose milling, drilling, or another supported operation mode.
  4. Review RPM, feed rate, and material removal rate before proving out the cut.

Frequently asked questions

What is the RPM formula for milling?

In inch units, RPM is surface feet per minute x 12 divided by pi x cutter diameter. Metric calculations use cutting speed in meters per minute and tool diameter in millimeters.

What does chip load mean?

Chip load is the thickness of material each cutting edge removes per revolution. Too little chip load can rub and heat the tool, while too much can overload the edge.

Where do recommended surface speeds come from?

Use the cutting tool manufacturer's data for the tool material, coating, work material, operation, and coolant condition. Generic charts are only starting points.

Why did a calculated feed chatter on my machine?

Chatter can come from tool stickout, weak workholding, spindle condition, toolpath engagement, machine rigidity, or poor chip evacuation. Reduce engagement or adjust speed and feed after checking setup.

Does more flutes always mean faster feed?

Only if chip evacuation and horsepower allow it. More flutes can raise feed rate mathematically, but they may pack chips in slots or soft materials.

References