CNC Machinist how-to

How to Calculate Spindle Speed (RPM)

Correct speeds extend tool life and improve finish.

Updated By the Career Education Institute Curriculum Team
Quick answer

Correct speeds extend tool life and improve finish. In short: find; measure; use, then complete the remaining steps below.

Step by step

  1. Find the recommended surface feet per minute (SFM) for the material and tool.

  2. Measure the tool or part diameter in inches.

  3. Use the formula RPM = (SFM × 3.82) ÷ diameter.

  4. Example: 400 SFM ÷ 0.5-inch end mill → (400 × 3.82) ÷ 0.5 ≈ 3,056 RPM.

  5. Calculate feed rate: RPM × number of flutes × chip load.

  6. Start conservatively and adjust based on sound, chips and finish.

Safety reminders for CNC machinists

  • Wear safety glasses and keep loose clothing, jewelry and hair away from spindles
  • Never reach into a running machine or clear chips by hand
  • Keep machine doors and guards closed during operation
  • Handle sharp chips and parts with gloves only when the machine is stopped

CNC · G-code · M-code · Work offset · Tool length offset · Feeds and speeds: see the full CNC machinist glossary.

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