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CNC Speeds and Feeds Formulas

The formulas you need at the machine, with a calculator that does them for you. Numbers are a starting point — listen to the cut and adjust.

Spindle speed—RPM
Feed rate—IPM
G84 tapping feed—F (G94)

Milling formulas

  • RPM = (SFM × 3.82) ÷ D
  • SFM = RPM × D ÷ 3.82
  • IPM = RPM × flutes × chip load
  • Chip load = IPM ÷ (RPM × flutes)
  • Metal removal rate = width × depth × IPM (in³/min)
  • Drill point length (118°) ≈ 0.3 × D

Turning & threading formulas

  • RPM = (SFM × 3.82) ÷ part D — or let G96 do it
  • IPM = RPM × IPR (feed per rev, G99)
  • Lead = 1 ÷ TPI (use as F in G76)
  • External UN thread height ≈ 0.6134 × pitch
  • Internal UN thread height ≈ 0.5413 × pitch
  • Surface finish (Ra, µin) ≈ (IPR² ÷ (32 × nose R)) × 10⁶

Formula questions

How do I calculate RPM from surface speed?

RPM = (SFM × 3.82) ÷ tool diameter in inches. In metric, RPM = (m/min × 1000) ÷ (π × diameter in mm).

How do I calculate feed rate for a mill?

IPM = RPM × number of flutes × chip load per tooth.

What is 3.82 in the RPM formula?

It is 12 ÷ π, converting surface feet per minute to revolutions per minute for a diameter in inches.