TAHA Engineering Toolsby TAHA Knowledge Lab

Machining & Workshop

Cutting Speed & Spindle RPM Calculator

Spindle speed from cutting speed and diameter — or cutting speed from RPM.

Inputs

Calculate from
End view of a workpiece of diameter D rotating at speed n, with a cutting tool at its surface where the cutting speed Vc acts.DnCutting toolVc
Turning: the workpiece of diameter D spins at n rpm; its surface passes the tool at cutting speed Vc. For milling and drilling, D is the cutter diameter. Diagram not to scale.

Take this from your cutting tool manufacturer’s data for the tool and workpiece material.

Turning: workpiece diameter being cut. Milling and drilling: tool diameter.

Result

Spindle speed
763.944

To cut at 120 m/min on a 50 mm diameter, set the spindle to about 763.944 rpm. Use the nearest available machine speed, rounding down if unsure.

Educational estimate — check the assumptions below.

About this calculator

Cutting speed (Vc) is how fast the cutting edge moves across the material. Tool makers give a recommended cutting speed for each tool and workpiece material. The machine, however, is set in revolutions per minute. This calculator converts between the two using the diameter — of the workpiece in turning, or of the cutter in milling and drilling.

Formula

Find spindle RPM

n = (1000 × Vc) ÷ (π × D)

n = spindle speed (rpm), Vc = cutting speed (m/min), D = diameter (mm). Imperial form: n = (12 × SFM) ÷ (π × D), with D in inches.

Find cutting speed

Vc = (π × D × n) ÷ 1000

Vc in m/min, D in mm, n in rpm.

Worked example

Turning a 50 mm diameter bar with a recommended cutting speed of 120 m/min:

  • n = (1000 × 120) ÷ (π × 50) = 120 000 ÷ 157.08 = 764 rpm

Imperial check: 400 SFM on a 2 in diameter: n = (12 × 400) ÷ (π × 2) = 764 rpm.

Assumptions and limitations

  • The calculator does not recommend a cutting speed — it only converts. Recommended speeds depend on tool material, coating, workpiece material, depth of cut, coolant and machine rigidity; use your tool supplier’s data.
  • In turning, the diameter changes as you cut (and is zero at the centre in facing). Calculate at the diameter being cut.
  • Never exceed the maximum speed rated for the chuck, spindle, tool or workpiece.

Safety note

High spindle speeds on large or unbalanced workpieces can cause parts to be thrown from the chuck. Always respect the machine and chuck speed limits and use guards.

References

  • Standard machining relation: surface speed = π × diameter × rotational speed.