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Constant Surface Speed G96 Spindle RPM Calculator

With G96 the control keeps the cutting speed constant by raising rpm as the tool moves to smaller diameters. This calculator shows the rpm you will get, where the limit set by G50 or G92 starts to cap it, and the time of a facing cut.

Your numbers

Surface speed (G96 S value)

m/min

Starting diameter

Ending diameter

Use 0 for a solid face to centre.

Spindle speed limit (G50 S or G92 S)

rpm

Feed per revolution

mm/rev

Your results

Spindle speed at the start

875

Spindle speed at the end

3,000

Held at the limit when the diameter gets small.

Diameter where the limit takes over

23.3427

mm. Below this, rpm is capped.

Feed rate at the start

105

mm/min.

Feed rate at the end

360

mm/min.

Facing time at constant surface speed

0.1874

Minutes, ignoring the rpm limit.

Facing time assumes the spindle never reaches the limit. If the rpm is capped, the real surface speed drops and the facing takes longer. Observe the machine's acceleration limits too.

Save results:

Spindle speed at the end diameter vs end diameter

Sweeps Ending diameter from 2 to 80, holding your other inputs fixed.

Keep lathe program standards in one place

Optional: a short review of how to manage CNC programs and standards alongside ERP data.

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How to use it

Enter the surface speed you program with G96, the start and end diameters of the cut, and the spindle limit you set with G50 S or G92 S on Fanuc style controls (or the equivalent on other controls). Add the feed per revolution. The results show the rpm at both ends, the feed rate in mm/min, the diameter where the limit takes over and the time for a facing pass.

The formulas

Spindle speed is n = 1000 x Vc / (pi x D), with Vc in m/min and D in mm. At smaller diameters n rises without bound, which is why the limit exists. The diameter where the limit is reached is D = 1000 x Vc / (pi x n_max). Facing time at constant surface speed has a neat closed form: t = pi x (R0^2 - R1^2) / (1000 x Vc x f), because the tool advances f per revolution and the revolutions per minute scale inversely with radius. For 80 mm down to 10 mm at 220 m/min and 0.12 mm/rev, the time is about 0.18 minutes.

Always set the rpm limit before G96, with G50 or G92 as your control requires.
Chuck rating, jaw type and bar capacity may set a lower limit.
On some controls, G96 is cancelled by G97 with an S rpm.

Safety and practical points

The limit protects the chuck and the part. Check the chuck's maximum rated speed and reduce for heavy workholding, long unsupported stock or soft jaws bored out large. When the rpm is capped in the centre of a facing pass, the surface speed drops, so the finish at the centre may differ from the outer area. Using a feed per revolution rather than per minute (G99) keeps the chip load correct as rpm changes.

Common mistakes

Forgetting the limit is the most dangerous error, because the spindle can accelerate to the machine maximum near the centre. Another mistake is using G96 with G98 feed per minute, which gives a feed that does not follow the speed, and the chip load changes wildly. A third is applying G96 to a drilling or tapping operation at the centre, where constant rpm is needed.

Frequently Asked Questions

What is the difference between G96 and G97?

G96 sets constant surface speed in m/min or ft/min, so the control varies rpm with diameter. G97 sets a fixed spindle speed in rpm. Use G97 for drilling, tapping and threading on the centre line.

Why do I need G50 or G92?

On Fanuc style controls, G50 S sets the maximum spindle speed used while G96 is active (G92 on some turning controls). Without it, rpm can rise to the machine limit as the diameter approaches zero.

Does the facing time formula work in inches?

Use consistent units. If you enter diameter in inches, surface speed in feet per minute and feed in inches per rev, the unit factor changes from 1000 to 12. This page is set for millimetres and m/min.

What happens to the time if the rpm limit is hit?

The spindle stops speeding up, so the part spins slower than the constant surface speed wants. Each revolution takes the same time but advances the tool by the same feed, so the tool goes slower than the calculated time. Add the extra time using the capped rpm.

If you document lathe programs and standards in an ERP or document system, we can help organise them. This calculator is independent.

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