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Constant Surface Speed (CSS) Reference

Understand why CNC lathes use Constant Surface Speed (G96) and how the RPM limit (G50) affects it. When facing a part, the cutting diameter changes continuously — CSS automatically adjusts RPM to hold a constant surface speed. This tool shows what happens when the spindle max RPM is reached.

Starting (largest) diameter — mm

Ending (smallest) diameter — mm

Desired cutting speed Vc — m/min

Max spindle RPM (G50 S limit)

RPM at largest diameter

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Ideal RPM at smallest diameter

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Actual RPM at smallest diameter

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Actual surface speed at small end

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What is Constant Surface Speed?

In conventional turning (G97), RPM is fixed. When you face a part from OD to center, the cutting diameter shrinks — so the surface speed drops, and you end up cutting very slowly near the center.

Constant Surface Speed (G96 on Fanuc) automatically increases RPM as the tool moves to smaller diameters, holding Vc constant. But the spindle has a maximum RPM limit — once the ideal RPM exceeds G50 S____, the machine caps RPM and the surface speed drops.

G96 S200 — CSS mode, target Vc = 200 m/min
G50 S3000 — cap RPM at 3000 (always set this when using G96!)
G97 S1500 — return to fixed RPM mode

The diameter where RPM hits the cap is called the "CSS crossover diameter." Below that diameter, you're cutting at lower-than-ideal surface speed. This is normal and unavoidable — it's why the G50 limit matters.