Concept diagram for review and learning. It is not a controller simulation or an executable program.

TURNMATE / TECHNICAL GUIDE · Updated 2026-10-28

G96 vs. G97: constant surface speed, fixed RPM, and limits

Constant-surface-speed mode adjusts spindle RPM as the cutting diameter changes; fixed-speed mode holds the commanded RPM. Code details, units, and speed limiting vary by control and must be checked in the matching manual.

Concept references: Haas NGC Lathe Operator’s Manual and Haas ST/TL Programming Workbook. This page explains the concepts in those Haas references; it does not prescribe G50 or other syntax for another controller.

Constant surface speed and fixed RPM

Cutting speed is the linear speed at the cutting edge relative to the work surface; spindle speed is revolutions per minute. At the same cutting speed, a smaller workpiece diameter implies a higher theoretical RPM. Constant-surface-speed control varies RPM with the cutting diameter; fixed-RPM mode does not automatically follow diameter.

RPM can be estimated from cutting speed and diameter. The calculator provides a unit-explicit browser-local conversion; it is not a material or tool-parameter recommendation.

Why spindle limits matter

In constant-surface-speed mode, the commanded RPM can rise toward the center. Actual permitted speed also depends on spindle, chuck, gripping method, workpiece balance, bar projection, and tooling. Check both the machine control limit and the shop process limit. Do not use a web calculation as a direct machine setting.

Code numbers and units are not universal templates

The Haas NGC lathe manual describes G96 as constant cutting speed and G97 as cancelling constant surface speed / fixed RPM; it also explains that the S word depends on unit settings. Other CNCs may differ in code groups, units, speed limiting, and modal behavior. Do not copy Haas G50 usage to another system.

When to use the calculator

The RPM tool helps check unit conversion and theoretical magnitude. Choose actual cutting speed from tool-maker data for the material and insert, accounting for machine rigidity, coolant, workholding, and finish requirements. Feed conversion also depends on whether feed is per revolution or per minute and on actual spindle speed.

Review checklist

Confirm G96/G97 meaning, code group, S units, and modal state for the target control.
Check spindle and workholding limits for the current balance and projection conditions.
Use tool-maker data for cutting speed and check material, depth, and coolant.
Validate through control graphics, dry run, and the machine’s prescribed procedure.

Sources and scope

Haas NGC Lathe Operator’s Manual (2024) — Haas NGC lathe reference; definitions apply to the corresponding Haas control and settings.
Haas Lathe Programming Workbook — Haas ST/TL explanation of constant surface speed, fixed RPM, and speed limits.
NTK Cutting Tools: Calculation Formula for Turning — Tool-maker reference for theoretical cutting-speed/RPM conversion; it does not recommend parameters for a particular material.