G96, Spindle Speed As SFM (G96Snnn)
See also G97, Spindle Speed As RPM

G96 is the Constant Surface Feet mode.
In this mode the S value will set the amount of surface feet the tool will see. The speed of the material passing the tool will stay constant, no matter what diameter the tool is turning or facing. As the tool gets closer to center the speed of the spindle will increase.

Many tool and material suppliers give suggested feeds and speeds in terms of surface feet. This mode is good for turning and facing operations.

To set the control so that the spindle speed is in Revolutions per Minute (RPM) see G97 Spindle Speed as RPM.

Using G96, G76, and G77 in a Program

The constant surface speed control is not intended to be turned on at the beginning of the program and then left on. If you do this the spindle speeds will vary greatly every time the machine moves in the X-axis.

Best practice is to set the spindle on (m03sn), call the tool, turn on g96, make the cut, switch back to g97.
The program fragment below is a simple facing example:

• G90G94F300
• M03S1500 (Turn spindle speed on)
• T1 (LH TURN TOOL .008 RADIUS)
• X.25Z.2
• G96S250 (Set spindle to SFM mode as 250sfm)
• G76S500 (Set min spindle speed as 500 rpm)
• G77S2500 (Set max spindle speed as 2500 rpm)
• Z0
• G95F.002X0
• G94F300Z2
• G97 (Switch to RPM mode)
• S2000 (Set spindle speed to 2000 rpm)
• T2 (DRILL)
• X0Z.2
• G95F.003Z-.5
• G94F300Z2
• M30