## What is Constant Surface Speed (G96 )?

In machining, constant surface cutting speed means programming a CNC Lathe to maintain a constant relative velocity between the cutter and the workpiece. It is achieved by letting the machine control the RPM according to the working diameter. As the cutting edge gets closer to the center of the workpiece, the RPM increases.

## What is Constant RPM (G97)?

In Machining, constant RPM means programing a CNC lathe to maintain the spindle’s RPM constant. Because of this, the Surface Speed (SFM / Vc), changes when Machining at different diameters. As the cutter gets further away from the center of the workpiece, the surface speed increases.

## G code G96 Vs G97

• The best practice is to use constant cutting speed (G96), since the cutting tool is “feeling” the cutting speed and not the spindle speed. However, there are a few exceptions that call for the opposite approach.
• When face turning or Parting Off to the center, the cutter gets closer and closer to the axis of the workpiece, eventually reaching zero diameter. Using G96, the RPM will increase as the cutting edge gets closer to the center, ultimately reaching the machine’s maximum spindle speed limit. Therefore, it is more common to use constant RPM (G97) in facing and parting operations.
• Vibrations are directly affected by the spindle speed and not the cutting speed. Therefore, when vibration issues occur, it is easier to control them when the RPM is constant.

## Examples: G Code G96 & G97

Choose units

Imperial Metric

Constant SFM (G96 G code Example)

N10 G20; (Imperial Inch Units)
N20 G96 S300; (CSS – Constant SFM=300)
N30 G00 Z0 X1.0; (Positioning)
N40 G01 Z2.0 F0.01 (Turning 1")
N50 G00 Z3 X2.0; (Positioning)
N60 G01 Z4.0 F0.01 (Turning 2")


Explanation:

• Line N20 – The cutting speed is defined as 300 SFM
• The spindle speed is calculed by the formulas:
$$\large RPM = \LARGE \frac {12 \,\times\, SFM}{ \pi \,\times\, d}$$
• Line N40 – As a result at a diameter of 1″, the spindle speed will be 1146 RPM.
• Line N60 – As a result at a diameter of 2″, the spindle speed will be 573 RPM.
• The Surface Speed (SFM) stayed constant and the spindle speed varied by 50%.
• Notice: In reality, the actual speed might be slower if the calculated RPM is higher than the machine’s maximum or is larger than the limit set in G50.

Constant SFM (G97 G code Example)

N10 G20; (Imperial Inch Units)
N20 G97 S700; (Constant RPM n=700)
N30 G00 Z0 X1.0; (Positioning)
N40 G01 Z2.0 F0.01 (Turning diameter 1")
N50 G00 Z3 X2.0; (Positioning)
N60 G01 Z4.0 F0.01 (Turning diamaeter 2")


Explanation:

• Line N20 – The Spindle Speed is defined as 700 RPM
• The cutting speed (SFM) at each diamater is calculated by the formula:
$$\large SFM = \LARGE \frac {n\,\times\,\pi \,\times\, d}{12}$$
• Line N40 – As a result at a diameter of 1″, the Surface Speed will be 183 SFM
• Line N60 – As a result at a diameter of 2″, the spindle speed will be 365 SFM
• As you can see, the cutter will “feel” a different cutting speed at different diameters.