Constant Surface Speed or CSS
Constant surface speed as applied to lathes is the technique of automatically varying the spindle speed as the tool moves along a contour. The goal of this technique is to maintain a constant speed as the cutter moves over the workpiece. Large diameter portions of the part much turn much slower than smaller diameter portions of the part which are close to the centerline.
The advantages of cutting at a consistent speed are as follows:
- Longer tool life
- More consistent surface finish
- Better cycle times since you can feed at constant rate without worrying about exceeding the recommended feeds in the sections with smaller diameters
- Programming is dead simple because all the programmer needs to think about is surface speed required on the particular material being machined.
The G96 code turns on CSS, while G97 turns it off. G96 is therefore modal.
It is desirable to cancel it as soon as possible when finished cutting as it will slow things down if used during rapid G0 travels.