CNC turning: What is Constant Surface Speed ?

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When you rub your palms together fast you’ll notice that they get heated up, and we actually do this to get warm when we are feeling cold.
This is because of friction, and the faster you rub your palms, the hotter they get. Basically, the higher the relative linear speed between your palms, the higher the heat generation. The same phenomenon occurs during metal cutting. The temperature rises as the cutting speed rises.
The tool material is designed to work at a certain temperature range.
Below this range, you are under-utilizing the tool and the cycle time will be unnecessarily high. Above this range the tool wear will be too high, and you end up with high tools costs and high machine downtime to keep replacing the tool. High cutting speeds and temperature also result in poor surface finish and unwanted metallurgical changes in the workpiece. Cutting speed depends on the RPM and the diameter at which you are cutting. If you are cutting at constant RPM, as you cut at various diameters the cutting speed fluctuates, so sometimes you are cutting below optimum speeds and sometimes at above optimum speeds that result in high tool wear and poor workpiece quality. This is why it is preferable to cut at constant cutting speed, also called constant surface speed, or CSS. This is how CSS works in facing. You can see that as the tool moves towards the axis, as the diameter reduces, the spindle RPM rises, so that the cutting speed is constant. Here is the same animation, in slow motion. This video shows constant surface speed on the machine. You can see and hear the RPM increasing as the tool moves towards the axis each time. The machine changes the RPM automatically according to the diameter, based on a command in the CNC program. The problem with Constant surface speed is that near the axis the RPM becomes extremely high, more than the machine's maximum limit. For example, at a cutting speed of 250 meters per minute, at a diameter of 10 mm. the RPM will be 8000, and at a diameter of 1 mm it will be 80,000 RPM. The machine may only be capable of 4500 RPM. So when you give a constant surface speed command, you also specify a limit of the spindle speed, which is called the limiting spindle speed.