Cutting speed and RPM – what is the difference, and how is it important ?
Cutting speed and RPM are different but related. Here’s an explanation.
When you rub your palms together fast, you’ll notice that they get heated up (we in fact do this to get warm when we are feeling cold). This is because of friction, mechanical energy being converted to thermal energy. The faster you rub your palms, the hotter they get. Basically, the higher the relative linear velocity between your palms, the higher the heat generation.
The same phenomenon occurs during metal cutting. Mechanical energy is used to cut metal by causing relative motion between the cutting tool and the workpiece. This is converted into thermal energy. Just like when we run our palms together, the thermal energy produced is proportional to the relative linear speed between the tool and workpiece. This relative linear speed is the Cutting speed. It is measured in meters per minute, different from the spindle speed, which is the number of revolutions per minute. The cutting speed is proportional to the spindle speed AND the diameter at which cutting is being done.
Tool wear rises as the temperature at the cutting edge 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.
With a constant RPM, as you perform various operations the cutting speed fluctuates, so sometimes you are cutting below optimum speeds and sometimes at speeds that result in high tool wear (and hence high tool cost) and poor workpiece quality.
This is why it is preferable to cut at constant cutting speed, also called constant surface speed (CSS).
Action point
Check if your CNC programmer knows about Constant cutting speed and is using it properly. A LARGE percentage of (I think most) programmers do not know what it is, and do not use it. Cadem CNC lathe programming software uses CSS and constant spindle speed automatically and appropriately.
