Constant surface speed – why does it always go with a Limiting spindle speed
When cutting in the constant cutting speed mode, as the tool moves towards the axis, the spindle speed increases. See these earlier posts for explanations: Constant surface speed – benefits and Cutting speed and RPM – difference. The animation below shows how the spindle speed changes as the tool moves towards the axis in facing.
https://www.youtube.com/watch?v=8uI2XcNKoG4
The spindle speed N in RPM is calculated using this equation, where V is the cutting speed and D is the diameter at which the tool is cutting.
At a cutting speed of 250, at 30 mm. diameter the RPM would be 2652. At 20 dia. the RPM would be 3978. At 1 mm. dia. the RPM would be 79,577.
At a certain diameter the spindle speed goes beyond the machine’s capability. At the axis of the part, in fact, the RPM would theoretically be infinity (D is zero). The machine however has a certain maximum spindle RPM, so in the CNC program we need to specify what this maximum is. This is specified as the Limiting spindle speed. When the spindle speed reaches this value, the controller clamps it at this speed and the rest of the motion is done at a constant spindle speed equal to the limiting speed.
E.g., if we want to cut at a constant cutting speed of 250 m/min and limit the RPM to 3000, for Fanuc we would write this: G96 S250 G92 S3000
So what should I program as the limiting spindle speed ?
If the part is held rigidly in the chuck and is circular, just set the limiting spindle speed to the machine’s maximum spindle RPM. If the part is non-circular or is held in a fixture that is not balanced, centrifugal forces might cause the part to fly off or damage the fixture. In such cases set the limiting spindle speed to a lower value determined by trial and error.
