Closed Loop Spindle Control

The spindle on this system has been used on two previous CNC machines. It is a Bosch 1617 that has been converted to be a shunt motor. The shunt motor has a far flatter torque curve than the universal motor. But, especially with heavier cuts, it does bog down a little. I decided to convert this to a closed loop system. The method I chose is a Super-PID 2. This changes this:

To this:

My previous cutting experiments were running in the 3000-4000 RPM region, but the lowest setting on this unit is 5000 RPM. So I set it to that and increased the feed rate a bit. It looked like I had some room anyway. The SuperPID does seem to be able to hold the speed ok, but it is hunting a bit when the cut load changes. What I am missing is an indication of power usage. I do have a Kill-A-Watt power meter connected, but this is a pretty blunt tool.

My test cut was on a 316 Stainless block, with a cutting depth of 0.02", a feed rate of 16 IPM and a step over distance of 0.125". The result was interesting. I noticed several things:

  • The RPM holds pretty steady when the cut parameters don't change
  • The RPM drops down to about 4750 RPM at the beginning of the cut. About 5%. I wonder how low low it would go for a heavy cut.
  • My test does not stress the power range of the spindle. The load bar only ever goes up to about 33%.
  • The load bar never goes below about 25%, even when the spindle is not cutting.
  • Thus, the load bar is not a power indicator; it probably indicates the firing angle of the TRIAC. One has to be VERY careful of diminishing returns past the half-way point.
  • My Kill A Watt seems to indicate roughly 170W extra power usage while cutting. Combined with the low TRIC duty cycle, this indicates that I can probably double or triple the cutting load - as long as the bearings can handle it.