The CNC spindle is supposed to operate its single shaft in two different modes:
- high RPM (30,000-80,000 RPM), variable, possibly quite high torque, a straightforward PID to maintain the RPM as torque varies ("work mode", RPM control).
- very low RPM (exact value doesn't matter), nearly no torque (just friction of bearings), rotation of shaft to precise angle. ("optical scan of the tool, from all sides", angle control).
The first is achievable through a good AC or DC motor. The latter would be achievable through a stepper motor with microstepping. I can cut off any of the motors (leave it idle) and drive the other to achieve either of the modes.
But I don't think typical stepper motors are capable of surviving 80,000 RPM applied externally - and I'd prefer to avoid a mechanical clutch decoupling the stepper.
Are there steppers that can just survive these kinds of speeds? Or other neat solutions that don't overly increase the mechanical complexity?