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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?

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  • $\begingroup$ Could you elaborate more about this low RPM usage of spindle? This is very interesting for me as I'm currently planning to make my own CNC milling machine. As for the stepper motor, most of them only rated for up to 2,000 RPM when I check the datasheet so I doubt they came with a bearing that can survive 30,000 RPM even if it's just a free rotation $\endgroup$
    – JohnDoe
    Commented Sep 13, 2017 at 4:27
  • $\begingroup$ @JohnDoe: The concept is to place the tool in a designated area (box with *-cut rubber seal on top) with optimal lighting, backdrop and a macro camera and scan the tool, rotating it in front of the camera. This way you 1) find the "zero" height (tip position), 2) after the work determine wear, find if the tool broke (abort job with other tools if it did, or pick replacement if available and redo) 3) determine if entered parameters are correct (errors in setting up the tool changer / wrong tool). Maybe even enter tool parameters into CAM software automatically, but that's more distant future. $\endgroup$
    – SF.
    Commented Sep 13, 2017 at 8:09

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Couple the stepper motor to the high speed spindle motor only indirectly by means of magnets rotating around the spindle motor shaft which, at low speeds, will align themselves with specially designed ferrous spots fixed to the high speed spindle shaft and sync with it. The magnets should create a detent force strong enough to move the spindle at slow speeds, but would break free at higher speeds, particularly if the stepper was held in place electrically.

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A one way clutch bearing on the stepper motor might give you the clutching action you want without much added mechanical complexity (note that this would require some experimentation). The high rpm motor would be attached to the tool, and would spin in the free wheeling direction. The stepper motor would be attached to the one way clutch bearing and index in the opposite (clutch locked) direction. This obviously limits stepper motor control to one direction. It also depends on the internal friction of the high rpm motor to keep from free wheeling after positioning.

Stepper motors are rated at lower rpm because they have less and less torque the faster they go; so they generally won't self power up to high speeds. However, it is also very possible that the bearings in the stepper would survive the 30,000 RPM. I am not sure about 80k though; the rotar will likely handle the load (not 100% sure though) and it should be reasonably balanced. You could possibly try tearing one down and pressing in 80k rpm rated bearings (sounds like a project in itself though). Cooling would be another possible road block.

Another possible option would be for the high rpm motor to be on the head and a high torque stepper motor (and large bearing) on the bed. Definitely some design compromises and software programming that direction, but something to consider.

And then there is the less attractive, but simple compromise of having two separate heads ;-)
Good Luck!

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  • $\begingroup$ The "two heads" and "motor on bed" aren't really doable, as the bottom end of the shaft, with the tool grip must remain obstruction free for video inspection and unchanged between inspection and work. I'm also worried about the one-way clutch surviving any considerable period of free wheeling operation at such high RPM. "Motor on bed" could rotate the camera rig around the tool, but it's really like trying to rotate the movie studio around the actor... plus it's a "dirty" environment and I'd rather avoid sealing yet another movable "clean zone" $\endgroup$
    – SF.
    Commented Sep 13, 2017 at 8:20

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