Stepper motor idle noise

I've mounted a NEMA 34 on my mill z-axis. When idling, it generates a noise like white-noise.

Never heard this while the motor was on the bench, and the noise goes away when I loosen the mounting bolts. It also disappears if I change the code to disable the controller with no input commands.

So, I assume this is the PWM holding current in the windings, mechanically coupling to the mill structure. I have the holding current set to 50%.

Nothing I can do about it ??

No much. It's the nature of steppers. If You can You disable power to the stepper, else it's powered and pulsed to keep its position.
Maybe rubber dampers on the frame parts could do something.

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Maybe if enough people demanded NEMA sized disk brakes... :grin:

Thanks.

I changed the code to disable the controller whenever user releases a move switch. This also allows use of the manual crank.

But I found that a delay was needed after re-enabling before allowing stepping.

I suspect to allow time to recharge motor coils?

Steppers require as much current to hold in place as they do to move. Obviously you are using some microstepping and the stepper controller is trying to hold that fractional step and it cannot do it with the low current setting.

The stepper controller is NOT "trying to hold" a fractional step. There is no feedback involved here!

The solution is to lower winding current when stopped. It is very common for steppers to hiss when stationary.

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