Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: garyhlucas on May 27, 2013, 08:46:14 PM

Title: Steppers as spindle motors
Post by: garyhlucas on May 27, 2013, 08:46:14 PM
I am setting up a fourth rotary axis for my mill, run by a stepper motor.  Most small mills and lathes use a DC motor or a VFD drive AC motor for the spindle.  If you look at the torque and horsepower curves for those motors and drives the torque is fairly flat across the useable speed range.  So at half speed you have the same torque, but that translates to half as much power too.  Which is why gears are needed to change the speed ranges without losing torque.  The thing I find interesting is that steppers are essentially constant power devices, and the slower you run them the the higher the torque, up to the motors holding torque rating. So it seems to me that a stepper matches the torque requirements of a variable speed spindle much better than a DC motor. If you have a high enough microstepping rate for smoothness and a high enough voltage to overcome the back EMF at high speeds the stepper seems like they could work pretty good.  The potential problem I see is that unlike a DC there is no overload capability that you may be using without even realizing it.  An overload on a stepper will stall it instantly.  My machine has a 1500 watt DC power supply at 68 vdc powering 80vdc 8.8 amp drives.  I have a Nema 34 800 oz/in. stepper so I am going the try it as a spindle drive.  I want to compare it to the Sherline motor currently running my mill spindle. For the fourth axis I need positioning anyway.

Gary H. Lucas
Title: Re: Steppers as spindle motors
Post by: HimyKabibble on May 27, 2013, 08:55:45 PM
The biggest problem you'll have is that steppers simple don't do high RPM, and their torque falls off much more quickly with increasing RPM than other types of motors.  The larger the stepper, the worse these characteristics become.  For large steppers, you may not get above 1000 RPM.

Ray L.
Title: Re: Steppers as spindle motors
Post by: garyhlucas on May 28, 2013, 09:26:11 PM
With the 800oz/in steppers I have that are low inductance I seem to be able to get 2500 rpm okay.  I'll have to mount one up and see how it works.  Everybody seems to focus on the torque falling off as the speed goes up.  However you also view this as the torque going up as the speed goes down, as if you had a gear box and were shifting gears.

Gary H. Lucas