Opinions Please: Stepper as a spindle motor

[Max Power]

Hello all,

I'm looking to add CNC capabilty to a small lathe- Taig actually. I want to be able to thread small fastners. I need to co-ordinate the spindle with the Z-axis obviously. This is a bit of a challenge when the spindle is driven by a 1/4 hp AC motor...

Could I use a large stepper motor (say NEMA 34 or 42) in place of my current motor? I was thinking NEMA 23s (~120 oz/in) for the X and Z as well as the bigger one for the three axis, all driven by Geckos. I'm leaning toward Mach3 as software as well...

What do you stepper gurus think? Sound plan or just fantasy?

Additionally, the parameters for the use of the motor are:

-Operation between 150-2500 RPM
-Most work would be below 1000 RPM
-No semi-industial usage planned, should be less than 100 hrs a year.

If you figure it can be done, how big would the stepper have to be to replace the 1/4 hp motor? Is a Gecko driver capable of the power required or suited to this size of application?

Thanks for your consideration...

Cheers

[NC Cams]

I would not expect a stepper to SMOOTHLY rotate at low speed as the "steps" that psuedo commutation generates will become MUCH more apparent.

We have a pulsed DC motor speed control on our commercial cam grinder and learned LONG ago that the use of PWM to regulate speed shows up in poor surface finish - the motor is constantly accelerating and decelerating due to the pulses.

I'd think this same sort of effect could be experienced with a stepper used as a spindle drive, especially at low speed where the "pulses" from the low frequency "steps" would be much more apparent.

I'd, frankly, find a different way to address the speed control that you're looking to achieve.

[Max Power]

NC Cams,

Thanks very much! Finish quality is something I never would have considered myself. Exactly the help I need in figuring something out....

I'll think I'll abandon this idea and look for a more conventional approach. Apparently some people have had success with treadmill motors and the like. Sort of outside the scope of this forum though. My search will continue...

Thanks again,

MP

[delstanton]

Put an incremental encoder on the spindle.

Also - apparantly TurboCNC CAM software can accept ONE pulse / spindle revolution and drive the lathe carriage for threading from that seemingly inadequate information. I just ran across this. So a simple optical interrupter ($3 US or less) actuated by a vane on the spindle would do the job. (I am still not believing this will work!)

I have just purchased a manual Taig lathe system that I intend to CNC within the next year or so. So I was pleased to see threading could be implemented so inexpensively.

Del Stanton

[jeffs555]

Mach3 also says it will accept 1 pulse/rev. Hopefully there is enough inertia in the spindle to keep the revs constant enough that the control software can interpolate between pulses.

[KPyro]

I would use the stepper to drive the z-axis and add a single pulse encoder
to the spindle.

I dont know about a Tiag lathe but I have done it on my 100 year old
10" engine lathe using TCNC and it works great.

If you do this make sure the signal going back to the paralell port is noise free or it will drive you crazy.

I didnt realize that Mach3 would would do it also with one pulse I may have to try that. a program to cut threads would easily fit in the 999 lines you get in the demo version.

Ken