Stepper as Spindle

[Bob La Londe]

Ok... first off this is a different application than my post about alternative spindles.

With an average small stepper, say 380oz, how fast (RPMs) can you turn it before torque drops significantly. I have an idea for a flexible small part cutting fixture that will allow me to make certain types of parts very quickly in a "lathe like" application where I can feed stock through and reclamp then hit start. I need to be able to drive the spindle I want to use with a stepper in order to make it work out.

[cameraguy]

Maximum RPM while maintaining torque has nothing to do with the oz/in rating, and everything to do with the voltage versus the inductance. We would need those before we could tell you.
For most uses steppers stay well under 1500 rpm. I will say that operating a stepper over 2000 rpm is very rare, and 3000 rpm almost unheard of. Totally the wrong tool for the job at that speed.

[Bob La Londe]

Ok, thanks. Actually I already did some math based on steppers, and how they have perfermed, and decided I really could get over about 900 reliabley, more like 400 to 500 at near peak torque/holding force. I wasn't looking for a high speed spindle from a stepper. I was just looking for limits to what they can do.

[Jeff-Birt]

The perfect stepper motor has a constant power output, as RPM goes up torque goes down and the result is that a torque/speed graph would be an striaght line from the maximum delivered torque (not holding torque) to zero sloping to teh right.

In reality the line is not striaght but can drop like a rock after a few RPM until it gets to a point where it declines at an even rate. The motors inductance, how it is wound, drive voltage, type of stepper drive, etc all contribute to this torque/speed curve.

A servo motor on the other hand will tend to have more of a constant torque output. Lots of factors involved of course including how the motor is made and the type of servo drive used.

[Zaggy]

Hi

How do you associate PPS (Pulses Per Second) and RPM (Revolution Per Minute) ?

I thought PPS could be meant as the number of full steps per second, no ?

I'm seeing torque curve of a nema23 stepper still delivering over 1Nm (142oz.in) @8000PPS = 2400rpm (Given it's a 1.8° stepper).

Are the maths correct ?

[cameraguy]

depends on type of stepping/microstepping. if you are at full step, it is indeed 200 pulses per full rotation. but at 1/2 stepping its 400, and at 10 microstep its 2000 pulses per full rotation, etc.

[keebler303]

Mariss has spun a stepper up over 10,000 RPM for fun but at that speed it barely has enough torque to keep itself going.

Matt

[hub]

Could work maybe.. Say the stepper has max 1000rpm. Add a 10:1 gearing and the spindle will spin at 10000rpm.

[Bob La Londe]

Most of us want to know in more practical terms based on what we have and know works ok.

I worked backwards.

Just the way I am setup with 380 oz/in motors from Ahren I can cut light cuts at upto 45 IPM. The limitation is the cutter not the stepper. Its also the speed I can run rapids at with 99.99999% reliability and accuracy. That is on a 20TPI Screw. I can take pretty heavy cuts at 8-20 IPM.

So my ranged based on load, inertia, cutting force, etc is

8 X 20 = 160 RPM

to

45 X 20 = 900 RPM

By going to full steps instead of micro steps using a different control I might be able to get more, but with what I have I know my range.

Of course as a spindle, a drive for a chuck, or on an indexer the load from the screw/drive/bearings may be more or less allowing for more use of torque/power for generating more RPM, but I think that difference is really minimal.

[cameraguy]

Look at the number of watts though. No matter how you might gear it to use the energy, there can't be any more energy than you put into it in the first place. Most steppers are running well under 100w and often much less than that, not the 500w+ range you would need for a decent spindle.

Get a far less efficient bigger stepper to handle power and heat dissipation, you increase inductance and so limit speed, so you gear down further, then you get into a vicious cycle.

This really is territory best left to a simple surplus treadmill motor or servo.