need to replace my large 1200oz-in steppers with smaller one with less cogging/detent

[Apples]

Hi all,

I need to downsize my stepper motors on my long axis from twin 1200oz-in to something else. Any ideas? How do I select the best stepper for my application.

The gantry weighs in at about 120lbs.
For plasma cutting
Pitch diameter at the moment is 24mm for the pinion, rack pitch 3.1416
They are setup with a 2.57:1 ratio at the moment.

I want to get rid of the big 1200's for some smaller steppers with less cogging and detent. When the big 1200's run reall slow at say 10-20rpm, the whole machine vibrates..

So a smaller stepper should not have this effect as much and that is the main reason why I'm getting smaller motors. My Y axis has a 270oz-in stepper and it does not vibrate at all.

Any ideas as to what size steppers I should get, remember it will be dual drive.

Cheers
Peter
Australia

[Splint]

Hi Apples,
I'm no expert on steppers but one thing that dose cross my mind is the question of whether you have the drives set up for micro stepping. I'm under the impression that if your drives are set up that way you should be able to get smooth motion, perhaps there's some tuning issues there that need to be looked at.

Cheers
Splint

[Apples]

Splint,

I have gecko's, 201's from memory. They come standard as x10 micro stepping(I think). And I do not think that you can change anything on them to make then not be x10 microstepping.

The 1200oz-in motors are just too big. Either I keep the current reduction ratio, and get smaller motors. Or I make a much larger reduction ratio. Maybe 6:1 or more. The faster the motors spin the smoother they get.

I changed the steps per unit in Mach3 from 68 to 200 just to see what happens. At 200 steps per unit, they run very smooth.

So having seen that, they need to be belted down more. Or I need a smaller motor with less detent/cogging.

The 270 are smooth when running, although a 3 or 4:1 reduction will be needed for ultimate smoothness.

Peter
Australia

[pastera]

Just drop the current - less force means less energy to pump into the resonance (vibration).

Aaron

[Apples]

Yeah, I'll give that a go!

Go one Aaron

[runinbymdnt]

I'm curious Peter, did you smooth out your big motors. I am going to a trade show day after tomorrow, and was planning to get 3 big motors.

Thanks Jim

[Pres]

Consider putting some flywheels (dampeners) on your motor shaft(s).
Fiddle with the size of the flywheel (i.e. inertia) and you might find your cogging problem goes away.

[Apples]

What do all these thing mean on a stepper motor?

Resistance (Ohms) +/- 10%
Induction (mH) +/- 20% typical
Rotor inertia (g-cm^2)

I'm trying to select a pair of stepper motors to push a 120lb gantry for plasma cutting. At the moment I have a pair of 1200oz-in on there, but the have too much cogging and vibrations. I know that 270oz-in motors do not have this problem.
So to save me buying stepper motors again that will have vibrations, do these resistance, induction and rotor inertia values give me a way of selecting a motor with out vibration/resonance. I have the 1200's from homeshopcnc.com you can see the specs there.
Can you work out just from the specs of the 1200's that they will have cogging and vibrations? If so how?

Cheers
Peter
Australia

[pastera]

Did you try lowering the current setting on the drives (change out the resistor)? What were the results

Have you tried adjusting the trimpot on the drive to null out low speed vibration?

A motor with a lower detent torque will run smoother.
You can increase inertia by adding a flywheel.
The resistance and inductance of the motor should not have that much effect on cogging

Aaron

[Apples]

pastera,

no havent tried changing the resistor to lower the amps.

Tried everything else though.

Did you see this post from the GeckoDrive group?
This is from Mariss.

10724 is the message number on the www.groups.yahoo geckodrive list.

Tried to cut an paste it here but it didn't work.

Peter
Australia