Newbie- I'm having a problem setting up my motor speeds.

[JasonCcnc]

Hello,

I am sure this is a simple fix, but I can't seem to figure it out. I have a cnc router table that I have built. The X and Z axis are run off of 1.8 degree stepper motors, and ball screw systems that are attached directly to the motor shaft. Basically they work fine with the six rotation per inch acme thread. The problem however, is that I couldn't get the Y axis to move smoothly with the acme thread driving it from center, so I replaced the acme thread with a rack and pinion set up. it works great, except the gears are also direct drive off of the motor. Because of the gear size, they move at close to 6 inches per rotation. I have used the motor tuning section of mach to get the steps per inch worked out and such, but the speeds are something I can't seem to figure out. My system will error out every time there is a straight Y axis movement because it moves too fast. Is there a way to run one motor at a different speed than the others? I would prefer not to try to gear down this motor because the ratio is so different. I was hoping there was some sort of set-up I could do in Mach to make this work.

Thanks

[ger21]

My system will error out every time there is a straight Y axis movement because it moves too fast.

What do you mean "Error Out"?

Is there a way to run one motor at a different speed than the others?

Whatever you set in motor tuning is the max for that axis. You can set them all different if you like.

[JasonCcnc]

Ger21,

Thanks for the response

When I say the system errors out, I mean that my stepper drivers lock up. Basically when it tries to go too fast, my power supply can't handle the load, and the drivers trip because they don't have enough power to go the specified speed. At least that is what I think it is. My power supply is undersized for sure. I built this thing on a very limited budget. it's not built for production, it built more for hobby stuff around the house.

I was wondering if I could set up that speed through the tuning, but it seems like if tune the motor to the same speed as the others it sounds terrible. Maybe I am doing something incorrectly at that stage. How bad is it if the motor sounds bad, or is it just that, a bad sound but no damage caused?

[ger21]

What kind of steppers, drives, and power supply are you using? What voltage are you running?

[JasonCcnc]

Ger21

I apologize for the delay in my response. I am running 3 pacific scientific stepper motors. Each is rated at 7.9amp 65 vdc and 337w. 1350 rpm

The drives are also pacific scientific model number 5410. Input 75vdc max

The power supply is from powersupply1.com. It is a 36vdc supply at 150 watts.

The power supply is the only part I purchased. The others were given to me. I selected this power supply with the assumption that the machine speed would be limited but would run. That is why I assume that the higher speeds error out the drives

Does that sound like it could be the problem?

[awerby]

Are you sure you've got the steps/unit set correctly? It's not important that all the steppers run at the same speed. What you want is for all of the axes to run at about the same speed, which is different. If your other steppers are going at 6 RPM, that's one inch per minute of travel, but if the one on the pinion is running at 6 RPM, that's 36 inches per minute. The steppers shouldn't mind it if the velocity is run way down; tune the motor by clicking on the appropriate axis, turning down the velocity so it sounds happy and doesn't stall when you press the up-down arrow keys. Then run the accelleration down to create a reasonable looking flat-topped mesa on the chart (usually, accelleration values are about 1/10 of velocity values, so if your velocity is 100, then try 10 for accelleration). When you're done, don't forget to click "save axis settings" or it will revert to the default.

The main problem with your R&P setup is loss of precision. The motors have 200 step positions per rotation, but if you divide that by six inches, that's just .030" per step. Microstepping will help some, but you can't really count on microsteps for accuracy. If you don't care about accuracy that much, your system will probably work okay, but if you do you might consider a smaller pinion.

Andrew Werby
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