X axis sticks - sometimes??

[becikeja]

I have no clue what kind of machine I have, but the crate says China on it so I'm posting here.
I have had the machine for about 12 years now, so for the money I spent, I'm doing pretty well. Recently the X axis starts randomly sticking in the middle of a run. Sometimes right away and sometimes not for 30 minutes, and sometimes works fine 1-2 hours before sticking. When it sticks it is temporary, and misses 1-2" then works fine, but at that point the piece is already ruined and the balance of the run is offset by that amount.

I'm not a machinist, and really have no idea what I'm doing but these are the things I've tried.
- I have noticed if I tweak the rails it seems to make a difference. Can't tell if tightening them or loosening them makes the difference but it only seems to last momentarily
- Cleaned the ball screws and rails, then applied a silicon spray lubricant. Didn't seem to help at all.
- Tightened and loosened the ball screw. No noticeable effect.
-Checked the wiring to make sure there were no loose connections

Ideas I am considering
-Buy a new Stepper motor for the X axis
-Replace the bearings on the rails

If anyone has run into this problem or has some insight in how to fix I sure would appreciate some guidance.

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[awerby]

It sounds like a mechanical issue. Have you tried moving the axis back and forth by hand, with the power off? You might be able to detect an area where it's more resistant to motion than other places. Does the problem happen in various different places, or is there one region where it often gets stuck? Another thing that can cause this is a coupler coming loose. Sometimes a set screw can slip on the shaft and then the axis will move again, only to repeat the fault when you change direction.

[Sterob]

What Awerby said....
Also, Post a pic of your machine so we have an idea of which one we are talking about.

I'd even suggest you disconnect your ball screw from the carriage so you can move the axis by hand in both directions.
You should not feel any changes in resistance. ( I had move rssitance in one direct when I checked mine.) Keep disassembling until you isolate the offending part.
Make sure your ball screw nut is not being deflected when re-attaching to the carriage.

[joeavaerage]

Hi,
it may be a mechanical problem but its not impossible that the controller or driver is faulty.

Does the fault happen on one axis or all of them?

Craig

[becikeja]

Appreciate the comments:

Awerby: I have disconnected the ball screw and it seems to slide back and forth with no issues at all. The sticking does seem to happen more often as it gets closer to the stepper motor side of the rail. I had not thought about the coupler coming loose. Seemed to re-tighten fine and the sticking is random, and assumed if it was the coupler it would be more consistent. I'll look into this. Seems to change direction just fine for several runs and then wham. I also thought about ramp up speed on direction changes but does not seem to matter. I can run very slow or moderate and I have the same issues.

Sterob, what does this mean? Make sure your ball screw nut is not being deflected when re-attaching to the carriage Not sure I understand. I'll try to post a pic this week.

Joeavaegare: I have never had any issue with the Z or Y axis.

[joeavaerage]

Hi,

I have never had any issue with the Z or Y axis.

That, in all likelyhood, rules out the controller. You could have a faulty drive.
Can you swap the X and Y axes drives? If the fault follows to the Y axis that suggests the driver, if it does not that suggests either the stepper
or a mechanical issue.

If its not too hard to swap drives its a reasonable test that may direct further diagnosis.

Craig

[Sterob]

Sterob, what does this mean? Make sure your ball screw nut is not being deflected when re-attaching to the carriage Not sure I understand.

Sorry...wording not the best....On my machine I had to fit packing between the ball nut mount and the gantry as the ballscrew was being deflected when the mount was being tightened. This was causing binding on my machine.

[awerby]

Appreciate the comments:

Awerby: I have disconnected the ball screw and it seems to slide back and forth with no issues at all. The sticking does seem to happen more often as it gets closer to the stepper motor side of the rail. I had not thought about the coupler coming loose. Seemed to re-tighten fine and the sticking is random, and assumed if it was the coupler it would be more consistent. I'll look into this. Seems to change direction just fine for several runs and then wham. I also thought about ramp up speed on direction changes but does not seem to matter. I can run very slow or moderate and I have the same issues.

If you're detecting stickyness on one side of the axis, it could be that the pair of rails it's riding on are slightly out of parallel. To check that, make a stick that fits tightly between them on one end, and see how it fits on the other. You might be able to fix it by loosening the fasteners that hold the rails down, adjusting them, and then (carefully) re-tightening them where the motion seems smoother.

Coupler failure can seem random, especially if there's a set screw bearing directly on a shaft. It can wear a track in the shaft that it moves along when stressed enough, but won't budge most of the time. Try making a mark across the shaft and coupler, run it until it exhibits the problem, and then check your mark for displacement.

[peteeng]

Hi Becik - As Awerby says check all set screw connections. They are difficult to diagnose due to them sticking and slipping and sticking. All set screw connections like motors to shafts and couplings need to be disassembled and checked. Then use loctite type adhesive on these when reassembled. With couplings replace with clamp types. Set screws should be banned!! Peter