X Axis Chattering and Snagging / Alignment Issues

Longmill wheel chatter - YouTube

Hey everyone. I just got my Longmill a few days ago and have been setting it up! I’m really excited to get started, but I cannot get it mounted correctly! I have made sure the table is level. I have tightened the wheels with the eccentric nuts to the point where they’re just unable to be turned with two fingers.

In this video it starts to chatter / vibrate towards the end of the zero movements. Sometimes one side of the axis will stop and get stuck, misaligning the axis with each other. It is worse on faster feed rates. I’ve tried adjusting the tension screws too.

I cannot get this aligned since this keeps happening. I’ve followed the instructions, and I just don’t know what to do anymore. I got my lead screws slightly bent / warped so they have some wiggle when turning a bit, so I don’t know if that matters. I tightened the ACME screws to be nice and snug too.

@Blunderpunk Welcome to the group, Jordan. In your video, it sounds like things get louder as you move in Y+. Check to make sure that your Y rails are parallel to each other. Even a little bit of toe in or out will cause the issue that I think you have.

@willemk Welcome to you, too, Willem. I believe you mean the Y rails, not X. There is only 1 x rail. You are correct, though, that if the Y rails are out of parallel by 3/16", you will have problems.


My bad, I mean Y-rails, yes.

Thanks for confirming my suspicions.

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Thanks for the reply!

I followed the instructions for making sure they are perfectly parallel, but I will see if I can measure them when I get off work tonight.

I noticed when Y was at its highest point, the right side of the rail will be at a bit of an angle to the right outwards. I didn’t think this would be a problem if the left side would be automatically aligned by moving the Y along it fastened down.

I’ll check my alignment before every job.

Hi, sorry for the delayed response. I got busy with work and setting up a better bench for the mill among other projects.

After making sure they’re pretty much perfectly aligned (within 1/64"), the Y Axis still chatters and becomes unsynced.

I am not sure what the cause is. I’ve played with the wheel’s eccentric nuts and no dice there. The rubber wheel material rubbed off on the rails, so I cleaned that off and still weird noises and unsyncing. Although it isn’t as bad as before, when doing:

G00 Y-100
G00 Y95

and measure the distance between the gantry and the 0 point they are different on either side. I assume the paint thickness is part of it at around 0.1-0.2mm, but sometimes it’s a little more dramatic, especially on higher feed rates.

I am not sure what the max speed the Longmill is supposed to be capable of. I have it at 3500mm/min on all axis and it still chatters on the Y axis.

I’d like to get this taken care of so I can actually start projects. I appreciate your help!

@Blunderpunk The noise would seem to indicate that this is a hardware issue rather than a software issue, I think. I just re-read all the posts here and in your first post, you said that you had tightened the delrin wheels so that you could not turn them at all. I would suggest that they are too tight. Mine are set so that I can turn them - not so easily that they are sloppy, but not so tight that I cannot move them at all. It’s far from an exact science, but you have nothing to lose by loosening them a bit and trying them out that way. One thing - if they are too loose, you will find that the bottom wheels will touch the feet supporting the Y rails.

All that said, you may want to try reversing the Y motor cables at the controller to see if the problem changes sides. If it does, you will know that everything I’ve said in the first paragraph here is irrelevant and you have an issue with one of the motors or its driver.

I will give this is a try. I must have misunderstood the instructions on that, as I watched a livestream from Longmill about setup and they went over this a bit in more detail as well.

On a side note, would swapping the V Wheels with a metal instead of rubber variant be better? It seems the rubber wears off on the rails a bit, something I would hope metal ones would not do. Or would they scratch the rails?

@Blunderpunk It’s your call, of course, Jordan, but I certainly would not change the delrin wheels for steel ones. The black that you see on the rails is not a big deal, is not causing your problem, and is easily cleaned off. IMHO, using steel wheels on an aluminum rail would not be a good idea at all. If anything were to wear in that scenario, it would be the rails. Then, you would be left with the cost and hassle of replacing them - not cheap, or fun. I would think that they would be noisy, too, but that would be the least of your problems. Finally, the rails would wear most where most of your projects are located on the bed. If you adjust the wheels over time to the narrowest part of the rails, when you do a larger project, the wheels would be too tight, riding on a section of the rails that is seldom used.

I’ll probably hold off on the wheel changes for now then.

I adjusted the wheels and they seem to be pretty perfect now. However, I am still getting the chattering.

I’m not sure why this is. The table is perfectly level. The mounting brackets are perfectly square, at least well within the 3/16" toe limit.

I’ve unscrewed the feet to give it free room to move around just in case to re-align them on a new board. It still chatters. And only on the side with the motors, and primarily only in the direction going towards the motors. It barely has any problems going away from the motors.

I’m not really sure what to do at this point. I don’t want to start doing projects with this until I can get this fixed. It’s becoming very frustrating. It will move up 100mm, come back 95mm, and be off by 0.05-0.15mm sometimes. Maybe this could be from poorly adjusted backlash screws too, since I haven’t messed with them much yet. But I don’t believe 0.15mm would make the wheels chatter this badly, especially since it’s only on one side.

What should I try next? Is it possible that the rail is warped? I’ve adjusted the wheels to be perfect for the part that chatters the most, yet it will still do it. I’m not sure what the maximum speed should be, but it starts chattering around 3500mm/min and doing anything higher (4000-4500mm/min) as a rapid is so bad with this that the X axis will become off square with the Y rails and eventually stall the motors.

@Blunderpunk What is this “3/16” toe limit", Jordan? I’ve never heard of it.

I think at this point you should contact Sienci support directly. They are very good at supporting their machines.

That said, you mentioned the anti-backlash nuts. To adjust them, back them off completely. You will have some forward and back movement in both of the Y axis leadscrews. Then, turn the adjuster a very small bit at a time until there is no play. Unfortunately, these should have nothing to do with a grinding noise.

Just to remind us, where is the grinding coming from?

I read somewhere that they must be aligned within 3/16" of an inch to be “aligned”. I don’t remember where I’ll have to search that’s again.

The chattering is coming from the Y rail on the side of the X axis that has the motor mounted to it, closest to the side of the Y axis that has the motor for it. It happens lightly travelling away from the motor, but heavily as it approaches within 300mm and very heavily in the last 50mm or so.

It happens regardless of alignment, rather it’s fastened down or not. If the feed rate is higher (4000-4500) this is the side that laggs behind the other causing an angled X axis.

@Blunderpunk I know that, early on, the guys at Sienci said that they had to slow the maximum feed rate down because they were experiencing issues. The default maximums now are 4000 for x and y and 3000 for z. I’ve not had any issues running at those maximums, and have never tried to increase them.
I’ve never heard of the 3/16" “allowance”. I would think that is quite a bit off square, but in your case, it does not seem to be the cause of the problem since you still get the grinding even when the rails are not mounted solid.
I’m fresh out of ideas. I’m sure that Sienci support will know much more than I do. I would be leaning now to something going wrong with the lead screws, but as long as you have the bearings, washers, and nuts installed correctly, I can’t think of what could be wrong with them.

I apologize for taking so long to reply. My CNC has been on hold due to some other things going on. I was not aware of these limits.

After re-reading the manual, I see those limits that I have missed. 4000 still causes some issues for me on the X, unfortunately. I have not had a lot of time to tinker with it lately.

A problem I am facing with my machine is that the X axis, even if slightly, will come un-aligned by a small amount progressively as it works. It’s not by much, but it’s enough that it would cause some noticeable variation through repeat jobs.

The way to fix this is to just “rehome” the machine by letting it run against the constraints and stall the motors until both are stalling. I am thinking of installing homing switches for these and making an AND gate between the Y Limits so that the homing operation will automate this process for me, and will run the motors until both limits are switched.

The 3/16" allowance was something I read on the forums and I never thought it’d made sense either. I do not have 4’ calipers to accurately measure the distance, though. I have the feet mounted to a strip of MDF against the straight edge as best as I can get them perfectly aligned, and a measuring tape has it within 1/16"

My biggest problem was how tightly the tension nuts being too tightly fastened to the XY gantry mounts, though! I loosened them a bit and it was able to handle 3500mm/min much better. Still tinkering, and hoping to have an even better table built for it soon. I appreciate your help!

@Blunderpunk At this point, I can only repeat a couple of suggestions that you may not have tried yet.

  1. Reverse the Y motor cables and see if the problem changes from one side to the other. If it does not, at least you will know that the motor drivers in the controller are not the problem. There is something mechanical going wrong.
    On the other hand, if the problem does move from one side to the other, you will know that the issue is not V wheel tension or the anti-backlash nut and you can proceed to look into the controller.
    Before going into a great deal of detail on what to do next, I’ll wait for your results.

  2. Have you contacted Sienci tech support? If not, I suggest that you do the cable swapping, then contact them. They will be interested in the results of that process.