Machine Troubleshooting Help Request

I have inconsistent results from one side of the machine to the other.
SO3 XXL. Originally came with steel core belts that failed, replacement belts installed last year. “Gates Carbon Drive” app tuning method used to properly tension them. I was never able to get them exactly the same but they are very close.
Much of the work my machine does is cutting multiple identical parts out of a single large panel. I’ve recently noticed that parts cut out on the left side of the machine were slightly different than parts cut out on the right. I’m not sure if this has always been the case and went unnoticed or it it popped up suddenly or if it has been slowly getting worse over time. I put together the following test to verify exactly what is happening.
I cut out a 16" long rectangle from an unpainted piece of 1/4" plywood mounted to the left side of the bed. And then ran the same file after resetting my zero on a second piece of 1/4" plywood mounted all the way on the left side of the bed, this one painted black.
The one cut on the right side is about 1/32" short of 16". That is perfectly acceptable for the type of work I do. But the one cut on the left side is a full 1/16" shorter than that, so 3/32" short total.

I’m not concerned about overall accuracy but consistent from one side to the other is important. Has anyone ever seen an issue like this before? Any ideas on what might be causing it? Do I need to go back to fighting for perfectly matched belt tension?

Left side result:

Right side result:

For this sort of thing see:

but if you don’t get the two Y-axis belts stretched to the same degree, then they won’t be consistent front–back for the left vs. right sides.

Some folks have purchased genuine Gates GT2 profile 2mm pitch 9mm wide from vendors such as BB Man., SDP/SI, or Texas Belting.

Has this only happened recently?

If so, how old are the belts on your machine?

Tensioning them both to the same note relies on the belts having the same tension modulus to give the same extension, and therefore pitch per tooth. If the belts are getting old and the internal reinforcement fibres are starting to let go then you may well get different extension for the same tension.

You can use the C3D belts. I’ve used the kevlar cored belts with no problems (they outlast the steel core) and would recommend those, or the regular fibreglass Gates belt, if you can’t find those then the fibreglass cored belts from Powge Transmission Parts on AliExpress are also perfectly good.


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Since this is an SO3 XXL you need to check all your v-wheels. If you have lose v-wheels or ones that are not tightened properly it will show as issues like this. The reason is the machine starts to move but the v-wheels are loose or binding and you lose steps but just a cog or two on the belt. That can cause your inconsistent cutting length. Now if every cut is exactly the same but the wrong distance then it is a calibration issue. If the cuts are inconsistent from one cut to another than it is likely mechanical in nature. Unless your belts are very loose I dont think that on an SO3 that makes much difference.

Besides the V-wheels check the other usual suspect and that is if a stepper motor pulley is loose. Make a mark across the motor shaft and extended to both sides of the pulley through the middle. Jog your machine side to side and front to back several times. Then look at your witness marks you made and see if they are still lined up as you marked them. If they are not aligned even slightly then you need to tighten up your set screws. One set screw must be on the flat of the stepper motor shaft and the other just tight.

Brian, here’s a post with some things to check.

I had an embarrassing number of issues with my S3 XXL due to deferred maintenance. Recommend going through literally every screw and checking it. Belts, v-wheels, and motor pulleys are the likely culprits.

If you read down that thread there’s a link to the kevlar belts I picked up from AliExpress, they’ve been great, and also setting up a test pattern square.

Thank you all. Of course I was hoping for some magic silver bullet fix. But all of this makes sense.
Get the two belts closer to matched tension and check everything else carefully.
And in thinking about all this I’m now realizing that when I replaced my belts I only bought a replacement set and not the replacement plus a backup. I need to have backup belts on hand. I will fix that right away.
Thanks again.

With noticing a difference side to side, definitely check those motor pulleys too. Could be one side slipping and the other holding, which is what was happening on mine. Here’s my fix:

Definitely something mechanical (possibly electrical, but more likely mechanical) and fixable. I get really nice, tight, repeatable cutting on my SO3 XXL now that it is maintained. :person_facepalming:

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