Help with diagnosing problem with plasma table

My shop partner has a 4x4 plasma table built by delusional design. He's had it for about two years and makes personalized signs and all kinds of one off jobs. He came to me saying that he's having problems with tolerances on like parts cut at the same time. So we did a measurement test. Programmed 50 inches of movement in both X and y from home point. On the X rails one side reads 49 7/8 other side 50. Y travel on gantry 49 3/4. Wtf
Machine frame is a .125 wall square tubing welded with a water tray. Extruded aluminum rails on each side of metal frame with a v wheel bearings 4 on each side. Both sides are driven by a nema17 92oz. Stepper motor with cog pulley and a cogged belt.
Y is also belt driven but by one stepper motor same specs. Z axis has a stepper that controls a ballscrew. Has height control too. Runs a hyperthermia 45 and uses repetierhost for programming.
I do fab and machining once I get into the programming and software I can't help him. So we did the test and only one motor gave the right distance so I asked him to cut the test length in half. So it should go 25". I forgot exact numbers but they were all off. Y ALWAYS being constant 1/4 off. Now the X was only like 1/8 off. I had him do the test again at 10". Y was like 3/16 off now and X WAS IN A 1/16 of each other but still not at 10. So the measurements were getting alittle smaller with less movement of each axis but never the distance called out. And the variance of measurements from each side on the X travel.
So my questions are the program should tell the steppers how many steps to go right. So my/his problem would be either the program or system isn't givin proper voltage to motors?
Stepper motors are losing efficiency and not counting correctly?
Belts are fatigued or worn between teeth causing measurement error.
Some of my preliminary checking was putting a mag base on z carriage and run the y travel. Had .002 deviance in 50" travel. Was consistent on both planes. Same with X carriage to rails was within .003. We actually replaced all the wheels and bearings on machine as a maint. Thing. Belts visually look good their like a nylon with metal strand reinforcements. We did tension all the belts though. Another question I had was how does the machine link up both steppers of the X travel to be exact. I would think that each belt would need to be timed with each other and same exact tension on both. I'm use to a gear rack system so that got me suspicious. Any thoughts or tips would be appreciated!

I ran a shear for many years and it had a rotary encoder to count the revs on the screws of the back gauge. Each morning I would have to re calibrate the back stop. I asked my daughter about she since she was in instruments and controls. Her response was Phht! "You are in an unheated building !" "Unless they are using military grade electronics the cold would knock everything out."
As soon as it was summer time I didn't have to re calibrate in the morning.
Is there a calibration procedure for the table?

Run anouther test, move any given axis forwards say 10 or 20" see were its at then run it back again programmeing effectifly a +-10 or 20" move, run this a few times each time it gets to the end of the move record were it is. If its going back and forwards the same amount and the end point remains constant you have a calibration issue, if far more likely based on your first post its not returning to the same point your simply loseing steps, stepper motors only generate so much tourque, exceed this and they just stall miss some steps and the travel randomly comes up short. New belts and guides that are too tight may well make the problem worse not better.

Normally for something like your twin driven x axis, with steppers you can simply stall them against stops for a few secounds and let it pull it true on start up.

Have you tried over at "The Zone" ?

This type of machine is discussed frequently over there.

Adam thanks for the response. When he first homes the torch he does bump it in both x and y kind of stallin Motors.
As for the drive and idler wheels we replace because they were getting wear marks from debris on rails. After Assy. Of wheels we removed belts and adjusted drive wheels for slight drag on rails. Kind of like how I would set gib drag. Very little resistance.
I did explain that temp is definitely gonna change the metal dimensions.
So would I be looking more at the steppers being lazy. Or something with the belts. Or could it be the stepper driver?
His calibration test is what I explained in the first post going to 50” in both x and y from the home point. I will have him do multiple runs and see if the motions stay the same or move around. If the measurements stay constant would that say theirs probably problem with Motors?
As for the “zone” the builder of this machine goes there and says theirs nothing wrong with his precision machine and is always has excuses. Won’t answer calls. Basically spent the money not his problem. And I feel more comfortable take n advice from professionals. Thanks

I don't have an answer for you about your machine, but this is why I buy machines from quality companies.

I was going to build my own machine buy it's not worth the savings if something goes haywire

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No calibration you need to go both out and back especially with open loop driven steppers, like this you know if its mantianing position, not just what its out by.

If the start and end are not the same point you have a very diffrent issue than a mear calibration problem, you have lost movment.

I really don’t think it’s a junk machine just a different client, application,price point. Now yes it’s not built for production 247 365 but its pretty accurate made its money back twice over and under 10k.
Thanks Adam I will do those test and see what’s going on. To help me understand a little clearer. When you say lost movement what would cause loss movement. Motors, drivers,programming. I really don’t think it’s dragging or binding. Or could it be a combination of all.
Thanks for the tips I’ll get back shortly thanks

Excessive rates of acceleration is normally the cause with steppers losing position or trying to move too fast, there’s certain speeds that stepper motors just won't run at.

The above posters coments about the zone are not being nasty, this is the kinda technology and drives most of thoes guys use.

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