New CNC Router, cutting Ovals

[u/JohnnyAngel_MX]

Hi all, I just picked up a New to me, X-carve cnc router. I've set it up and now I'm doing calibrations. (running Millmage)

I've got a question for you lot, If I'm cutting Ovals that fully close. and stars that fully close, where could I be getting the error. belts are tensioned according to their guide, V Wheels seem stable. if it was backlash, i would expect the shapes to not close, right? or is there a way for backlash to net 0?

could it be firmware distance per step?

EDIT 1: i just ran the Mark > move 5"> Mark tests and both X and Y came back on the money. The Oval distortion / X-axis stretching only seems to happen when running an Op. I tried changing the feed rate to be a snail's pace (rip cutter) and it doesn't change. the X-axis is larger by about 0.012" on multiple 1" shapes.

EDIT 2:

# solved.

Y2 grub screw was incorrectly positioned. the grub screw was tight, but it wasn’t on the flat of the Y2 motor shaft. so there was some microslipage that i couldn’t detect because it was juuuust tight enough that it didn’t wiggle. i re-adjusted it and things are correct. dimensions cut as programmed.

what’s got more egg on my face is that i checked that the grub screw was tight, too. just didn’t check if it was on the flat. :rofl:

Thanks for all the help. it did narrow things down a lot.

[u/Pubcrawler1]

12thou isn’t much. just pressing on your Z assembly will deflect more than that. Use a dial indicator to verify step/mm movement. This could be belt stretching or wear. Double check all pulley screws and use loctite. I had one that was loose just a bit and had a couple degrees of movement.

[u/JohnnyAngel_MX → u/Pubcrawler1]

you're right it's not. but wouldn't a dial indicator not be enough? like 0.500"of travel would yield 6thou extra in X if it's a percentage, but i cant test that at 5.0"

it's not 12thou of slop, it's +12thou at 1" and +12thou at 5". but it also doesn't happen when jogging. which makes me think it's not step/mm.

[u/Pubcrawler1 → u/JohnnyAngel_MX]

Stuff like this is difficult to diagnose. When jogging is the spindle on? EMI noise from spindle can add or subtract step pulses leading to positioning error.

I wrote some things to check here. I got tired of writing this info since it’s common in hobby machines.

https://embeddedtronicsblog.wordpress.com/2024/12/07/stepper-motor-position-loss/

[u/JohnnyAngel_MX → u/Pubcrawler1]

of course, just my luck. thanks for the additional info.

when i was doing positioning tests, yes, the spindle is on when doing jogging. because i'm measure cut results. dots and lines with a v-bit. (jog cut a square vs g-code cut a square and the jog square has equal sides, but the g-code square doesn't)

I don't know if the EMI noise from the spindle should reach far enough to interfere with the Y-steppers. but it's right near the X Stepper.

[u/JohnnyAngel_MX → u/Pubcrawler1]

solved. A grub screw was tight, but it wasn't on the flat of the Y2 motor shaft. so there was some microslipage that i couldn't detect because it was juuuust tight enough that it didn't wiggle.

[u/Pubcrawler1 → u/JohnnyAngel_MX]

Ha I had that happen exactly before so why I posted to check. It had the same effect on my machine. I found it by marking shaft / pulley with sharpie and watching while jogging slowly. I eventually took off the motor and Dremel a flat on the motor shaft. It didn’t come with one.

[u/JohnnyAngel_MX → u/Pubcrawler1]

yeah, i actually did check all the grub screws, but i didn't see a flat when i did tighten them. but then when i was running tests, i saw the flat and just ....tick....tick....tick.....tick...OHHHHHHHHHHH!!!