I am making a 3D Lake Tahoe in acrylic. After just a few passes I am getting

vertical banding. I know it is from my belt because it is spaced the same as my belt notches but I cannot figure out the mechanism by which it happens.
At 1000mm/sec or say at 400 I still get it so I can't imagine it is a resonance...or is it?
Is the head moving faster and then slower because of the belt notches?

The engraving I am doing is 52 layers at 30 power on my 150W and takes over 10 hours or more depending on size of course. I want to up it to over 100 layers for larger more detailed projects...but not with these results.Attachment 381065Attachment 381066

Any ideas at all? (Many hundreds of hours invested into this project already.)

Should I go to a rail and ball screw gantry system? Anyone have one?

Whatever the cause, I need it gone. I don't care what the cost is.....within reason.

One thought I had, and it would be a hassle, would be to to move every layer 1mm to the left and move my piece 1mm to the left as well. : (

edit: Oh, I forgot to add, I have tightened the belt without any difference.

IF the belt is the cause, you'll want to loosen it, not tighten it. While you're loosening it, remove it and check the cogs for rolling smoothness, you might have a flat-spotted bearing that's thumping. Check the cogs teeth for crud... Also, check the cogs for trueness of rotation--the cog on my X stepper has a slight wobble to it, it's very slight but still, it will create a slight accel-decel effect while moving. For what I engrave and cut I've never noticed a problem so I haven't worried about mine...

Back to belt tightness- when my LS900 was new I got an acrylic job and had severe banding issues engraving large expanses. Service guy came out, worked on it all day to try to alleviate the problem. He actually did, got it to engrave a 6" square perfectly-- however, to do this the X belt was so loose the backlash was terrible at anything above 20% speed! I couldn't keep it that loose but it did work--

Your Chinese software should have some backlash adjustment settings; if you do loosen the belt and find it helps, you should be able to adjust for the extra backlash you'll have and keep the left and right sweeps engraving in the same place :)

>edit> - do you have linear guide rails on that thing? If so, check the rail bearing for smoothness- sometime they spit out bearings, how I don't know but my GCC did it!

Alright, I fee REEEAL SILLY right now but I'm going to share anyway. I solved the problem. In fact, I had already solved the problem before I started this thread.

I HAD the banding as you saw above and thought I'd clean everything thoroughly. I cleaned my lens, my mirrors and the glass on the laser itself. Mirrors are perfectly aligned and table is level.
I ran the SAME job as above on the same size of acrylic. When it was completed, it was rather coated with acrylic "dust". I washed it up and took it out to the shop to do a final glaze/polish. But, before I did I had
check the settings on some scrap. After getting it right I picked up the piece and glazed it.....THE WRONG PIECE. I grabbed the 1st one from a few days ago that is shown in the picture above.
I just found the new one and it is OH SO MUCH BETTER! No vertical banding. I still don't know why dirty mirrors and lenses would cause the banding but somehow there was repetitive deficit of power in sync with my belt notches.

HERE is what it is supposed to look like. Next I'll edge light it but before I run it through and out of focus glaze treatment.

Unglazed still:
Attachment 381083

Thanks Kev. I will try loosening the belt as well but for now I'm all smiles. Next MDF for a woodier look. I don't think wood grain will allow me to get a decent result.

I read the rest of your post. I don't know if you'd read my past post about taking my bearing apart. I took one ball out of each and it really does slide much smoother and still without play. Thanks for taking all time to share your knowledge : )

Yup, I remember that thread about removing the bearing :)

FWIW the new bearing I put on my GCC way back then is still extremely tight, and sounds like a fingers across a washboard, and the bearing tightness does show up on the engraving. Something I found out too from the bearing mfg (forget who at the moment) via email that you're not supposed to lube them, other than maybe wiping the rail with some light machine oil once a year. The GCC instructions say to use 'their' white lithium grease. The mfr said any grease is too thick and can cause the bearings to spit out. I actually have another bearing I got on ebay I bought at the same time, only it's a longer version- might work but I think it may too long to allow the X rail to reach the limit switch. Still, I should put it on the rail just to see if it's as tight as the other one. My only other frame of reference for rail bearings is my LS900- it's 14 years old and I've never touched the bearings but once; changed the belt a few months ago and checked the X bearing movement...I was expecting a bit of slop and maybe some gritty-ness, but nope-- it glided along the rail smooth as silk, I could blow on it and move it, and yet I couldn't discern even a tiny bit of slop in any direction. I put the new belt on and left everything else alone ;)

Sorry for the hijack :D