try to do a test where you engrave the same shape, simple on a piece of wood in the middle of your workbench. Repeat the same operation a few times without moving your material, just press start…
Does the shape “move”?, if so, then there is something mechanical with your machine. (But if so, then it should also be visible on all shapes you make)
Are you absolutely sure your workpiece is exactly the size you think it is and cut absolutely square? Even a small amount out of true on the workpiece will affect the front to back registration.
Your red images are lower down than your blue ones. So simply flipping the image will engrave them lower, just as you have presented. Best solution. Put the red image over the blue one and centre them in the same circle. Then using the cuts and layers tab, hide the red ones and set them to no output. Lock the sheet to the bed by clips or tape, so it does not Move.Engrave the blue images, cut the discs out. Leave the sheet clipped to the bed. Take out the discs, flip them over and put them back in the holes. Turn the blue layer off. Turn the red layer on, engrave the red layer. I do this often. Works perfectly every time
Thank you everyone! I think I did figure it out… once I set the back cut to “flood fill” everything worked fine!
I can accept operator error but I am one of those people who will work at something until I get it! If it’s my error, I will work it out until fixed! So no I can’t accept my own errors until I fix the problem!
Trying to save time is what cost me the issue. It was slower to do flood fill!
Yes the project area was correct!
Thank you So So much to everyone who provided suggestions! I truly appreciate them and your willingness to take the time to help!
Blessing to you all!
A cut shouldn’t be able to use flood fill, it’s just in line mode.
Wonder why this would fix it?
Guilty here too! 
There is not even a button for it in Line mode. I suspect it was one of those wide (2 parallel) lines that can be filled. Flood Fill should still not have fixed a front-back alignment. Unless there was actually no misalignment after all, but the front and back modes did not match (one line and one fill).
Of course, this is all speculation based on speculation.
My theory is that you might have your speeds set too high:
- High Speed and Acceleration Cause Slippage
The main culprit for a belt slipping (or a stepper motor skipping steps) is high inertia during rapid movements, acceleration, and deceleration.
- When the laser head makes long, rapid passes (like in a standard fill), the G-code commands often push the motor speed and acceleration settings to their limits.
- The mechanical stress from a sudden start or stop can cause the belt to momentarily jump a tooth on the pulley, resulting in a permanent loss of position (a “lost step” or “slippage”).
- This lost position would be cumulative, leading to the second side of the engraving being misaligned relative to the first.
- How “Flood Fill” Can Mask the Issue
The “Flood Fill” setting in LightBurn generates a different toolpath designed to skip empty areas and only engrave where necessary.
- This often results in a series of shorter, more segmented travel moves compared to the long, continuous scans of a standard fill.
- Shorter movements inherently mean the laser head may not reach the same high peak speed or is not subjected to the same rapid acceleration and deceleration over long distances.
- By reducing the physical stress on the belt, the Flood Fill mode inadvertently prevents the belt from slipping, making the issue appear to be fixed.
There’s a facility in Lightburn to allow you to cut things larger than your work area. It involves putting in registration marks which you use to align your work for the second cut. You could use that feature as long as you cut through with the marks rather than just marking the surface.
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