LaserUser.com Forum (formerly RDWorksLab)

Hi! I've been struggling for a few weeks trying to find the cause of an alignment problem on my laser. I've been getting some feedback on the LightBurn forum but haven't nailed it yet, so I hope it's okay that I run it by y'all here.

I have a new Chinese laser cutter and I'm having alignment problems related to the direction of x-travel. It appears to move different distances in the x-direction depending on whether it is moving left or right (but the error "catches up" by the end of the cut pattern... strange). Here's what it looks like:

In the first picture below is two sets of 10 lines, one set above the other. The top set cuts first, in the order and direction as shown with the numbers 1-10 and in the directions shown by the arrows.

Next it cuts 11-20, going in the opposite direction, as the numbers and arrows show. They miss alignment in the middle.

(The lines were made by drawing one 15 mm line and then using the grid tool to create an array with 10 lines evenly spaced, 10 mm apart. I assigned these a cutting priority from 1-10 to control the order so they cut left to right. Next, I copied the entire group and placed the copy underneath. I then changed the priorities from 11-20 so that this group would cut in the opposite direction. They are dead accurate on top of each other in the file, the top half an exact copy, moved 15 mm down, unrotated.)



Close-up:


As you can see, the cuts do not align in the middle, but they align on both ends. Strangely, the misalignment gets worse and worse as it reaches the middle, and then better and better until it lines up again at the end!

Next, I started with the same line segments 1-10, copied them and placed them underneath, and then set the priority so they would cut left to right, the same direction as the top part.

Here is the result: it cuts fine. No alignment problem.



So, there is a difference in the laser position when moving left to right compared to moving right to left!

Just for kicks, I was interested in how the error gets worse and worse to the middle, and then better and better until it aligns again at the end of the row. So I did a version with 40 double lines. It also gets worse and worse, but only for the first 3-4 lines and then it holds a steady error of about 0.1 mm until near then end, where it mysteriously realigns again.

(Underneath that 40 line test is a another test. This is rotated 90° ... I cut with long lines in the y-direction. This was to see if a long y-travel made a difference, but it did not.)



What I've tried:
Rising Edge on and off, tightened every screws I could find, loosened and tightened the belt (up to very very tight), cleaned and lubricated the rails, adjusted acceleration and speed (including all the way down to 10 mm/s2 and 10 mm/s) with no effect, cut all all 4 corners of the bed with no effect.

Also, every file I cut is identical -- the problem is 100% reproducible for any given design.

Any ideas? I'm going crazy with this problem! Thank you!

viewtopic.php?f=105&t=3528

https://cartonus.com/how-to-improve-eng ... r-machine/

Try doing the alignment in these 2 links and reperform you test

Mike Naylor wrote: Sun May 16, 2021
What I've tried:
Rising Edge on and off, tightened every screws I could find, loosened and tightened the belt (up to very very tight), cleaned and lubricated the rails, adjusted acceleration and speed (including all the way down to 10 mm/s2 and 10 mm/s) with no effect, cut all all 4 corners of the bed with no effect.

I suspect it's the acceleration and speed. Can you please tell us WHICH SPECIFIC acceleration and speed parameters you adjusted? You might infer from my question that there are several. For example, the speed we usually adjust (to accommodate specific materials) is the speed of the head WHEN THE LASER IS ON. There's a different parameter that controls the speed when the laser is OFF (Idle Speed). Ditto for acceleration (Idle Acceleration). I hypothesize that the laser is losing steps when it is off (and moving between your drawn lines).

These parameters may have different names in LightBurn; so far, I only use RD Works.

Pete Cyr wrote: Sun May 16, 2021 viewtopic.php?f=105&t=3528

https://cartonus.com/how-to-improve-eng ... r-machine/

Try doing the alignment in these 2 links and reperform you test

Hi Pete, the first link is about adjusting step size to match with cut dimensions. A 100 mm x 100 mm square cuts exactly the right size. The total width of the multiple-line design is correct. Something is happening in between. The second link is about engraving and I'm cutting, but "reverse interval" does sound promising. I'll give this a go tomorrow!

Jeffrey Aley wrote: Mon May 17, 2021

Mike Naylor wrote: Sun May 16, 2021
What I've tried:
Rising Edge on and off, tightened every screws I could find, loosened and tightened the belt (up to very very tight), cleaned and lubricated the rails, adjusted acceleration and speed (including all the way down to 10 mm/s2 and 10 mm/s) with no effect, cut all all 4 corners of the bed with no effect.

I suspect it's the acceleration and speed. Can you please tell us WHICH SPECIFIC acceleration and speed parameters you adjusted? You might infer from my question that there are several. For example, the speed we usually adjust (to accommodate specific materials) is the speed of the head WHEN THE LASER IS ON. There's a different parameter that controls the speed when the laser is OFF (Idle Speed). Ditto for acceleration (Idle Acceleration). I hypothesize that the laser is losing steps when it is off (and moving between your drawn lines).

These parameters may have different names in LightBurn; so far, I only use RD Works.

Hi Jeff, thanks for taking the time to write! This was my first thought, too, that it was moving too fast and missing steps. I've tried, I think, just about every combination, both idle speed and acceleration, max acceleration both overall and in the x- and y- parameter settings, cutting speed, I've adjusted directly on the Ruida controller, I've cranked down start speed. I haven't messed with the engraving settings, but I think I got everything else. And it was quite obvious when cutting if I got speed and acceleration correct for idle and cutting speeds, it moved at a crawl both when the laser was off and on.

I have a two-head system. I removed one of the heads to reduce mass on the gantry, thinking that might be a factor, but it was not.

Not sure there are any more adjustment - sounds like a controller issue

Mike Naylor wrote: Mon May 17, 2021
And it was quite obvious when cutting if I got speed and acceleration correct for idle and cutting speeds, it moved at a crawl both when the laser was off and on.

Hmm. Okay, hypothesis rejected

Next hypothesis - something is wrong in the file you created. You could try reading the G-code yourself to see if the X-coordinates match. Or if LB has a simulator (RD Works does), you could look to see if the simulator thinks the lines should align.

By the way, what happens if you delete all the lines except for 1, 5, 10, 11, 15, and 20? Do 5 and 15 still misalign by the same amount?

Hi Jeffrey, thank you for the troubleshooting ideas. I deleted all except for 1, 5, 10, 11, 15 and 20 and the misalignments were in exactly the same places. So strange that the first and last line segments on the second row line up correctly... I looked at the G-Code and the coordinates are exact.

Mike Naylor wrote: Thu May 20, 2021 Hi Jeffrey, thank you for the troubleshooting ideas. I deleted all except for 1, 5, 10, 11, 15 and 20 and the misalignments were in exactly the same places. So strange that the first and last line segments on the second row line up correctly... I looked at the G-Code and the coordinates are exact.

Hmm. Just to be thorough, were you able to look at the G-code that actually gets sent to the controller, or is it the G-Code for the file before LB processes it? I wonder (hypothesize) that LB might be doing something to the file (e.g. kerf compensation or something).

Here's a simple way to eliminate "software" (as a general term) from the equation: can you send the file to someone else who is running the same version of LB and have them see if they have the same issue?

I notice in your first picture that the top lines are to the left of the bottom lines. If you cut the lines from right to left (on the top) and left to right (on the bottom), does the misalignment switch so that the top lines are to the right of the bottom lines? I'm not sure what that would tell us, but it seems like it'd be an interesting data point.

Do you have a pair of calipers? Is it possible to measure the distance between the lines accurately enough to tell us how the line spacing is changing?

Does LB work natively in inches or in metric? Is it possible that there's some kind of rounding error in the conversion between units, or the conversion to step angle in the stepper motor (I assume you ARE microstepping - please check).

Jeffrey Aley wrote: Thu May 20, 2021

Mike Naylor wrote: Thu May 20, 2021 Hi Jeffrey, thank you for the troubleshooting ideas. I deleted all except for 1, 5, 10, 11, 15 and 20 and the misalignments were in exactly the same places. So strange that the first and last line segments on the second row line up correctly... I looked at the G-Code and the coordinates are exact.

Hmm. Just to be thorough, were you able to look at the G-code that actually gets sent to the controller, or is it the G-Code for the file before LB processes it? I wonder (hypothesize) that LB might be doing something to the file (e.g. kerf compensation or something).

Here's a simple way to eliminate "software" (as a general term) from the equation: can you send the file to someone else who is running the same version of LB and have them see if they have the same issue?

I notice in your first picture that the top lines are to the left of the bottom lines. If you cut the lines from right to left (on the top) and left to right (on the bottom), does the misalignment switch so that the top lines are to the right of the bottom lines? I'm not sure what that would tell us, but it seems like it'd be an interesting data point.

Do you have a pair of calipers? Is it possible to measure the distance between the lines accurately enough to tell us how the line spacing is changing?

Does LB work natively in inches or in metric? Is it possible that there's some kind of rounding error in the conversion between units, or the conversion to step angle in the stepper motor (I assume you ARE microstepping - please check).

Hi Jeff, the G-code looks good. Of course, I'm not experienced in reading G-code, but the coordinates are all whole numbers and doesn't look like any corruption.

BUT! I have great progress, and will post it as a general reply. Thank you so much Jeff!