Rotary output center not aligning with flat material center, always under rotating

LightBurn Software Questions
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1

I am using a generic Chinese 40W galvo laser setup.
It has a 175mm lens which is setup in LB.
Flat stock material calibrates and aligns to the output to the dot center perfectly.
When doing rotary with the chuck, the dot center is aligned with the cylinder physically.
When I show the output center, it lines up perfectly with the dot center, down the Y axis of the cylinder perfectly.
When I have the Output center set to half (87.5) the lens field (175mm/2=87.5) the Show output center aligns perfectly with the red dot center.
The image is always grouped and “move to center” is clicked as a pre-fire checklist item.

The Framing boundary also aligns perfectly with the Shown output center and red dot.

But when I actually fire, the rotary Under rotates at the start and off centers the image, not at the red dot center or at the shown output center.
It consistently under rotates and off centers by the same distance each time.

The default setting for Output Center was 100mm and that seems to center a rotary image, but it’s burning at an angle not perpendicular to the cylindrical center, and a few mm out of focus, getting worse with smaller diameter cylinders.
So skewing the output center by 12.5mm seems more of a hack to get LB to do correct rotation because of the other problems it creates.

Engraving at a skewed output center position causes paint to chip off and visibly blurry lines presumably because the laser angle of incidence is not perpendicular to the tangent of the cylinder and mm out of focus.
This defeats the purpose of having a rotary chuck in the first place.
It’s like using LB Cylinder Correction instead of the rotary chuck. Effectively engraving at non-tangent angles and causing the same off center and out of focus situation.

LB also seems to randomly reset and save the Output Center as 87.5mm after changing it from 100mm as a hack, and it ruins cylinder after cylinder if not manually checked as one of 11 pre-fire check list steps that randomly reset themselves in LB. But that might be another issue related to saving or user error, not related to this rotary centering problem.
Just thought I’d mention it to add context if it turns out to be a related known bug.

My thought is that there is the laser model lens field setting of 175mm, but there is also some other setting in LB that relates to the image canvas size, or maybe a canvas size specifically applicable for rotary only (not sure what the cross grid image drawing area is called in LB, but financial charting software calls it a “chart canvas”, CAD software calls them “sketch fields”).
That dedicated rotary canvas size is maybe somewhere set to 200mm in a non-obvious place in LB, but I have no idea where that setting may live.
(LB is presumably using a rotary canvas size calculation of 200mm / 2 = 100mm)

The flat material settings seem to use the lens setting of 175mm, but the rotary output center seems to use this other “rotary canvas size” 200mm setting.
Where is that other setting? I’ve been searching for hours in this forum and inside LB.

Maybe I’m off base and there is no secondary rotary canvas size setting.
Maybe when rotary is enabled the LB code just shifts everything 12.5mm and that’s normal?
And the output center is meant to correct for a hard coded offset or bug in LB somewhere?
But then calculating and setting the focal distance becomes a problem because it’s not at the center line of the cylinder, so I’m not sure what the design intent of that skew would have been.

2

That’s a lot to digest, could you post a video?
Couple things, whether the image is centered or way off center, the rotary rotates to the lower edge, and then burns and rotates towards the upper edge of the image. Only if you move the bottom edge of the image to the output center, it would not back rotate. I don’t think there is a different setting for canvas that would cause the issue you describe. I use a 175 x 175 and a chuck, actually a couple different chucks for 2 years since LB first supported galvo, and have not experienced what I think you describe. My output center is within a couple mm of the 87.5 you note. Never noticed the output center changing, maybe when I update I’ll have to check after next upgrade.
Back to your issue, how close is your laser center to the center of your rotary? I’ll be down by my laser later today and the 175 lens and rotary are set up, we could try a couple things.

3

Thank you for the help.
I’ll try to answer best I can to clarify.
I won’t be back at the machine site until Thursday night so I can record a video and post a URL Friday morning / afternoon.
Forgot to mention that the Rotary is a 3 jaw Chuck.

The image is centered on the canvas with the “move to center” button / menu selection.
The laser setup is set to 175mm field / canvas. This matches the printed lettering on the lens and seems correct from doing flat material testing.

When I Enable Rotary and set the Output Center to 87.5, all 4 references of the Visual Physical Y axis of the cylinder (aligned via fixture), Red Dot, Output Center and Bounding Box centers are all aligned perfectly.
If the image is a Rectangle, I can trace the outside of the bounding box with a pen to see where the Rectangle ‘should’ burn. Call this “Pen Mark A”.
The expected / predicted output with all 4 references lining up at Y center would be to simply burn a Rectangle right at the edges of the perfectly centered Bounding Box. (That’s my Ockham’s Razor expectation anyway, I accept that I could be misunderstanding LB)

If I then Fire the image, the rotary Under rotates and burns the image Off Center.
Since the image I fired was a rectangle, I then measure ‘Pen Mark A’ of the bounding box with the Actual burned box. This is off by 12.5mm.

The rotary is set to “Return to center” so it returns to center at the completion of the burn.

If I then change the Output Center to 100mm, the Visual Y center of the cylinder, Red dot center are still aligned. But the Output Center and Bounding Box are both off centered when I click the “Show” button.

If I then Fire the same rectangle with the Output Center now set to 100mm, the burned Rectangle lines up on top of ‘Pen Mark A’.

4

You are welcome

That’s part of what confuses me

So is the chuck lined directly under the lens – 87.5mm, or is it lined up 12.5mm high, 100mm?

I guess another way to ask, forget about the red dot or the bounding box, if you have a 1" cylinder in the jaws, and you project a 1" high by say 4" long rectangle centered in the work area, does it line up with the cylinder or do you need to shift the rectangle center to 100mm?
Last question for now, what is your split size?

Also, all this is assuming the lens has been properly calibrated.

5

I spent Thurs and Fri evening doing the exact test examples again I outlined with the rectangles above.
And no matter what I try now, with an output center of 87.5 (the 175mm of the lens / 2) it burns boxes perfectly under the visual center, bounding box, and center dot. This is great news, as all seems to be perfect. But I also can’t recreate the problem I was having. Arrrrgh!
I have about a dozen pics showing what I was doing and they all end up with a perfect output result. So I guess problem solved? Magically by itself?

I should say that I’m not using my own laptop. It’s someone else’s who takes it home every night. And I’m only at the laser site 4 days a week. So anything could have restarted, rebooted, etc in the time before last Thurs when I started documenting my testing.

To answer the last question though, yes the chuck is lined up directly under the red dot and “show output center” line. It is using a 3D printed fixture that registers into the hole pattern on the bed and allows the 4th axis to slide in the Y axis manually to adjust for different height / length mugs and tumblers. But it is fixed in the X both linearly and in Z rotation (yaw if the cylinder was an airplane).

Forgetting about the red dot and bounding box, yes the bounding box would line up with the edges of the 1" cylinder (a bit wider actually and projecting onto the bed because cylinder correction would be off with rotary on logically. With cylinder correction on, I imagine a 1" bounding box would hit exactly at the edges of a 1" cylinder). No shift is needed with output center set to 87.5.

Split size is 0.2mm. The model # of the stepper says it’s 1.8 deg, so revolutions is set to 12800.
I’ll try to upload pics of the setup and screen if I can figure out how the forum software does it.

6

So the big question, did it work properly when you tried to engrave on the cylinder shaped object this time?
You shouldn’t absolutely need a dedicated laptop or computer, but as cheap as they are man why not. Any, and I mean any cheapo laptop seems to work with LB, only reason I upgraded from the cheapo I started with was the videos were using up all the storage.
Good Luck!

7

Yes, I tape an index card to the mugs and do a 15% power burn to make absolutely sure of the positioning. Takes extra time, but making an error really sux . Then I fire it for real at 80% and slower speed and the images burn exactly where I aligned.

The option to use another laptop isn’t really there. This is for a seasonal Christmas holiday venue that’s offering customized acrylic and wood ornaments, badges, and painted steel mugs and tumblers. I do bring my own measurement and hand tools, design and 3D print fairly intricate alignment fixtures for the targets and 4th axis, but the environment is kinda dirty, dusty since it’s semi-outdoors; and the security isn’t best. Especially with all the public customers roaming around. So risking theft or dirt damage to my own laptop (which is a mid range gaming capable rig with a higher end graphics card to power CAD software) is really not what I want to do. Certainly not for what I’m earning there.

I don’t own a laser personally (yet). Maybe one day. I keep busy with a CNC mill, lathe, 3D Printer, 3D scanner. Maybe I’ll build a CNC router table one day, but I haven’t needed it yet with the printer able to make alignment jigs and fixtures.

8

All’s well that ends well! Prices are coming down…