Some times 1-5 mm materials are far from flat and holding them down is real pain. I’m thinking about attaching length sensor to the head and constantly adjust Z-axis to keep laser in focus.
As far as I know Lightburn is not supporting such thing, but maybe this would be worth considering?
Data as z-offset may come on another serial port, so all the calculations would be done in external box (averaging and reporting only when z-tuning is required).
Would you find such accessory useful?
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Not sure what kind of machine you are running, but generally, if it has a Ruida type of controller, LightBurn’s job is over as soon as they send the code over to the Ruida. The Ruida takes over from there, not LightBurn. Perhaps on Grbl or such, LightBurn has more control.
I run a Ruida 100 W, and would have loved to have continuous adjustable Z axis. In fact, I had designed a ESP8266 controller that would allow pass through on the focus from the Ruida, then modify the Z axis based on measurements from a laser rangefinder. What I did worked, because I did not want to touch the work with a probe, but it was very expensive, and to be fair, not fast enough to handle the x and y moving at the same time. I ended up averaging, but I still had to drive the laser slower than I wanted to, ran into power issues, and decided it wasn’t worth it. So, YAPA ( Yet Another Project Abandoned ). A very complex but solvable issue, and bottom line, the laser, with proper lens, could handle 1 to 2 mm without readjusting… Good Luck!
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There are many issues about this.
- what kind of sensor?
- it would have to sense the area around the lase point, as the head could move in any direction
- waiting for the table to move would slow the whole engraving project.
Sounds nice and some cnc machines that cut metal have a hall sensor to help them change the Z axes… It didn’t take into account much of a Z range…
The right way would be where the laser actually struck the material, meaning you really couldn’t scan anything without waiting for the Z to respond.
Besides that… @shipmodeller is right, that would end up being a controller firmware issue…
Even with a grbl machine, it’d be very difficult… Just sensing properly for any material is difficult.
Good luck
I have an LFS-ANM-T43-V2 non-metal live focus distance controller. It might be what you are looking for. Mine came with the machine, Though I still have not got around to setting it up.
That looks pretty slick… How does it know the height above the material?
Looks like you have to drag around a lot of ‘stuff’ on that axes…
Kind of pricey … $3,200… I paid $2k for my whole machine…
- Don’t know yet but there are some pretty accurate (0,1mm), short range (2-10cm), fast (300Hz) sensors for nonmetalic (!) surfaces for less then $20.
- It is not about avoiding obstacles hits, just fine distance tunning during work
- It would tune once per 2cm or any adjustable distance. Plywood waves are like 20-30cm and I have movable Z-axis (so the laser head, not the table) which can by moved simultaneously with X/Y - should not influence work time.
The only idea I have so far, is to make my own “controller” for one Z-axis motor. So when “leveling” would be enabled, motor would be electrically switched to separate controller driven by sensor data.
It is… patchwork but as my controller is not capable to do such things, and LB wont do this neither then what are other options for wasting couple evenings in the garage? 
This is likely the easiest/most effective path forward. That would make your focusing device independent of the controller as well as LightBurn.
Attempting to move the table would likely be far too slow anyhow but you could likely get very fast focus adjustments at the head. But then you’re likely now in the custom head territory. Or you could use something like Russ Sadler’s design where the lens is independent of the mirror so that you can adjust distance between lens and mirror relatively easily.
How does this effect focus?
This would allow you to move the lens to keep focus with the material without affecting mirror location. So only changing the elevation of the lens itself. So the proximity sensor would determine distance to material and the height adjustment mechanism would only change lens height, not mirror height.
Got it… Thinking down some other path…
try using cheap masking tape along the edges. works great to keep that thin 3mm flat even sticking it to the honeycomb…
Probably the right way to ultimately approach this would be a LiDAR scan of the material which (in theory) LightBurn could use to create z axis moves in the output file. Potentially not cheap but would be very effective.
I am using tape! And some flat 2x30mm neodymium magents. Problem with magnets is well known: they really like laser head more then steel below the material.
Using tape is usually enough for smaller workareas, 30x30cm. Anything bigger tends to bend in the middle, no matter how strong it is mounted on the edges. Of course there is better plywood, and worst plywood.
I’m not sure how accurate is LiDAR but even having unlimited amount of… resources, potentially it is not the best solution. Plywood holded down with some force, wants to go back to its own shape as soon as possible. Very often bigger, openwork shapes, tends to bend after cutted out. So still dynamic measurement could be more effective.
By the way: Having unlimited resources…
I came across these things on youtube called “Magclips” and you can download the 3d printable file on Thingiverse. The simple low-profile design eliminates you from crashing into them with your laser. Just do a search on magclips on Thingiverse. His video is on that site as well. Hope this helps.
It doesn’t sound like there is interest/need for any “low-tech” solutions here. Why not just buy a ready-made “professional” solution?
It might not be as fast as the one in the video, but for plywood it sure does, and all for under $100.
Do you have any idea how close to the work the laser would be at it’s strarting point ? Thank You
Uh, that’s the shipping. The tracking head is closer to a kilobuck.
I can buy a lot of really flat plywood for that money … 
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****, that’s a typo missing a 0