Laser losing focus every 3rd or 4th session

I have issues with my SM2 350 1600mW laser module. Using the default settings on Luban for 1.5mm basswood cutting. Laser focus calibration completes no problem.

There seems to be no problem cutting a single one-off item but if I try to create multiple copies of the same item, same settings, same batch of wood, same toolpath etc… it seems to fail after the 3rd or 4th copy.

I have my design, I camera capture, I place the design, I create toolpath (using the default basswood 1.5mm setting), export to workspace and start on Luban, Auto Mode, Thickness is 1.5mm, laser works…fine all good…

Next copy I change the wood out, delete the old toolpath, camera capture, position the design, create a new toolpath, export to workspace, start on Luba, Auto Focus, Thickness 1.5mm…

Works fine for around 3 or 4 copies then suddenly find that cut line is thick and not cut through (out of focus). If I recalibrate the laser it seems to work for another 3 or 4 copies.

My question is… Why does my laser lose focus after 3 or 4 items? Does the Auto Mode change the z height of the focus at random? My tool is tightly screwed in, no play in table or any of the axes. It’s pure basswood and not laminated so the material should be fairly consistent (you would think)

I think I solved this.

I manually calibrated and checked against the auto calibration. Same result. 25mm laser height. So it confirmed my calibrations weren’t the problem.

I found that the different bits of 1.5mm basswood were curling up ever so slightly after the first pass so the work was lifting off the board a millimetre or so which I think was putting it out of focus. My work peices are relatively large and fill the base plate and my cutting was at the outer edge so the problem was worse at the other edges of the plate with the material bending.

I resorted to screwing down the edges of the work mechanically (the bundled m5 bolts tap themselves in between the aluminium plate grooves quite nicely)

So far my cuts have been fairly consistent.