[resolved, but mysterious?] Tarkin cutting issues today 03-29


I could not get Tarkin to consistently cut… pretty much anything earlier today. Tried several variants of plywood. Clearest example of something that should cut just fine was good-quality 6mm Baltic birch. At one point, I slowed it all the way down to 10mm/s and still couldn’t get consistent cuts all the way through, and when it did cut it was very char-y. Same piece of birch had no issues cutting previously.

@jamesfreeman was there toward the end of my session and took a brief look at the lens and such, but didn’t see anything obviously wrong. A few specific notes from basic troubleshooting:

  • Lens was clean (in fact, looked very clean)
  • Chiller read under 20C
  • Air seemed to be flowing fine
  • I didn’t notice any specific patterns w.r.t. location on the bed where I was trying to cut. I do tend to cut toward the right side of the bed since it is close to the workstation, but I tried a fair amount of cutting near the bed center overall as well.
  • At first, I presumed a focus issue, but eventually tested a fast text line engraving at standard 7mm focus distance and it was crisp.
    • Along the way, I tried to vary be height +/- about 2 mm just to see if it had an affect, but nothing consistent happened.

Example image showing the difference between a fast surface line engrave (crisp focus) and the sloppy/charry effect of trying to slow down enough to cut:

Along the way, I did observe an odd sort of “optical artifact” that I don’t remember seeing before on Tarkin. There was a sort of “lens flare” (wrong term, perhaps, but that’s the visual impression it gave) just to the right of where the beam should be focused. In other words, a much wider area of the wood was lit up than just the area where the beam was cutting. See example image… I wondered if perhaps it could be reflection off of the edge of the cone if that were misaligned? Or some other optical aberration? Note that this flare only showed up intermittently, not consistently during every cut / movement.

Finally, I wonder if what I observed might be related to mysterious sub-par performance observed by @Bwarez a couple weeks ago here: [Resolved] Tarkin not working

If it goes away just as mysteriously, that’s all well and good, but this inability to cut cleanly really delayed my project.

We were using it and it appeared fine.

There was a problem in that someone had created an alternate Library and set LB over to use that for everyone (please don’t ever do that). If you used Library settings, they might be wrong.

Nope, this definitely wasn’t layer library issues. I was using my own pre-established settings, generally speaking. I also double-checked the advanced settings, etc. I used settings that should cut these materials just fine and it did not.

Hmm. I did some cuts in thick 9mm acrylic and it went right along at the regular speed and gave a nice cut

I did replace the #2 mirror with a higher quality one, but everything checks out

Hmm. Well, perhaps it’s possible that when @jamesfreeman did a dismount/remount of the lens tube, that fixed whatever was out of whack? I’m just guessing at this point. He came by as I was rushing out the door, so didn’t do any significant testing after that — we just agreed “weird, looks fine”.

I’m still curious about the “lens flare” spot. Is that actually normal and I’ve just never noticed? I am used to some amount of extra light and flashes, but this was a very well-defined spot at a fixed offset from the focal point.

I thought about it and I think we need to inspect that lens. Tarkin is uniquely hard on lenses because its power density is crazy high, although this can, and does, happen with any laser.

It could have stressed the lens crystal structure, but the beam’s not totally on the center of the lens. This stress will not be evident on visual inspection in normal light but readily shows up on cross-polarization inspection. If the damage isn’t centered- typically the case- and the lens is rotated, the damaged portion may be rotated away from the active beam (slightly off-center) and it will “miss” the damaged area. This could explain why it kind of “fixed” between the time you saw probs and when I looked at it.

The theory will be immediately, conclusively provable with a cross-polarized inspection on that lens.

Very interesting point @dannym. Curious of the test results now.

Marking this thread as “ [resolved, but mysterious?]”. I’ve had two good cutting sessions on Tarkin since these issues, including cutting 0.47” Columbia Purebond plywood reliably at 18mm/s (seems to be a great alternative to Baltic Birch, BTW). Did not see the odd “focus flare” visual artifact.

The “mysterious?” bit remains because AFIAK, we never found a solid explanation for why it had an off day. I believe that Danny said that he examined the lens more closely and didn’t see any off-axis issues.