Lasering 102 - Different focal lengths?


So, I see that we have new focus squares on the Rabbit Lasers. Two, in fact - one marked “39mm” and one marked “41mm”. Clearly there is some next-level work going on. I see that the 39mm is the same as the previous (unlabled) ones. What is the purpose of using 41mm, other than the obvious outcome of getting a “different” focus?

I’m also thinking that the 39mm square should say “Normal focus” or “Sharpest Focus” on it.


Uh, yeah, I cut those new tiles yesterday since laser 2 seems like it might still be a little off of the factory 39mm focal length. Someone else had cut out an unlabelled 41mm tile that I ran across while cleaning up the area, so I printed some new tiles that had labels on them just so we had some known values to work with. Both the 39 and 41 were giving me generally clean lines, so the actual focus might be somewhere around 40.
I’d meant to put a “default” marker on the 39. If someone beats me to it, please write that onto the existing tiles and I’ll update the file.


Hey guys - let’s limit the tools at the machine to only factory default. If something has gotten weird with the machine let’s fix that, but having a lot of ad hoc fixes is going to be confusing.


I’ll tuck the 41s into the cabinet when I stop in today. We’re currently much closer to ‘normal’ after the lens cleaning than we were before. Still, there’s not a standard ‘perfect distance’ across all lenses, and laser 2 has always (in my experience) needed a slightly longer focal distance than 39. I’ve always adjusted manually for it, but the 39 block has always been small. A 40 is probably the correct size for cutting, and a 41 for crisp engraving. I recommend bumping the standard block to 40, and have a 41 on hand for advanced users. Testing from others to back up that theory would be appreciated so we can get the proper size available for general use.

I’m planning on machining up a tool to remove the lens retaining ring, as well as a special ring-clamp that can be used to hold the lens by its edge, allowing access to both faces while cleaning in order to help avoid scratches & lint. I’ll type up some instructions for general cleaning as well and make up a fully stocked kit to put in the cabinet.


Thanks! I think the biggest improvement will come with the regular maintenance schedule and tracking you’ve been advocating. I think we’ll make progress on that at the next Area Lead meeting. :slight_smile:

Question on the lens cleaning procedure: how does this compare to the procedure provided by the manufacturer? The “Convention over Configuration” principle from Ruby on Rails comes to mind here. If manufacturer procedures tend to be good enough, using them on our dozens of tools that require maintenance would leave more time for the maintenance itself.


Not sure what the ‘this’ is that you’re asking about.

  1. The lens removal tool is 100% necessary to remove the retention ring properly and safely. We’ve been doing it in a wrong, and unsafe manner so far, with a high likelihood of scratching the lens.
  2. The clamp would just be for ease of use and would eliminate the need for a microfiber cloth to be on-hand and in a dirt-free state.
  3. The rabbit cleaning instructions do not indicate how to remove the lens from the housing tube, nor what orientation to use when reinstalling the lens. A few sentences of explanation, and a diagram would be very helpful as a reminder for someone who needs a refresher before performing maintenance.
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So, as a follow-up to Thorn’s intent, just-for-now we need a tile marked “Standard for Laser 1” and a different tile for “Standard for Laser 2”??

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Sorry about that. I was referring to the method for cleaning the lens, and whether it called for the lens to be removed at all. I just looked back at the “Maintenance Schedule” instructions and it does call for the lens to be removed - - but it you’re right that it doesn’t say how.

I was sure I had seen pictures of this being done somewhere and found them starting on step 30 of the “Beam Alignment” document - - it looks like they use a small screwdriver and dental pick. It also mentions putting the lens in flat side down. But this document isn’t referenced by the maintenance document and definitely should be.

Do you think those documents combined are sufficient?


Just-for-now we’re back to only having 39mm tiles at the machines. Potentially each machine has a different focal length, but that’s unlikely to be functionally different. I haven’t done any checking on laser 1’s focal length. I’d planned to do some cleaning and testing on it today, but there wasn’t open time on it. In a perfect world, the same focal block will work on both. In an imperfect world, there may be enough difference that separate focus blocks would be needed to get proper cuts out of the machines.


Thanks for the beam alignment link. I hadn’t seen that one.

For my own benefit I would like to consolidate all of the useful information into one page of instructions that can be printed out and kept with the cleaning kit for ease of use. One document for lens and mirror cleaning with no extra information.

The document provided by Rabbit says “This method is used by Ray Scott and should NOT be considered to be the only method or even considered as a professional method.” The usage of a screwdriver to remove the retaining ring WILL someday lead to someone slipping and gouging the lens, the threads of the focal tube will be damaged by repeated scraping, and may leave aluminum filings on the lens. Even just adding the proper spanner wrench to the cleaning kit makes the existing rabbit instructions obsolete.


So, we found the original acrylic reference square and it was 40mm, not 39mm. Both lenses have been cleaned, and new 40mm blocks are now the only focus aids at the machines. Both machines seem to be cutting well on that setting. Further testing may happen, but for now we’re OK.


I have a lasersaur (manual focus only) and we are always adjusting the focus based on what we’re cutting and the cutting power. We focus on the middle of what we’re cutting and on the surface of what we’re etching, so I made these to start the focus procedure:


Per Devin, I drew up a 40mm focus tool in Corel Draw, since I couldn’t locate the original file. The AI file is currently on Laser 2 (unless that was lost on reboot), as well as the saved RD Works file. I have all three files saved in my Google Drive should anyone want them (or better yet, I can upload them to the Area Leads folder in the shop’s Google account).

I noticed that despite the drawn tool being exactly square, the laser (or RD Works) cut the focusing tool precisely in the X direction and somewhat imprecisely in the Y direction. That’s why the drawing is on its side!

I cut half a dozen 40mm focusing tools (“FOCUS!” indicates which end is up) out of some scrap 1/4" birch plywood and dispersed them among the two lasers.


There used to be a folder on the laser desktops with the old file in it (as well as a couple other shop-related files like the tool status tag holder). Has that been erased?


I don’t know. I just couldn’t find them (not knowing where to look or what they’re called). I poked around unsuccessfully, then decided just to make new ones.