My A250 laser sometimes loses focus randomly. And when I re-calibrate it, the focus length changes, why is that? Also I noticed that sometimes swapping the heads (e.g. to a CNC, then back to laser) help bring the focus back to normal…
Any advice? Thanks!
Sounds as if something may be loose or easily moved. Try turning the silver knob of the lens housing as far as it will go to the right (twisting it inwards), then you may find the laser beam stays more consistent. I would also look at how you are screwing the module to the rail attachment. Is it tight enough. Finally look at the Z axis rail and make sure that it is securely fastened to the machine.
Thanks for the response!
I was being lazy and only using 2 screws (was swapping heads often), but it feels pretty tight. I’ll double check the module and the Z axis rail.
The stresses induced by movement in the rails is likely to be magnified by the laser beam. Try tightening the screws properly (using all four set screws) and you will probably find the issue has miraculously disappeared.
Thanks I’ll try that. I also have a question and not sure if this makes sense since I’m newbie to laser. Will the focus length change for different material (same thickness)? I was cutting some 1.5mm plywood this morning, on one piece it was able to cut through, on the other piece (from a different brand) the same setting didn’t make it through…
No. Focus is always the same - distance from lens that you get the optimal size beam/dot.
Plywood is always going to behave differently from piece to piece and especially between brands. They use scrap wood for inner layers and you never know what’s in there. Even if it’s all the same wood, they may use more knotty pieces. I think the type and amount of glue can vary.
You may have to slow down or use more passes and lessen your path depth.
Always make sure when you’re ordering materials that you have extra to run tests on.
-S
The focal length of the laser diode should not change. Turning that silver ring as far as you can to the right (just visible in my image) on the bottom of the laser module will tighten the focus available to you in the shortest possible Z axis distance. Use the calibration card that has a dot next to the arrow on it. The laser beam should be able to get as small as the dot. When the laser beam is precisely focussed and aimed at that dot, you should see the end of the laser beam darken slightly as the dot prevents a small amount of the reflected light from shining back.
Now you have a little work to do to find out what the beam focus distance actually is. You are going to have to jog the module head down on the Z axis rail until it just touches the workpiece. You can jog the laser module head in steps of 10mm, 1mm or 0.1mm. Start with the 10mm setting once you have focussed the beam as narrowly as you can. Move the head down in 10mm steps until you are just under 10mm away from the workpiece. (in other words, if you jog one more 10mm step, you will hit the workpiece with the laser module) now count those jog steps which will usually be around 3 steps = 30mm.
Do the same with the jog control until you are just under 1mm away from the workpiece and count those jog steps. (in my own particular case it was 8mm) Add the measured count of of 1mm steps to the first total count of 10mm steps.
Finally, you need to jog the laser module head down in 0.1mm steps and make sure to place a piece of paper (or the calibration card) under the lens and move the module head until it just touches the paper and makes it feel harder to pull out from under the lens.
Count those steps and add them to the total millimetre count you have already measured. In my own case this was another 0.4mm. The total number is what the focal length of the lens should be set to whenever you are using the laser module.
Regardless of the workpiece type, you place the lens just resting upon it then count the distance that you have previously measured, while raising the module on the Z axis, and the laser beam will always be in focus. My focus distance for the 1600mW laser module turned out to be 38.4mm.
The image below shows you how a well-focussed convergent laser beam should appear.
Thanks for the detailed walkthrough, really appreciate it!
Are you saying do not use the auto-calibration? I’ve always and only used that functionality. My laser focus was 20.5mm (from the auto-calibration) but a few days ago it randomly changed to 17.4mm now…
Also the laser module I have (also 1600mW) looks a little different from yours in that there’s no silver ring, but rather a black head (see below gif) - but I’m guessing it’s the same thing? And I should screw it as tight as possible?
Thanks for the explanation! I had no idea that plywoods could be so different!
I have no idea what auto calibration is… I am using SM1. If that black ring is knurled, you should be able to turn it and change the focal length of the laser diode. The ring contains a lens array so I would be surprised if it does not turn. People may sometimes choose to change the lens array for a high resolution lens set, so it ought to move and bringing the lens objective nearer to the laser beam should adjust the total focal length of the laser beam. Another reason for doing up the ring until it stops, is that the position will always be the same no matter whether the lens array has been removed or not (a cheap kind of consistent measurement if you will).
Being a bit of a Luddite, I don’t start out doing anything automatically… I prefer to use a manual method so I actually understand and know what is going on, only then am I happy to let the automatics take over. 
On SM2 it’s not a focus ring. Just a lens hood. Focus is set and you focus by adjusting distance from workpiece.
-S
I see… thanks, S, for the information. What do you have to do to change the lens array for a high resolution lens set?
Buy a glowforge? 
I don’t think you can.
-S
Wow! They are not especially cheap. 
You are correct,S.
I certainly could not afford even the basic model with them being priced at the equivalent cost of a small country’s GDP. Good to see that they are also available with free delivery. 
As for a 51kg machine weight… it would a chore to find a normal household table strong enough to stand that on.
Now if I was in the market for a cheaper than Glowforge laser printer, I might consider an Ortur…
400 x 430mm work area and cheap for 20 Watts blue diode laser at just £401 including all taxes and delivery paid. For the price of one Glowforge one could purchase 13 of the Ortur machines.
I was looking for the weight of the Ortur and came across this advertisement…
At $269.99 + $18.47 shipping which equates to $288.46 which translates to £223.25 at today’s US dollar to UK pound rate. So I was wrong, one could buy 18 Ortur machines for the price of one Glowforge. The advertising campaign looks rather Apple like and I suspect that there is far more marketing push than substance and convenience. It surely would have to be an absolutely fantastic piece of kit to justify being 18 times more expensive than what appear to be a reasonable competitor.
