Hi Guys
My laser lens is 18mm ( Inch 0.708 ) and was wondering if there any major difference compared to the Larger lens like the 20mm & 25mm in diameter. I know the focal length makes a difference but I am talking about the diameter?
Thanks
No, diameter is just for the lens tube you have. I have both 18mm and 20mm lens’, no difference. You will see some difference if you get plano vs meniscus, but that is physics, not size.
Smaller diameter is equal to less weight gives faster movements - in theory. So why not use only 18mm if there is not an optical difference? (this is a serious question)
It’s not the lens diameter, it’s what it’s held in.
A 20mm lens is held in a lens tube and nozzle that will usually have an M22 nozzle thread, and usually a 24mm or 25mm OD lens tube. Those numbers let you buy upgrade parts just about anywhere and in any combination you want. Finding a cutting arrangement for an 18mm lens in a M20 nozzle is near impossible. With an M22 nozzle thread and a 24mm or 25mm OD lens tube, making the change is an $8 nozzle and / or a $10 lens tube. Most holders designed for a 20mm lens, will have an 18mm shelf set back 1mm from the 20mm shelf, making them usable for either.
That’s one of the HUGE lies of omission for most laser advertisements. Just about anything under 100W is configured for engraving, not cutting. That puts you at a rough 40% disadvantage in cutting before you ever open the crate. Been there, done that, burned the t-shirt…
How big is the diameter of the laser beam from e.g. a 100Watt tube, of a reasonable quality when it hits the mirror no.3? Could it be that it is too big for an 18mm lens? Some laser machines on the market have no nozzle at all but only an open lens holder device but still a large lens. That’s pretty interesting I think. I may have to revisit some of Russ’s videos 
Beam diameter at the #3 mirror is the critical more than the final lens size. You loose height, like you would width at the #1 and #2, because the mirrors are turned at a forty five to create a ninety degree bend in the beam. I want to say a 20mm mirror drops to around 12mm usable mirror when turned and the edge of the holder are accounted for.
The spec on a 100W beam is around 5mm, but that is as it leaves the tube. We all know how exacting the laser tube manufacturers are in keeping a coherent beam along the whole of the beam path… ROTF LMAO. So with expansion, that is why you see a lot of machines over 60W with 25mm mirrors and 20mm lens. I thought I saw 35mm lens advertised on Cloudray in the CO2 section, but don’t know just what they fit.
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^^^ this guy gets it.
As to lens sizes, a smaller lens gets hotter and deforms more than a larger one. It’s tiny, but measurable.
And when your gantry is at 0,0 vs max, max, that can mean a beam path of a couple of metres more on a larger machine (~1M axis), which results in more diffusion at the edges, losing some of the beam’s power due to scatter.
For that reason, all my calibration is done at maximum extents - always align at the furthest extents - it will only get better closer to the tube the gantry moves.
I’ve gotten to the point I don’t put a target on the #2 when aligning the #1 to #2, I put it on the #3 at the furthest end of travel. The extra length of the beam going all the way across the bed acts like an error multiplier. I fought tiny #1 to #2 errors until I started doing this. The only thing that slips my mind once in a while is reversing the left to right adjustments since they get reversed going through the bend at the #2. But the first time the spot goes the wrong direction you will cuss yourself and remember it for at least the next few calibrations.
Now when I build the tube on gantry machine the tube to #1 to #2 will be around 200mm, but the #2 to #3 will be around 1350mm at max travel, so that will more than make up for it. And for damn sure it will have an SPT TR series tube with the integrated red dot.
Exactly this. doing it the other way round just means twice as much work 
And for damn sure it will have an SPT TR series tube with the integrated red dot.
I’ve used them - they make it easy to align, but how often do you need to do that? Post-it-notes are my go-to
Alignment as in fitting pieces on scrap from other things. A nice red dot on the work without the need for a combiner. And the SPT ratings are after the red dot, not before.
Ruida is in the beta of a camera feature, and it is very much a beta this point. Chinese instructions I have to run through Google Translate. It works, but it is still a beta. Get that running and I won’t worry about the red dot so much. I broke my side mount red dot about a year ago, and since I had converted from the eye in the sky engraving set up to a down close cutting set up I don’t really miss it unless I’m trying to fit something on a scrap piece that doesn’t have much margin for error.
I have Russ’ MK2 lightweight head, and love it. But that also gives me the opportunity for true targets with cross hatches. I have a target that catches the screws on the #3 mirror, another that sits in the corner of the holder on top of the lens tube, and with a holder, another that catches the bottom of the lens tube. Even without moving the lens tube, I get a rough 100mm separation between the top and bottom targets. Move the lens tube and I’m near 150mm in travel all without having to move the cutting bed.
I have a diode that does that. $15.
You have an up the lens tube diode, glad it works for you. Or are you talking about a combiner so it can be used when using the laser?
It is interesting reading, thanks for sharing your experiences and opinions.
Insignificant weight, insignificant. Shouldn’t be a factor. You’re talking micrograms. It did make me think about that though.
Ok guys , so would you reckon that there is no real good reason to upgrade to 20mm ? I have 80w laser and its working fine. Thanks for the info guys and some real good info. Thanks
Upgrading to up grade, hell no. Upgrading as part of a head and or lens tube upgrade for longer focal lengths, I’d do it every time. The 20mm lens practically guarantee you the M22 nozzle thread.
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No, my diode is mounted outside the laser tube. It’s adjusted so that when the dot is central to the tube, the object is at the correct height. I don’t use a spacer to set focus, just the diode spot.
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Ah, the diode clamped on the side of the lens tube. I had one and used it pretty regularly until I switched to the cutting set up, and then broke a wire a few weeks later. It’s been zip tied back ever since. With the nozzle tip down around 7mm above the work, I can get pretty accurate in placement. A quick pulse tells me just where I am compared to where I need to be. If I ever used something like the print and cut I’d have to get it operating again, but I don’t see that happening in the near future.
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