Length of the Laser Head Lens Tube

[Roger Lueck]

What, if any, are the limiting factors in regard to maximum length of a laser head lens tube?

I suspect vibration is a potential problem.

The question above is to inquire if the are any optical, mechanical or other factors that must be considered when making a long laser head lens tube?

Does anyone make extra long laser head lens tubes?

[Mike Null]

I believe you can buy lenses with a focal length of 1.5" to 4.0" for CO2 lasers. 2.0" is generally the standard and when you move to a 4.0" lens the spot size is larger and the power at the focal point is less intense. As you change the focal length of the lens the holder sizes change as well.

The way your question is posed leads me to think that you want just lengthen the holder for the lens. That would take your laser out of focus and probably render it useless.

[Kev Williams]

Funny you should ask--

I have a Triumph 1390, 6-1/2 years old. It has no moving table, aside from it's in or it's out ;) -

Because I engrave really goofy stuff at times I found out quickly I needed a whole selection of lens tubes. Triumph made me a set of 3 for $100 shipped...
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The second from the right is the original. I needed a shorty so I can engrave things taller than 1-1/4" on the table, and the 2 longer ones really help with reach and lens changes.

This is one job for the long tube-
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One of the nice things about a non-moving table, is you're able to cut the bottom out of the machine and engrave things most other machines can't... the box being engraved is 28" tall-! It's standing on two 2x8 planks sitting on the floor! With that hole and my lens tube selection I can engrave up to 19" x 12" x 35" tall stuff :)

--I've found zero issues with the longer tubes, BUT I don't use them for cutting. But I'm sure they'll work fine, the X-factor would be using a cone (I never do, cutting or not), whether the beam would hit the center or not. But that should take no more than a mirror adjustment...

And just today, I had lens problem; I need to Cermark-engrave 800 aluminum panels. My 80w Triumph has enough poop to do it, but I need two machines to get them done by the due date. My little LS100 Gravograph is running a measured 47 watts, which does do a fair job, but it's right on the edge of 'a GOOD job' on aluminum. I have a 20mm dia. 1.5" lens that I figured would supply the added beam spot power needed to get GOOD, consistent results. Problem is, the LS100 lens tube fits a 19mm diameter, plus, the way the machine and lens tube is designed, I can't use a 1.5" lens anyway, main problem is when focused 1/2" lower than normal, the auto-focus plunger will hit the workpiece! If Gravograph DOES sell a longer, compatible lens tube, I'm pretty sure it'll be a bit spendy. And I'd have to wait for it, and a new lens. Don't have that much time...

So, since I live by Goldberg/Macgyver rules, I just fixed the problem this way:
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--I took my 2 shortest Triumph tubes (it's all I had that will work) taped them together short-end-to-end with T-Rex 'Ferociously Strong' clear tape, added enough double-sided tape around the threaded end until the lens clamp would reasonably tighten it all up, put my 1.5" lens in, did a ramp test to find the focus sweet spot, and Voila! Works like a charm!
--Engraving is very crisp, very black, and it's ON there :D
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Granted- this machine is running quite slow to do Cermark on aluminum, but even as slow as it is, the space changes are done at full speed and each X direction change is pretty abrupt. The lens is only reasonably 'firm' mounted this way, I can move it fairly easily, but after about 60 panels so far today nothing seems to have changed.

The Triumph uses 2 thumbscrews to anchor the lens tubes in place, even if my 8" tube was twice as long, I don't think I'd notice any issues. It is possible a longer tube would aggravate a vibrating belt or stepper, but really, those 2 thumbscrews hold them TIGHT! :)


.

[Mike Null]

Kev

You're showing me something new (to me). Where is the lens and what focal length are you using?

[Roger Lueck]

Thanks Kev, you confirmed whar I suspected.

[Kev Williams]

Kev

You're showing me something new (to me). Where is the lens and what focal length are you using?
Hey Mike- this is the machine's 'actual' lens holder, very short, and the lens is about right in the middle.
notice the bottom of focus plunger; with the lens that far up into the barrel, when focused with
the 2" lens, the bottom of the plunger is only about 3/8" from the work...
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So if I put a 1-1/2" lens in the same barrel, the focus point will be 1/8" ABOVE the plunger!
I'd have to remove the bottom half of the plunger from the stud. Which honestly isn't a big
deal, but still ;)

Note the lens in the long tube, it's a 1-1/2" focus lens, and extra power density from the
smaller spot really helps! Focus is more critical than with a 2" but the plates are very
uniform and flat.
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--the 2 ends are the same, just wrapped enough tape on this end to fit & stick :)
These are just typical 'cone' lens tubes, only I've never used the cone...
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As noted above, I just took my 2 shortest Triumph lens tubes, and taped
them together! FWIW, this clear T-Rex tape has zero lateral(?) stretch or flex,
it's like really really thin, sticky plexiglas, I only made one wrap-around, and
it's like a solid tube...
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Whatever it takes to get the job out! :D
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[Kev Williams]

duplicate post... that was weird! http://www.engraver1.com/gifs/thinking.gif

[Mike Null]

Kev
Thank you. Never having used a fixed table I wasn't aware of those options.

[Dave Sheldrake]

On tube length, there is no real limit (other than beam divergence) BEFORE it hits the final lens (on many lasers the beam has already traveled several feet before getting to the tube, in the case of a 1600 x 1200 machine that can be up to over 9 feet before it hits the lens)

After the final lens tube or cone length is controlled by focal length

Vibration is already a problem in optical trains, it will cause all sorts of distortions on the mirrors and lens's before worrying about it's effect on the length of the tube so not too much to worry about there.

In short, there are some potential limiting factors...but probably less than you may believe :)

cheers Dave