I have a OMtech 100 Watt CO2 Laser. According to original specs it has a lens with a 2" focal length. Since new I always used the tool that came with it to set the focus. Which measured 20mm. And everything was fine. The other day I tried cutting some unknown material looked like plexiglass. It just melted and smoke went everywhere. Since it did not cut well after that I looked and found a black spot on the lens. The lens is 2 years old so I ordered one from Cloudray through Amazon. I checked alignment and cleaned all mirrors and installed the new lens. Now According to the ramp test the focus is best at about 7mm. I did not have time to do much testing but it does not seem to have as much power as before.
I installed it the new lens the same way the other one was. Is it possibly the wrong lens or something I did.
So now after some more research It looks like my laser actually had a 1.5" focal length lens not a 2"
Do I need to get a different tube to be able to use the 1.5" Lens
You don’t have much control over your focal lengths 
A 2" has 50.8mm for its focus and a 1.5" has 38.1mm for its focus.
The distance is from the underside of the lens to the material.
However, once you have done a ramp test you should be in focus, no matter which lens you use. Remember, focal point and focal length are different things.
If your 1.5" lens fits in the tube and it reaches with its focus to the material, it should be ok. (but if it doesn’t reach out of the nozzle tip, you have a problem)
Probably polycarbonate.
IME, very thin sheets cut poorly, thicker sheets cut terribly, and the surface engraves weirdly:
Always know what you’re burning!
…it doesn’t sound good to my ears, I just ordered 3mm clear and opal PC… never worked with it, is it really that bad to work with???
I tried cutting up scrap CDs and DVDs, with results that convinced me to never do that again:
The edge discoloration may buff out, but it’s not going to be any fun at all.
Turn on plenty of ventilation to handle The Big Stink™.
This is more of an FYI…
When you go to a shorter beam has to fit through the nozzle… It will probably be ok, since you’re probably using a similar nozzle for a shorter lens… A longer nozzle works well for cut operations.
Just be aware that a change in lens length (focal) and what you are doing will effect what nozzle you may want to use.
This is for my tube type, but it’s the same idea. You 2" will focus further from the lens.
Make sense?
This is a pdf (remove the .txt) on how my lens/tube combinations work…
Lens Tubes, Lenses and Nozzles Chart.pdf.txt (30.0 KB)
Too bad I didn’t investigate before, now I have material that I won’t use, it costs me a reasonable amount of money.
But thanks for the info
(Tim, Sorry for going off topic)
Very thin…