hi all, newish user here just stepping into CO2 lasers from diode type. I have a question for the more experienced people / users on here regarding best value for money lens for cutting wood and acrylics.
Which type / brand would you guys recomend? My machine currently has, what looks to be a standard, 20mm dia, Fl 38.1 which looks like it has scorch marks on it and I am looking to replace. My problem is I don’t know what type I should go for with regards cutting wood and acrylic and maybe getting into engraving later. I have heard a longer focal length would be better for the thicker materials but what is the general thoughts in here??
I have been looking at Cloudray but the variety is almost staggering! Does the thickness of the lens 2mm to 3mm make any difference? I have seen also that they do two different types one being USA CVD ZnSe Meniscus and a more expensive on called Cloudray II-VI ZnSe USA CVD. Does anyone know the difference in these??
Thanks for the assist with these questions, reds Kev
For thick materials, what you need is a long beam “waist”. That is the area where the beam is in best focus. Otherwise you can punt by repeating passes and changing the Z downward by the appropriate amount for your lens. Hopefully the lens documentation shows the waist in some useful manner.
The ideal would be a perfectly collimated beam as narrow as possible which would effectively have infinite waist, but that would be an expensive lens.
I have and thoroughly love the American Photonics 3 piece set 1.5, 2.5 & 4" lenses. Their focusing system is unique in that they all use the same focus distance form the workpiece, a AAA battery in it’s side. The lenses are glued into the tube so there is no way to install upside down or to have them fall out of the holder while handling.
I do a trick with the 4" lens when engraving tumblers. I made a custom focus gauge and use the 4" lens with the cone for the 2.5". That gives me plenty of clearance so I don’t have to worry about hitting the handle tabs.
Not affiliated at all, just a satisfied customer.
thanks Tim, I will look into those👍
Thanks David but I was hoping to get users opinions on what is available as much as the theory behind lens choice👍
I understand, but this is the parameter you’re looking to find.
Pick your lenses on how much resolution you expect for engraving or how thick of material you need to cut. Shorter lenses have smaller spots and shorter depth of focus (dof). Longer lenses larger spot and deeper dof.
You can compute various options via on-line calculators such as this or this. See which one works for you best.
Here is the 2nd calculator where I used a co2 frequency and a 2" and 4" lenses.
I cut 3mm acrylic and basswood with my 50.8mm (2") lens. The spot size is 160 microns (0.160mm) You can see it only has a dof <3mm, so you could see a slight taper in the closet <0.15mm of the edges.
If I move to the 4" (101.6mm) lens, I have a spot size of 320 microns, but a depth of field of over 10mm. I use the 4" for cutting thicker material.
The best resolution from the 2" would be it’s spot size or and interval of 0.160 for a possible dpi of 158.75. – 25.4/0.160 = 158.75 A 4" lens would be 25.4/.320 = 79.375dpi.
The most common lens I use is the 50.8mm (2") Seems to work best for general use.
I have four lenses also. I have a tube for each lens, so I marked the tube (using my fiber) so I know pertinent information. This is the 4" (101.6mm).
Make sense?
Thanks Jack 
Agree completely, love their products..
They even made me a 3" lens with no problems
great folks to deal with..