Hey guys i am looking at a new focausing lens for my k40 looking

Hey guys i am looking at a new focausing lens for my k40 looking at LO i see a 15 MM lens and a 18 MM is it better to go with a larger dia dose it make a difference

Which ever fits the head on your machine, so same diameter as the original.

Focal length will change the characteristics of your engraving, not lens diameter.

Gee thanks for your response but LO only offers a 15 MM and an 18 MM lens so i guess its the 15 MM lens i can make the hole any size i want as i have access to a machine shop

I would get the 18mm Laser head w/ air assisted. Ideal for K40 machine

With the High quality 18mm ZnSe Focus lens (F50.8mm)

I have this combo and it made a works of difference.

All from light object

Thanks for the info i don’t know if i like the way the air assist is set up but i may go with it anyway thanks

The bigger the lens, the bigger the “sweet spot” thru the center so that alignment in all axis will be less critical.

All else being equal, bigger is better.

I went from stock to 18mm (in several focal lengths) and the larger lens is indeed easier to set up and burns a deeper/thicker material than the smaller one.

It IS possible the factory lens was of much poorer quality(less uniform grinding, reflective coatings and beam scattering defects) than the replacements (the base line from Saite Cutter - 20 bucks), but I tend to think it’s the larger dia & thickness that’s making the biggest difference.


Thanks Scott that’s the info i was looking for 18 MM it is

Any of you guys notice the lightobject air assist head slightly scratches your lens where the holder tightens up? I have two and notice it on both. :frowning:

@I_Laser yes. But I figure its not in an active area of the beam so no big deal.

Thanks, figured the same, just having some issues with power and @Phillip_Conroy suggested it may be the focal lens. Trying to cover all bases! :slight_smile:

I noticed scraches around the mirrors where the ring screws on but it’s outside of the working area used

@Dennis_Fuente My mirror mounts have some little white plastic rings, kind of like a washer, that go between the mirror & the metal screw-ring.

@Venuto_Woodwork I agree, got the same from LOL and cutting 3mm plywood at 10mm/sec & 5ma. Very happy with that combo.

Well my machine was built the day after the chinese new year if you get what i mean

It’s standard practice in most better quality optics to use a fiber washer between any metal and glass. Once you nick a glass or crystal lens or mirror, it’s almost a guaranteed crack sooner or later.

I use a paper or cardstock washer (just cut out of index card with punches or Xacto) between lens and holder (Saite Air Assist).

The fibre washer also provides a little compressiblity which glass/crystal does not have and you get a better air seal as well.

If you do tighten the head directly on the lens, go VERY lightly.


I’ve always wanted to cut a washer with the laser, I just never remember to write the dimensions down before I reassemble the head.

@Gee_Willikers http://www.thingiverse.com/thing:1145526

The washer ID for a Saite cutter air assist head (the aluminum one with the square body and adjustable focus - 18mm lens) is 20mm.

I have Corel and Draftsight (Acad - .dwg) files I can send, but it only takes a minute to draw the 2 concentric circles. 30mm is a good OD. I made mine out of regular green felt (mostly because I had some within reach at the time) and it cuts great with 2-3ma. and 18mm/s. Make a bunch and stack them, saving the rest for next time you ‘adjust’ it.
That gives you pretty fine adjustment and a little final height adjustment using final torque.
Cork gasket material (thin), hobby foam or even Corrugated cardboard would probably all work ok too, anything somewhat compressible and laserable ought to do it.

The advantage of the compressible gasket is that you make, in effect a friction held radial adjustment.

The final radial adjustment can be made by simply turning the assembly against the friction, and you don’t have to try to tighten it down and “hit” the proper radial adjustment at the exact time it pulls to torque, a nearly impossible and frustrating endeavor…

If you are concerned about it slipping after the fact (mine has not) you could add a couple drops of CA (super glue) to the felt gasket once you’re sure it’s dead on. That will make it require replacement if you disassemble it, but you’ve got extra gaskets.