American Photonics(APC) does some great videos on laser optics and I found they have a reasonably priced 25mm combiner lens so I modified my K40 Combiner Mount to accommodate their combiner.
the APC combiner:
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I love the idea of a K40 with a beam combiner, so classy. 
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I’d only used an Epilog which had a combiner before I purchased the K40 and it just felt right to have the thru-the-lens red dot pointer. Back then just the combiner lens was almost $100 for CO2/IR lasers and this was a $350 laser cutter. Searching ebay I ran across some used medical equipment with IR combiners installed for $30. The first had damage to the surface of the combiner and the seller sent me a 2nd one which was better.
The combiner just needs to be at 45 deg to the laser beam so that part was pretty easily designed. Getting the part lined up with the beam was a bit more effort. So I created a target/cross-hair in Inkscape and printed on paper then taped it centered over the K40 laser exit hole and fired the laser.
@tim.peachey will appreciate this part, I parameterize everything so by measuring the offsets in X and Y from the center of the target, both the combiner mount and the thru-hole plug could be adjusted so the laser beam was perfectly centered on the Combiner lens.
Thingiverse uses OpenSCAD behind the scenes and it has a feature called Customizer which exposes certain parameters to the GUI with sliders, check boxes, drop-downs and text fields so anyone can customize the design to fit their machine, generate the appropriate 3D models and it’ll work.
Finding a vendor of a reasonably priced combiner lens means anyone can repeat this process as long as APC keeps selling that lens and it’s currently $59 so a decently priced upgrade.
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Very useful upgrade! I plan to add a laser to my shop in a few months, will be looking at which laser to get and how to get the most out of it.
If you’re open to hacking a K40 then be sure to talk with Doug. He’s San Diego’s most knowledgeable expert on this platform. You could also corner Pat who singlehandedly upgraded our Shop K40. Just meeting him there for a verbal tour would be useful.
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I’m CERTAIN that Tim is proud. 
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I’m only knowledgeable of the K40 because of all the research I did before purchasing, then implementing many upgrades others recommended and creating some of my own based on observation and long discussions. The K40 is a great laser for the price and even if you update it to the deluxe version it’s still half the price of the next commercial machine in that size. The K40 does not hold your hand for you so if you don’t pay attention to what others have learned, you can easily over drive the laser and end the tube life in just a few months.
Most of the same principals apply to what’s known as the K50/K60 which are the larger machines like the shop has. example, without visual cooling water flow indicator you can easily burst your tube even if you have an inline flow switch installed.
Any laser you get will need some form of water circulation and possibly cooling depending on usage pattern, will need smoke extraction system and good air assist. Did you know you can nearly double your cutting capabilities just by directing a small high velocity air stream(tube type) at the burn as opposed to a lower high volume air flow(nozzle type)?
Easily 3 beers worth of discussion on the subject of laser cutter needs, choices, sizes, setup. Available after 2:30 most days and evenings.
Could you put more words (or a drawing) around this?
I can’t quite visualize the different air flows.
Have a look at this combination lens cone and air assist tube. A lens cone with a 3 mm diameter hole in it and over 30mm from the cut takes lots of air pressure and air flow to do much and most of it is blowing all over the board and only a little air is affecting the cut itself.
I’ve only seen one use of a lens cone which was effective and it was/is in a video by American Photonics who are geeks when it comes to laser products. Many just use the cone nozzle because it’s easy but the better solution is a small tube, with a small hole, blowing a sharp stream directly at the cut.
Here is the APC video which showed a lens cone used for optimal air assist. I would not be surprised if they answered any questions you might have on why they have/use that extended nozzle cone.
