Say I have a piece of 5mm plywood with a 5mm bow from one side of substrate to the opposite side. I normally set focal point 2.5mm below top surface. Top surface being left bottom corner. When laser reaches the high point of the bowed wood FL is now 7.5mm below top of substrate. Theoretically the laser should cut all the way through the plywood with the only anomaly being wider kerf at top and bottom side of plywood. Does the laser lose so much power as it passes over hump in same that it cannot cut completely?
Why not have the laser with powered “Z” have the ability to read the distance from nozzle to substrate surface ???
This setup would eliminate the problem with having hold downs of clamps or magnets.
I normally focus on the top of the material. Keep in mind that the longer the lens the less you have to worry about kerf width changing. So with longer lenses you can cut thicker stock and variation in the stock matter less. The down side is they do a poorer job of engraving fine lines.
What length lens are you using?
I am using 2.0” lens 50.8mm
I have that same lens. It’s best to cut flat materials and I hold corners down using magnets. That lens seems to cut with a taper on the side of the wood, but again I focus at the top. I have a 75mm lens that works much better.
Of course, the perfect focus is always the best, but …
If you do a ramp test and have to decide where your line is thinnest, it is often difficult to decide, especially if the angle of the ramp test is relatively flat. At the same time, the length of the very fine narrow section of the line gives an indication of how far your focus area is. With a 60Watt laser (which I also have), I do not think it is crucial whether you are plus-minus a few percent out of the focus center. By the way, I also always focus on the top of my material.
But I do not use material that I can not get relatively flat on my work bed, by relatv I mean if 4mm material curves 4mm then I do not use this piece until I have got it more flat.
A 3 "lens is fine, I just did the first tests with mine and am happy to have it lying around for very thick foam and similar materials.
PS. a life focus system exists, look at Cloudray for example, it’s just a matter of economics 
The laser does not lose power per say when the focal distance changes. Rather it is a matter of power density…stated as watts per centimeter squared. When you change the focal point or change to a different lens etc we change the size of the spot. A spot size of 1mm has a much higher power density per centimeter square than a Spot size of 1.5mm. Seems like it wouldn’t be a big difference but if you did calculate it you would see. The formula is not complicated but a bit time consuming…but this formula below is a shortcut method using the beam in mm verses the old school method where we had to convert area to cm…
irregardless this is what happens. You defocus the beam power density decreases even though actual power is constant.
Use a 3 inch focal length lens to cut. The beam “waist” is longer and the beam diameter won’t change as much with a minor change in distance.
You can focus halfway between the top and bottom of the substrate and get a good cut.
A 2 inch lens is not a real dedicated cutting lens it is rather an in between lens…meaning good for engraving and cutting as long as the material isn’t too thick. If you are cutting anything over 1/4 inch I would use a 2.5 or 3 inch lens. You can also swap to a 2.5 inch meniscus lens and have a bit better beam waist as well but they are more money.
As for auto tracking Z this does not exist for our lasers and if it did no one here would be able to afford it. It is not practical. I used to be an engineering tech on large 3000 watt lasers for the aerospace industry. These are over a million dollars each and they do not auto focus. To me autofocus on our laser is a gimmick for the untrained. We always manually focus our lasers.
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I will add on kerf. I would guess many people have an introduced kerf due to misalignment especially on thicker materials.
I have three co2 lasers and all three were not perpendicular to the work surface after fully aligned.
Simple check…remove your focusing lens. Place a piece of wood under the nozzle. Run Z up until you are about.5 to 1 inch away from the tube. Do a burn with the raw beam. Now run Z down to its lower range. WIthout moving the wood…do a burn. $100 says this second burn is off from the first one whereZ was at the top. Unless the beam is dead on both burns your beam is not perpendicular to the plane of the work surface. You will get non perpendicular edges and these edge slants will change direction relative to the cut edge as you cut around the part
I’m not saying it’s worthwhile or that it’s something for the soho market, but it’s possible. Take the price and price of a 100Watt OMT machine, then you are still nicely cheaper than a standard 100Watt from an American or German laser machine manufacturer, without any accessories. A lot has happened in the last few years in the industry.
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Hmmm…well I stand corrected…cloudray makes one…cool. There you go. Although my point was these Chinese lasers do not have this ability Via OEM
I wonder though does it fit the OM tech rail setup…most cloud ray nozzle assemblies do not without a lot of mods….doesn’t look like this is a direct fit but doesn’t mean it couldn’t be done. I would like to see them make more of their nozzle assemblies fit the omtech style rails as I like their stuff but so much looks like it needs to be modded.
Actually would be useful for 3d engraving as well. Not a bad price if you need it.
No it is not plug and play for our OMT machines (yet) but it will surprise me a lot if it does not become an option in the not so distant future. The videos I’ve seen, here it’s reasonably normal consuming machines, not some fancy big or metal cutting machines.
I seriously doubt my wife would approve. She almost went hysterical when they delivered my 60W Omtech. Lol
Thanks for all the comments. I really appreciate that
Another note:
If I am using from a pile of precut plywood I keep them stacked and use from the bottom.
I keep my stacks pressed together with clamps.
I’ve been tempted by this a few times. Also tempted to buy their metal cutting head which I think is a “kissing cousin” to this product.
I used to go through the Sears catalog as kid and dream of the cool stuff I could buy. Now I use my phone or tablet and browse Cloudray’s online store. 
What I would buy in a heart beat is something that magically detects and fixes the random issues cutting all the way through birch plywood due to glue and another anomalies.
At a very young age my older brother had an Erector set. I would go through their catalog for extra pieces and mark them. Most were only a few cents then. If I had money I didn’t know what to do with, I would have that “rig” in a heart beat. LOL
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When I was in high school I sat with a few guys and one had the last name Olsen He or they convinced me that he was connected to a catalog named Olsen electronics. So I marked up the Olsen electronics catalog with all the stuff I could get a discount on. That day turned out to be a memorable experience when they finally let me in on the joke. 
Back to the warped ply, too bad a sheet of glass (or some heavy material) lying on the ply wouldn’t work. Something that’s transparent to the beam.
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