This is an interesting concept.
You’d probably have issues with ‘exact’ alignment, but you could probably get it close enough !
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Clever idea, yes I think alignment is an issue.
I eventually got rid of my dot and add a target into the source that I align with a test pulse.
Do the same on my cnc router
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what?! That is dangerous! Putting plastic and electronics right in the laser’s path. If you ever forget to move it aside, you’ll fry the plastics in seconds.
We are running a beam combiner, much much safer option and also static so only requires aligning at the install.
check out https://youtu.be/Eh1ZSW4hMSs
Well he does have a cut off so the laser can’t fire if its in position.
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As @Jammy says, it has an interlock, and he said he did it because it was cheaper than a beam combiner. 
I see @donkjr’s point about alignment; that’s also what I would have expected to be problematic. Hard to argue with results, but that’s what surprised me.
I took a beam combiner out of the BOM for my laser because folks here kindly suggested that a physical object that indexes to the laser head and provides either a pointer or reticle for aiming might be easier to position perfectly than a red dot and would be an easy 3d print. @Steven_FF have you compared a physical alignment tool to your red dot? I’m a nutcase who is building a large laser as his first laser, so I don’t have any practical experience yet, and would love to hear from folks who have tried both…
I admit, I missed the cutt off-microswitch in his movie since I scanned the video (way too) briefly - in hindsight. Mcdanlj, just to make sure I understand correctly you do mean a physical alignment tool to align your material with the laserbeam, or do you mean with alignment of the mirrors?
On our previous lasercutter my workaround was doing an engraved line (just a few percent power on a cut) of a square on a piece of MDF. That marked the larger size of my to-be-cut stock material. So for instance engrave a 420x297mm (A3 paper size) square and have a piece of plywood cut in that size with a full-print sticker on top. Alignment is easy since that square is also physical visible - centered in the square onscreen is also centered in your material in real life.
Not mirror alignment, that’s separate. Only thinking about aligning cut location to object.
What I had in mind was a piece that fits around part of the lens tube and has some sort of pointer that shows where the laser would hit; that would get the focus distance as well as location. It could have a pointer or a reticle to show the location where the laser would hit when firing, and the height would be set for the focus point. I haven’t designed it yet but ought to be able to after I create a model for the head. (I have the two mirror stages modeled so far, but not yet the head.)
But your suggestion aligns not only the point but the whole object, avoiding registration failure from skewing the object around that one point.
But your suggestion aligns not only the point but the whole object, avoiding registration failure from skewing the object around that one point.
My point exactly, only with about a tenth of the amount of words 
As height alignment we’ve used a 3D printed block in the correct thickness for years. just put that on top and raise the bed just so that the block is still able to wiggle. If you have air assist, turn it on and you will hear the pitch of the hissing air change as well.
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I don’t know why, but somehow this popped into my head recently. Some software has options to tackle this problem, like for instance in Lightburn there is a whole feature dedicated to aligning with printed media.
check out his tutorial (on facebook, but still it shows an example)
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