Kerfalicious!!
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Thank you @dan and @deirdrebeth. And @trually, I did watch the video, but it still confused me.
So now I know to hold it flat. 
And although I don’t understand how it works (like how it knows to stop at the proper kerf), I will trust it to do its thing. Yay! Thanks again @dan for such a handy tool!
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My assumption is that the slot is wider at one end and narrower at the other, so the slider gets stuck at a certain point where the slot narrows too much to allow further passage. The overall width of the slot (as well as the width of the slider itself) depends on the kerf, therefore how far it travels also depends on the kerf. With zero kerf, it wouldn’t be able to move at all. As the kerf gets bigger, there’s more of a gap between the slider and the sides of the slot. Therefore it can travel further up the slot before it hits the sides and becomes stuck.
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Tim is correct. The slider and slot are sloped - the top of the slot is .235", and the bottom is .347". So depending on the kerf, the slider will stop such that the carefully-calibrated scale indicates the actual kerf value.
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Thank you @tim1724 and @eflyguy for explaining it to me. I understand it now! Who says you can’t teach an old dog new tricks? 
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This is great! is there any chance of getting the file for a non-GF laser? We have a few different ones we use and need to check the kerf on each one.
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This is a fairly simple project, maybe it’s a good way to develop your design skills?
It just takes a little logic to figure out how to design it. We can talk about it some more if you’re not sure how to proceed.
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I could recreate it for sure. My concern is getting the scale correct on the right side. Just thought I’d save some time if possible.
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The scale is set by the angle you choose. Make a grid at 0.001” in the X direction and whatever you want your label height to be in the y direction, probably 0.15” or so.
Draw a vertical line starting on one of your grid intersections and then drag one end of it to an intersection along a “diagonal” of your grid. The line will be nearly vertical but the slope will be perfect.
Now make your labels at about 0.1” tall and snap their centers to the horizontal lines of your grid. Each horizontal grid line would represent 0.001” of horizontal distance.
Mirror it for the other side, and you’re good to go! It’s a little fiddly but it shouldn’t take too much time.
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Everything about this (the neat shape, incorporating the Glowforge logo into it, the flick of the wrist, the math behind how it works) is clever in the absolute best way.
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@dan Are there any plans to automate kerf calculations in similar ways as the Snapmarks feature (which is great)?
Would be nice to have the Glowforge use its optics to add a test as part of the job, calculate kerf, and then follow settings for each cut that could be defined as “precise inner, precise outer, default/center”. 
Thinking out loud here.
Baking it into the process and the software would be nicer instead of printing a tool for each material. It could also avoid any variances with the same material over time.
Regardless thanks to you and the team for putting this helper tool together and sharing it with us all.
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This is awesome and so much easier than using a caliper! Is there anyway to use this for non GF lasers? As much as I love my GF it makes you want to try and engraving everything. Unfortunately as a CO2 laser it can only do so much. Been playing with the diode & fiber laser at my college since they dont have a CO2 one and it’s fun to explore. Is there anyway to compare the kerf differences between the lasers? As well as cutting performance?
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I don’t see why you couldn’t use this file on any laser with any material (within reason) and get an accurate result.
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One reason is that it’s a Glowforge catalog design and you can’t download it.
As we’ve discussed before this is a very simple design to replicate. Honestly this is a great beginner project to learn the fundamentals of design for the laser.
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Evan I missed the discussion above. Could you elaborate more on how you figure out the measurements? The design is a simple one and fairly easy to create. My OCD about getting the measurement results the gauge is supposed to give you right is driving me nuts.
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Thank you so much @tim1724 for explaining how it works. I hadn’t had a chance to delve into it and was wondering also. That’s awesome!!
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Print it, using your chosen settings, then do like the video clip shows. Read off the kerf. Adjust your design as necessary.
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I got how it operates eflyguy, my question is in reference to recreating it from scratch. This will allow me to use it on a non gf laser, at my college, thats not CO2.
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In that case, just replicate the design (screenshot at high res. and trace using whatever tools your preferred app has available and you are familiar with), then print a test on your own laser and material. Compare the results from using it to the actual kerf (measured using any of the techniques that have been discussed here for years), and adjust the scale as necessary.
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Ok, so I printed it on .125 Baltic Birch Plywood, and got a reading of .004 inches on the kerfometer. I’ve run tests before, measuring a square and the opening, and have got readings with a calipers of .008 inches for kerf on the baltic birch. Is there a outer dimension size of the kerfometer, and it’s possibly not importing as the right size?
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