Kerf wider at the top? Surely everybody can't be wrong?

Thanks, tjones! … but all those diagrams are apparently pointing out that the kerf should be narrower at the top, if it’s shaped the same as the beam, regardless whether the focus is accurate or too high.


I bet you and I were looking for the same one! I couldn’t find it either :-/

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And again, that could be a focusing issue. If it focuses that point lower than the surface, then the triangle is upside down. A lot of people do that when they use thicker material. Set the focus halfway deep and the top and bottom have about the same kerf. I would guess that you should get the best (narrow top/wide bottom) by using Set Focus and not putting in any numbers manually. Not sure if there is a way they can check your beam “height” remotely, but if set focus doesn’t work, you might cut a Gift of Good Fortune and then contact support.

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I’ve seen that advice too — set it halfway down to get the top and bottom to have the same kerf — but that doesn’t work for me either; the kerf is always wider at the top.

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I don’t know either - it’s just not how it works, unless you’re setting the focal point off from where the actual top of your item is! Very odd. Can you cut a few lines off the edge of a piece and then take a photo off the edge? That should pretty clearly show the kerf triangle.

It occurs to me that if you’re overpowering your cuts, you could literally be burning a trench in your material - which could definitely look like a wider-at-the-top kerf…



“overpowering your cuts” … I get the same result if I use barely enough power to get all the way through to the bottom … in fact, with that minimal power, the kerf at the bottom is almost zero.

I tried taking a photo but it’s kind of hard to see it in the photo.

The escaping gases and materials end up widening the top of the channel as the laser penetrates.


Yeah, I guess that’s what’s happening… though obviously other people are getting the “right” result…

Getting to a kiss-cut takes a lot of experimentation, but I imagine if you tried you could find one that more closely matched your expectation. The PG settings are designed to succeed every time so they are most definitely overpowered - but they are dependable!

Yes, by cutting face down.

(laser theory follows)

There is no way to get a inverted kerf profile with the glowforge, focus height won’t do it, nothing will. It’s all down to the physics of the laser and the circular cross section of the light beam.

It can help to think of it like a series of “dots” of light. You can model it pretty effectively with semitransparent circles in your favorite image editor:

Imagine the laser is a circle of uniform light density (it is, mostly), you get this sort of arragement. The one circle at the top is a representative laser beam. As you run that beam left to right, the “circles” overlap and you get a natural gradient of how much light is hitting the material as the laser travels:

Zooming in it’s easier to see:

You can see that the amount of power that hits the material drops off on the edges of the laser path. The top and bottom are lighter, which is roughly our model for how much laser energy is actually being applied. Also of interest is the ends of the paths, they get significantly less energy. Sharp corners get a lot more, check it out:

That significant overlap plus the acceleration effects that make constant speed through corners impossible was a tricky thing for glowforge to improve… they worked hard on this problem to get what they call “clean corners”.

Anyway, long story short, the center of the path always gets more laser energy, and so it’ll always be “deeper”. This effect outweighs the hourglass shape of the beam, as that is very slight 1. Since you’re trying to just barely cut through, you will always notice an angled cut profile. Physics!

And we even have empirical evidence to bear it out, from @kittski:

So, case seems pretty closed, yeah?

1 We can calculate the hourglass angle. From this post, we see that the beam is about 0.05" wide without a lens. The lens height above the tray is roughly 2 inches, so that leaves us with this sort of arrangement: You can see that the rough angle of the hourglass (represented by the red triangle) is about 1.4 degrees or 0.7 degrees for each side. That’s less than the observed kerf angle, so the hourglass can’t really reverse it even in ideal conditions.

Supporting research:
Extreme width scoring


You can cut upside side and the visible separation will appear to be smaller, but you’ll have an ever looser fitting piece despite the appearance.


That’s pretty cool. Never thought about it like that really. So those exaggerated drawings showing the X shape made by the beam focus is really irrelevant? And it basically just has to do with the beam shining “on” the top surface longer than the bottom? ::mindblown::


It’s theory, since I can’t really measure it… but the theory fits the data as far as I can tell.


I think evensd2’s explanation is pretty compelling. Thanks everyone for helping out!


Yeah so something I didn’t make clear: the interface of laser and material is very complex and analog, and the dynamics of heat and material/chemical changes like combustion/ablation/sublimation all join together in a way that can’t be separated.

My explanation of the laser ignores hourglass shape and the messy interactions that @timjedwards pointed out in order to arrive at a simple “ideal best case” reason why the cut profile would be angled. The real truth of what’s happening involves a lot more, but since every other factor only increases the effect it’s a useful way for me to think of it.

Ultimately, exactly why isn’t so important: we’ve all seen it in action, whatever the reasons that cause it. For a uniform material, kerf’s always wider at the top surface.

Now…. For less uniform stuff all bets are off. See: foam core board cut profiles.


Oh and PS:

In addition to all of the combustive/chemical processes, coefficient of linear expansion (i.e. things tend to get larger as they get warmer) can also come into play, especially with acrylic. Check it out, it’s wild – it doesn’t affect kerf profile exactly, but it does show that the material+laser party is complicated:


This old discussion about acrylic kerf and thermal expansion has some applicable info in it too I think. (It’s one one of my favorite discussions on the forum becasue science is cool, even if I never got all the pieces to fit well without mechanical adjustments and/or glue. :grin:)


Oh yeah that’s good. Not sure if I ever saw that one.

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