I’ve noticed when I cut thicker pieces of material (1/4") there is a slight variance from the top to the bottom. It’s not much about 0.2 MM, I didn’t even notice it until I was cutting circles and they came out a bit oblong on the bottom. Unfortunately it means that I can’t use the glow forge for this particular job, as I was trying to create a drill guide, and that bit of difference matters.
Is there a way to calibrate the laser to it’s (more) perfectly vertical?
The laser beam is conical, not vertical like a saw blade. You can try using a focus point that is different than the surface (midpoint in material) but in general you will not be able to accomplish a totally vertical cut.
Are you referring to the shape of the circle on the surface, or the shape of the depth of the cut?
If depth, what @dklgood said, a laser beam is triangular in shape (by micrometers), but setting your focus to the center of the width of your material will lessen the expression of that.
If you’re referring to the shape of your circle on the surface, that shouldn’t be happening. You should check the tension of your belts, verify that there’s no damage to any of your wheels, and make sure there isn’t any debris in your tracks slowing things down. If you need instructions for any of that go to support.glowforge.com
I’m so sorry to hear that there is an issue when printing circles
I see that the community has provided some good advice on the next best steps. I’ll keep this thread open a little longer, and see if any of those steps helped.
If you can post some photos of the prints I would also be happy to take a look and see if I can suggest any other next steps.
Let us know how it goes!
A surface focus puts the beam waist there, and below that the beam diverges, so the surface should be very close to the requirement. 
Awesome thank you. I think this is what I was looking at. I knew that the beam was conical, I just didn’t realize it would make that much of a difference in such a short depth.
Yeah, the answers were great. I think perhaps lasercutting is not a good option for this particular project. But that’s OK I have other options. It does raise an interesting question for me though. If I set the glow Forge to three passes does the focus remain the same for all three passes or does it adjust deeper for the second and third pass?
It doesn’t adjust automatically, but you can use the manual settings to do so.
Between passes? Or do you mean doing the same cut three times and adjusting the focus in between?
You can either adjust between cuts, or copy the cut line over three times and have each one cut once with varied height between them. Whichever way works better for your brain 
Yeah I just meant there is no way to manually do it if you set the cut to 3 passes.
Correct - if you set a single operation to three passes, the machine will make each of those three with the same settings.
I’m happy to see that the community was able to help you! I’m going to close this thread - if you have more questions in the future, go ahead and post a new topic.