When i first received my Glowforge I made some Gifts of good Measure for friends and that was about it.
More recently I have had more projects suited to the machine and am using it quite a lot. I built a box with tabbed corners for a fan I wanted to use for the exhaust, and was amazed all over again with the dimensional accuracy of the machine.
One aspect of that project was just a bit less than perfect; as I was assembling the box i noticed it would not lie flat on the table. That made me question whether it was building with a twist in it. So out to the granite surface plate in my shop and sure enough the faces of the assembled box were twisted. As a cabinet maker I knew that if dimensionally accurate panels are assembled into a twisted box - assuming the stock is flat and not warped or twisted - then the panels are almost certainly out of square ie. parallelograms. Note: My proofgrade Draftboard was dead flat - not warped or bowed
Sure enough, all the pieces were out of square. About 1mm in 200mm. Checked against a Mitutoyo machinst square.
One design choice made by the Glowforge team was to use 2 stepper motors to drive the Y axis (move the gantry). Great choice. The gantry has quite a bit of mass and could use a bit more motor to accelerate it and travel. Unfortunately the two timing belts driven by the steppers are not mechanically connected to each other. The gantry itself is somewhat self aligning on the rails and wheel it travels on. On my unit the gantry can be deflected off square about 3mm along its length without very much force. If you move the gantry by hand (power off of course) pushing near one end and not in the center it also easily goes off square the same amount.
Now as users and readers of manuals and forum posts, we have been told to gently move the gantry to the rearof the machine (power off), and square it initially by contacting the the back of the case. I have done this frequently - it makes sense. The problem is that doing this on my machine does not result in a square gantry. It’s close, but not square. Once the steppers are powered this alignment is held by the snychronised steps of the two motors. I do not believe that my gantry is wandering off this initial alignment during cutting.
If you are still reading you are probably very patient! LOL.
I have made a crude but effective test arrangement to see whether the alignment of the gantry can be maintained by the addition of some pulleys and non elastic cord. Picture attached. I have not tested this extensively yet, but early on it seems to provide a significant improvement to maintaing square. I estimate that the misalignment potential has been reduced to about 25% of what it was before. I will update when I know more.
In order to prevent readers from misunderstanding I will summarise:
The procedure to square the gantry as posted in mauals and on this forum is very sound. It certainly creates a repeatable initial alignment.
The above procedure may not be initializing the gantry perfectly square on all machines. Mine was close but not dead on.
I love my Glowforge, and do not want anyone to think it is substandard in design or build.
If you are finding that sometimes parts you cut do not fit well, check that your machine is cutting square. Perhaps adding some shims at the back of the machine could correct initial square if your unit is a bit off.
My arrangement is to satisfy the designer in me that always wants to improve things.