I have my XXL all set up and am to the point of squaring. I fiddled with it for hours last night trying to get it square and had no luck. When I pull the gantry to the back of the machine on the back right corner the Y plate does not touch and there is about 1/8 inch gap then when pulling to the front the front left corner has the same gap so it’s easy to see which way it is out of square and I have seen many people talk about using shims but what sort of shims would I use? I believe the shims would need to be placed in the corners where the plates are making contact so opposite the corners with gaps?
Hi Eric,
I have a standard SO3 not XXL, but I used (kitchen) aluminum foil folder over N times as shims, and inserted them between the extrusion and the steel plates (on opposite sides indeed at each end of the extrusion), worked perfectly to square my machine.
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Great Idea! I’m assuming that you folded it so that Covered the entire area between the extrusion and the end plate where you were applying it?
i did the same with a piece of thin plastic cut from an ice cream container… worked like a charm! it was just a strip 1/2" wide x ~3" (width of the rail) and i just slid it hard up against the bolts that hold the extrusion to the end plates (obviously between the extrusion and the end plate) 
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Did you need it on 2 opposite corners or just 1?
just one corner for me. then checked that the gantry plates hit the end plates at the same time, and checked with a square to make sure the extrusions are perpendicular
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I folded it then cut it so that it is as long (as high) as the extrusion, and around 5-10mm wide, then pushed it between the extrusion and the end plate against the bolts (as Stuart mentioned too). I did both sides as initially my machine had a slightly trapezoidal shape, i.e. gap on one side when pushed to the back, and a similar gap on the other side when brought to the front.
Just be patient, because to know for sure whether you have put the right thickness of shimming, you’ll need to fasten everything tightly, then check, then most likely realize it is slight too thick / not thick enough, and start over. Took me 5 or 6 times to get it perfect 
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I have a 6"x12" sheet of .016 aluminum i think would work well.
The ends of the rails are only square within a certain tolerance. (I don’t know what the tolerance is.)
I carefully filed the ends of my rails to square them up.
As for shim material - soda pop (or beer) cans make great shim material. Brass shim stock is the standard material and is sold many places. Aluminum roof flashing also works well.
I would also point out that ensuring that your X and Y rails are at 90 degrees with a precision square is more important than whether the carriage hits the front and back of the machine square.
The rails and end plates can form a parallelogram and it will still be accurate as long as the rails are parallel and the cross rail is at 90 degrees to them.
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You make good points. I had not checked x to y. The instructilns were just pull the gantry to the rear, tighten, now to the front, tighten. Nothing about what to do if it is not square. I appreciate the feedback.
I had a similar problem. 1/8" gap on the right. I tried just about every option but a good square and a little bit of filing on the front lower left of the x axis did the trick. Machining is slightly off. Took ten minutes. I suggest all new owners check the squareness of the 3 rails before assembly. Saves disassembly later.
Great way to know your machine before using it.
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