I realized some of the parts I was making weren't square. It wasn't a big deal, these particular units can be rhombus shaped, but I did make a note that when I got a chance, I'd investigate my lack of squareness.
So last night I pulled the gantry all the way forward and noticed there was almost a 1/4" different from left to right, when it came to reaching the ends of the Y-axis rails.
I had read a comment somewhere about the extrusions not being cut square at the factory, so figured I'd shim the X-axis gantry and I got rid of the 1/4". But what bothered me was that I checked for square between the left and right gantry plates, and the extrusion, and it was now a rhombus.
So I started checking my extrusions to the front and rear frames. Very nominal out-of-square (though I did shim here and there with some receipt printer, about .002").
I then noticed that the rear of the right gantry plate was toed-in slightly towards the center, about .1-MM. Huh. Stopped at the hardware store and picked-up some additional 5-MM washers (I sorted a bunch of them in the store for thickness and got my usual rolling of the eyes by the staff). Added a washer behind each rear wheel (top and bottom) and checked the machine for square, voila.
I still have about a .05" difference when pulling the gantry all the way forward, but when I power the unit up (so the motors lock) and check for square between the X and Y-axis extrusions, I'm pretty spot-on according to both a Starrett 12" combination square and a 12" drafting triangle. I will have to cut some material to get a better idea of whether I'm close enough, but at least I'm on the right track.
That is sort of the nice thing about the Shapeoko, I can effect these sorts of adjustments easily and inexpensively. I have a benchtop K2 prototype that cannot be squared, they didn't make any allowance for squaring the machine when they designed it. Adjustments have to be performed in Mach.
Oh well, thought I'd share my squaring story.
I don't know how typical this is, I got the machine used and abused.