Following the squaring instructions I have the y carriage bolts loose and the Y rail to the front and touching the front plates. As soon as I tighten one bolt on the left side it goes out of square and there is a 1/4 inch gap on the left.
What is it that I should try to adjust for this?
This is on a XL
Probably to take out the remaining unsquareness you’ll have to do one or more of the following:
loosen all the bolts and re-tighten them gradually, working systematically around the machine and each fastening point in an X pattern as one would on a wheel hub, not allowing the machine to go out of square (if it begins to on one bolt, back off and switch to another) — if one can’t tighten all bolts, see below
clear the powder coat out of the holes
drill out / enlarge the holes to allow for adjustment of the plates
shim the ends of the extrusion
file / machine the ends of the extrusion flat
Work systematically from large-to-small — ensure that the table is level, the endplates square to each other, the Y-axis rails level and parallel to each other, the X-axis gantry square to the Y-axis and level, the Z-axis carriage square and plumb. Sometimes it’s necessary to start over from the beginning making every smaller adjustments.
That said, if you can get any unsquareness or gap to occur at the back left corner of the machine it won’t interfere with homing, and so long as:
the bed is level
the extrusions are level to the bed
the Y-axis extrusions are parallel to each other
the X-axis extrusion is square to the Y-axis
the Z-axis is plumb and square
the machine will run true to itself and cut squarely (given the proper toolpaths and feeds and speeds).
Check that the end of the extrusion is actually cut square - there are some people that have found them a little out, and it can make a big difference.