I recently changed out my spoil board and then had the machine bore some 3/8" holes for plastic
dowels so I could have alignment pegs. After I bored these holes I checked square and it was off slightly. So I measured corner to corner and it was nearly perfect. Then I looked at the X axis and it was off by about 0.05" left to right. So I got a handy dandy beer can, cut a slice off and shimmed the left rear / right front of the X rail and it brought it to 0.009" left to right.
My question is - is this close enough or should I try chasing 0? How close are your machines? This is measuring from the front cross member to the Y mounting plate.
If you keep chasing you may well find that the machine doesn’t always start up with the X rail at the same angle, despite shimming.
I checked that my X beam was square to the Y rails when the Y plates were pushed all the way back contacting the rear steel plate and now just gently push it back into that position before power up.
Also, if you don’t balance the tension in your Y belts the X rail will change angle as it moves back and forth.
My Y rails are square and parallel. But my gantry is skewed. The steel plates on my machine are warped. I generally push the gantry into the end stops and energize. I suppose this isn’t ideal for v wheels or bearings, but it works.