Quote Originally Posted by Jonathan View Post
I think the spring method will be better for longer as wear with the double nut will lead to more backlash, whereas wear with the sprung nuts will not have much effect. It will reduce the preload a bit, but you can adjust it so it guarantees that backlash is eliminated. Which is pretty pointless on a router anyway...but I know which method I'll be using when I convert my lathe/mill.
Either way for hobby machines they are used so little (relative to in industry) than wear on the ballscrews/nuts is negligible. If you've used it for long enough to wear out the ballscrews then you've probably made enough money to replace them.
Ok jonathan you seem to know something all the top Ballscrew manufacturers in the world don't so get on with it and good luck.!!

James first I would double check the square is accurate.? Those type of wood working squares are not the best for accurecy.! Ideally you want a good engineers square, longer the better.

The gantry is not neccesorally out of square, check the angle between the cross brace and gantry sides.! . . . Are they 90deg.?
Like as been suggested the cross brace could be short or the rebates in the gantry sides could have been cut fractionly deeper than intended creating the same affect.
If the difference is only small then you could easily place a shim/s in the rebate effectively widening the gantry back to original width and/or use them to bring back into square if rebate is uneven depth or wedged shape.?

Because your using round rail you can accomodate this slight angle difference were the bearing plate meets the gantry side. (Easily fixed with with triangle braces like Jonathan suggests)
Thou it's actually not a big problem if the gantry sides are not exactly 90deg to the cross brace because your using round rail. (If was using profiled rails then yes it would be a big deal.)
Yes ideally you would like absolute square and true but So long as the error is not so great that it affects the screws then you'll be ok

Far far more important is the gantry runs square to the X axis.

I say get your self a good accurate square and some shim steel, It's not un-common to have to use shims to bring things into line or back to square so dont consider it a bodge.!
Far better to hold something square with shims than force it square.!!!