Luke11CNC Build Log ,I'm so naïve

[JAZZCNC]

James if your only 1.5mm out then thats only 0.75mm across the diagonal. If the machine is not bolted securely to the floor then the frame could very easily bend or move this amount just from moving the gantry back n forth.
If you intend to move it around or it bothers you then put some diagonol adjustable tie rods braces so it can be adjusted and held square. I did this with my machine while building then re-adjusted when I eventually found the place in shop it was going to live then bolted it to the floor . Only then did I tighten and true the gantry/rails etc.

Will it affect the accurecy of cut.? . . Well Yes if your working to high precision then this amount would be considered massive but for oridinery woodworking etc then it's acceptable to some degree.?
I say some degree because there's another issue and IMO better far more important reason to find the inaccurecy and square the frame to near perfect as can be.!

Like jonathan says you can effectively cheat and square the Gantry by forcing it square using the motors.!. . . Problem with this route is that it puts components under constant stress, bearings and even the screws are fractionly twisted this over time leads to premature wear or failing. . . . Much better to find the cause NOW than replace parts or worse still be constantly chasing inaccurecys or even missed steps etc from binding as parts slowly wear away losing performance.!

Taking the time now will pay dividends later down, don't fall into the trap of rushing the machine into service only to pay a big price over the coming months.
Promise you even if it takes a few days to sort this inaccurecy now it will save you weeks of hassle and unneccesary expense months further down the road.

Pulling and holding the frame square with Diagonal tie rods (M12 Threaded rod works good) then bolting the frame down BEFORE finally trueing and bolting the Gantry and rails etc makes the process so much easier and accurate without stressing important expensive components like Ball-screws and linear bearings. This way Only frame is then under tension.!

Edit: I know the T nuts did but have any of those Jigs turned up yet.?