Lay the Y Axis C-Beam profiles on top of the X-Base 2040 profiles. You're about to mount the XZ Axis on the XY Carriages. Either ask for a friend's help or get a couple of boxes as temporary support for your XZ Axis.
X Base 2040 profiles (they are 5mm shorter than your X Axis)
We can't overstate how important this step is. Take your time to get it right or your machine's performance and lifespan will be affected.
Measure the diagonals of the square formed by the machine's frame. They need to be exactly the same length.
To make a precise measurement, get 2 pointy sticks. Each stick needs to have more than half of the diagonal length. Position each stick on opposite corners of the diagonal. They will meet in the middle, overlapping each other.
Mark the position where they overlap, remove the sticks from the frame, and measure their combined length.
Move the XZ Gantry back and forth across the entire range of movement, so the Y plates settle in the right positions.
Move the gantry all the way to the front and give the final tightening on the front Y plates.
Move the gantry all the way to the back and give the final tightening on the back Y plates.