First, you will need to measure the inside dimension of the gantry sides. Bring the sides closer together to provide a tight assembly, but not too much. Try to get the gantry sides as parallel as possible, it's not all that important, but it should be somewhat tight to appropriate support for the load of the gantry. Cut the y-axis rail support to the measured length. I'm using a router to cut the board. I would rather use a power saw, but I don't have one. I bought the router because I would need it for the CNC anyway. It's an expense my wife can justify... yeah right!
