X - Y Axis
One of the biggest flaws in my opinion of the X5S are the belt pulley mounts, as the load placed on them when the belts are in tension is located in one point on each fixing.
You will Need
6 x 6mm Belt Pulleys 20 Teeth 5mm bore.
2 x 6mm Belt Pulleys smooth 5mm bore.
2mm and 4mm high 5mm inner bore 7mm outer Stainless Steel Spacers
M4 Slide in T-Nuts
You will need to file down the X - Axis 2020 by around 2mm to 3mm to fit the X to Y Gantry mounts, But I would wait till you install this part of the gantry as you only need to file enough of the 2020 to allow smooth running of the MGN-12 Blocks.
Print off the Motor mounts and the two comers, and the Y gantry.
Run an M5 Tap down the 5mm hole in the holes of the exposed ends of the 2020 (this is important for the motor mounts. you will need to add 24 slide in T-Nuts to mount the motor mounts and corners. (just make sure they do not disappear behind the feet if you have installed them by blocking the extrude.) If you intend on using the belt tensioner you will need to add 6 extra slide in T-Nuts to the front of the frame.
Before you install the corners you need to add the 20 teeth pulleys to the corners. Start to fit the M5 Bolt into one of the corners so around 3mm of the bolt is protruding into the pulley aperture, plus a 4mm spacer into the bottom recess of the pulley aperture. Now fit the spacers and pulleys in this order 2mm spacer, pulley, 2mm spacer, pulley. Then finish off with an M5 nut into the nut slot below the aperture. Install the corners to the frame.
Install the motor mounts to the frame using an M5 bolt prior to using the T-Nuts.
Once these parts are installed you can install the linear rails to the frame in the same fasion as the Z-Axis. The rail will slide just under the corner mounts and butt up against the motor mounts.