A CNC router machine is probably the most useful tool a hobbyist can own, but the price for a CNC machine on the market is way more than the average hobbyist is willing to spend. You can build your own CNC with very basic tools, little knowledge of machinery, mechanics, or electronics, but be warned, these machines are inherently dangerous, so wear the proper protection and use common sense. At the very least, read the instructions and precautions on every tool you use.
Now, a CNC stands for Computer Numerical Control, but that is gibberish. It does hint you to the fact that something is controlled, and probably by a computer. It's really a mechanism that interfaces to a computer to control its movements, similar to a robot, but this machine has a specific function. It cuts or shapes things with this control. In essence, a computer precisely controls a cutting tool, like a router for instance, to cut materials such as wood, plastic or metals into shapes only bounded by your imagination.
This video tutorial will show you how to build the y-axis gantry rail support for a CNC router. The y-axis rail support will provide two functions. It's primary function is to serve as the rail for the z-axis assembly. The secondary, but really important function is to help support the gantry tighten around the cutting surface x-axis rail support.
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.