Step 3 - CNC Z-Axis Part 1
It's time to start buying our major structural materials for the CNC machine. The base for this machine will be the size of a standard 24" x 48" x 3/4" sheet of MDF. In all, you will most likely use a total of three sheets for this build (I would buy one for now). Your local hardware store should have this in stock. While you're there, you might as well have someone at the hardware store cut it too. Hopefully there will be an assistant that can cut the sheets pretty accurately.
Illustration shown is the cut measurements for the first board.
The cuts will be used for various parts of the machine including the z-axis, y-axis and a part of the x-axis. The cuts that measure 2' 3" in length will be used later on for a part of the y-axis and gantry supports. Save them for now.
I have decided to go with aluminum angles instead of round bars to serve as rails. The beginning of the video shows an example of what I'm talking about. These angles will be able to secure itself around the edge of an MDF board allowing no play or bending. It also brings the cost way down. If you are a little skeptical of the aluminum angles, you can still go with the round bars, or maybe steel angles, however, these aluminum angles I feel will be superior to the round bar approach. In addition, the measurement for the back of the z-axis (supporting the two linear bearing supports) is not the correct length at 5.5". The fourth video explains the correct method to get the length in a more logical way.
What is a Z-axis you ask? Do you remember 4th grade geometry? I only remember using the x and the y axis in that class, but it's the one in the 3rd dimension. Still confused? Ok... for the sake of simplicity, I will use the CNC machine as an example. Pretend you are looking at a router... Got the image in your head? Ok, imaging that the router is moving up and down... up towards the sky and down towards the ground... all with the bit pointing downwards of course. And if you didn't know, the bit is the thing in the router that spins and cuts stuff. Well, the up and down motion is motion on the z-axis. The x axis moved forward and backward, and the y axis moves left and right. I hope that helped... back to the build.
In the video, I will only use one of the 4" x 2' cuts from the hardware store for this part of the CNC z-axis assembly. In this part (Part 1), we will build (not sure what to call it) the part of the z-axis that hugs the rail. It consist of three sides (for now, the fourth side is the holder for the router). Two of the sides will hold one slide bearing each. The other piece will serve as the back to this assembly. It is critical that the back is measured correctly. In fact, if you are using round bars instead, the measurement of this piece will vary. If you are using round bars, wait for the z-axis part 2 to see how the rails are constructed. This way, you can clamp the two sides against the rail and measure the length that the back will need to be. Really, this can be done with any rail configuration. It's up to you.
The z-axis slide bearings on each side will fit into grooves. This is to insure that they are perfectly perpendicular to the side pieces so that the two slide bearings are parallel and there will not be any rocking. It's also possible to make the holes for all of the connections a little bit larger than 1/4" so there is some flexibility in the machine. This way all of the alignments can be tweaked. Definitely use lock washers. These are things that look like donuts, some are a little spirally, and some are spiky. I'm not sure which ones are better, but I use the spirally ones and I haven't been let down yet.
The overall idea is that this assembly of the z-axis will use tension and compression to stay tight to the rail for smooth flawless operation. To elaborate, the two slide rails will be under pressure from the back of this assembly. The back will be under tension, where the rails and the rail support will be under compression. This will keep the rail nice, aligned and tight. My first machine uses the same concept, but I used round bars instead.