The Z Axis
OK here’s what makes the construction of the machine so special. The thought of a linear motion stage or platform that will move your modeling stock accurately can cause a person to stress out over dollar signs followed by large numbers.
The good news is that in many cases is this stress is entirely unnecessary. It helps to follow a few simple principals. Use materials that are straight and flat and pay attention to where the forces will be applied.The materials that I recommend are Extruded Aluminum Channel, Thick Plexiglas, and Ball Bearings.
The Plexiglas will be cut to various rectangles. The Extruded Aluminum will be the rail way that the Plexiglas will slide on. And the ball bearings will hold the Plexiglas firm and straight to the rails as it slides. Let me take a moment to explain my choice of materials. This aluminum channel stock is something you can get at the hardware store. It is very consistent in its dimensions through out its length (i.e. it’s straight). The rail will be slightly compresses in our application so the U shape allows it to give a little. Above all it’s relatively cheap. The Plexiglas I used is about 3/8″ thick but it’s not necessary. Any material that is firm and flat and easy to work with will do. This part might be a little hard to find. If you do find it at the hardware store it may cost you an arm and a leg. Scrap pieces of solid acrylic counter top material are good, phenolic, or even aluminum or steal. I would not recommend wood, particle board, or masonite because they might tend to change shape over time especially if they encounter moisture.
So lets get with it and build something. We’ll start with the up and down movement, the Zaxis. We want to make it wide enough for the milling tool and the stepper motor, see the picture. I used the counter top material for the base and screwed two aluminum rails to it. Put a small washer under the rail at each screw when mounting. Then screw one rail to another piece of flat material and line it up with the base like so.
Now we’ll make a bearing fixture. Cut a little piece of aluminum and drill two holes into it.
Then screw a ball bearing to one side, like so. Then we want to carefully drill a hole in the movable plate so that when we screw the bearing to the plate it will tighten down against the stationary rail.
Now since this movement will be up and down we will need to add a few more of these bearing fixtures to both sides to keep the moving plate from falling off.
Next we want to mount the motor. It dosen’t mater so much how you do it just so that it is held firm and somewhere near the side where the berring is holding the rails together. Here’s another area where you can develop a lot of unnecessary stress. You might think when connecting the ¼ by 20 all thread (screw shaft) to the motor shaft it would have to be perfectly straight. And your right, it does help, but don’t worry too much about it. There’s a market for this kind of stress called shaft couplers, a very expensive solution for a simple problem. But just take a short piece of hose, rubber or plastic, actually plastic is better, and thread the drill rod into it with a little super glue. Then jam the other end of the hose on to the motor shaft with some super glue and you’re done. If the rod is not straight with the motor shaft the hose will bend enough to compensate.
Now were going to thread a nut on to the rod and the trick here is; don’t run the nut al the way up to the motor. The farther away the nut is from the motor will help our little shaft coupler (hose) do it’s compensating. The rod should be lined up more or less with the aluminum channel of our sliding Z axis. Mark the points on either side of the rod just above the nut and on either side of the nut. Then drill some small holes and press some steal pins (nails or wire) into the holes. The reason for using the pins is if you want to make some minor adjustment to the height during the milling operation you can just slide the platform up, turn the nut, and slide it back down. Also if your milling process gets into trouble you can just lift up the Z axis platform.
Next we’ll make the frame and mount the Z axis to it.