This week I spent some time in the Sculpture lab at my campus and attempted to fabricate a simple axis based on things I’ve learned while doing research for my own CNC machine. In particular, there is one crucial technique that may determine any future success or failure in my CNC machine project – match-drilling. To me, the logic is obvious: fasten two pieces of wood together and drill two holes through them at the same time, with one complete motion using a drill press. However, I had never used a drill press before, and was advised to try to make a couple of prototypes before diving fully into the rest of the fabrication, just to get an understanding of the machinery and gain some experience in the shop.
I started off with a big of extra 1/2″ MDF stock I had laying around (from my wall plotter project), which I cut up into pieces. Next, I went out to one of my local hardware shop and purchased a cheap stainless steel rod for about $5. I cut the rod in half and deburred the edges. Next, I match-drilled holes into the MDF using, if I remember correctly, a 3/8″ drill bit, which the rods fit into quite snugly. Finally, I ordered some bronze sleeve bearings, P/N 2868T72 from McMaster-Carr (ordered 8 for about $8 with shipping) and put them onto the rods. Finally I fastened all the pieces together with wood screws and epoxied the bearings to a piece of wood to create a moving carriage.
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As you can see in the following video, the bed does indeed move across most of the axis, but not with uniform smoothness. It gets slightly stuck in a couple of spots, and really stuck at the extreme ends. I believe this is due to two main reasons, possibly more:
- In my excitement, I removed and re-seated the rods into the holes several times (about a half dozen). The last time I did this, I noticed that the fit was a little bit more loose, and the rods were easier to rotate and push through the holes. Close inspection of the MDF shows that the holes have developed small burrs, probably due to the rods pushing small amounts of MDF upon entry.
- The holes I pre-drilled to fasten the ends to the large bed were not as precise as I’d like. Without a laser sight or jigs/clamps, locating a feature with a drill press can be a little difficult. Hopefully I will get better with this or find a way to work around it.
I plan to build another, very similar axis next week and see if I can get the carriage to move more smoothly. Otherwise, I will have to rethink my approach and maybe even spring for some self aligning bearings.