Second attempt at building a CNC axis prototype

Added on April 20, 2012

Earlier this week I took another stab at fabricating another prototype CNC axis, just like the one I made a couple weeks ago. This time I was hoping to improve on some of the areas that I felt I didn’t get quite right the first time.

Fabricating the axis

Just like the first attempt, I started out by cutting down 1/2″ MDF stock into two end pieces and a flat bed. Next, I took a piece of smooth steel rod and cut it in half, being more careful to go too far with deburring the resulting pieces.

On the first attempt, I felt that I didn’t have good enough control over the alignment of the mating holes to connect the end pieces to the flat bed piece, so I tried to create a simple jig to help me out. I clamped a spare piece of wood to the bed of the drill press about 1/4″ away from the center of the drill bit center, then moved my pieces along it as a guide to create a straight line of holes.

Finally, just like on the first attempt, I added two bronze sintered bushings onto each rod and epoxied them to a bed. This time, however, I checked each bearing on each rod before installing them, to make sure that they all slide well on their own. Below the photos is a video of me testing them out. By doing this, I now know that the source of the binding is not due to the mating of the bearings to the rods.

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Believe it or not, this axis turned out to be almost exactly as sticky as the first one. At first, this was disheartening, but I soon realized that this is actually a GOOD thing! Because the main difference between this attempt and the previous one was in the quality of the shop work, the fact that the axes are almost identical tells me that something else is causing the problem. Perhaps the quality of the rods wasn’t good enough, or the bearings are too picky for this kind of project.

Thoughts on the source of the jamming and possible solutions

If the quality of the shop work isn’t the problem, then what is? In general, jamming occurs when the two smooth rods are not perfectly parallel, and by “perfectly parallel” I mean to within 5 thousandths (.005) of an inch. At least, that is with the bearings I am currently using. The bearings I am using are basically small rigid tubes of bronze, which are only about 1 thousandths of an inch larger than the diameter of the smooth rods. This means that they do not move in any direction except axially around and along the rods (not left/right or up/down). When the bed is epoxied to both sets of bearings on each rod, it is now rigidly constrained to these rods and cannot move any more than the bearings can. If I had the ability to fabricate everything in this axis to within .001″, this may not be a problem, but instead, it seems that some amount of misalignment is unavoidable for me using shop tools.

Now it seems to me that there are too many rigid components and connections that I am hoping to be precisely aligned. Therefore, I think now is a good time to start introducing some self-aligning elements and implement some cheap tricks that I’ve been picking up from the Makerbot machines and the RepRap community. I will be trying out each idea in order of cheapest to most expensive, and stop when the axis works the way I want it to.

First, I will try to replace the bronze sintered bushings with self-aligning bearings. Specifically, the exact same self-aligning bearings used by the Makerbot 3D printers. These bearings are essentially the same sintered bushings I am using now, held captive inside a slightly loose plastic housing such that the bushing can move a small amount relative to the housing. The spec sheet for these parts states that they can handle up to 5 degrees of mis-alignment. So, I ordered 8 of them last night!

In addition to buying these bushings, I will also be trying out a trick I learned from the Makerbot design files. Rather than creating holes for the precision rods to press-fit into, I will make these holes slightly larger so that the rods can move a little bit around. To prevent them from falling out, the Makerbots actually have small pieces of wood at each end of the rods to act as a sort of cap. Since this doesn’t require any extra money, and allows the rods to ‘settle’ into alignment, I think I’ll give it a go.

Overall, I’m very optimistic and feel like I’m making good progress, I just wish I was able to do it faster!