Bearings!

In the previous post I showed bearings mounted to the stages, but I didn't go into particular detail on how I did it. This is rather important because the bearings have a big hole in the middle (approx 8mm ID), much wider than any fastener I have to go through it. A 3/4" washer on each side is perfectly sized to squeeze the inside ring, but if the screw gets even slightly loose then the assembly fails.

Therefore I needed to pack the centre of the bearing with something, and I immediately reached for my stash of polycaprolactone. For those not familiar with capa, it's a low temperature thermoplastic variously known as Polymorph, Shapelock and Friendly Plastic. It's getting easier to obtain here in the antipodes, but a couple of years ago I only managed to get some from www.thethreadstudio.com, which I have subsequently hoarded like it was gold and I was some sort of troll or leprechaun. (although my Irish Foreign Birth Registration hasn't come through yet, grr)

At normal temperatures capa is like nylon - hard, smooth and slippery. But stick it in a pot of hot water on the stove and you get this:


Yes, it's goth chewing gum (what flavour would that be? Answers on a postcard and sent to someone else; I think I'm better off not knowing). I wouldn't suggest actually putting it in your mouth, but gum is exactly the consistency it obtains. It's pretty sticky in this state; it especially likes certain plastics and stainless steel, but of course it's not too hard to peel off once it's cool.

First I filled the centre of a bearing with a small amount of capa and let it solidify. This very quickly showed itself to be a bad idea; not only is capa touchy about being drilled (drill bit gets hot, capa melts, things jam up) but this particular bit of capa was in the middle of a bearing! When I tried to drill it the bearing span freely and I got nowhere. Oops!

Good thing about capa is that it's almost infinitely reusable. I popped the slug out of the bearing and put it back into my stash for attempt two.

Attempt two involved putting a screw into the bearing and packing capa around it. Or more accurately, putting capa around the screw thread and sliding the bearing over that while it was still soft. Then the capa could be squeezed into the gaps and the screw centred as close as possible with fingers. Once the capa went hard, a hobby knife/scalpel was used to cut the excess capa away. It's much fiddlier than it sounds; the capa had to be squeezed into a thin tube to wrap around the screw, which made it cool down much faster - if I took more than 5-10 seconds then the surface of the capa would stop sticking to itself and I'd have to start over. And I never did get a screw perfectly centred - but at least that turned out not to matter.

The previous post showed two of the three bearings mounted to each stage; here is what I'm using for the third:


It starts out the same as Tom's design here; I cut a couple of strips of aluminium from some spare U channel (which is then turned into L channel - good for when I come to mount the stepper motors later on). The bearing and the screw were self-tapped into 3mm holes at the end of the strip; here's the underside:


I've since cut off the end of the bearing screw with the dremel and ground it close to flush.

These bearings go on the other side of the stage's primary rail and hold it tight. Well, that's the theory, but as things stood even a small lateral push would twist the bearing loose. But I already had just the thing to fix it from my previous trip to Reverse Garbage; a spring! It's a tightly wound tension spring, about 0.9" in diameter and 15cm long, and exceedingly stiff.

I originally was going to use the spring on the Z axis like in Tom's design, to hold the dremel down and provide some protection to the dremel if it's jammed down into the stage by accident. I've since gotten an incredible deal on a clone flex shaft off Ebay ($20 including shipping!) which I will use on the Z stage instead and won't need a spring for.

The dremel made short work of cutting two 5cm springs, but bending a couple of turns of spring perpendicular at each end was a nightmare. However once that was done, and a small tab dremelled out of each aluminium strip above, I was in business:



Nearly all my screws were much too large to use for anchoring the spring; in the end I'm using a couple of very long screws that came from a surplus stepper motor. It's far too long though; it sits about 1cm proud of the surface, so I'll have to find a suitable replacement. I am a little worried about the spring pulling the screw out of the plastic, so it's been angled away from the spring line to try and secure the spring more firmly.

It seems to work splendidly - the stages simply don't budge when I try to twist them manually. I have to be careful over-extending the stage positions though - once a bearing gets to the end of a rail the spring flips the stage around pretty violently :) I'll be adding limit switches at the end of the rails later to prevent that from happening.

Next time: coupling nuts. This post just got a *bit* too long to include it here...