Sunday, 21 February 2010

Shafting the Slides

On Saturday, after much deliberation - meaning, staring at things for hours while assembling in my head - and doing my eBay Pooja, I decided to get shafts and linear bearings for the z-axis to see if this would be a better design than bearing slides. --- Is that a question? This is more of an experiment, as my exposure to bearings and shafts if effectively zero.

CNC and Cupcake World on eBay had offers for rails and bearings that I could resist less than afford. It turned out they are driving distance from my place, plus, they gave really friendly and supportive service. I ended up buying 2 x 400mm x 12mm hardened shafts and 3 matching linear bearings (SC12UU) for around $46, just to play and see.

The next morning back home in the garage I put together a quick setup to check on my new friends and it didn't take much convincing, but shafts and bearings are the way to go. It's simply a much cleaner, more aesthetic, and a more precise solution. In short, it's what you would expect your neighbour to see when he - enviously and with sawdust in his hair - peeps at your workmanship through your chamfer routed fence hole.


Let's do this nice and smooth, baby!

A number of sub-plots worth mentioning:

I got 400mm bearings and I'm planning to space the bearings 150mm outside-to-outside, giving me a run-length of 250mm. I want this much z because I think in 3D, not just cutting flat sheets of wood. The 12mm rail does show some bending when I press it down with my hand, but for one, the z-axis won't have too much side-pressure, and secondly, I visit a gym, so it's not fair. 16mm or 20mm might have been better, but it'll do.

I started with 3 bearings in the belief that 3 contact points should be enough. It does work, but somehow it lacks natural beauty and I ended up buying a 4th bearing just to make it look right. I'd be curious to get the maths on that theory.

Open question: How much more stable are 4 points vs 3 ?

Those bearings work real nicely even when a fair bit of weight is on them, though when you twist them out of axis and then push them along, they tend to grind and lock up quickly. Hence, you will want to end up with at least 2 bearings on each rail. Also, each individual bearing has a tiny tiny bit of play. You can't see it, but you can feel it. So again, 2 bearings take care of that.

CNC Cupcake also had "long" bearings on offer, but I went with 2 individual bearings per rail so I could place them further apart, just in case.

As the astute reader would have concluded by now, the convincing result for the z-axis got me into a spin and after some more "thinking" (translate to agonizing) about the actual total size of the desired work area, rail sizes, basic design framework, etc, I took the plunge and my wallet, returned to the store, and bought rails and bearings for my x and y axis for around $180.


20mm, 12mm, 20mm slotted.
We're all oiled and ready for you.

One great relief with all this is that since I now bought the stuff, I am locked in to the basic design and in particular the total size, which helps getting on with it. So, in summary:
  • x-axis: 2 shafts at 1000mm x 20mm; 4 slotted bearings (to be supported by aluminium angles).
  • y-axis: 2 shafts at 600mm x 20mm; 4 linear bearings.
  • z-axis: 2 shafts at 400mm x 12mm; 4 linear bearings.
Isaac Hayes knew it all along:
SHAFT!
Can you dig it?
...
SHAFT!
Right On!

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