Sliders

The main consideration for the gantry and Z motion is the sliding motion. It needs to be smooth, accurate and strong. I'll save the drive mechanisms for a separate post, I want to consider the supports first.

The normal approach is linear bearings or bushings on polished round bar or tube. This is a cheap option, and good for 3d printers. Problem is as the strength is turned up, so does the price. Linear bearings in particular can get very expensive very quickly. China specials are £5 each for a 20mm tube, 30mm are £14 each, 40mm are £30 each. I'd need four minimum for each axis, that's 12 bearings. £360 just for bearings!!

The one option that did occur to me was a 'rollercoaster wheel' approach. Three nylon wheels at the top, middle and bottom of the mounting bar. It would be a larger set up than the linear bearing, and more complex to set up, but it would be very rigid. It would also give me an option of making the bars more rigid as I could reinforce them midway across.

Having said all that, apparently nylon bearings on a steel or aluminium platform does cause things to wear and lose tolerance. This is a criticism of the openbuilds cncmill, where they use nylon wheels on the v slot of the extrusion.

And then on to This Old Tony. TOT has built a fixed gantry CNC mill from scratch, and while the build is massive overkill (it is built to cut steel and the construction and cost reflects this), some of the techniques can be miniaturised. His approach is linear rails;

This is what he's used for his CNC mill. 15 mm doesn't sound very big but apparently it's enough. Unfortunately while they are Ebay specials (TRH15B for my future reference) it's £71 for two rails and four bearings. That's basically enough for a single axis, so multiply that by three and I'm back up to bearing prices. I'd like to go down this route though, if I can find a smaller version then that would work. But I need to make sure that it can handle loads in all three axes. For example, the rails above have quite a deep incut on the side which suggests they are quite strong in every direction. Whereas smaller ones seem to have less of an incut which might be an issue. Which brings me back to my rollercoaster plan as that would have good strength in every direction.

Oh, and while I remember... I was planning on having this machine get close to 400 mm of axis range, but I've just realised. I can't do a 400 mm depth, that would be useless. I reckon I only need 100 mm of travel. And even then, most cuts will be 10 mm deep at most, with metal cuts being 1 mm. But I'll keep 100 mm so it can cope with deeper materials. And it's only this shorter Z axis that needs to cope with forces in three directions, the other two axes only need to worry about two directions of force.