On Mon, 6 Oct 2003, Les Newell wrote:
> I have worked on and built a few CNC machines and I would recommend
> supported rails rather than unsupported rods or pipe. You will be amazed
> by how much an unsupported rod will flex. I used to use a CNC engraver
> for routing shapes out of PCB with a 1.6mm cutter. It used 25mm
> unsupported rails and I have seen it deflect by more than 0.5mm!
I'll second this. If rod is all you can get your hands on, all's not
lost. You can cross-drill and tap drill rod, and make your own supports.
> If I was building a machine as cheap as possible I would use skateboard
> bearings running on rectangular section cold rolled mild steel bar (AKA
> bright mild steel). The bar can be bolted to your chassis (wood or
> steel) for a really rigid setup. Bright mild steel is cheap and can be
> reasonably accurate especially if you can go to your local steel
> supplier and select the straightest piece they have got. If you don't
> have a local steel supplier look for small engineering companies. They
> are often quite willing to sell you small quantities of steel.
I'd also like to second the use of skateboard bearings. In case you
haven't priced bearings recently, they're uncomfortably expensive. This
is even more true if you're looking at using an uncommon size. Skateboard
bearings (and inline skate bearings, which have the same dimensions) are
very very common, and people doing competitive skating have helped drop
the cost on the things tremendously. You can get a nice set of ABEC7
ceramic skateboard bearings for about $25US, and they're massive overkill.
A nice set of ABEC5 bearings will work quite well, and definitely won't
break the bank.
Tom