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!
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.
Allthread quite accurate enough for drilling and PCB milling. If you use two
preloaded nuts then backlash should not be a problem. For the larger sizes
of allthread you can buy extra long nuts that are used as couplers. They
seem to have less backlash then ordinary nuts.
Les
> --- In Homebrew_PCBs@yahoogroups.com, Stefan Trethan
> <stefan_trethan@g...> wrote:
> > John,
> >
> > thanks, but this is way of what i asked.
> >
> > i know there are other options, i know there is drill rod.
> >
> > but i hoped you may tell me which surface your gas pipes have.
>
>
> The problem with galvanized or cast pipes is that the surface is very
> rough. a few minutes with a file followed by some sandpaper can
> offer a smooth surface.
>
> fixed upper bearings and spring loaded lower units can allow for the
> differences in pipe diameter.
>
> My point of view is that if you can get your hands on something, do
> it. then look for better stuff as you go !
>
> Dave
>