On Mon, 6 Oct 2003, Dave Mucha wrote:
> --- In Homebrew_PCBs@yahoogroups.com, Tom Benedict <benedict@h...>
> wrote:
> > 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
>
> There are other ways to support a machine than to have ridgid rails,
> but come clever engineering is needed.
There are lots of ways. If people are looking at building a fairly robust
machine, take a look at the books by Dave Gingery (published by Lindsay
Publications <
http://www.lindsaybks.com>. It's overkill for PCB drilling,
but even if you don't build to Gingery's specs, the sections on how to
make accurate and robust machine ways (sliding parts) are invaluable.
> btw, you didn't mention that a set of ABEC5 bearing is less than
> $20.00 US and a set is 16 bearings !
DOH! Sorry, I didn't mention the number. Yeah, it winds up being about a
dollar a bearing. Compared to some of the bearings I've had to price in
the past, that's dirt dirt cheap. Some projects I've had to skip because
they required a couple hundred in bearings.
One last thing regarding what "weight" machine to build: Attack what you
desperately need. If drilling PCB holes is what you desperately need,
attack that. But also look to future needs. A beefier machine can also
do front-panel cutouts. More importantly, a beefier machine can make the
parts you'd need to make an even beefier machine. If the price was close,
I'd opt for something that would do milling as well as drilling, simply
because when that machine got to be too small or too limited, I could use
it to make the next machine.
Tom