On Tue, 7 Oct 2003, Dave Mucha wrote:
> lubricating such a shaft will create the problem of the lubricating
> oils picking up dust. and the dust will become a problem.
>
> a soft metal like brass will wear out from use.
>
> Stainless is preferred because it eleminates many of these problems.
>
> the note about not using a stainless leadscrew on a stainless nut is
> correct. I preferr a nylon or some plastic type nut. I find they are
> much better for a number of reasons. and making your own is much
> easier.
There are two classes of machines in use in industry that can potentially
answer this question: One is a surface grinder, which combines very
precise ways and leadscrews with extremely abrasive dust (aluminum oxide
or silicon carbide). The other is (you guessed it) commercial PCB
drilling and milling machines.
I don't own either one of these, but one word of advice I've heard about
surface grinders is that you don't oil or grease the ways or leadscrew.
The guy I was talking to uses a molybdenum disulfide grease (aka
anti-seize compound, available in the local hardware store). He rubs the
thing down, runs the axis back and forth a couple of times, then wipes it
clean. The moly gets into the ways, screw, and nut, and will act as a
lubricant for about a year of intermittent to medium use. After a year,
thoroughly clean the machine and repeat the treatment.
I don't think this will stop rust, though. I live in a humid environment,
so my machines need oil or a dry lube like Boeshield T7. Nothing else
stops the rust for me.
For leadscrew nut material, take a look at any of several self-lubricating
plastics. Brass on steel does work well, but it does wear. You'll need
to adjust the nut several times over its lifetime. (For what it's worth,
I've got a brass nut on all my leadscrews, and after almost four years of
daily use I've still got 0.0025" backlash on each axis, same as when it
first went into service.) Other options include molybdenum filled nylon
rod (Small Parts, Inc. has a 6" length of 1" diameter bar for $6.90), or
something like Rulon J (same source, same size is $85.05, and yes it's
overkill). Teflon is not a great choice because it has a tendency to
creep under load. The nuts will wear out very very quickly.
A thought on all this: If you're building a drilling-only machine as your
first machine, and plan to build another machine to replace it, don't go
too nuts on making sure parts can survive for 20+ years of uninterrupted
service.
Tom