Tim Wescott wrote:

I have seen (held in my hands, actually), metal parts that are made by
etching thin steel with acid (presumably after printing on a resist).
It's great stuff for making optical stops, encoder wheels, and other
things where the worst that the metal has to resist is a stream of
photons smacking into it.

The steel in question appears to be either stainless steel or tin plate,
dead soft, and is maybe 5 or 10 mils thick.

What's the processing called in the industry? What sort of shop should I
direct someone to look for in their yellow pages or their Thomas
Register? What sort accuracies can one expect, and setup costs, and
fabrication costs?

This stuff isn't that hard to do yourself, even, if you have a way to
make the master films. I make solder paste stencils this way. I use
..003" brass shim stock, and laminate dry film photoresist to both sides.
I make mirror-image films with a photoplotter and align them to each other.
I then slip the sensitized brass between the films and expose both sides
to UV from the filtered black light bulbs. Develop the photoresist in
sodium carbonate and then etch with ferric chloride in a double-sided
spray etcher. I've gotten down to .010" apertures or thereabouts.

positional accuracy of the apertures can be quite high, easily to
..001", maybe better. SIZE accuracy depends on how well-controlled their
process is. Mine isn't so great, I get a lot of undercutting where
the etchant gets between the resist layers and opens up the apertures,
even with short etch times. But, I guess a professional vendor of
such work can hold tighter tolerances.

There are outfits that will make "solder stencils" for $50 each, up to
maybe a foot square. If you can get them the artwork in Gerber file
format they do it pretty much by the process I described above, or
by laser-cutting the apertures. This is done for the printed circuit
assembly industry, you should get a million hits googling for
"solder stencil".

Jon