NavList:

It's an interesting question, the best way to fabricate a slide rule in prototype quantities.  3D printing might be marginally useful for the bulk sliding parts, but it would not do all that well with engraved scales, even with high-resolution processes like multijet.

A few possibilities:

- the Pickett route.  Fabricate aluminum strips (with a CNC prototyping service), then engrave the scales (with a CNC engraving machine).  Fill with black ink (or colored ink as desired).  I think the actual Pickett slide rules were done with photolithography for the scales, though, which is better for mass production.  I have an N4-ES which is a pleasure to use.

- the PCB route.  Render your parts as small printed circuit boards (either single-sided or double-sided), then send to a PCB fabricator.  You want the ENIG finish (electroless nickel immersion gold)---it is slightly more expensive but provides a beautiful flat gold surface on the copper traces.  PCBs routinely go down to 75 micron traces these days (3 mil), so fine lines are no problem.  I'm not sure how durable the surface might be under hard use---it might be prudent to overcoat with an acrylic clear coat.  Dimensional accuracy is good to excellent, and the PCB material, a glass-fiber/epoxy composite, is very strong.  The milled edges will be quite straight.  Assemble the tongue-and-groove from multiple PCB fragments for a sliding fit, then epoxy.

PCB rulers are the fashion these days.  Infinitely customizable.  Here's one:

- the Kinko's route (or whatever they're called these days).  Find your local print shop, given them a PDF, and ask for a high-resolution film phototool at 2400 dpi.  Bond the film to a suitable substrate like aluminum or plastic, emulsion down.

- the inkjet route.  Inkjets produce good images at surprisingly good dimensional accuracy in the x direction, and the PDF can be compensated to remove any detected distortion along that axis if needed.  Print onto one of the opaque white plastic inkjet sheets (such as are used for durable labels), then affix to substrate.  Arbitrary color is a plus.

- the laser route.  Laser engraving is convenient onto a number of substrates, such as metal or ceramic, with suitable marking films or fluids (which are wiped off post-laser).  The laser's high heat fuses the marking substance to the substrate, and the laser spot size determines the resolution.  Line quality may not be quite as good as with engraving, but it might be good enough.  Also anodized aluminum can be laser marked---the laser zaps the dye, exposing the aluminum below.

Cheers,

Peter