Pennsylvania Railroad Cast Sign Project

[marklayton]

The Pennsylvania Railroad deployed standard cast iron signs to mark stations and towers, whistle and ring indicator signs, trespassing signs, etc.  Modeling in 1:32 scale, can't buy such detail parts from the hobby shop.  So I'm starting off on a project to make some in 1:32 scale.

The plan is to print them on a 30-micron resolution printer in a hard, heat-resistant resin, make vulcanized rubber molds, inject wax to make copies, and then have them cast in brass by a shop with a good pressure-vacuum casting machine.  There's method to my madness - while these signs could be printed in multiples to avoid the casting process, I want to graduate to making freight car trucks, so by doing the easier signa, hopefully I'll master the details of shrinkage rates, surface finishing castings, etc.  Already have a vulcanizer and vacuum wax injector, so I can do the most tedious work.

Started work in making CAD models.  First step was to import a pdf of the drawing of the standard PRR alphabet into a layer in AutoCAD.  Then working in another layer, created closed polylines outlining each letter.  The alphabet drawing and the drawings for the signs are available on line at http://prr.railfan.net/standards/.

Here are some renderings of mu models.  My skills in manipulating the light sources for renderings is not as good as my rapidly improving 3D drafting!

Attached are rendered images of the front and rear of a grade crossing sign, and the front of a trespass sign for bridges.  More to follow.

Happy New Year,
Mark

[marklayton]

Here is the front and rear of another trespass sign:

[marklayton]

Front and rear of the ring sign board.  I reduced the height - the prototype extended 4 feet into the ground, which would make installation on a layout somewhat difficult.  This model was time consuming.  Lots of fillets to add to recreate a typical casting.  Of course once it's reduced to 1:32 scale, some of that labor will be hard to see!

[marklayton]

Here's the front and rear of the yard limit and post-mount whistle signs.  The post-mount whistle sign was a later incarnation (1920s) - probably cheaper to make and install.

[Hydrostat]

Mark,

which wall thickness do those pieces have in 1:32 scale? To me it looks like this may be too thin for brass casting.

Cheers,
Volker

[finescalerr]

The lettering and ridges also may be too fine to print in 1:32. If not, please post the results! -- Russ

[marklayton]

Russ, Volker -

The prototype signs are 1/2" thick, which will scale down to 0.0156 inches.  I have confidence these can be cast.  Of course this is a learning process, so I'll keep you posted as to the trials and tribulations!  I did exaggerate the lettering height by 50% as I figured they would be too difficult to paint if they hardly stood proud of the sign face.

Have some sample models printed on the B9Creator printer at its finest resolution, and there was crisp lettering I could only read under a stereo microscope.  So I believe these models will print.  As for the wax injection step, I have some molds of jewelry with sections thinner than 0.01 inches, and the wax has no problem filling the mold.  Naturally, must take care removing the wax copy from the mold, as they are quite fragile.

The signs are definitely too thin for spin casting.  I'm planning to go the jewelry casting route - attach a bunch of wax models to a tree, invest with investing plaster, burn out the wax, and cast.  Modern casting machines pull a strong vacuum around the invested flask, and push the molten metal with an inert gas under pressure.  The fine filigree that is being cast is amazing - some is lacier than these signs.  And the yield of good castings is now very high, compared to older technologies.

First step will be some test casts to learn about shrinkage.  Will make a washer shape from brass, vulcanize a mold, inject wax, and take it to the casting shop.  Shrinkage is best determined from the actual process.  The rubber mold shrinks a tad when it cools.  The injected wax, if the injection pressure is too high, can slightly expand the mold cavity by compressing the rubber.  The wax shrinks as it cools.  Investment shrinks a tiny amount when it dries.  And of course, the molten metal shrinks when it cools.  Guidelines range from 4-12% oversize models, so I'm better off measuring the model and the resulting brass part.  I know the OD and thickness will be reduced, but not really sure about the ID.  While I'm at it, will create a couple of models with comparable cross section to the signs to see how those cast.

My sanity check is a co-worker who worked a summer job at Cal-Scale way back in the early 1970s.  He was spin casting HO scale parts.  He recalls the yield was awful, but some of the parts had very fine detail, such as the cooling fins on locomotive air compressors.

Mark

[lab-dad]

I look forward to seeing more. Can't wait to see them cast.
The cad drawings look great!
Marty

[Ray Dunakin]

Very interesting project, I look forward to seeing how it turns out.