Re: Model brake component size comparison to prototype

Dennis Storzek

A couple of things are not being accounted for in this discussion. Yes, metal shrinks as it solidifies, as does most things, water excepted. But in the investment casting process, the molds are at somewhere around 1000 deg. F when the metal is poured, so thermal expansion has made the molds bigger.

Shrinkage does not have to be equal in all directions; if the part is constrained as it solidifies, most of the shrinkage will be in the thickness of the part. An example would be painting your house. Paint shrinks as it cures, but that doesn't mean that you house gets smaller as it dries. Since the house is sturdier than the shrinkage forces, 100% of the shrinkage is in the thickness of the paint film.

The big problem calculating size with the traditional lost wax process is accounting for the size change in all the steps; the master is used to make a rubber mold, and traditionally it wasn't RTV, but some heat cure rubber, which shrinks. Wax is poured into the mold, and it shrinks. The wax part is then used to make a plaster mold, which is then heated to burn out the wax and drive any moisture out of the plaster, so it doesn't flash to steam when the metal is poured, causing voids, or worst case, causing the mold to burst open. Finally, the metal shrinks as it cools. The shrink rate of each of these steps is only an estimation unless the job is such a long run that actual data was tracked for the purpose of adjusting the final part size. That just doesn't happen in model railroading. 

Another story, told to my by a professional modelmaker who was a friend of Bill Clouser:

Clouser modeled in 1/4" scale using the prototype track gauge, what we now call P:48. One of the nicest freightcar trucks available in those days was made by Carl Auel (sp?), but it was five foot gauge. Bill took one of the bolsters and used ti as a pattern, had enough cast in brass to provide multiple patterns to fill the typical spin casting mold, and then used that mold for waxes to have parts cast for himself and some friends who also modeled to exact scale. The resulting parts were 5% shorter than the original, which corrected the width of the truck.

Dennis Storzek

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