[jasonwatkinspdx]

At the ill fated Portland TechShop I took woodworking classes from a retired gentleman, who professionally was a pattern maker for molding cast metal parts. This made his approach to woodworking really interesting. He had a huge array of freestanding sander machines, including a disc sander with more than a yard diameter.

For anyone unfamiliar, pattern makers would make wooden model versions of parts that were to be cast in metal. The pattern would be used to make the mold. He could use these various sanding machines to get 1/64" precision for complex geometries. It was fascinating to watch how he approached things, especially in comparison to modern CNC.

His major project outside of teaching the classes? Making patterns for a local steam locomotive restoration project. He had all these wooden versions of various parts of a locomotive sitting around.

[dekhn]

Does 1/64" precision really mean anything in wood, where small fluctuations in air moisture can cause > 1/64" distortion? I guess it's OK if you stay within a climate controlled area.

[jasonwatkinspdx]

So he would build parts by first making an oversize rough blank of bonded layers of marine grade plywood in a big press. Then he'd rough cut it various ways on a big band saw. Then he'd work his way through using all the sanders to slowly approach the net shape. He used precision squares to measure bigger stuff and calipers for smaller stuff.

I can't tell you the exact stability of marine grade plywood, but I know it's about as good as you can get for a wooden material, and I doubt he'd go to the effort of such precise measurements if it didn't matter.

[dekhn]

Plywood is good for dimensional stability, but I'm pretty sure all this work must have been done in and around a toolroom with stable moisture content or the part was used immediately and then consumed/destroyed before it "moves" too much. However, he sounds like he's pretty knowledgeable so I'm going to guess this isn't just garage woodworking where 1/32" doesn't really matter when the wood is going to shrink/expand by 5-10% over the course of a year.

[jasonwatkinspdx]

So the patterns once done would be taken to a foundry where they'd be used to make molds. I'm not totally up to speed on that process but I know it involves surrounding the pattern with a combination of refractory sand and binder. Where it gets tricky is complex parts that have multiple cores and so on.

And yeah, this guy was retired at the time but he'd been doing it for like 50 years. I'm very sure he knew what did and did not matter.