Crank pin proof of concept

The M0.6 screws and nuts arrived just before the Railway Modellers’ Meet, and now that most of the dust has settled, I’ve spent a part of a glorious spring morning trying out my idea for a crank pin. When the weather’s as nice as this, progress on the locomotive has to slow down. Vancouver winters are too long to miss a beautiful day like this.

The Sharman wheels for #10 had crankpin screws cast in place. That decision committed me to having nuts out in the open, and while I know loads of modellers thin down the nuts so they don’t look too conspicuous, I never had the guts to reduce the nuts to their minimum depth. The result is not as fine as I would have liked.

It makes more sense to me to use the back of the wheel to house all those non-scale threads, but then you would lose depth in the wheel, which we want so we can hold the crank pin perpendicular. So, I designed a pin that fills an oversized hole in the wheel, and the nut is somehow going to be fixed into a recess in the back of the pin. If I could have found an M0.6 tap, the design would be much simpler, but sadly none has turned up yet.

Turning the pin and the washer was a bit of a fiddle, to say the least. The 1mm end of the pin wants a mini form tool to get it crisp. Moreover, I should have known better than to try to part it with the parting tool, as it deformed as it ripped apart. It would, in fact be better to use steel for the part. Even so, once all was said and done, the parts fit together as planned. And with a little truing, the rods rotate nicely.

I should mention how glad I am that I bought the little 1mm socket that works with these screws. It felt like a bit of a luxury when I added it to my cart. However, I would still be chasing nuts that had sprung under the workbench were it not for this tool.

Also, while waiting for the screws, I had ample time to think about alternatives, and I believe I have an elegant solution to secure the rods to the crank pins. It is inspired by the prototype, and yields a smooth face on the outside of the rod, rather than a hexagon. Like Fermat, though, I’ll have to write it up later.

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