622 on her wheels

You might expect that at this point it would be a simple matter of dropping the drivers into the chassis, screwing the engine truck on and sending the whole assembly out for a test roll. Sadly, no.

Somehow, the design for 622´s springs got changed, and a whole lot of extra metal got added. So, before I could get the engine on her drivers properly, I had to thin them down. This design flaw may turn out to be serendipitous, as the springs may be too soft; I can already see them taking the load up when I drop the boiler on, and they may bottom out once I’ve filled the firebox with lead and tungsten.

Speaking of design flaws, somehow the little pockets in the wheel covers for the top of the springs got missed on the etch. Unlike the change to the springs themselves, the flaw is not apparent in OnShape. It must have crept in when moving to InkScape. The pockets were a challenge to recreate on the assembled frame, and took a lot of rifler filing and carving before the springs would run smoothly.

A cruel closeup: There is not much clearance for the rear driver under the cab! Notice the spring, which is just barely visible under the top chord of the frame against the firebox. It looks like someone needs to clean the swarf off the wheel, and fill a bubble or two. That screw that passes through the running board is 00-80, but looks giant here!

Then there is the matter of the missing quarter millimetre. The design calls for only 1/2 mm of clearance between the driver flange and the underside of the cab (there were perhaps three inches of clearance on the prototype, but I used two of them to support the running board). However, when I sat the engine on her wheels for the first time, I found they rubbed. It appears that the running board has come out 1/4 mm lower compared to the frame, likely due to inaccuracies in folding; I don’t know where the other 1/4 mm went. It doesn’t especially matter, though, and so I didn’t spend too much time trying to find it. Out came the Dremel tool to carve a little relief.

Once all these modifications were complete, I dropped the wheels into their slots and found… the front wheel still didn’t rotate very easily.  The width of the frame was virtually right on its spec, and there’s not much to file away there, but file away I did all the same.  Under heavy magnification, I also detected a tiny fillet where the wheel meets the axle, and this proved to be the culprit. 

Once I took care of that fillet, the chassis rolled pleasingly on the test track.  It even keeps going if you give it a shove. I believe that is better than #10 ever did.

5 thoughts on “622 on her wheels

  1. Do you make your own drive wheels? If not, would you be able to recommend a manufacturer for Proto:87 drive wheels? Does one even exist?

    1. Yes, I made my own. If you click on the tag, “wheels” on this post, you can find the whole sordid development. The only easily accessible source for P87 drivers I’m aware of is Apogee Vapeur. Check the suppliers page on proto87.org; whenever I learn of product availability, I put it there. However P4 wheels are very close and a little sanding with fine emery paper makes a very passable P87 driver; that’s how I made the wheels for #10.

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