Scrap Tank - valve gear
Entry posted by antyeates1983
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This is the first time I've attempted outside motion, so apologies to the experts. I've concluded that it's rather small and fiddly, but no more so than other fine details.
First off, I drilled, sawed and filed up the connecting rods in the same way as the coupling rods, from two sheets of 15-thou nickel silver sweated together. Here you see them once the paper pattern has been removed. They're shape is supposed to be a simple approximation of the real thing. As you can see in this enlargement, one of the crank-pin holes is a bit bigger than necessary (I don't remember how that happened now). But hopefully it will still work ok.
Next, the slide bars were filed up from pieces of code 40 plain rail. The left-hand ends (as seen here) will be attached somehow to the cylinders. The drawing here shows how the crosshead hangs from a single slide bar on each side. I omitted the piston rod itself from this print (must have been on a different layer!).
There then followed some experimentation with how on Earth to make the crossheads. In the end I settled on a variant of a design described on Henk Oversloot's website (http://www.fs160.eu/fiNeweb/Lconstruction/walschaerts/walschaerts.php). In my case the main body of the crosshead is made from a front piece of 10 thou nickel silver, and a rear piece of thicker brass (I didn't have any thicker nickel silver). The rear piece has a channel milled into it for the slide bar, and another channel milled part of the way along for the little end of the connecting rod. The photo below shows me after milling the first two channels:
This is the resulting piece of brass (containing both crosshead backs as mirror-images of one another). The other two slots (top and bottom here) are just to give me a guide for cutting out. The holes will take the pivot pin for the connecting rod.
Here you see one of the crosshead backs cut oversize, with the slide bar and connecting rod in to give an idea. The little end of the connecting rod still needs thinning down here:
I then soldered on (temporarily) some 10 thou brass for the front layer, drilled the hole through this, and shaped both pieces. The photo below shows these and the 0.5mm diameter crank pin that forms the pivot. I had to thin down the head to make a better representation of the real thing (from photos).
Next, I drilled a hole in the side for locating the piston rod, before separating the two layers and soldering the piston rod onto the rear layer. It is just 0.5mm nickel silver rod with a collar of brass tube (again just for cosmetic detailing). In fact, I originally tried to solder on the piston rod after soldering up all of the other bits, but this was a bad idea as the connecting rod jammed up solid and I had to heat it all up and separate all the bits again. In this picture you can also see that I soldered the crankpin to the front layer. (Again, this was the result of experience showing that it was better to attach it without the con rod being in sight. No need to actually solder the pin to both front and back.)
The next photo shows my "lash up" for soldering the two layers together with the connecting rod in between (sorry about the focus). I just stuck the rear layer to my work board with double-sided tape, and held the piston rod firmly in place with masking tape. There is a layer of Rizla paper on each side of the con rod, and these are soaked in oil. As a final precaution, the con rod and the insides of the front and back layers are coated with black marker pen where they are not supposed to be soldered. Although in practice this seems to have little effect at stopping the solder. This lash up worked, however, and allowed me to solder front and back together without gumming up the rod or unsoldering the piston. Of course, working this all out took much longer for the first one than the second!
And the final result:
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