drmditch wrote:I was going to wait until one engine was finished before I posted this, but it’s taken quite a lot of work so far, and from (when viewed from some angles) I am quite pleased with it.
There are two J21s on my workbench at the moment. One is a new build Alexander kit, which will be built as a non-superheated, slide-valve, Joy geared version. I developed a plan to give it a representation of its internal motion, adapted the frames, and built some of the components. Then I realised that the gearbox I had planned wouldn’t fit, so while I was waiting for a new one to arrive I started to re-work a much older Nucast kit.
I had had doubts about the accuracy of this kit (at least as I had first built it), but after a putting it into a glass of paint stripper, I was pleased to find that the basic dimensions (especially boiler diameter) seem to be correct. I’m working to a Beattie drawing from RM, and a drawing reproduced from ‘The Engineer’ of 1887 reproduced in Ken Hoole’s ‘Illustrated History of NER Locomotives’. This drawing, which scales to 7.5mm to the foot, is of course of the original compound locomotive, with Joy gear, of which more later. The Nucast kit will be superheated with Stephenson gear, and having started the ‘works’ for the earlier loco, I decided to try and see if the same techniques would work with this one.
Three weeks (and a lot of fiddling later) here are the results.
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The basic idea is to make a sleeve for the centre axle (the driven one on the prototype) with two eccentrics, one on each side. These were soldered up out of brass tube I had in stock, 4mm and 7.5mm OD respectively. There are two large washer plates inside the eccentrics and in the gap between them is a hole drilled through the sleeve and through the axle. A locating pin can then be used to fix one to the other.
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Connecting rods were then made up from brass, with an effective ‘eccentric strap’ to fit onto the axle sleeve. (The lower parts in the photograph.) This means that as the axle rotates, the rods have 3.5mm of backward and forward movement, this being the difference between the centre of rotation of the axle and the eccentric. For connecting rods this is obviously cheating. The scale movement should be 8mm, and they should bear onto a crank not an eccentric. However, my micro-engineering skills are not up to making a crank axle, whereas my chosen technique only demands bench tools and a little patience!
More properly driven by same eccentrics, two other rods (correctly the Forward Eccentric Straps) were made up in the same way. These have forked ends, and a short rod representing the Lifting Links are riveted (loosely to allow movement) inside the fork.
The trickiest part to make was the Reversing Bar arrangement, which was made out of more brass tube, rod, and sheet. The frames need considerable work as well, with a new motion plate with slots supporting the end of the connecting rods, and a dummy cylinder back plate. This last is in the wrong place, but this was dictated by the rest of the kit. It does allow slide bars, and even (and these are simple to make and very tricky to fit) a representation of crossheads.
Well, you can see the result, if you can interpret my photography.
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I have left out the Backward Extension Straps, the Expansion Links, and the Piston Rods. Obviously the Forward Extension Straps and the Connecting Rods should not have the same centre of rotation on the axle.
(There is also the offset bend in one of the straps is more pronounced in the photograph than I had realised – and no I’m not going to dismantle the whole thing to straighten it. It will only be visible from directly underneath anyway!)
However the result is moving components under the boiler, although you do have to watch closely to see them. You can even see the crossheads shuttling two and fro between the front splashers.
Most of what is left to do is conventional white metal kit assembly, although I will have to make a representation of the Piston Valves between the frames at the front.
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So, when I’ve finished this loco, I can revert to the Slide Valve engine, and I think there will be more obvious movement with that one. (Except that I used the motor I’d planned for that one on the Piston Valve version – so it may have to wait until I can afford another Mashima can motor.)
Anyway, after all that I thought that other people might like to see this technique. It is adaptable for any other internal motion, and would be really good for a loco with outside cylinders, internal Stephenson valve gear, and a small enough boiler pitched high enough to give a good view. Anyone fancy re-working an O4?
(Apologies again for my very limited photography!)