Entry posted by Ian Smith
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Work has begun in earnest on the GWR Small Metro Tank that I wanted to build, some of you may already have seen the little gearbox that I had built for the project in a reply I had posted in Missy's blog http://www.rmweb.co....-2mm-finescale/.
Well as the chassis was being built I decided that the gearbox already built was a little too large and would encroach into the cab space (the model I am attempting to make will be an open cabbed Metro Tank to suit my preferred modelling period of c1905). So a rummage in my parts box yielded an even smaller worm/gear combo (38:1 I think) than the one used previously.
The first stage was to sweat together the worm gear to a spur gear that would drive onto a further gear on the driven axle, then the sides and ends of the gearbox were "fettled" from some phosphor bronze sheet (actually the material sold by the 2mmFS Association for chassis sideframes), the shape of these parts can be seen in the photos below.
Given my inability to accurately mark out and drill the holes for the worm driveshaft and the gear layshaft acurately, I do what I always do and drill the hole for the layshaft, and file out a slot for the worm driveshaft - the driveshaft will eventually ride in small phosphor bronze bushes that are soldered in place, as this allows the driveshaft to be manipulated to the correct mesh. Note that since the gearbox is built in two halves accuracy in drilling the layshaft holes is not that important as the holes willl be aligned by the layshaft later. Each half of the gearbox is folded and the joint strengthened with solder, hopefully the photo will illustrate this far better than any words would be able to.
Assembly is simply the 2 halves of the gearbox soldered together with the gears on the layshaft sandwiched between, the worm is super-glued onto the driveshaft which then has the bushings fitted either end of the worm and that assembly too is soldered into place (I put a drop of oil on the driveshaft between the worm and the bushing just to make sure the I don't solder the whole thing solid 
).
Once the whole is complete and seems to run without binding, any excess material is carefully filed off (making sure that no damage is inflicted on the worm or gears), the whole lot is washed thoroughly to remove any flux residue, and finally I blast brake cleaner through it to flush out any remaining bits of swarf. The last operation is to flood the thing with oil, and pop the driveshaft in a mini drill to give it a good run in.
In the lower photo the Metro Tank chassis can be seen, the spur gear on the layshaft will eventually engage with the gear on the rear drive axle. The final drive will be 38:1 from the worm then 22:12 giving about 70:1 overall.
Ian
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