During the enforced shutdown I've started work on a loco to shunt my 2mm Diamond Jubilee Challenge entry: a Drummond 0-6-0T "scrap" tank, so called because they were put together by Lochgorm works from leftover bits of earlier engines. I decided to "borrow" the wheels and gears that were earmarked for a Barney 0-6-0. That loco will have an etched body and chassis, for which I have nearly finished the artwork. For a change, I decided that this one would be completely scratchbuilt.

My starting point was Peter Tatlow's drawing (https://www.amazon.co.uk/History-Highland-Locomotives-Peter-Tatlow/dp/B000GTEMYU), which I scanned into AutoCAD and traced over to identify the key dimensions. The different layers in the drawing are a bit hard to see in the sketch below, but essentially they are (i) body and wheels [for guidance]; (ii) frames; (iii) chassis block [which will be milled from solid brass]; (iv) gears; (v) coupling rods; (vi) outside motion [my first attempt at this!].

To maximise the weight of the loco, I decided to mill the chassis from solid brass (6mm square section). Following a survey of the metal I had in stock, I decided to build the side frames from 0.64mm thick brass. Since this will be a standard split-frame loco, the side frames will screw into plastic "plugs" in the chassis block, and the frames will be separated from the block by thin layers of plasticard.

Some experimenting with 2mm Association gear sizes on the drawing led me to opt for the 30:1 worm and then a 14:25 reduction with the M0.3 spur gears. My logic was to use the largest possible gear on the driven axle (constrained by the wheel diameter of 10mm), then to use the largest possible worm wheel that would clear the axle muff. I also wanted to restrict to gears that I already had in my bits box! The motor will be a cheap coreless one that came from eBay. With 7mm diameter and 16.5mm length, it will go in the cab and drive the middle axle, as shown in the drawing. Ultimately I will try to cram in a DCC decoder wherever it will fit.

Another advantage of drawing out on the computer is that I can then print as many paper templates of different bits as I like, which saves a lot of measuring and marking out when it comes to cutting metal.