Tanat Valley 3 by Peter Drost
Drawings to any scale by James Harwood
From the drawing, which may be a GA or a detail drawing if you are lucky enough to have one (there are plenty of these becoming available now if you happen to model the Midland Railway, through Midland Record or its supplements), choose a simple dimension. By this I mean X feet, or at worst X feet 6 inches. The wheelbase is a good place to start—tenders were often 6ft x 6ft 6in. Choose the simplest dimension and transfer it to the edge of a piece of plain paper. Divide it into equal parts, ie 6 parts for 6ft, and then divide the left hand one into 12 parts. You should have something that looks like this:
You can divide the inches into halves or quarters if you please. I usually divide into quarters. Add at least another six feet to the six you already have, and you are now ready to wander all over your drawing taking measurements in feet, inches and fractions thereof.
We now need to convert these to the dimension of the model to be built. These will obviously depend on the scale the model is being built to. If we are working to 7mm:1ft, 1in of prototype equals 7/12ths of a millimetre.
From now on most of the work is done on a simple calculator, and I mean simple—plus, minus, divide, multiply, equals and a memory function is all that is necessary. Divide 7 by 12 and put the result in the memory. This will be 0.5833333, and is our model’s size in millimetres for 1in of prototype. Now measure from the drawing in feet and inches, key into the calculator in inches and parts thereof in decimal, multiply by the memory and the display will show the model's size in millimetres.
1ft 10-1/2in = 22-1/2in =
22.5 x memory = 13.125mm
8-7/8in = 8.875 x memory =
5.177083mm = 5.2 or 5.18mm
This works well for any scale:
4mm/ft
4 ÷ 12, 1in = 0.3333333mm
S scale 3/16in:1ft
4.7625 ÷ 12, 1in = 0.396875mm
10mm/ft
10 ÷ 12, 1in = 0.8333333mm
3/8in:1ft
9.525 ÷ 12, 1in = 0.79375mm
The reason for converting the two imperial scales to metric is that a millimetre is a much more useful unit of measure on a model, being quite small. If you work in imperial, the amount of zeros preceding any useful numbers is, I find, a bit of a strain on the brain. It’s much easier to have some figures in front of the decimal point as well.
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