Quite often on a drawing of a loco, coach or van the roof radius is left off. This means that you need to find the radius, which is actually quite simple. The dimensions that you need are the width of the vehicle, and the height difference between the sides of the vehicle and the total height, that is the rise above the coach sides.
To find the radius;
a = half the width of the vehicle
b = maximum rise above the top of the coach side
r = radius of the roof
So, r = a2 + b2 (a squared plus b squared*)
2b
(*I can't find a way of showing squared properly on this laptop.)
On the build shown below the width is 17mm and the height b is 2mm. So to find a we divide the width (17mm) by two to get 8.5mm. Putting that into the formula;
r = 8.5 squared + 2 squared i.e. 76.25 = 19.06mm
4 4
So, from that the roof radius is 19.06mm. To mark out the radius I marked a line from the top of the end 19mm down, found the centre of the end, then used a pair of dividers to mark the roof profile. This worked well;
I hope the above is easy to follow, however this post is more of an aide-memoir for myself. This method was taken from Narrow Gauge Railway Modelling by D.A. Boreham, an excellent book that I would recommend if only to show how easy we have it these days!
Comments welcome.
Paul.
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