Cautionary Tales
Accuracy of Drawings
In an earlier post, I wrote: “I used the same method that I described in my previous post to extrude the saddle tank from a drawing – this time a pencil sketch by F.J.Roche, reproduced in the ‘Broadsheet’ article. This drawing was useful for the front elevation but I feel the drawing in Mike Sharman’s compilation by the Oakwood Press is more dependable for the side elevation.”
Some recent correspondence within the Broad Gauge Society (BGS) e-group suggested that the drawings in the Mike Sharman compilation may not in fact be that accurate, I quote: “The Sharman's book drawings were transcribed from originals published in the Loco Magazine. The one in question here* was published in 1903. The transcribers varied greatly. The originals are believed to be accurate but ... this particular transcription was one of the least accurate.”
* This quote refers to a drawing of a Dean 2-4-0 convertible of the 3501 class, not my engine, but it sowed seeds of doubt in my mind.
On looking more closely, I noticed for example, that the spokes of the bogie wheels on ‘Aurora’ were not placed accurately on the drawing, which showed up clearly when I designed my own wheel, using ‘Fusion 360’s Pattern command to produce nine equally spaced spokes.
3D-printed wheel laid over Drawing
It’s a small point but a warning not to believe the correctness of all the details on the drawing. I have now measured the wheel base and other key dimensions on my 3D-printed model and have been relieved to find that they are all accurate. I shall pay more attention to the accuracy of any drawings I use as a basis for extruding models in the future.
Front Elevation
Looking through my own small collection of drawings, I found three showing the front elevation of one of these 4-4-0ST engines. Two of these, by F.J.Roche and by Ian Beattie are said to be of ‘Lance’ (both drawings are from the BGS magazine ‘Broadsheet’ no.17), whereas the one by Alan Prior (in his book ‘19th Century Railway Drawings') is of ‘Corsair’. Putting these three alongside one another shows that there are many significant differences between them:
Front elevation drawings compared
Faced with discrepancies such as these, I turned to photographs and, in particular, the one of ‘Aurora’ that I showed in an earlier post The tank seen in this photo appears to have the more rounded profile shown in the Roche drawing above, although the sand pipes are not so arched. Of course, those early engines often showed a iot of individual variation, so the dictum to work from photographs as far as possible is very sound but can be difficult to apply, when photos were so far and few between.
Following ‘rules’ may not be best
Another interesting point appeared when I started to create the trial prints of the wheels that I designed for my model of ‘Aurora’
I made the initial prints by laying the inside of the wheel flat on my printer bed. This meant that the widest part of the wheel, with the flange, lay on the bed so that there were no overhangs as the printing progressed, which is the ‘preferred’ method.
In practice, the flanges came out thinner than expected and were damaged when I removed them from the printer. This may be a result of using a printer with an unheated bed but it was another case where disobeying the rule book yielded a better result!
My later prints were made with the outer face of the wheel on the bed, so that the flanges actually overhung the main part of the wheel, as it printed. Nevertheless, the flanges printed cleanly and, by printing in this orientation, I could include sleeves extending from the backs of the wheels, to guide the pin-point axles and ensure the correct back-to-back measurement between the wheels.
3D-printed wheel-sets with integral axle sleeves
Each sleeve contains a clearance hole for the 2mm axle, while the wheel itself is an interference fit onto the axle. I assembled each wheel-set by dropping a pin-point axle into the sleeve and then tapping it gently home into the wheel with a light hammer, as shown below. Once one wheel had been attached, I turned the part over and tapped the axle into the other wheel, until the two spacing sleeves meet at the centre line.
Assembling a wheel-set
Once I had produced a set of wheels, I could place them under my model, so that it began to look like a real engine! It has a purposeful look, well matched to its task of hauling important passenger trains over the South Devon banks. The famous Gooch singles may have stolen the limelight but it was these tank engines that maintained services across the more difficult routes of the South Western peninsular – they must be lauded for that capability
‘Aurora’ on her Wheels
A few years ago, I could not have contemplated making a model like this and I am pleased that, during its design and construction, I have gained an appreciation of the prototype’s remarkable qualities. No longer shall I call it an ‘ugly duckling’ 
As usual, there’s a lot of finishing still to be done. One day, I must get together my collection of unfinished Broad Gauge locomotives and have a session of handrail fixing,-plus all those additional fittings and polished brass-work that make make them into beautiful swans.
Mike
Edited by MikeOxon
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