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Fire Fly Class – Part Four




Angled Spokes


I ended the previous part of this series by noting that my next subject would be wheels. This subject, once again, raised questions as to what were the real facts! An ‘official’ GWR drawing shows alternate spokes sloping in opposite directions between hub and rim, as below :




Drawing of ‘Firefly’ Driving Wheel


On the other hand, another ‘official’ plan view of the frames shows one set of spokes fitted radially, while the other set is angled. Commenting on this latter drawing, shown in his book on Broad Gauge Engines (Part Two), Brian Arman writes that “the inner [spokes] are visible, inclined inwards to take advantage of the extra space permitted by the broad gauge and provide a deep strong hub. The outer spokes are vertical but concealed by the tyre


Whichever is the correct view, the sources agree that the spokes were inclined in some way,which presents problems for making a 3D print by the ‘Fused Deposition’ method.


Before considering the printing problem, I also had to prepare a 3D drawing of a wheel of this type. For the record, I used the above drawing of the wheel as a ‘canvas’ in ‘Fusion 360’, drew a single spoke and then used the ‘pattern’ tool to create a ring of 14 spokes. I then drew an intermediate spoke and used the ‘move/copy’ tool to set it to an appropriate angle. I repeated the pattern to create the second ring of angled spokes. These steps are illustrated in a clockwise direction below:



Drawing spokes in two rings, using the ‘pattern’ tool in ‘Fusion 360’


Having found that my printer could handle a limited amount of overhang, I tried printing this wheel but it failed to complete the raised ends of the angled spokes, as I had expected it would  :(


Two-part Wheel


Another pause for thought and I hit on the idea of splitting the wheel into two halves. I found that I could flex the rim sufficiently against the hub to vary the angle of the spokes, so I printed two sets of flat spokes. What would become the outer part of the wheel carried the main depth of the rim. The inner part had a shallower rim of larger diameter, to represent the wheel flange. I raised the hub of the inner part to act as a spacer from the front part and then pressed the flange outward to meet the main rim of the front part.



My design for two wheel ‘halves’ in ‘Fusion 360’


The two parts printed successfully but I could not use my favoured jointing method of applying a 200°C soldering iron in the confined spaces between the spokes. Instead, I threaded the front part onto an axle, added a thin film of superglue on the inside face of the hub and slid the inside part of the wheel along the axle until the hub was firmly against the front part. Once the glue had ‘set’, I worked around the rim, pressing the two parts together over a thin film of superglue. I found that superglue does appear to be very effective when used on the PLA plastic. The only tricky part lay in ensuring that the two rings of spokes were in the correct alignment.


Once assembled, my 3D-printed wheel looked as shown below. The angled spokes can be seen at the back of the wheel:



My 3D-printed Wheel and Profile


I was pleased to achieve this result, although I shall admit that there was a period of trial and error, while I adjusted the dimensions to achieve the correct-looking profile.


Since each pair of ‘halves’ only took around 8 minutes to print, the experimentation was not too arduous. I found that the inner section tended to print slightly thinner than the planned dimensions, probably because of the effect of adhesion to the printer bed. I had planned to make the wheel 2 mm depth but my first prototype measured only about 1.65 mm on my micrometer, so I increased the depth of the flange to compensate.


Contemporary Sketch


In replying to comments on my previous entry, I referred to the work of E.T. Lane, which I repeat here:


E.T. Lane was a pupil at Swindon in the 1840s, who sadly died at the age of 20. Fortunately for us, he made many sketches of the engines he saw in the yard at Swindon during the late 1840s. I can imagine the young lad standing there, looking at these new engines and transmitting his vision to posterity.  They seem to convey more 'feel' for their subjects than finished technical drawings.  The inclusion of dimensions makes them very useful to modellers, although some are hard to read.  I like his youthful flamboyant signature too. Amongst the engines he sketched was ‘Gorgon’, one of the ‘Fire Fly’ group built by Fenton, Murray and Jackson and delivered in November 1841:


‘Gorgon’, sketched by E.T. Lane in 1848


I feel that he has left us a real link with the past, to stand alongside the early photographs by Fox Talbot and others.


Smokebox Rivets


To conclude this phase of my modelling, I added some of the details to the front of the smokebox. While there are several drawings of the plan and side elevation of these engines, there is less information about the front elevation. I turned to the lithographs of J.C. Bourne and to my own photographs of the replica at Didcot (remembering that this is not a totally reliable source of prototype information).



My photo of the Didcot ‘Fire Fly’ Replica


Following my usual method, I overlaid a drawing of the components on the front surface of the firebox, allowing for the perspective from which my photograph was taken. Note that the curved top to the door also appears in lithographs by J.C Bourne.


I overlaid my drawing onto the front of my 3D model in ‘Fusion 360’ and used the drawing tools to copy and then extrude the main features. For the rivets around the edge, I used the ‘pattern on path’ command, after drawing the outline as an initial guide. This is a very effective way of adding repetitive details to a drawing in ‘Fusion 360’.   I know that I have added too few rivets but I have to allow for the limitations of my 3D printer in resolving fine details. My intention is to convey the impression of a riveted edge. The current version of my model is shown below:



My model with Smokebox Door details


There’s still quite a way to go and I must think about putting something between those widely-separated Broad Gauge frames! I’m getting so absorbed in this model that other household tasks are becoming neglected :)



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Fantastic progress. The assembled loco is looking great. The interesting thing about those wheels is that they have a prototypically fine rim and tyre, something that is lost on commercial wheels with chunky rims and tyres. I always feel even the best wheels can still be too chunky for 1840s locomotives.

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A problem is that working models rely on the tyres for electrical pick-up.  I'm now thinking about using radio control, in conjunction with on-board lithium power.  I feel it could solve a lot of running problems although I expect it will introduce new ones of its own!


After a period of hectic research and modelling, I now need to think about getting out more as lock-down measures start to ease :)


Thank you for your kind remarks,


Edited by MikeOxon
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Brilliant Mike, I admire your problem solving mindset. As usual, when faced with a challenge it helps to break down things into smaller parts.


Thanks for another E.T. Lane sketch.  Looking across Rising Star, Evening Stare and Gorgon, the lines seem to be getting straighter! And then there is Royal Star, which seems to suggest that once the initial sketches were done, they were developed into something more advanced?


Regarding the wheels, even with radio control, would the printed material slip on rails? Some sort of adhesion needed?






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11 hours ago, Mikkel said:

Regarding the wheels, even with radio control, would the printed material slip on rails? Some sort of adhesion needed?

You've been listening to those people who kept telling Stephenson that it was not possible to pull a train by revolving iron wheels above an iron rail.  They said the wheel would just spin round.


We shall see :)

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