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‘Fire Fly’ class – Part Three




It has taken me a considerable amount of thought before deciding how to proceed with the next stage of building my my ‘Fire Fly’ class model. The obvious method would be to construct a strong frame around the outside, as I have done for previous models, but it doesn’t really work with this prototype.




Fire-Fly replica at Didcot showing Boiler Support


As I showed in the previous post, the prototype was built with four short frames linking the smokebox and firebox. Two of these frames can be seen in my photo of the Didcot replica, above. These frames carried bearings for the driving axle and all the ‘motion’. The drag bar at the back of the engine was attached to the rear end of the fire box, The only links between this ‘inner’ structure and the outside frames were six diagonal brackets between boiler, smokebox, and firebox. The role of the outside frame appears to have been limited to distributing the weight of the engine across the two pairs of carrying wheels.


Brass Inner Frame


I decided to adapt this approach to my model, by constructing a strong brass frame that follows the outermost pair of the four inner frames of the prototype, extended to the drag box at the rear of the engine and with a cross member behind the smokebox. This would give me a strong centre-section for the model, with the outside frames being largely ‘cosmetic’. This idea is illustrated below, overlaid on the frame plan of the prototype.




My plan for an inner brass frame


I constructed the inner frames from 6 mm x 1 mm brass strip, cut to length, with reference to the frame drawing and soldered into a rectangular ‘box’, as shown below:


My soldered brass inner frame



3D-printed components


I realised as I started to build this model, how much my ideas have changed since I started using my 3D-printer, a couple of years ago. In those early days, it always seemed ‘touch and go’ whether a print would work or not – models detached themselves from the base-plate, filament failed to feed properly and so on, such that I always felt the need to ‘baby-sit’ the process during printing.


Gradually, I learned to adjust the various parameters, to get more reliable printing and much smoother surfaces. A breakthrough occurred when I developed the concept of printing a model in sections, rather like a kit, that would be assembled after printing the various parts. In this way, individual print times reduced from several hours, for a complete model, to less than one hour and, often, around 20 minutes or less for individual parts. This changed my approach to modelling completely, since I could lay out each part on the printer bed, in a way that optimised the print quality. It also gave me confidence to leave the printer to itself, while I enjoyed a coffee break :) The printer became just another useful tool, rather than something requiring special attention.


The shorter print times encouraged me to experiment with different techniques, before finalising a model. An example of this arose when I printed the footplate for my ‘Fire Fly’ model. The foot-plating follows a rising curve in the outside frames, over the axle boxes for the driving wheels, as shown below, where I have marked out the 3D model over an imported image of the frame plan, imported into ‘Fusion 360’.


My 3D model footplate, built over an imported drawing in ‘Fusion 360’


According to the ‘rules’, I should provide ‘support’ for this raised section when printing but, since the print time was very short, I decided to take the risk and see what would happen if I omitted these supports.   I don’t like supports, because they can be difficult to remove without damage or leaving traces on the surfaces where they were attached. My temerity was rewarded by cleanly-printed arches :)   It’s good to try these things, as it helps one to learn the true limits of the printer. 




Another recurring problem with modelling early broad-gauge engines lies in producing those close-fitting bicycle-like splashers.  For ‘Rob-Roy I fabricated the splashers from brass strip, with Silhouette cut outside faces, finished with gold-coloured foil.  For ‘Tantalus’, I used 3D-printed wheel arches, finished with etched brass facings from the Broad Gauge Society (BGS) kit.


This time, having learned that I could ‘get away with’ some overhangs, I tried 3D printing the entire splasher, laying the back of the splasher on the printer bed and crossing my fingers that the front faces would print, without filling the cavity behind with spurious filament.  Well, it worked very well - just a few individual threads of filament crossing the void, where the print head had traversed from one side to the other. These strands were easily removed to leave remarkably clean prints, as shown below (cruelly enlarged):


my 3D-printed Driving Wheel Splasher


Considering that each splasher took just 6 minutes to print, I feel that the result was well worth the time spent on this test.




I have already described the footplate. It’s sufficient to say that it printed exactly as expected.


Outside Frames


I obtained dimensions for these frames from a sketch in Gooch’s notebook, reproduced in Brian Arman’s book ‘The Broad Gauge Engines of the GWR – Part Two’. I discovered that these sketches are by no means to scale but are a useful source of dimensional information. I chose a different scale drawing to use as a template for building my 3D model in ‘Fusion 360’. This software has a useful feature to ‘calibrate’ an imported ‘Canvas’ from a single known measurement.


Then I followed my usual method of tracing the outlines over the imported drawing (canvas), using ‘line’ and ‘arc’ drawing tools. I then extruded the side frame using the ‘push-pull’ tool in ‘Fusion 360’ and continued to add details, such as rivets, spring and axle boxes, extruding these parts as appropriate. Drawing all those rivets was assisted by using the ‘pattern’ tool described in my previous post.


My 3D model of Outside Frames


It is worth pointing out that it is not necessary to design separate left and right frames, as the ‘mirror’ command can be used to create the opposite side.


Assembly of 3D-printed parts


I brought together all the separately designed parts on the screen of ‘Fusion 360’, with each ‘body’ labelled separately. Each of these parts could be transferred individually to my ‘Cura’ slicer software and laid out in the optimum orientation for 3D printing.


Individual Components identified in ‘Fusion 360’


For the record, the print times for each part were as follows:


Firebox – 51 minutes

Boiler cladding – 45 minutes (not shown above)

Smokebox – 43 minutes

Footplate – 23 minutes

Haycock – 15 minutes

Outside frame – 8 minutes (2 needed)

Wheel splasher – 6 minutes (2 needed)


After printing, I assembled the parts together, using a soldering iron set to 200°C to attach the outside frames and the splashers to the footplate. This is a very quick and easy method to fuse together separate 3D-printed components. The smokebox, firebox, and boiler cladding are currently just slipped loosely over the brass boiler tube.


Once assembled around the brass boiler and inner frame, the model so far looks as shown below:



My 1st assembly of 3D-printed components on brass frames


I’m very pleased with the progress so far but there’s still a long way to go. The front of the smokebox awaits details and several lines of rivets need to be added.


There is also the matter of wheels – the prototype wheels had a set of straight spokes alternating with spokes splayed toward the inside of the hub. At present I have no idea how I shall create such a wheel but ‘thinking is in progress’ :)


Then there are the more mundane tasks, such as support brackets, handrails, buffer beam, buffers …



Edited by MikeOxon
Restore images

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  • RMweb Gold

Posted (edited)

I feel there's a quiet revolution going on here Mike, the way you're doing the loco part by part. It seems a bit strange to me that everyone else are so bent on printing whole bodies. 


Edited by Mikkel
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An interesting comment, Mikkel.  I think it's partly because I am trying to overcome limitations of a low-cost simple printer.  I have no doubt that UV resin printers can produce much finer results but at the cost of needing hazardous chemicals and post-processing.  I know the resolution of my prints is not up to current standards but I find it adequate for 4 mm scale models at normal viewing distances.  Most important of all, I enjoy the freedom it has given me to explore the 'lost world' of earlier railways.


I have some other revolutions in mind but I'll keep those under wraps for now :)





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This is superb work and absolutely fascinating. I wonder if you would be interested in printing further splashers? I could do with some of a slightly smaller radius for my Rennie locos and your solution is so good.

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Thank you 5&9.  I do this modelling for my own interest and in the hope that I might encourage others to try 3D printing for themselves.   My personal licence for 'Fusion 360' prohibits any commercial use, so I don't supply anything beyond the thoughts that I share in my blogs.

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