Jump to content

‘Fire Fly’ class – Part Two




As I carried out my research for this project in Part One (Research), I realised just how difficult it can be to decide what these old engines were actually like!


One of the problems seems to be that, in those days, engines were hand-built by craftsmen who. perhaps, were not so keen on trying to follow a drawing but knew how things should be done!   I noticed, for example, that the shape of the firebox casing on the ‘Fire Fly’ replica is actually quite different from that seen in an early photograph of the original.  I suspect the Didcot team worked to drawings, whereas the original builders did not. The more I looked at old drawings and photographs, the more differences I discovered.


I shall derive as many details as possible from the early photograph of ‘Argus’ that I first showed in my previous post.


Fire-Fly class engine ‘Argus’


The boilers of these early engines did not rest on their frames in the way more modern designs do but were supported on brackets from the outside frames.   In the case of the ‘Fire Fly’ class, there were also four inner ‘frames’, which were slender bars between the smokebox and the firebox that carried multiple bearings for the long Broad Gauge axles. The ‘machinery’ was laid out in the spaces between these inner frames, as indicated below.


Fire Fly Frame Plan (from below)


The smokebox, firebox, and boiler seem somewhat lost within the width of the Broad Gauge outside frames. Since my intention was to build these three body components around a length of brass tubing to represent the boiler, this was where I decided to commence building my model.


My Construction Plan


I decided to start from the idea that the smokebox and firebox would be attached to the ends of a brass tube representing the boiler. These two components will be 3D-printed and the boiler itself will be clad by a 3D-printed outer sleeve.


At the outset, I thought that the greatest design problems would lie in the design of the ‘haycock’ cover over the outer firebox. I have already described how I created the shape of this component in my previous post.


Haycock Top


I have added the type of manhole cover that was characteristic of those engines built by Fenton, Murray, and Jackson, such as ‘Argus’, to my existing design of the ‘haycock’ top to the firebox.


I thought it was going to be difficult. until I remembered that, in the 3D computer world, solid objects can happily slide through each other! All I needed to do was to create a small cylinder, tilt it to the appropriate angle and merge it into the haycock body, as shown below:


My 3D Model with Manhole Cover



Lower Firebox,


The lower firebox is essentially a rectangular box, stretching upwards from the ash-pan to the base of the ‘haycock’ top. The sides of this box carry vertically-planked lagging, while the front has a circular opening at the junction to the boiler tube.


I created the appearance of vertical planking by drawing a series of narrow rectangles, to represent gaps between the planks, on each side of the box and used the ‘push-pull’ tool in Fusion 360 to recess these by 0.25 mm.


The circular hole for the boiler was a little more difficult, since the top of the boiler connects to the sloping front surface of the ‘haycock’. I joined together the 3D models of the upper and lower fireboxes in ‘Fusion 360’ and then set up a vertical construction plane, parallel to the front face of the lower section. By using this as the reference for the ‘hole’ tool, I could extend the hole through the sloping top of the ‘haycock’ as well as through the lower box. I then separated the two parts again for the actual printing.


My 3D- Model of Firebox


Smoke Box


I drew the front elevation of the smoke box with ‘Autosketch’ and imported the drawing into ‘Fusion 360’, where I extruded it to the appropriate length. I intend to create the details of the front, with its door and rivets, on a separate overlay. The three parts, as designed in ‘Fusion 360’, are shown below:




My 3D-models for Boiler End Components


Boiler Cladding


At first, this proved easier than expected, since I had already designed the cladding for my Gooch Goods boiler. All that I needed to do was to re-scale the cladding, to suit the ‘Fire Fly’ dimensions, using the ‘scale’ tool in ‘Fusion 360’    BUT what about that wooden cladding?


Fortunately, I found the answer in another of the series of excellent videos by Lars Christensen from ‘Autodesk’.  This one is about How To Use The Pattern Function and the relevant bit for me starts about 20 minutes from the beginning.


It turned out that all I needed to do was to draw one plank along the side of the boiler cladding and then select ‘Pattern’ from the ‘Create’ menu.  There are options for ‘rectangular’ or ‘circular’ patterns.  I chose ‘circular’ and, after selecting my single ‘plank’, I then selected the outer surface of my cylindrical boiler-cladding. and increased the ‘quantity’ until I had a whole array of appropriately spaced ‘planks’.


This proved far easier than I had anticipated and is a very powerful feature of ‘Fusion 360’. I expect that this ‘pattern’ tool will also be very useful for adding arrays of rivets and the like.  I must acknowledge that it was @Timber who first drew my attention to the ‘pattern’ tool in ‘Fusion 360’, when he commented on one of my earlier posts.


Creating the Wooden Cladding





After printing the various components, assembly is simply a matter of sliding the printed smokebox and firebox onto the ends of the length of brass tube representing the boiler. I have shown the printed boiler-cladding separately, so that the brass tube, which gives rigidity and weight to the whole assembly is visible:




Boiler Tube with Cladding


Once I had fitted together the various parts , the complete boiler assembly appears as shown below:


Main components of my Boiler, Assembled



Next Steps


These assemblies were all initial test-prints and I expect to add more details in due course, including an overlay for the front of the smokebox and lots of rivets :)


It’s good to feel that this project is now well under way :)


I now need to start thinking how to tackle the underframes and how to incorporate brass parts for additional strength. There is so much space between the outside frames of these Broad Gauge engines that I feel it necessary to include more details of the motion than I would do on a standard-gauge engine. I hope that 3D- printing will be a help in producing the necessary components,




I created the ‘header’ illustration from one of J C Bourne’s lithographs, using the Dynamic Auto Painter software.


Edit: replaced last photo with improved 2nd. print.

Edited by MikeOxon
Restore images

  • Like 8
  • Informative/Useful 1
  • Interesting/Thought-provoking 1
  • Craftsmanship/clever 6


Recommended Comments

  • RMweb Gold

Very nice, I'm looking forward to the rivets. Won't that be tricky for the printer?


That boiler support arrangement for the early locos sounds bit dodgy!

  • Like 1
  • Thanks 1
Link to comment
1 hour ago, Timber said:

wonderful CAD work (and modelling)......what do you do for wheels?

Thank you, Timber.  It's a good question about wheels since I would like to represent the alternating spoke pattern of the prototype.  This will need more thought :)

  • Like 1
Link to comment

if you look at some of my S Scale blog posts I have examples of driving wheels I have made using Fusion 360......you may need to use a top end 3D printer via shapeways to ensure concentricity but anything is possible.  There are other examples on RMWeb of people doing similar things.



  • Thanks 1
  • Interesting/Thought-provoking 1
Link to comment

Fantastic and absolutely fascinating as always.


I’m going through a very similar process at the moment but my firebox/boiler/smokebox assemblies are resin cast from scratch built masters. These are for my Rennie locos, but Satellite was basically a standard gauge version of their Mazeppa and Arab built to Gooch’s patterns.


Do you have Brian Arman’s two volumes on early BG locos? I’m reading them at the moment and have to say they’re superb.


  • Like 4
  • Craftsmanship/clever 1
Link to comment
2 hours ago, 5&9Models said:

Fantastic and absolutely fascinating as always.


Do you have Brian Arman’s two volumes on early BG locos? I’m reading them at the moment and have to say they’re superb.


Many thanks and I'm really pleased that you find my write-ups fascinating.  I realise that there are many experts in 3D design but I try to write for people who, like me, are trying to get going in a new technology.  It's always nice when things turn out to be easier than expected!  The planking round the boiler is not quite right yet but I'm up against limitations of my printer.  I tried thinner planks but they ddn't appear at all and now I think I've gone a bit too far.  I'm hoping to find a happy medium by trial and error.


Yes, I have Brian Arman's books and i believe that Part Three is on the way.  He is creating an amazing resource, by bringing together information from a variety of sources.  As I have been discovering, though, a lot of the actual practices are not recorded, so engines do not always look like their 'official' designs.

  • Like 3
Link to comment
On 23/04/2021 at 05:24, Mikkel said:

Very nice, I'm looking forward to the rivets. Won't that be tricky for the printer?


That boiler support arrangement for the early locos sounds bit dodgy!

The Fusion 360 pattern tool allows you to ‘follow a path’ eg the edge of a saddle tank and you can tell it how far apart you want the rivets.  I am using it on the locos on my 3D workbench with some success (but not on one particular design and I can’t fathom why).


  • Informative/Useful 2
Link to comment
1 hour ago, drduncan said:

The Fusion 360 pattern tool allows you to ‘follow a path’ eg the edge of a saddle tank and you can tell it how far apart you want the rivets.  I am using it on the locos on my 3D workbench with some success (but not on one particular design and I can’t fathom why).


Thank you Duncan - it does seem to be a very powerful tool!  Regarding 'Fusion 360' sometimes refusing to do things - I encounter this quite often, especially with imported drawings.


  • Thanks 1
Link to comment
  • RMweb Premium

It seems to have taken locomotive engineers about 30 years and any number of catastrophic failures to appreciate that treating the boiler as the main structural element of the locomotive wasn't such a good idea. But in mitigation it has to be recognised that they were using wrought iron boilerplates which corrode much less quickly than steel, so the boiler was expected to have as long a life as the rest of the engine, in contrast to the practice from the 1880s onwards where boilers were standardised interchangeable components.


In the 1860s, Matthew Kirtley had some 'Jenny Lind'-type 2-2-2s of late 1840s vintage "rebuilt" as 0-6-0WTs, which sounds rather unlikely until you learn that it was the iron boiler that was the main part re-used from the old engines.

  • Like 1
  • Informative/Useful 2
Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...