There is one problem that has been lurking in the background throughout my design of this outside-framed locomotive – how to fit the wheels? The wheels need to be pressed onto their axles and set to gauge, before fitting them to the locomotive.
I had already decided that this engine will be tender driven, so I do not have to make provision for a gear train. After considering various options, I decided to adopt the method used in the tender-drive ‘Mainline’ Dean Goods model. Since I have previously converted one of these models into an outside-framed ‘Stella’ 2-4-0, I already knew that this was a feasible approach. [see : GWR 'Stella' 2-4-0', Railway Modeller, April 2013]
I decided to reproduce the ‘Mainline’ method, by creating a ‘sub-chassis’ to hold the driving wheels in their correct alignment while also allowing them to be removed from the rest of the model for any servicing that might be required.
To do this, I designed a hollow box girder to run the length of the locomotive with transverse holes to retain the axles in their correct locations. After creating this simple 3D object in ‘Fusion 360’, I split the ‘body’ in two, along the centre line through all the axles, as shown below:
Sub-chassis to carry axles in correct alignment
The two split ‘bodies’ could then be transferred separately to my ‘Cura’ slicing software and printed as upper and lower halves of the complete sub-chassis. Nuts and bolts can be used to hold the halves together around the axles, with the wheels already attached, although for my initial tests I have simply wrapped adhesive tape around the two halves, between the axles.
I needed to open out the axle holes into slots on my 3D-printed outside frames, so that these could be lowered over the extended axles. The rest of the superstructure, starting from the footplate, could then be added in layers above the brass frame structure. The process of assembly is shown below:
Assembly of Major Components of my Armstrong Goods
By making the body as an assembly of separate components, wrapped around a brass cylinder to represent the boiler, it was easy to paint the parts in their appropriate colours, without any need for masking between differently coloured areas. Although I hadn’t realised at the outset, this ‘layered’ method of construction also facilitated alignment and rapid assembly into a complete locomotive.
I now feel that my model is a viable ‘rolling’ engine. Of course, there is plenty more to be done, including assembling the final form of the running gear and then adding a lot of detailing, including lining and lettering, for which I will use the techniques I have previously described in this blog.
There is also the matter of the powered tender. I have a variety of different designs all of which were designed to be transferable between different engines. All the ones I have built so far have been based on Dean designs, so I shall create a new body in the Armstrong style to fit around one of my existing power plants. My adoption of 3D-printing has greatly increased the scope for making different tender types.