In Part One , I wrote that “this engine had several very unusual features” and, in regard to building a model, “I had to start somewhere and, with so many peculiarities, it was hard to choose. As a ‘gentle introduction’, I decided to start with the two bogies.”

 

I intend to continue, as far as possible, to follow a line of ‘least resistance’ but before going any further, I collected as much potentially useful information , photos, and drawings as I could.

 

In his book ‘The British Steam Locomotive 1825 – 1925’ p.106, Ahrons noted that “no description of them, beyond the meagre details in Colburn's " Locomotive Engineering," page 73, has ever been given, and the following account of their constructional details may therefore be of interest.”

 

 

He continued with quite an extensive article, providing many key dimensions and descriptions of the rubber suspension system. This suspension anticipated Alex Moulton’s work for the Austin Mini by more than a century! Ahron’s also referenced an article in ‘The Engineer’ supplement, 16 Dec 1910 , which provided various sectional views of the engine from which I could make a start.

 

 

Using these drawings, I started by creating some of those major components for which I had already gained experience in modelling other engines.

 

Ahrons gave the boiler dimensions as 10ft. 9in. long by 4ft. 0½ in. inside diameter, so I created a tube of length 43 mm, inside dia. 16 mm, with 1 mm wall thickness, which provided a good match to an ‘The Engineer’ drawing. I then added firebox and smokebox by tracing over the drawing and extruding as required, to create solid ‘bodies’ in ‘Fusion 360’. The results were as shown below:

 

3D model of Boiler Assy. Referenced from an illustration in ‘The Engineer’.

 

The next ‘familiar’ item on the agenda was a pair of driving wheels. although these are a little larger than usual, at 36 mm diameter, and flangeless. The stages of my usual method are shown below:

 

My steps in 3D modelling the Driving Wheels

 

It always surprises me how rapidly something resembling an engine emerges, especially once the platform, chimney and safety valve housing have been added.  The chimney and safety valve cover were created by my usual method of tracing the profile and then using the ‘Revolve’ tool to create the cylindrical ‘bodies’.

 

The platform was a simple rectangular extrusion from the plan drawing, with the exception that small ‘humps’ had to be raised over each of the bogie wheels. Those early designers did nothing to make life easier for the workmen having to fabricate these shapes by hand!

 

First impression of my evolving model

 

That has completed most of the straightforward parts of this engine and it is now time to start tackling its (many) peculiarities!

 

Outside ‘Frame’

 

According to Ahrons “The inside bearings were only 5in. long, and therefore additional outside bearings, 9in. long, were provided, the hornblocks of which were riveted to the triangular queen truss " frame," shown outside the driving wheel”.

 

This ‘frame’ was my next subject for modelling. I created the truss and the outside rim of the splasher as a single ‘body’ in ‘Fusion 360’ by extruding from a drawing, as shown below:

 

My sketch of the outside frame supporting the driving axle

 

Most of the sketch was made by using the ‘three point arc’ tool in ‘Fusion 360’ and then I extruded the area coloured blue to form the frame. Next, I extruded the rectangle representing the axlebox, to house the outside bearing. I used another useful tool – the ‘offset’ tool – to create an outer rim around the frame and then ‘pushed’ the annulus (coloured yellow) backwards, to surround the driving wheel as the splasher.

 

Yoke and Boiler Support Plates

 

Now it was time to address two more peculiarities – the yoke across the top of the boiler and the curvaceous plates that supported the boiler from the platform. I created both these features by sketching over one of the front-elevation illustrations from ‘The Engineer’.   Again, the most useful sketching tool was the ‘three point arc’ and I then extruded the outlined area to form plates of 0.5 mm thickness, as shown below:

 

There were two identical ‘yoke’ plates in tandem (blue in my sketch) straddling the top of the boiler, with pivoted brackets between them, to carry the vertical suspension rods above the riving wheels.  Two curved support plates (green) were placed, one on either side of the boiler, immediately ahead of the driving wheel splashers.

 

Tracing the shapes of the Curved Plates around the Boiler

 

I then move and rotate the parts I have made and align them against my reference 'canvas'.  I have hidden the rest of the model for clarity:

 

 

After producing models of these plates, I moved on to the suspension units for the driving axle. These were illustrated and described by Ahrons.  I created my models of the suspension units by sketching the profile over the drawing from ‘The Engineer’ and then using the ‘Revolve’ tool in ‘Fusion 360’ to create the cylindrical rods and the brass pots that contain the india rubber ‘springs, as shown below:

 

 

My extrusion of a suspension arm alongside ‘The Engineer’ Drawing

 

Motion Plate

 

The motion plate was, again, unusual in that its outer edges were shaped to follow similar curves to those of the boiler support plates. For details of their appearance, I had to turn to another drawing, shown in the Broad Gauge Society (BGS) journal ‘Broadsheet’ No.49 (Spring 2003).

 

The original is one of those ‘split’ drawings, with halves of two different cross sections shown together. To help my visualisation of the engine, I 'mirrored' each half to provide two separate complete cross sections, although I cannot guarantee that all the details on the two sides were perfectly matched.

 

Two cross-sections created from drawings in BGS ‘Broadsheet’ No.49

 

In my drawing ‘B’, I have shaded the split motion plate, which has square apertures for the connecting rods to pass through. I copied the outlines of these motion plates, as for the other boiler support plates, and extruded them to 0.5 mm thickness.  The valves were placed between the cylinders and the valve rods are shown with supporting slide bars. There are many other unusual features in these drawings that I shall return to later, including the well tank suspended below the boiler on brackets from the motion plate.

 

Cylinders

 

The cylinders themselves were too long to fit within the very short smokebox, so they extended for about 1 foot forwards of the smokebox. They were connected across the width of the engine by a box, which enclosed most of the front end of the valve gear, although there were two tail rods protruding from the front face of this box. A photograph of the front end of No.42 also shows a small steam cock on the centre of his cover

 

I constructed this box and the cylinder ends as a separate ‘body’, which I then fixed to the front of the smokebox.

 

My model of the cylinder ‘front end’

 

That has completed all the main components needed for a ‘top-side’ view of the engine. I have added buffer beams, copied from a different Broad Gauge engine, and extruded the outer sides of the coke bunker. Although simple in external appearance, this bunker has many unusual internal features but I shall come to these later.

 

In the meantime, my 3D model in ‘Fusion 360’ now looks as shown below:

 

 

I think this model is beginning to capture something of the ‘presence’ that the original engines must had. I still have a lot of work to do on the underpinnings – especially the attachment points for the bogies and the well tanks but right now I feel it’s time to pause for the Easter break!

 

Mike