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‘Sir Watkin’ for Bullo Pill




As the end of another year approaches, I’ve been looking back over the last few rather strange years, very much influenced by the Covid-19 virus. As it happens, 2019 was also the year when I acquired my 3D-printer and embarked on a new phase of model-making. Lock-down provided me with ample opportunity to practise 3D-model making.


A couple of years before that, I had moved my attention to the Broad Gauge era of the GWR, following the discovery that several of my wife’s ancestors worked for the railway over many years. The family arrived at Soudley, in the Forest of Dean, in the early 1850s and the sons started their railway careers at Bullo Pill in the early 1860s. They were there at the time of a serious accident, when a Mail Train ran into the back of a slow-moving Cattle Train. That event inspired me to start modelling the various vehicles described in the BoT Report on the Accident.


More information about the family emerged from their GWR employment records and, from one of these, I learned that my wife’s Gt. Grandfather was involved in a collision between two engines of the ‘Sir Watkin’ class, as described in his record:



Extract from GWR Employment Record


This is the only record I know of any of these engines being used in the Gloucestershire area. The ‘Sir Watkin’ class were originally built as condensing engines for use on the Metropolitan Railway but they were moved when the gauge was narrowed. Later, when all the lines west of Gloucester were narrowed, they were transferred to the South Devon Railway, where they were converted into more conventional saddle tanks.


There are few photos of these engines in near-original condition, although ‘Miles’ was photographed, still with its large side tanks but after removal of the condensing apparatus. I have ‘colourised’ this photo, as shown below:



Sir Watkin class engine ‘Miles’


I found that I have some rather low-quality drawings of these engines in my collection but cannot remember where I found them. I had considered building a model some years ago but decided at the time that it lay in the ‘too difficult’ box. Having gained some familiarity with 3D-drawing and printing, I decided that I was ready to make a fresh attempt.


When I looked at the boiler dimensions, I realised that they were very similar to the Gooch Standard Goods, which I have already modelled, so I started my design by opening my earlier model of the Gooch Goods in ‘Fusion 360’. I also imported my drawings of ‘Sir Watkin’ as a ‘canvas’, which confirmed that the existing parts could be re-used with only minor modifications.


I still need to create a new chassis and to re-design the front of the smokebox and add side tanks and a coal bunker. The smokebox doors were particularly unusual in comprising a pair of rectangular doors, rather resembling oven doors, as seen in the drawing below:



Sketches of 'Sir Watkin'


I added the new parts to my existing Gooch Goods components, using my well-tried methods of extruding them from the 2D drawings. The chimney, dome, and safety-valve cover were all created by using the ‘Rotate’ command in ‘Fusion 360, as described in an earlier post.



3D-modelled parts for my ‘Sir Watkin’ model



Exploring new Modelling Techniques


Over the last couple of years I have had plenty of time at home, under Covid restrictions, to develop my simple use of extrusion, to create 3D models from 2D drawings. I felt that it’s now time for a new challenge, by exploring some of the slightly more advanced features of the ‘Fusion 360’ software.


A suitable challenge was presented by the pipework associated with the condensing apparatus originally fitted to the ‘Sir Watkin’ class. I have a sketch of the front-end, showing how the exhaust pipes could be diverted to direct steam into the side tanks, where it would hopefully condense. In early trials on the Metropolitan Railway, it was soon found that the water in the side tanks rapidly approached boiling point, so the method was not very successful!



Condensing Apparatus Dwg


There is a tool in ‘Fusion 360’ called ‘Sweep’, which can move a profile (such as a drawn circle) along a path, drawn by using the sketch tools. It wasn’t immediately obvious to me how to use this feature but I found a useful tutorial called Getting Started with the SWEEP TOOL in Autodesk Fusion 360’ on the web, which soon got me started. As this tutorial shows, it’s possible to construct a huge variety of curved shapes by using this tool and I felt that it opened up new avenues for my own model-making, extending beyond simple linear extrusions. All the examples in the video, however, only showed paths that were confined to a single plane, whereas the pipework on this engine needed pipes curved in two different planes.


My first attempt was to make two half lengths of pipe and after creating two curved sections as separate bodies, I could rotate one with the ‘move’ tool and then use the ‘join’ tool to combine the two bodies with their curves in different planes.



My 4 steps to create a double curved steam pipe


After joining the two sections of pipe into a single body, I moved the steam pipe and a mirror-image copy into their appropriate locations on the 3D-model of the engine.




3D drawing of steam pipes in Fusion 360


This provided a method to create the pipes I wanted but it also made me want to explore the apparent limitation caused by only sketching paths in one plane. So, I set off on another learning exercise.


3D Drawing in ‘Fusion 360’


Fortuitously, I found another training video on 'YouTube' entitled ‘How to Create a 3D Sketch in Fusion 360, which looked like the breakthrough I needed!


It’s actually very simple but requires two options to be enabled in ‘Fusion 360’. First, in ‘Preferences’, under the ‘Design’ tab, the ‘Allow 3D sketching of lines and splines’ checkbox needs to be ticked and, second, when entering the ‘Create Sketch’ mode, the ‘3D Sketch’ options needs to be selected in the ‘Sketch Palette’. I assume that these options are not enabled by default because the resulting appearance on the screen can be very confusing, if you do not actually need the facility!



Drawing tools in 3D sketch mode


This capability to sketch in 3D provides more food for thought, when planning future models.


3D Printing vs Modelling


Another consideration to be borne in mind is whether a 3D model can actually be printed on a particular type of 3D printer. My Fused Deposition Printer (FDP) builds up a solid model by adding successive layers of plastic filament. There always has to be something underneath the point at which current deposition is taking place. So, for example, the printer cannot ‘bridge’ across large openings in a vertical wall, such as window frames.


In view of these limitation, I try to lay out the parts of my models in such a way that there is a flat surface on the printer bed, above which the main body of the model rises with no significant overhangs. That option, though, is simply not available when I look at my steam pipe, which is curved in two planes! There just aren’t any flat surfaces from which to start.


The answer is to use support structures, which the ‘Cura’ slicing software can generate automatically, with a choice of parameters, such as density and thickness of the support structures. I had to try a few options before I settled on one that worked reasonably well on my pipes. Fortunately, each print only takes about 10 minutes, so it doesn’t take too long to discover if the result is an amorphous blob of plastic! In addition to supports, I found that the model needed a ‘raft’ to start from, so that the structure was firmly anchored to the Printer bed. After making my choices, the preview screen in ‘Cura’ looked as below. The actual print looks a little messier, since the printer does not suppress the transits between the support lines:



My method of 3D-printing a curved steam pip


I will admit that I made this pipe as a design exercise and 3d-printing may not be the best method for creating pipework of this type.


Bonsai Wire


An alternative would be to use ‘Bonsai Wire’, to which my wife, an enthusiastic plant-grower, introduced me. Bonsai wire is made from annealed aluminium, which has been coated in copper for the colour. It is available in a large range of diameters and has the virtue that it is easily bent to a chosen form and then holds its shape. I have used smaller diameters for pipework on some of my other models and it could well be applied here.



Bonsai Wire


After printing all the separate components, I assembled my model as shown below. The side tanks and bunker are glued to the footplate, while the firebox, boiler, and smokebox are threaded over a length of 18 mm diameter brass tubing and are simply resting in position. The chimney, dome, and safety-valve cover were printed separately and glued into position on plinths that I provided on the relevant components. The 3D-printed steam pipes are only ‘tacked’ in position, as they need to be easily removed when I dis-assemble the components for painting. I’ve not decided yet whether to add any form of cab.


Some of the printing is a little rough, especially around the boiler, so I shall probably try re-printing to improve the surface finish. Other parts have come out well.



My 3D-printed ‘Sir Watkin’ model


Old and New Comparison


I have mentioned before that one of my objectives in modelling 19th century prototypes is to gain a better understanding of their proportions, in the context of more recent (and more familiar) vehicles. As an example of such a comparison, I show below my model of a Broad Gauge coal wagon against a more modern type of standard gauge wagon. I like to make these comparisons.



Broad and Standard Gauge Comparison



Edited by MikeOxon
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I do like that extract from the employment record. Do I understand correctly that the event was in 1870, and the fine was 1£ ?  If this inflation calculator is correct, then that corresponds to just 125£ today. Does not seem a lot, but there may be other factors than inflation to consider there, I suppose.


Sir Watkin is looking good. Thanks for the Bonsai wire tip. Do you think it is easy to get 100% straight?



Edited by Mikkel
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I suspect that it was a fairly gentle bump, so more a fine for passing a signal than actual damage cost.  My interest was that it showed that at least two 'Sir Watkin'-class engines found their way to the Forest of Dean.


Bonsai wire is not too easy to straighten but I've inly used smaller diameters.  Perhaps printing is better after all - it printed better than i had expected.


The engine is now in primer and awaiting paint.  Side tanks on the GWR were rare birds :)

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1 hour ago, MikeOxon said:

Bonsai wire is not too easy to straighten

Aluminium wire which is used as armatures in sculpture and similar (I think florists use it) is quite soft and short lengths can be straightened by rolling on a flat surface (cutting mat is ideal) with a small square of, for instance, 6mm mdf or a block of flat steel. I get it from Tiranti: https://www.tiranti.co.uk/Products/aluminium-armature-wire


Kit PW

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On 28/11/2021 at 16:22, Mikkel said:

I do like that extract from the employment record. ...

I like the florid handwritten £ sign.  It oozes the feeling that a pound was worth something in those days :)

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That was a supremely interesting post to read Mike. I’m just about about to start moving over from TinkerCAD to TurboCAD for ease of use. I tried Fusion 360 but didn’t have the time to learn it. Sir Watkin is looking very good.



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