Thursday, 14 May 2020

Articulated Geared Engine

Revised 2020 to include funnel options

    ©  2020   David Hurst     All  Rights Reserved

The idea for this engine came from an original Hunslet catalogue. Although the design was, I believe originally produced by the Bristol-based Avonside Engine Co.  I was struck by the unusual format of the engine and the possibility of developing a design to run on a Bo-Bo chassis. I found that a good option for the chassis was the BANDAI B-Train Shorty Powered Motorized Chassis 3, which gave me the opportunity to design coupling rods and hubs to add to the wheels, as they did not have fixed axle boxes covering the wheelsets.


The bodywork and details
Note, in 2020 I added a straight funnel as well as the original with the spark arrester, to give a different feel to the model that you can build.  
The bodywork is printed in one main part with separate parts, which I think makes it easier to paint the model. The approach to finishing the print, which in this case is done in Fine Detailed Plastic, was to remove the wax support material from the print by soaking in petroleum paint thinners (white spirits in the UK) and then wash in warm (not hot) soapy water, leaving the model to dry overnight. I left all the parts attached together when I did this, so I did not lose any of the smaller parts.

©  2020   David Hurst     All  Rights Reserved

The first parts I removed from the bodywork was the frame printed directly under the footplate, which included the steam pipework, lamps, and couplings. This will expose the connecting rods and hubs. The cylinders are printed within the cab area and removed last.
At this stage, it is an advantage to spray the model in a grey primer which will hi-light any distortions to the surface caused in the printing process. I further separated the hubs and the cylinders from the main body now to allow me to clean the surface of the print removing any furring and surface lining caused in the printing process of the model. Once this is completed a further couple of coats of spray primer will give a good surface to work with.

The chassis adaption

The design of the coupling rods and hubs needed to be done so that the new hubs could be aligned and quartered to each set of the existing wheels. To achieve this I had the idea to print the rods and hubs as a set, which were at the correct centres and orientation to be glued directly to the wheels.

©  2020   David Hurst     All  Rights Reserved

The connection of the coupling rods to the new wheel hubs would be done using 14BA brass machine screws, nuts, and washers. These would be cut to remove the head and bonded into pre-printed pilot holes in the new extended wheel hubs, leaving them long so they can be trimmed off after final fitting.

The final print was set up with the four sets of hubs and coupling rods printed together off a central sprue. When fitting the hubs to the wheelsets, each pair of hubs still connected to a coupling rod has the 14BA threads put in place, which are glued in place and then glued to the wheel centres with a Cyanoacrylate (super) glue. A delicate job that needs to be done the right first time. Once this is done on all of the wheelsets the coupling rods can be separated from the hubs. The sprues will need to be cleaned off these and the holes in the coupling rods drilled out to give a loose fit to the 14BA threads. This loose fit is essential for the coupling rods to work without causing the whole mechanism to seize up. The washers are placed between the coupling rods and the nuts. These are not tightened but will be locked in place with a little contact adhesive when the final assembly is completed. You will find that all of the holes will need to be drilled out on both the hubs and the coupling rods. This is a restriction in the printing process the hole size can dictate the overall size of the printed parts.

©  2020   David Hurst     All  Rights Reserved

The other items to be fitted to each bogie are the coupling assemblies, which are fitted within the existing N gauge coupling pockets. These are printed as a frame and the rear part of this is pushed into the U shapes housing left when the N gauge couplings are removed. I used a contact adhesive to bond the print into the pockets. I found it easier to complete the painting of these before finally gluing them in place.

©  2020   David Hurst     All  Rights Reserved

There were a few bits to add to the model that I had sourced from suppliers. Smokebox door
darts were from RT models and the 0.45 mm brass wire I used for the cab handrails were from eBay. These were all added before the final coat of primer.

©  2020   David Hurst     All  Rights Reserved
Painting and lining

It is an advantage to fit all the body parts before painting and final fitting in place. The main body of the engine was painted in a Revell enamel paint, ref matt 56 blue. Several thinned coats were applied to achieve a smooth surface to the paint finish. Before applying the lining transfers to the model, a coat of gloss varnish was applied to the surfaces and left to dry. This gives a much better surface to apply the transfers to as it also allows them to be moved into place much easier.

©  2020   David Hurst     All  Rights Reserved

I also added a set of brass etched nameplates to the model, which were obtained from Narrow Planet who supply a large range of names and works plates are etched brass. They were fixed in place with a small amount of Cyanoacrylate (super) glue.
Other areas of the model such as the footplate, smokebox, funnel, couplings and bogie hubs were painted in a charcoal Revell enamel paint ref matt 9. In areas such as the steam pipes and cylinders, I wanted a metallic look to the finish so I mixed matt 9 with a silver 91 to get metallic gunmetal. I also hi-lighted some areas such as window frames and the dome with a brass finish paint. The buffer beams, hub faces, and coupling rods were finished in a Revell enamel paint ref matt 36 red.
Once all the painting was completed, a coat of matt varnish was applied over all areas, which has the effect of sealing the transfers and smoothing the joint between the etched plate and the model. If any area needed a gloss or shiny finish like the dome, I applied a little more gloss varnish to them. 

I sorted out some figures for the crew and adjusted them to fit within the cab. It turned out better to add these to a small base plate and then paint them and was also much easier to add them into the cab afterwards.

©  2020   David Hurst     All  Rights Reserved

The final touches to complete the model was the add some real coal into the bunker at the rear using PVA adhesive and real crushed up coal. Then the lamps were given some sparkle with some clear plastic rhinestones of a suitable size.  

If you would like a 3D print of this model visit Shapeways at:




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