I’ve now managed to produce a set of ‘printable’ parts from the original download from the ‘Printables’ website described in my previous post.

 

I’m still puzzled by the theories on exactly which of Trevithick’s locomotive engines was actually used for the Penydarren trial, which was the moment in history that put the steam locomotive on the map.

 

There is a print in the Science Museum collection, said to be the Penydarren engine but it has been discredited because, according to the attached scale, it shown an engine on a 3-foot gauge track.

 

 

When I compared this print with the illustration in Francis Trevithick’s book, however, it was obvious that they were extremely similar, although the book states that gauge was 4’ 4”. On further inspection, I realised that the Science Museum print became a very good match (not identical but very similar), if I increased the scale of the print to correspond with a 4’ 4” gauge. Can it really be the case that subsequent authors have been misled by the scale appended to the drawing? I’ve seen so many examples of a wrong scale on a drawing to feel that it is not that unlikely an occurrence!

 

Incidentally Francis Trevithick, son of Richard, was himself a railway engineer, having been appointed Locomotive Superintendent to the Grand Junction Railway at Edge Hill, Liverpool, in 1841. I mention this to show that he was very knowledgeable about engines

 

Printing my model parts

 

Making the parts downloaded from the ‘Printables’ website printable at the smaller scale of 4mm/ft did not prove to be too arduous.

 

I made all the gear wheels a little thicker by a simple scale change in the ‘Cura’ software. The boiler and chimney were both too thin-walled to print, so I created new versions as simple cylinders in ‘Fusion 360’. The triangular front support for the piston-rod was also too fragile, so I re-drew a stronger version. After assembling all the parts together, I made a ‘rendered’ view of the model within ‘Fusion 360’. The piston-rod and slide bars are indicated as brass rods, which is how I intended to create them for the physical model.

 

Model of Trevithick’s engine rendered in Fusion 360

 

Apart from these rods, I transferred all the revised files from ‘Fusion 360’ to the ‘Cura’ slicer program and thence to my printer. In the case of small parts, which take only a few minutes to print, I collected several items together as a single ‘print job’. For example, all the gear wheels were printed together in the printer build plate.

 

Various wheels and gears printed together – on my printer bed

 

A curious observation was that, in a couple of cases, small parts which did not print cleanly the first time, gave a far better result when I immediately repeated the print, over the same location on the printer bed (i.e. no change to any of the print parameters). Could the fact that the print head was still warm have resulted in a cleaner start to the re-print?

 

Assembling my model

 

When I first viewed the collection of tiny parts that I had printed, I quailed at the thought of ever being able to connect them all together! Fortunately, I have a good illuminated magnifier and, very importantly, have recently had cataract surgery that has restored my acuity of vision!  I’m also pleased to have found that my hands are still steady enough to thread small-diameter rods through barely visible holes in tiny parts!!!

 

Of course all you 2mm FS modellers will wonder what I’m talking about but it was a delicate-seeming operation for me to attempt. I don’t know if Julia Adams still visits this site but it was seeing her demonstrations at various shows that assured me that such fine work is possible.

 

So, where to begin?

 

I decided to try threading the main drive-shaft through the lugs on the boiler end-plate, to connect to the flywheel on one side and the primary gear wheel on the other. It turned out that a 1 mm diameter brass rod slid through the lugs, after just a slight touch with a reamer, so I soon had these parts joined together:

 

My Primary Driveshaft Assembly

 

I realised that there is a boiler extension below the drive shaft, which may explain why the boiler length is described in the references as being 6 feet. This extension presumably accommodated the curved end of the return flue inside the boiler.

 

This initial step demonstrated that there was a good chance that other shafts and gears would all fit together so, with a little more confidence, I assembled the main driving wheels onto their 2mm diameter steel axles. In the prototype, the axles were fitted to a very basic wooden ‘boiler truck’, to support the main components of the engine. This illustrates the concept that Trevithick had of designing his engines as ‘multi purpose’ machines. In a letter to Mr. Giddy dated February 22nd 1804, Trevithick wrote “An engine is ordered for the West India Docks, to travel itself from ship to ship, to unload and to take up the goods to the upper floors of the storehouses by the crane, and in case of fire to force water on the storehouses. The fire is to be kept constantly burning in the engine, so as to be ready at all times.”

 

I fitted a second 1 mm diameter rod into the raised mounting on the side of the boiler, to carry the ‘intermediate gear’, which conveyed the drive from the primary gear to the gears on the faces of the driving wheels. After setting the model boiler onto its ‘boiler truck’, this part of the assembly appeared as shown below:

 

My 3D-printed Boiler on its wheeled carriage

 

I was pleased to find that the gears meshed together well, so the next step was to attach the boiler end plate, carrying the primary drive shaft: and its gear wheel.

 

Assembly of the Main Gear-train

 

These parts all fitted together well, so it was time to look at the thorny issue of the cross-head and its associated support structure.

 

I decided to attach all the relevant parts to the end-plate of the boiler, before adding the completed assembly to the boiler itself and the main gear-train.

 

I started by cleaning up the details on the end plate of the boiler since, in the small scale, some of the detailing had become a little obscured by stray filament, where the printer had attempted to negotiate some very convoluted paths.

 

Once this was done, I used a reamer to open out the central hole in the end-plate to accept the 1 mm diameter brass piston-rod. I thought that the printed cross head might prove too frail but, in fact, the central hole opened out successfully, by careful use of a 1 mm drill in a pin-vice. I slid the end of the piston-rod into this hole, adding a drop of super glue to keep it firmly in place.

 

For the two slide bars on either side of the piston-rod, I used 0.5 mm diameter brass rods. Again, I used a reamer to help insert the rods into the appropriate sockets on the boiler end-plate and in the cross head. I cut two slightly over-length rods and slid them into their places alongside the piston-rod.

 

As the last part of this assembly, I added the rather fragile print of the triangular support structure. The two arms fitted into two notches on the sides of the end-plate. I put a small dot of gelled-superglue into each of these notches and then manoeuvred each of the arms into place, in turn, holding them until the glue ‘caught’. After re-checking the alignment of all these parts, I set the whole assembly on one side, to allow the glue to harden.

 

My assembly of the Cross Head and its associated Rods.

 

To call this a delicate operation would be something of an under-statement! The end bracket holding the outer ends of the slide bars is the smallest item I've ever printed and the main problem was keeping it from the clutches of the carpet monster😆 I show it being held on the shaft of a reamer, against my thumb for scale:

 

End support held on a reamer

 

Note the two small ‘ears’ above the bracket, which replicate those on the cross-head. These are supports for rods to operate the four-way cock by means of tappets, moved by the cross-head to change over the cock as the piston nears the ends of the cylinder. They were also used when starting, to ensure that this single-cylinder locomotive started to move in the desired direction. I’m afraid I shall omit these parts, as they are simply too fine (for me) to create at 4mm/ft scale.

 

The next items to add to the end-plate were the chimney and its right-angle connecting pipe. A key part of the original design was that the exhaust steam from the four-way cock was directed up the chimney, so that the steam blast helped to ‘draw’ the fire and so improve the steaming of the engine. As I had anticipated, the tall slender chimney did not print very smoothly and, a length of metal tube would be a better solution. The printed one serves the purpose of showing the overall appearance of the engine.

 

After gluing this assembly to the end of the boiler, the remaining task was to add the long connecting rods, between the cross-head and the drive-shaft at the opposite end of the boiler. I couldn’t manage to fix pins into the end of the cross head so the rods are glued together here. The other ends are pinned to the flywheel and primary gear crank. In practice, although the cross -head will slide, the movement is too tight for this to be considered as a working model … perhaps, next time, with a few improvements to those components that are really insufficiently robust at this scale 🙂.

 

So, at last, the rather spidery contraption looks as shown below:

 

My 4 mm/ft scale model of a Trevithick locomotive c.1803

 

So, why did I do it? Partly of course because it was there but also, I wanted to place the earliest GWR engines within the context of what had gone before. Even the earliest GWR engines were rather late on the scene – 35 years later than this Trevithick engine, which is rather a long time in a period of rapid technological development.

 

I can write about dimensions and show models against a ruler but a much more ‘real’ impression is formed when I place my models from different periods alongside one another.

 

Below I show the Trevithick locomotive in context with some of my early GWR models placed on the same table. I believe this brings home much more vividly what progress was made during the first half of the 19th century!

 

 

Mike