June 2019 – Penrhyn Tractor No 5

By Bob Gamble.
Header photo: Ted Robinson

It all started when the Association of 16mm Narrow Gauge Modellers published their “Penrhyn Quarry Railway Modeller’s Guide”. I had not really paid much attention to Penrhyn Quarry before, but the guide inspired me to acquire several steam locomotive models of the originals used on the system. I also kept returning to the charming “Bullnose” tractors which were built by the quarry over a period of years. Further investigation revealed that quite a few were constructed and they were all modified and/or deteriorated over many years. The thought struck me – why not build one to act as a standby when the steamers give trouble?

With the help of fellow 16mm member Ken Reade, who has made extensive researches into the Penrhyn Quarry and the tractors in particular I drew up a drawing as close as I could using the few dimensions available. I do not claim it to be a close to scale drawing, rather I hope it captures the spirit of the original. It was clear from the drawing and photos of the original that the radiator shell would be the most difficult component to make, so I set about this task to see if the job was feasible.

I have access to a CNC mill, but the software is only 2D – it will machine along and across to any variable length within its capacity, but depth can only be set to one value per cut. Using my schoolboy geometry in CAD I worked out a set of contours to cut close to the actual shape. Bear in mind that each cut visible in the photo above is about 1mm deep, so it’s quite small.

The contours were then blended to form a smooth shape, filing down till the step just starts to vanish.

Eventually all the corners were blended and after fine abrasive paper was used a passable radiator shell emerged. I did two at the same time, one for Ken. Needless to say, they were not identical!

The photo above shows the CNC cut nickel silver (about 1.5 mm thick). The sheet had a CAD drawing of the final object pasted to it. This enabled me to drill 12 holding down holes and establish the start point of machining (origin) and thus, hopefully, miss the screws once machining commenced. The sheet was screwed to thick MDF. To form the channel, a 1.5 mm end mill was used, set to half depth to form a fold line.

The two photos above show the fold. Getting a sharp fold in nickel silver with a lip of only a couple of mm’s can be quite a job, but the half depth cut made it easy and being nickel silver it was surprisingly rigid despite the cut.

The axle boxes are 6.3 mm thick. The blank was soft soldered to a backing piece. A 1.5 dia end mill was used to cut the profile, and a 4 mm dia mill was used to relieve the centre boss. A carbide 0.8 dia printed circuit board drill did the holes. The centre hole was used to line up the box with the centre line of the axle.

A wooden former gave the shape of bonnet, and also enabled me to hold the bonnet whilst filing the edges. A 1 mm thick plate was soldered to the front and two screws pass through the radiator recess and are threaded into the plate.

The motor mount is just a piece of brass angle soldered to a plate which is screwed to a block under the footplate. The footplate itself is slotted to allow correct meshing with the worm and wheel. A plain brass strap with draw bolt holds the motor upright whilst slackening the bolt allows vertical movement of the motor and worm.

A 25:1 worm and wheel give suitable motor speed reduction, both axles are driven via delrin chain and sprockets, all supplied by “Swift Sixteen”.

The rest of the construction used conventional techniques, which resulted in the finished but unpainted model shown above. It was then stripped down and all parts degreased and grit blasted. A coat of etch primer followed immediately after. The complete model was sprayed, wood and all. Rattle can satin and matt completed the metal work; the wood was coloured with acrylic washes.

I had hoped to fit batteries into the tractor but could not find space enough for a useful capacity. I therefore put the batteries into a Penrhyn coal waggon (Wood Valley Works). Unfortunately the on/off and direction switches are all too evident, must do something about that! On 3V the tractor is a little lively, so I have reduced the voltage by about 0.6v with a diode. The driver is due for replacement. He scales in at 6 foot and is built like a sumo wrestler, but he does show how small these tractors were. Originally he was an articulated one man army covered in body armour and weaponry and stood 7 foot tall. A craft knife and assorted files reduced the hardware and some of the excessive bulk, and a saw reduced his trunk and lower legs. The joints were filled with “Plastic Padding” and suitable matt enamels almost made a convincing slate worker.

Since writing this description, I was presented with a driver from the Jons People range. This was a much better fit than the overblown action figure, but, to my relief I had at least managed to scale the original for height!

Acknowledgements

Ken Reade for his extensive research.

Ted Robinson for his photographs of No 5 running on the Ridgmont Track.

Clive Nobbs of the Bullnose Morris Club for photographing and measuring the bonnet and radiator shell of his Bullnose Morris.

Penrhyn Quarry Railway Modeller’s Guide.