This is a work in progress around the research, design and building of a miniature traction engine, in particular a Burrell 6NHP traction engine.
My son bought me this book for my birthday as I’ve seen a number of Cherry Hill’s models and they are inspirational.
This book has made me think about my approach, my research and as I’ve got older how I can more calmly make parts and remake parts.
The book is quite difficult to find now, but there are copies listed on Amazon:
The main image shows the elevation view of this traction engine printed on a sheet of A4 paper. This is meant to be a small traction engine that works and make to enchant.
- Length: 263mm
- Width: 122mm
- Height: 176mm
- Bore / Stroke: 12/17mm
- Rear wheels:
- diameter: 97mm
- width: 22mm
- Front wheels:
- diameter: 70mm
- width: 12.5mm
I wanted to work out the scale of this miniature traction engine. So, I’ve looked at a number of drawings, images and specifications. All information that I can find online. My conclusion is it is 1/20th scale.
The front axle was quite simple to design and make. A tapered 3/16″ square mild steel with machined ends to accept the wheels directly.
This axle then rocks and rotates on the axle pivot that is located under the smokebox.
The rear axle bearings are phosphor bronze. These fit into an 8mm hole in the tender and each is bolted into place with 3 off 12BA bolts.
A brass tube sits over the bearing as it protrudes through the brass sideplate and bridges the space inside the tender. Thus protecting the rotating shaft.
The boiler design is to heat partly from underneath and with through tubes from the firebox. This means I can make a simple copper boiler and fix this into a steel firebox. Firstly I need to learn how to make the boiler endplates.
What is the right angle for the boiler to sit relative to the road surface?
The materials used in boiler construction is important, brass fittings need to be avoided.
The chimney was produced in a number of sections. The lower section has some compound curves and was machined on the lathe and milling machine.
The mid-section of the chimney was made from 3/4″ round mild steel and machined to a thin 1.2mm thick wall at 2.5°, this was quite tricky but rewarding.
The conrod has come together from 3 parts. This has meant that I have had to split the crankshaft. Hopefully this is the right approach.
One of the initial design questions was around the clearance between the crankshaft and the top of the firebox, in particular the clearance to the conrod bigend. However, making the crankshaft as two parts that splits at the pin allows the conrod big-end to be smaller. This then allowed me to increase the stroke to 14mm and still have clearance.
The double eccentric was made from one piece of mild steel. This will give me forward and reverse.
The firebox on this small model has been designed to be separate to the boiler. Manufactured from mild steel and silver soldered together.
The boiler is going to be clamped to the firebox.
The firebox door is a simple single hinge and latch design.
I just had to compare this model that I’m making with the Mamod engine. OK, it’s a steam wagon, but still comparable in terms of the front end design.
Plans and Kit of Parts
The parts made and bought laying on top of the engine plans.
Could this be a set of plans for sale or a kit of parts?
There are some models that at some point during the build take shape and I end up just staring at them. Pondering the design and dreaming how to solve certain issues with the design. This 1/20th scale Burrell is one of those models. Wonderful, engaging, frustrating and intriguing.Nigel Taylor firstname.lastname@example.org
The rear axle is fixed and made from a solid piece of phosphor bronze. The wheels then rotate on the axle and are driven independently from the layshaft.
The smokebox is gradually taking shape with some basic dimensions being sketched out and then I’ve machined the main ring out of mild steel.
The lower chimney is curved and some of the turning was done freehand, with this you need to keep tool overhangs to an absolute minimum. Plus take your time and use very fine cuts. I wear a pair of heavy duty gardening gloves when I’m turning like this to minimise damage to my hands.
For the lettering around the door I bought a Genmitsu 3018pro CNC engraver, but after a few upgrades I still reverted to a tried and tested etching technique using Ferric Chloride.
I then started again with the door lettering and etching.
The door lock works as per a full size engine with a T-headed bolt and two handles. One handle rotates the bolt to lock it into the smokebox, the other handle pulls the bolt up tight.
After lots of soul searching as it’s not a scale solution I opted for a remote valve for the Burrell. The space for a scale steam valve machined into the saddle was just too tight.
I started to make a mould for the steering wheel, this worked out reasonable as a first attempt. Then I thought this would be better just machined from solid brass.
The steering wheel and handle with an 8BA threaded central boss. I can just about operate the wheel with my rather large hands.
Made from a length of 1/4 inch square brass with a machined groove. This was then riveted to the tender.
The multiple towing points show that this engine really was for use on the road and maneuvering trailers in the yard. A real workhorse.
I’ve roughed the Burrell valve chest out of brass. The valve pushrod bearings were made as one part, silvered soldered into the valve chest. The centre part was then removed on the milling machine. This part is quite small as the Burrell is just 1/20th scale. The main valve chest is 22 x 20 x 7mm.
The front wheels are just 70mm OD and the rear wheels are 97mm OD. A total of 64 parts and 60 rivets go into making the four wheels.
The miniature traction engine has not quite made it onto the wheels, but I do now have a Burrell on “Matchboxes”.
This have given me the ability to visualise it more clearly and think carefully about the next parts.