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Project to design and build a mini version of the 7.25”g Wren

Having finished the 7.25”g Wren it seemed a good idea to use the drawings to scale down to make a 1” gauge version!  Whilst many of the drawing parts needed to either be re-scaled or completely re-designed the were some parts where it was impossible to simply copy and scale down, an example being the boiler and fittings.  As far as possible, this model is a faithful replica in gauge 1 of the 7-1/4" 'Ken Swan' original. The overall length is just 10" and all drawings have been scaled down and redrawn to run on 1.75" or 45mm track gauge. The scale of the model is approximately 1/12 full size The design of the boiler has to divert from that of the larger model but is based on the same concept. The main problem is the limited space for pipe connections from the engines to the exhaust and inlet. The safety valve fits in the dome space whilst the regulator is designed to be as small as possible. The boiler is fabricated from thin copper sheet and all joints are silver soldered. I decided to include the option of a methylated spirit burner and alternatively incorporate a grate and ash pan so that the model is a true to the larger model. This means that with luck it will be able to run using coal as well! The hardest aspect of a small boiler this size is the design of the regulator and other fittings. The pipe work in the smoke box is another example of having to fit a lot of unions and pipes in a tiny space. At the back head, in order to preserve the scale appearance of the larger model I decided to avoid fitting an oversize water gauge. As this model is probably only going to be filled with water and run until empty I have instead used one of the Try cocks which has a removable handle. On filling, as soon as water dribbles from the Try cock then the boiler is full. Although the model is mostly finished it has yet to be tested to make steam and run.  Using compressed air it ran well at an amazing speed so as long as the boiler can maintain steam then it should work. This has been a winter project and thankfully the cost of materials is negligible.  I decided to make the motion from stainless steel and fabricate brass plate to replace castings.  It looks rather good with all that brass and stainless steel so it is likely to remain cility lacking on the original lathe

GWH Engineering

creative engineering in a home workshop