Basic Training for Model Engineers Part II
Building the “Polly” steam engine
For this six day course (one Saturday a month) we selected Tubal Cain’s design for an oscillating steam engine as the basis for this course for a number of reasons.
It allows us to introduce you to a wide range of skills but each component is not too complicated.
Only stock materials are used. This means that the materials (which you must buy yourself) are relatively cheap and the penalty for a mistake is not high. Incidentally all students learn that mistakes are inevitable, we all make them and learn from them.
The final result actually works – on the last day of the course you will be encouraged to raise steam and demonstrate the fruits of your labours (you can also impress your family when you get home).
We try to demonstrate as many of the techniques required as possible. After each day you will be given as “homework” the next stage of construction. You should complete this in your own workshop and we exchange experiences at the next session.
After an introduction to SMEE and the practicalities of the course we discuss steam engines in general, and introduce the concepts behind oscillating engines such as “Polly”.
We discuss boiler construction, choice of materials and choice of solder/braze metals. The “Polly” boiler is silver soldered using copper with bronze fittings and flanged end plates. Skills covered include turning, thread tapping, flanging using a former and silver brazing.
Constructed from sheet steel you can make this from scratch or we can supply a laser cut piece. Skills covered include sheet metal cutting, drilling and forming plus soft soldering.
The burner is made from sheet brass. Skills covered include tube forming, flanging, turning and silver brazing.
Basically a shallow box made from sheet steel with a weight to stabilise the model when running. Skills covered include sheet metal bending, soft soldering.
Boiler Fittings and Safety Valve
Constructed from sheet steel and brass this supports the crankshaft and cylinder of the engine itself. It will be bolted to the firebox in the finished engine. Skills covered include sheet metal bending, turning, accurate marking out and drilling. One part requires milling or filing.
Cylinder Piston and Crankshaft
By now most of the required skills have been learned but the cylinder and piston must be made accurately if they are to work well. Skills covered include boring, lapping and the use of adhesives.
Painting and Finishing
Many engineers dislike painting their models and many spoil their work in consequence. We discuss the process of painting, which paints to use and why they work (or not).
This document contains a list of the materials you will need- Polly Shopping list