Double Oscillator

April 6th 2010

Here’s a small model steam engine which is an ideal first project for the newcomer to model engineering.

Whilst I have been extolling the virtues of Elmers Wobbler as a good starter project this engine is just slightly more advanced in that it is a double acting oscillating engine rather than a single which is the case with Elmer’s Wobbler.

With a single acting engine the steam (or compressed air) powers the piston in one direction only and relies on the flywheel to carry the piston through to the top of the next stroke. With a double acting engine the steam is also used to power the piston back. All that is required is to provide an extra pair of ports at the opposite end of the stroke and seal off the base of the cylinder whilst allowing the piston rod to pass through it without allowing the pressure to escape.

Making a start

So, first things first. Download the plans from http://www.john-tom.com/html/SteamPlans.html Scroll down to ‘Model Steam Engine Plans – Page 1′. Just below you will find ‘The first steam engine I built is Here – a good first engine’. This link will take you to a downloadable set of very clear well dimensioned plans in metric.  Whilst I am a great fan of Elmer’s engines I am aware that some beginners find the mixture of imperial fractions and decimalised dimensions a little confusing at first.

On looking over the plans two things might concern you, particularly if you haven’t invested in a milling machine at this stage. First the main column features stepped sides, which does add a certain something to the styling. If you prefer to keep things simple this main column can be left as a 105 x 60 mm rectangle.

You will also see that the base has a cutaway well for the lower part of the flywheel. If, like me, you like the idea of the well and you are working without a milling machine then you could make the base in two layers. The opening on the top layer could be cut right through by chain drilling and filing. The two layers could then be bolted together from beneath using four M5 countersunk screws. If you don’t fancy that suggestion you could simply raise the crank shaft hole and porting so the flywheel clears the base without any unecessary machining.

Choice of materials

My advice is to work in brass for the cylinder and flywheel and aluminium for the base and column. Items such as crankshaft, crankwheel and  piston can be machined in free cutting stainless steel. One possible pitfall to be wary of is the drilling of the steam passageways in the column. Without care it would be all too easy to snap off a drill during this delicate operation which would mean all your hard work in fashioning the column would be wasted – more on this later. I mention it here to support my suggestion to use aluminium rather than steel.

I usually prefer to start by making the base and pillar. In other words the basic framework on which the other components are added. Lack of suitable material for the base decided me on making the pillar first. I have shaped this pretty much to plan using my milling machine but, as I said, a plain and simple rectangle would do the job just as well. By using a backstop on the vice it was a simple matter to shape one side of the pillar then flip it over to mirror the milling of the first side.

My local monthly autojumble at Rufforth was on this morning and there are always a number of model engineering tool and material suppliers in attendance so a piece of 12mm aluminium plate was aquired. I cut the plate to size using my hand held Black and Decker jigsaw and plenty of WD40 as a cutting and cooling agent. The two plates were drilled and tapped then bolted together to give me a total depth of 24mm and the edges milled square and flush. The next stage will be to machine the well, or cutout, to provide clearance for the flywheel.

A ball endmill was used to form the rounded corners of the pillar steps adding a stylish touch to the pillar.

After machining side one the vice backstop ensured accurate positioning when work piece is turned over for side two.

The completed pillar machined according to the plan. If you don’t have a milling machine simply cut a rectangle.

The 12mm engine base plates were cut using a Black & Decker jig-saw with copious amounts of WD40.

The double layer engine base plates were screwed together and are now ready to be trued up.

A flycutter provided the width of cut in one pass to provide a good surface finish to the edges of the engine baseplate.

April 14th 2010

A little bit of progress over the last week.