The Siamese Twin Model Steam Engine
A friend who had seen a few of my simple models on a forum sent me the plans for this two cylinder model via e-mail. I had never attempted a two cylinder, so had to give it a shot. The Siamese Twin as it was named by it's designer, Dave Goodfellow, was a challenging model to build. Dave's web pages and downloadable plans were a great help.
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I thought the crankshaft would be the toughest part to make, so that's where I started.
The plans call for the throws to be made from brass. With the current cost of brass I made mine completely of steel. I used 3/16 mild steel rod for the shafts and 5/16 key stock for the throws. Actually I made 4 of them before I got one that was straight enough to be usable in the model.
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The plans called for the connecting rods to be made in pieces that would be soldered together. I'm not that good with soldering brass, so I decided to make mine all threaded together.
The bearing ends were made from 1/4" thick cuts of brass ground to shape and drilled for the 4-40 holes in the end. After that the cap end was cut off and the cap hole were drill slightly larger for clearance on the 4-40 screws. The bearing it's self was tapped and the two pieces were reassembled with the 4-40 screws. At that point the 3/16 hole to fit the crankshaft was drilled and reamed to size. The rod end was drilled and tapped 10-32 and 3/16 rod was threaded to match. I later made the rods out of steel so I could pound the ends flat to fit the slots in the pistons.
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Aluminum isn't cheap these days either, but I happened across a buy on a 3" Dia X 7" long bar end on eBay for $9.00 I made the flywheels from that stock with plenty left over for future projects.
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This pictures shows the completed cylinder block, air valves, the two eccentrics that operate the air valves, one modified steel connecting rod and one piston. The pistons were turned out of some small scrap aluminum rods for a slip fit in the cylinder. The plans called for the cylinder to be made of brass. Again, the cost was prohibitive so I made it of steel. Two flat valve rods were made from 1/32 thick steel strapping. They will run between the eccentrics and the valves.
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Here it's starting to go together and issues are arising. First off, the flywheels are loose on the crankshaft. To fix that I drilled and tapped the hubs for tiny 4-40 set screws. The cylinder cover was also made of steel, drilled and tapped to close off the end of the through drilled holes of the cylinder.
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The engine will get it's air supply through this brass manifold. For lack of a milling machine, it was shaped with hacksaws and files. It was then drilled and tapped for 10-32 thread nipples. The nipples were made by cutting the heads off of 10-32 machine screws and drilling a 1/8" hold through them.
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And here it is all finished up. The valves were timed and I put the air to it. Nothing! No matter what I tried I could not get it to run. I decided the air valves were not allowing the cylinder to vent properly.
The original design was allowing too much air pressure back into the cylinder on the exhaust stroke. To fix that I made two new valves with flats cut facing the cylinder. That would completely block incoming air on the exhaust stroke while allowing the cylinder to vent pressure along the flat.
Then I put the air to it again and got this.
It runs good and can be throttled down to slow speeds without stalling. Great design Mr. Goodfellow!