The Siamese Twins

I'm presently building a twin-cylinder single-acting mill-type steam engine from scratch, with the old "by guess and by gosh" method. I know what I want, but I'm "free-handing" construction. The bore will be 7/16", because that's the largest reamer I have. Stroke will be 3/4". All 4 cylinders (2 piston cylinders and 2 valve cylinders) are bored and reamed in one 1-3/4" x 2" brass block.

There seems to be a convention for naming steam engines. Usually some descriptive name. Since two cylinders share the same block, I'll call this the "Siamese Twins."

I'm writing this as I do the project, mainly to "solidify" my thinking. I need all the help I can get!

April 29-30, 2000. Here is the brass block from which the cylinder block was cut, with the cut off piece alongside. Thanks to Grizzly's cheap bandsaw, the cut was relatively square and done without breaking a sweat!

And here's the cylinder block. The two larger holes (reamed 7/16") are for the pistons. The smaller holes (reamed 3/16") are for the valves. All four holes go completely through the block, so the cylinder block will have to be mounted on a backing plate to seal the back end of the piston cylinders. The valve cylinders will be left open for steam exhaust.

I will be rounding the top surface for cosmetic reasons (after I have a working engine) and will be taking an as-yet undetermined amount of material off the bottom to achieve the correct height of the cylinders in relation to the crankshaft.

May 3-7, 2000

Here is the material for the crankshaft. The first two tries for a crankshaft were failures. Finally I changed the three steel pieces to brass and used a lower temperature plumber's solder.

Here's the result:

At right is the crankshaft mounted in the bearing blocks, which still need cleanup and finishing, and oil holes on top.

May 14, 2000: Completed the crankshaft couplings (for want of a better term - I'm brain-dead this evening) and soldered them to the piston rods. Then I mounted them onto the crankshaft to see how they worked. They work just fine. Steps to complete, in order, were:

1. Lay out and drill the holes for the four clamp screws (2 for each coupling).
2. Cut the bar lengthwise and cut off the two couplings.
3. Tap the lower half of each 2-56.
4. Enlarge the top holes to clear 2-56 screws.
5. Screw together the two halves of each coupling.
6. Drill and ream 3/16" crankshaft hole in each coupling.
7. Mill each coupling to 1/4"x1/4"x3/4".
8. Slit the end of each coupling to accept piston rod.
9. Silver solder the lower half of each coupling to its piston rod.
10.Mount the couplings/piston rods to the crankshaft.

Here are a few pictures:

1/2" brass bar stock. I would have liked to have started out with something smaller - like 3/8", but this is what I had. The photo shows the prick-punches for the clamp screws.

Couplings near completion. I still have to mill them to their outside dimensions, slit the ends and silver solder them to their crankshafts. Time to build and install the couplings - about 10 hours.

The far coupling is mounted. The near one is in place, with the top piece still to be screwed on.

It's beginning to take shape! I need to take 1/4" off the bottom of the cylinder block to bring the cylinders in line with the crankshaft.

Also, I will shorten the piston rods to 3", to bring the cylinder closer to the crankshaft. Length shown is 6". I went with the long lengths at first because I wasn't sure what I wanted the final length to be. Same thinking with the crankshaft. The pictures don't show it, but it extends 2" beyond each bearing. I won't trim those ends until I decide what kind of flywheel(s) I'm going to hang on it.

So what's next? Eccentrics (2) for the valves. These will go on the crankshaft just inboard of each bearing block. Then the valve rods and then the valves themselves. And finally, the air intake. Oh yeah, and the flywheel(s). Maybe one, maybe two. When I get it all done and operating, I'll take it apart and clean up each piece. I'm leaving the base plate long, as I may want to build a boiler and mount it behind the engine.

May 20, 2000: A busy day today. Mounted eccentrics and built/mounted flywheels.

Eccentrics: I cheated. I had two eccentrics from a previous project, and their offset is right for this one. Here's a shot of one of them.

Flywheels: This was fun. I started with two 3" diameter aluminum rounds, each 1" thick. I faced one end and reduced the diameter by 1/16", to true the round. Then I drilled the center hole and reamed to 3/16", to fit the crankshaft. From then on, the operation was sort of like whittling.

You remove everything that doesn't look like a flywheel, and what's left is a flywheel. I reduced the inner section to a depth of .2", leaving a hub and a 1/4" rim. Then I took about .005" off the rim, leaving the hub extending beyond the rim so the rim would have clearance from the bearing block. Then I reversed the piece in the chuck, and with facing cuts reduced the rim to .6", and cut that side the same way as the other. The result was a .6" overall thickness, with the "inner" thickness of .2".

This would have been the perfect opportunity to learn how to center my rotary table, which has been gathering dust since I bought it. But I forgot about it. Instead, I painted one side with Dykem, and with a piece mounted in the chuck I scribed a circle to be the centerline of the hole pattern. Then I located 3 of the 6 holes by using the chuck jaws as stops. I located the other 3 holes with dividers, scribing an arc from hole A to hole B and another arc from hole B to hole A and scribing a line between the two intersections of the arcs. The center of the 4th hole was where that line crossed the scribed circle. I did the 5th and 6th holes in a like manner. I drilled the holes progressively larger, with the final size of 1/2".

Here's the overall progress so far. It's beginning to look like a steam engine! Note the two eccentrics located just inboard of the bearing blocks. They will drive the valves that port the steam (or compressed air) to compression and exhaust.

May 20-21, 2000

Today I built one valve and roughed out the two valve rods. The picture shows the valve and one of the valve rods. The valve was made of 3/16" brass rod. It was slightly oversize, so I spun it in the lathe with a sandpaper wrap, testing it frequently in the hole. About five minutes of this brought it down to a tight fit. Then I scrubbed it with S.O.S. That removed the sandpaper scarring and brought it down to a nice sliding fit. I slit and drilled one end for the valve rod to fit with a wrist pin (small brad for the moment) and shaped the other end as shown. The last half-inch is strictly for show; it has nothing to do with the operation of the valve. But I think it will look kind of neat, going in and out the back end of the cylinder.

The valve rod will be shortened to 2-3/4", with a tiny hole at the narrow end to accept a wrist pin connecting it to the valve.

The half-inch hole was drilled with a stepping bit. I learned about those after slightly mangling my hand trying to drill 1/16" thick material with a regular drill bit. The piece was sucked up the bit and spun with it. A stepping bit helps prevent this.

After drilling the hole, I shaped the piece with the rotary part of a rotary/belt sander. A tough job, but easier than using a file!

I'll build the other valve after I've found that this one works. If not, it's back to the drawing board!

May 22, 2000: It runs! I finished the valve and linkage for number 1 cylinder, and decided to test it before building the other valve. Since I don't yet have a plate on the back of the cylinders, I put my thumb over the back of the "operating" cylinder, and punched the air to it. It went like a banshee at 30 p.s.i., and ran sedately when I throttled back to 5 p.s.i.

In the next couple of days I'll build the other valve/linkage assembly and put on a backing plate. Then I'll have to build a "divider" to send air to both valve ports at the same time.

When I'm satisfied with it's operation, I'll tear it all down and clean and polish the parts -- possibly paint some of them; I haven't decided on that yet. Meantime, here's a picture of the engine in its present state:

Notice the empty space on the base behind the engine. I'll mount some sort of a load there to make it earn its keep. I'm thinking of a toy electric motor operating as a generator to drive a model train set's street light.

May 23, 2000: I built the other valve, with a slight dimension change. When I tested it, that cylinder ran stronger than #1. Now I'll have to make a new valve for #1. Cut the back plate, out of 1/16" brass plate. Haven't drilled the holes for the mounting screws yet, because I ran out of time. I shut down at 9:00 p.m. because my downstairs neighbor's bedroom is right below. Sometimes I'd prefer a house to an apartment -- not often, but sometimes! No point in adding another picture; it would only show a 4th rod between the crankshaft and the cylinders.

Tomorrow (if I'm still this pumped) I'll attach the backing plate and make a "T" divider to put air on both cylinders. If I'm lucky I'll have time to test it before "quiet hours."

May 24, 2000: I made the backing plate and installed it. Strictly a no brainer. I dimensioned it to fit between the valve cylinders but covering the piston cylinders, as shown. After drilling the screw holes oversize, I glued the plate to the cylinder block and drilled through the plate holes into the block, and tapped the cylinder block holes 6-32.

Then I broke the glue bond, cleaned up the inner side of the 1/16" brass plate and smeared a light coating of epoxy glue in places appropriate to make a gasket. Then I screwed the plate into place, as shown.

May 25, 2000: "T" junction box (manifold) to send air to both cylinders. This took a while. Like about 3 hours (ok, I'm slow!). I made it out of a piece of 1/2"x1"x2" aluminum as shown.

If I'd had a piece of 1"x1"x2", I could have made it as one piece. As it is, I had to settle for two integral output ports and a "screw-in" input port. As they used to say -- many years ago -- "That's the way the cookie crumbles." I made the input port out of 1/4" brass round.

Here's the way it will mount to the cylinder block, held down by two 4-40 screws. The output ports will be joined to the cylinder input ports with plastic tubing. It would have been nice to join it with copper tubing, but then I would not have been able to tune the engine.

To tune a cylinder you have to be able to look through the input port and observe the valve position. If I later have to retune it, I need to be able to remove the input tube to do this. That's the reason for the plastic tubing.

June 1, 2000: It's done, up and running! I'm really pretty tickled with it. By stopwatch, it's ticking along at just a little over 100 rpm, on about 5 p.s.i. I suspect when it's broken in I'll be able to keep it going on lung power - as long as the lungs hold out, that is.

Anyway, here's the final photo, with the little guy chugging along. I'll be bringing a camera home from work tomorrow that will let me take movies. If I can get the file size down to a reasonable level, I'll post a half-minute or so of it in operation.

Click HERE to see it go. (2MB).

Plans for The Siamese Twins in .pdf can be downloaded through the link below. To download, right-click on the link and follow the directions displayed.

Siamese Twins Plans

Attempting to read the plans without downloading can give you an error message.

Other Builder's Versions

Several people have built the Siamese Twins from my plans, and this is a real rush and an education as well, because their modifications have given me ideas for future projects. Some of these versions can be seen at the following link:

OtherSiamese