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Truing a Piston to a Piston Rod

I started building steam engines with single-acting wobblers. My first ones had piston/rod combinations that were a single piece of brass. Later I made them of two pieces -- a piston and a threaded rod screwed into it. I carried this construction over into double-acting mill engines, and immediately ran into a major problem.

I'd turn the piston to size, then drill and tap the hole for the piston rod. Then I'd cut threads in the rod, screw the rod into the piston and seal it, either with LocTite or jamming the threads. This worked just fine, except for one thing: alignment. The threading operation would skew the piston rod just enough so that it would jam in the hole, as shown in the drawing. I could rotate the piston and rod to find a "sweet spot" in which the rod would move freely, but that spot was hard to find and hard to keep during the rest of construction.

I'm a slow learner.I built several engines this way until I finally got fed up and figured out another, better way of doing it. It turned out that the "better way" was a simple change in the sequence of machining operations -- such a simple change, with such dramatic results! This is how I do it now:

1. Drill and tap the hole in the base of the piece that will eventually be the piston.

2. Thread the piston rod to fit the hole made in step 1.

3. Screw the rod into the piston and seal it.

4. Chuck the assembly by the piston rod and turn the piston to size.

When the assembly is built in this sequence, skew is eliminated, and the "sweet spot" is all 360 degrees.

So simple I'm embarrassed to have not seen it from the beginning.

Building a Crankshaft

An engine (in my case, a steam engine) with multiple cylinders in parallel needs a crankshaft with a number of throws equal to the number of its cylinders. Such a crankshaft can be turned from one piece on the lathe, or built up from several pieces. With the limitations of my shop (mainly, between my ears) I find building up from several pieces is the better option.

The major problem in building up a crankshaft is making sure it runs true. Here's how I do it: (Note that this is not my original idea; it came from a book or magazine article I read years ago. I cannot find that write-up anymore, so I decided to write this in the hope that it might be of benefit to someone.

This crankshaft is designed for a two-cylinder engine with a 1/2-inch throw. I'm making it entirely of brass, because that is what I had in the dimensions I needed. The crankshaft diameter is 3/16 inch, and its "throw pins" are 1/8 inch diameter.

I started with a 2-inch piece of 5/8" diameter brass round, and located the offset holes as shown to the right, using a blue "Sharpie" pen as layout dye. (I've found these pens are more convenient than traditional layout dye.")

Then I placed the workpiece in the 3-jaw and drilled and reamed the center hole 3/16" as shown below:

Next came the offset holes. I transferred the workpiece to the 4-jaw, and offset it for these reamed 1/8" holes.

When these were done, I sliced off three pieces, each 1/8" wide, as shown below. There was enough of the workpiece left over for me to make a similar crankshaft for a future project.

The finished pieces looked like this (below). Since all three holes were drilled through one piece, they have no choice but to line up during assembly. Note the slight champher in the holes. This will facilitate soldering, allowing a strong joint without flowing out onto the shafts.

Then I soldered what was to be the "center" disk to the shaft, and spaced the other two disks 5/16" to either side.

The next step was to space the two "outboard" disks 5/16" off from the center disk, and insert 1/8" pins as shown at left. The pins forced the disks into alignment for soldering to the main shaft. I soldered the disks to the shaft, and then soldered the pins to the disks. At that point, all that remained to be done was to trim the pins and cut out the two sections of the main shaft.

And here's the crankshaft complete. It still needs a little file work to make the disks and pins smooth, as any roughness would tend to bind the connecting rods in operation.

It took me several tries to make my first crankshaft this way, but it has since become routine.