A Tiny, Spoked Flywheel

September 28, 2003

My flywheels for small steam engines have always been simple affairs -- a single piece of aluminum or brass round with a centered hole for the crankshaft, and a few other holes to break up the monotony. The good thing about them is they take just minutes to build. But boring.

This weekend I decided to try out my new Sherline indexer. The idea was an aluminum hub, eight brass spokes and an aluminum rim. Couldn't figure out how to drill from the inside of the rim to accept the spokes, so the spokes wouldn't show on the outside of the rim. (Still haven't figured that one out; I suspect it's not possible.) Finally (I'm a slow thinker) I got the bright idea of drilling the holes from the outside, but adding a steel tire to the rim. This would cover the holes and add weight to the flywheel, which would smooth out the operation of whatever steam engine I may choose to put it on.

This stuff is old hat to experienced machinists. But a rundown on its construction might be of interest to beginners (like me).

The Hub

I turned the hub to 7/16" and center-drilled/reamed a 1/4" hole for the crankshaft. I left the piece 1" long, to allow drilling for a setscrew to hold the piece to a 1/4" shaft that would be mounted between centers in the Sherline indexer. When the piece was properly mounted, I used a center drill in my Taig mill to start eight holes at 15-degree intervals. Then I went around again, drilling 1/8" holes through and into the sacrificial shaft.

Then I cut off the piece to 5/16" long. Probably I should have gone to 1/2", to make the hub wider than the rim. This would have allowed clearance for the flywheel mounted on an engine. But what can I say? I was planning this as I went along!

The Rim

At this point I started to take pictures. (Some are better than others.) I prepared the rim in much the same way as the hub, except I used a piece of 1-1/4" aluminum round. Turned it down to 1-3/16" to ensure concentiricity, and drilled a 1/4" center hole to mount the piece in the indexer. As you can see in the picture, I left some scrap material to anchor the workpiece to the shaft.

I drilled the eight holes as described above, then bored it out to 1-1/8". Finally, I cut it off at 3/8" wide.

Then I cut eight pieces of 1/8" brass to 3/8" long. These were long enough to seat in the hub all the way to the crankshaft, but short enough to not protrude through the outer surface of the rim.

Assembly

While building these pieces I was trying to figure out how I was going to assemble them to keep the hub and rim concentric. The answer was like a bolt from the blue, and made me feel "intellectually challenged" because it was so simple. I built a jig, as shown below:

	I already had the piece on the right. I'd made it as a workpiece holder for powder-coating another project. I turned the other piece down for a slip fit with the inside of the rim
The hub went on the shaft and the rim fitted over the jig. Voilla! The two pieces were held in alignment while I inserted the spokes and anchored them with Loktite.

This left only the tire to do. I turned this out of 2-1/4" steel round, down to 2-1/16". Couldn't seem to mount it straight in the 3-jaw, so turned it down that far to get rid of the "wobblies." Then I bored it out to slip-fit the outside of the rim, cut it off to 3/8", and powder-coated it red. I bored it slip-fit rather than press-fit because powder-coating would reduce the size of the bore. Press-fit would become no-fit!

And here's the finished product. The job, including this writeup, took 15 hours all told (ok, I'm slow) and pretty much ate up the weekend. But what the heck? It kept me out of bars and off the streets. What more can you ask? Besides, this stuff is fun!