The most important aesthetic criteria I have for a flywheel is that it runs truly true. I want my flywheel to appear to be a stationary blur, if that makes sense. So I strive to machine all surfaces in one setup, and a setup representative of how the flywheel mounts to the engine. As far as the functional design, I follow George Britnell’s advice: “The flywheel in total doesn’t need to be heavy but rather just the weight at the rim. Most flywheels (hit and miss type) are spoked. This is to reduce the overall weight while still providing the force needed to overcome the compression. If the flywheel is too light the engine will still run but it won’t run slowly. If the flywheel is too heavy the engine will run but won’t respond to throttle openings properly.”
I liked the look of my finished gears with the six lightning holes, so I went with that design here. I also like to use a separate split taper collar as opposed to tapering the crankshaft. In the picture below I have highlighted the taper collar, unfortunately it is almost the same color as the flywheel and hard to discern. I use the compound to cut the 10 degree inclusive angle on the flywheel and split taper collar, and use the same setup to do both to insure the angle is identical. I make the taper collar first, then leave the 5 degree offset in the compound, machining the flywheel using just the cross slide until the internal taper is cut.
(Bonus question,”What is wrong with the pistons in the above model?”)
Design of the flywheel and split taper collar.
I face the front of the flywheel to create a relatively flat side that I then mount in the centering 4 jaw chuck. I drill the center hole to the small diameter of the taper collar, then cut the taper into the flywheel, using the collar to test fit until it is just even with the faced surface (back side).
I fabricate a mandrel to the same dimensions as the crankshaft. Then I mount the flywheel just as it will be mounted on the engine.
At this point I machine the complete flywheel in this one setup. I even take a tiny skim off the back of the flywheel as deep in the X direction as I can without hitting the collet nut, to true this edge as well.
Then it is over to the mill to drill out the large lightning holes. I drill these in five increments not including the center drill. I use parallels underneath to support the work piece, moving them to insure they don’t get hit with the drill bit. I use aluminum packing to protect the outer surface.
Here are a couple of pictures of the flywheel test fit on the engine.
The flywheel ended up weighing 19 ounces. I still need to balance it and I think I would like a larger chamfer on the center mounting spigot.