One of my first high school metal shop projects was to make a flywheel
weight for my Suzuki trials bike. I had found a 5 lb barbell weight
while delivering my paper route.
Took it to the shop and started hogging away on that chunk of cast iron.
Mostly getting little chips I asked the teacher how to speed it up.
Slow down the lathe and increase the pressure. That worked. Took a
couple days to get the contour just right so it would fit in the side
cover. It cameout nicely balanced and the bike was much more fun to ride