When I was making the Kinematic Coupling for Lab-4, I was wondering how I would manufacture the wooden balls. In the first day of class, Prof. Slocum had given us a wooden hemisphere which looked almost perfect.
The most number of balls that are used anywhere in the world is in ball bearings. They are mostly made out of steel, but ceramics bearings also exist.
Here is a very nice YouTube video describing the entire ball-bearing manufacturing process for a deep-groove ball bearing:
Making of steel balls in more detail:
Also, this would be challenging for me to do in the MIT Hobby shop, so I found another way to be able to turn the wood into a sphere:
Finally, I used wooden balls given to me by a class-mate Oscar Viquez, which he had left over from his 2.75 class last semester.