Drill bit geometry has several characteristics: The spiral (or rate of twist) in the drill bit controls the rate of chip removal. A fast spiral (high twist rate or "compact flute") drill bit is used in high feed rate applications under low spindle speeds, where removal of a large volume of chips is required.
Cut a cube of rock and then trim off all the corners with your saw. Use a coarse grinding wheel to smooth it more towards a sphere. Put your lumpy ball on the plug (that is on the spindle, bolt end down and round edge, which should be cup shaped, up) and hold it down with one end of the brass elbow.