Hidden from sight, ball bearings are key to almost all devices that spin or roll: power plant turbines, steering columns, wheels, skateboards, yo-yos, dentist drills, and the electric motors in everything from refrigerators and can openers to computer hard drives and CD players. In each case, the balls allow efficient, low-friction movement of rotating parts. Each sphere must be perfect, or the motion it facilitates will come to a grinding halt. Yet manufacturers produce them by the millions for pennies apiece. How do they make the balls so incredibly round and smooth?

Metal, ceramic and plastic balls ranging from an ounce to hundreds of pounds share a similar genesis. An initial plug is cut from wire or extruded and is then stamped by a press into an approximate sphere. That globe is massaged into greater roundness between heavy plates and honed to exact sphericity and smoothness by rolling through grindstones hundreds, if not thousands, of times. The same process creates pellets for air guns and balls for ballpoint pens and roll-on deodorant.