Mar 10, 2017

[2.70] Seek and Geek #5: Ball Bearings

I have two unanswered questions about ball bearings:
  1. How do you mass-manufacture precision balls and rings cheaply enough to sell them inexpensively?
  2. How do they press-ft the balls in, anyway?
Random photo from

  1. A ton of grinding
  2. They actually rivet the two sides of the cage together, so there's actually not a press fit with the internals.

Discovery Channel's How It's Made provides all your educational needs

Grinding seems to be the default manufacturing process to achieve good surface finish with close tolerances. Grinding wheels themselves are made from pressed and bonded-together coarse aggregate, so you'd expect them to be the least precise tool to create a precise surface. 

What saves the grinder is that the grinding wheel doesn't have to be precise; it just needs to be trued to make the wheel concentric. Having low-spots on the wheel that don't make contact with the part surface is fine; as long as something is making contact, the grinding wheel is working. There's also a bunch of documentation on acoustic-based methods for determining whether the grinding wheel is making contact with the part surface, so CNC operations can compensate for positioning errors.

Ball bearing balls start out as thick steel wire, cut into short bits and smashed together in a press to form a ball.
The flash gets torn off in a coarse grinding process that rattles the balls around between cast-iron disks (the flash is a skinny cantilever, so it fails before the rounded parts of the balls are affected). Then the balls get heat treated, and fed through progressively finer grinding disks until they are mirror-smooth and round. A convenient thing about desiring perfect spheres is that allowing the balls to randomly rotate within the grinding process only helps to remove high-spots. The longer balls are left in the grinding/polishing process, the more precise they will be.

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