Fits, Limits, and Tolerances

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  • This page goes over Fits, Limits, and Tolerances, and the application of them in design
  • Essentially nothing is perfect, and getting something closer to perfect is costly/time intensive (see degrees of flatness / Lapping and whatnot) so you define what is acceptable for the application (ie specific hole for a shaft, etc)
  • By doing this ahead of time, you take the need for "fiddling" out of the assembly / iterative design process
  • Press Fit is a common example
  • A Fit is a relationship between two components, typically based on a Hole and a Shaft, such as a bolt or rivet holding components together, a rod in a bearing, or a piston in an engine
  • The Limits are the upper and lower extremes of what is within the desired tolerance
  • The Tolerance is the total permissible variation of size
  • The Nominal Size is what is used in the CAD (ie the perfect/impossible dimension that you get as close to as the tolerance dictates)

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