I’m writing this post to help those in the audience that aren’t familiar with detailed mechanical design. A basic understanding of tolerances is essential to follow subsequent discussions here.
First, a quick definition:
Engineering tolerance is the permissible limit of variation in
- a physical dimension,
- a measured value or physical property of a material, manufactured object, system, or service,
- other measured values (such as temperature, humidity, etc).
- in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge).
Every manufacturing process has some variation on dimensional output and a sound mechanical design needs to account for these variations. If you ask a machinist to make you a block that’s 1″ by 1″ by 1″ you might get a block that’s 1.012″ by 0.923″ by 1.103″. Is that close enough?
Could the machinist have done a better job getting closer to the 1″ target? Probably, but since we didn’t specify a tolerance, technically it’s close enough. If we wanted something closer to 1″ per side we’d need to specify how close. That’s the tolerance. We’d say 1″ plus or minus 0.010″, for example. The 1″ dimension is called the nominal value and the 0.010″ is called the tolerance.
Later I’ll talk about tolerances associated with different manufacturing processes and environmental conditions and how mechanical engineers and product designers account for them in their designs.