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Why no measurement is perfectly exact — and how you control the deviation
A measurement on a drawing looks exact but in reality no production process can hit a measurement exactly. There will always be a small deviation. A tolerance specifies how large that deviation may be before the part is no longer usable. As a technical designer it is your responsibility to set tolerances so parts can be manufactured realistically and still work together.
Imagine a shaft that must fit in a hole. If both parts were required to be made completely accurately, production would be unreasonably expensive — and still impossible. Instead, a permitted interval is specified for each dimension. As long as the shaft and hole each stay within their interval, they will fit together in the desired way.
When two parts must be assembled, the fit describes the relationship between them. In a clearance fit there's always space between the parts so they can move. In a press fit the parts are always slightly too large for each other so they sit tight. A transition fit sits between and can give either a bit of clearance or slight overlap. Which fit you choose depends entirely on what the joint must be able to do.
| Fit type | Proportion of parts | Typical purpose |
|---|---|---|
| Game passing | There is always space between | Parts that must be able to rotate or slide |
| Transition fit | Can give a little play or a little overlap | Exact placement without much need for movement. |
| Press fit | The parts are too large for each other | Fixed assemblies that don't need to be taken apart often. |
Most dimensions on a drawing do not need an individual tolerance. Instead the drawing header refers to a general tolerance standard that applies to all dimensions without special specification. Only dimensions critical to function receive a specific tolerance. Similarly surface requirements (roughness) are only specified where it matters — for example where two parts slide against each other.
When multiple measurements are chained together, their deviations add up. This is called tolerance stack-up and can mean the overall length varies more than expected. That's why you set critical dimensions against a common reference (baseline) instead of in a long chain, so one deviation doesn't build on the previous.
“A tolerance is not a sign of inaccuracy but a deliberate decision about how precise a part needs to be.”
— Principle in tolerance theory