Tolerances and fits: when dimensions must fit together
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.
§Why tolerances exist at all
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.
§Basic concepts
- 01Non-invasive blood pressure (NIBT)
- 02Upper and lower dimensions: the permitted deviations up and down from the nominal dimension.
- 03Tolerance: difference between upper and lower limit dimensions — that is width of allowed interval
- 04Limit dimensions: the largest and smallest dimensions the part may have.
§Fits: clearance, transition and press
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 clearance (e.g. H7/f7) — there is always clearance; the parts move freely | 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. |
§General tolerances and surface requirements
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.
§Tolerance stack-up
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