Tolerances measurement systems and quality control — ISO standards in practice

A drawing without tolerances is useless in precision mechanics — because without them you do not know how much a dimension can deviate before the part no longer fits or works. Tolerances are not bureaucracy; they are the precise description of what 'correct' means for this particular part.

§General tolerances per ISO 2768

Not all dimensions on a drawing have an individual tolerance marked. For them, a general tolerance usually applies, often indicated according to ISO 2768, which divides precision into the classes fine, medium, coarse and very coarse. The class is typically written in or near the drawing's title block (for example 'ISO 2768-f' for the fine class), and it tells you how much an unmarked dimension may deviate depending on the dimension's size.

§The ISO fit system: fits between holes and shafts

When a part needs to fit with another—a shaft in a bearing, a pin in a hole—the ISO fitting system (ISO 286) is used, which specifies tolerance classes with letter and number, for example H7 for a hole or g6 for a shaft. The letter indicates the tolerance deviation's position, the number its size (IT grade). The combination H7/g6 is a classic example of a loose running fit, while other combinations give press or shrink fits. Being able to read and choose the correct fit is a core competence when you design or assess an assembly yourself.

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No measuring instrument measures completely accurately – there is always measurement uncertainty. The rule of thumb in quality engineering is that the instrument's uncertainty should be significantly smaller than the tolerance you need to check, a factor of around ten is often recommended. That's why you use a micrometer screw for a tolerance of a few hundredths, while a calliper is sufficient for rougher dimensions. Using an instrument that's too imprecise for a tight tolerance gives a falsely secure result.

§Calibration – the basis for being able to trust the measurement

A measuring instrument that is not calibrated can show a wrong number without anyone noticing it. That is why measuring equipment is calibrated at regular intervals against reference standards with known traceable accuracy and the result is recorded in a calibration log. At companies with quality certification (e.g. ISO 9001) calibration intervals and documentation are a fixed requirement — but even without certification it is good practice to know when your calliper was last checked.

To master tolerances is ultimately about being able to read a drawing and convert it into a clear decision: is this part good enough or should it be scrapped? That decision must be based on a calibrated accurate measurement result — not on a sense that it probably fits.