ISO tolerances and fits: why Ø30H7 matters

A drawing that just says Ø30 is useless in production — so how precisely should the 30 mm be hit? All dimensions have a tolerance and the standard for that is DS/ISO 286. The system makes it possible to describe precisely how much a dimension can deviate and to get a hole and a shaft to fit together in a predictable way. Being able to read and apply tolerances is a fundamental skill you use from the first part to the journeyman exam.

§Letter and number: position and width

A tolerance designation consists of a nominal dimension, a letter and a number — for example 30f7. The 30 is the nominal diameter. The letter f specifies the tolerance position relative to the zero line that is whether the tolerance range is above, below or around the nominal size. The number 7 is the tolerance class (IT7) — the width of the permitted margin. Capital letters are used for holes (H, G, F …) small letters for shafts (h, g, f …). It is a simple but important convention to keep track of.

§IT grades: from coarse to fine

Tolerance classes run from IT1 to IT18. Smaller number finer tolerance narrower allowed clearance. IT01 and IT1 belong to measuring blocks and gauges while IT5 to IT10 is what normally work with in machine manufacturing. Coarse classes from IT11 onward used for parts where dimension is not critical. Note that same IT grade gives larger actual tolerance in millimeters the larger the part is — an Ø200 H7 has wider window than Ø10 H7

§H-hullet som referencesystem

I det mest brugte system — enhedshul — er hullet altid H. For alle H-huller er den nedre afvigelse EI = 0, altså placeret præcis på det nominelle mål, mens den øvre afvigelse ES svarer til toleranceklassens bredde. Et Ø50H7 har for eksempel grænserne 50,000 til 50,025 mm. Så holder man hullet fast som reference og vælger akslens toleranceposition efter, hvilken pasning man vil have. Det er langt lettere at bore og reife et standardhul end at ramme et skævt tolerancevindue.

§Three types of fits

The combination of hole and shaft gives one of three fit types. A clearance fit always has air between the parts — they can slide like a shaft in a bearing. A transition fit is on the border: the parts can have either a small clearance or a small press, depending on where in the tolerance they actually end up. A press fit always has excess — the shaft is larger than the hole and must be pressed or shrunk in like a gear wheel that must not rotate on its shaft.

• 01Game clearance (e.g. H7/f7) — there is always clearance; the parts move freely
• 02Transition fit (e.g. H7/k6) — slight clearance or slight press; precise centring
• 03Press fit (e.g. H7/p6) — always oversized; parts fit securely without a key
• 04Always choose the fit based on the function — not the other way around

§Tolerance costs money

A finer tolerance is more expensive to manufacture: it requires better machines sharper tools slower finishing and more measuring. Therefore it is poor craftsmanship to put IT6 on everything for safety's sake. The good designer and good technician choose the coarsest tolerance function allows and only tighten where it really matters — typically on fitting surfaces and bearing seats. Understanding the economics of tolerances is just as important as being able to achieve them.

“A tolerance is a promise to the next person in the chain: this surface will be this precise — no more and no less.”