Fits & Limits (ISO 286)

Fits and limits under the ISO 286 standard are how an engineering drawing controls the relationship between two parts that must go together — a shaft in a hole, a pin in a bore, a bearing on its seat. A plain hole dimension and a plain shaft dimension, each with its own tolerance, do not guarantee anything about how they will assemble: two loose tolerances could meet as a sloppy clearance or as a press that will not go together. ISO 286 solves this by defining pairs of tolerance zones, one for the hole and one for the shaft, chosen so that the assembled fit is a known thing. A call-out such as 25 H7/g6 names the whole arrangement at a glance, and reading it — knowing what the letters and numbers mean, what fit the pair makes, and what operations will hold it — is a daily skill of machining.

What the letters and numbers mean

Every ISO 286 tolerance zone is named by a letter and a number, and each does a different job. The letter — the fundamental deviation — fixes where the tolerance zone sits relative to the nominal size: whether the allowed sizes lie above, below or straddling the nominal dimension. The number — the IT grade — fixes how wide the zone is: the size of the tolerance band itself, running from the very fine IT grades of gauge work to the loose grades of rough casting. Lowercase letters name the shaft, uppercase the hole, so in H7/g6 the H is the hole and the g the shaft. Two conventions do much of the work. The letter H denotes the basic hole, whose tolerance zone starts exactly at the nominal size and extends above it; and the grade 7 sets how far above — at 25 millimetres, some 21 micrometres, giving the hole limits of 25.000 to 25.021.

Reading the pair

Reading the full call-out means resolving both partners into their actual limits. The shaft letter g places the shaft zone a little below nominal, and its grade 6 gives a band of about 13 micrometres at 25 millimetres, so the shaft’s limits fall out as 24.980 to 24.993. The fit between the pair is then arithmetic: the largest shaft meets the smallest hole at a clearance of seven micrometres, and the smallest shaft meets the largest hole at one of 41, so 25 H7/g6 always assembles with a small running clearance. That is the meaning of a fit call-out — not two separate tolerances but one controlled relationship, expressed through limits a machinist can cut to and an inspector can measure. The drawing should carry both the code and the resulting limit values, so the pair always agrees and the shop does not need a table in hand to set up the job.

Clearance, transition and interference

The three families of fit describe every possible meeting of hole and shaft. A clearance fit leaves the shaft always smaller than the hole, with a guaranteed gap — the world of parts that must move, slide or run: the g, f and h shaft letters against the basic H hole. A transition fit lets the two zones overlap, so a particular pair may assemble with a whisper of clearance or a whisper of interference depending on the actual sizes that happen to meet — the j, k and n letters, chosen where a part must be located precisely without play, and accepted for the small uncertainty it carries. An interference fit makes the shaft always larger than the hole, so assembly needs force, heat or cold — the p and beyond letters, chosen where the joint itself must carry load through friction. The drawing’s choice among the three is a functional decision: does this pair slide, locate, or grip?

Hole basis and the common fits

Most machining drawings use the hole-basis system, the basic H hole mated with whatever shaft letter gives the needed fit, and the reason is practical economy. A hole is made with fixed tooling — a drill, a reamer, a boring bar set to one size — while a shaft on a lathe can be turned to any size the drawing asks, so it is cheaper to hold the hole at one standard grade and adjust the shaft to suit the fit than the reverse. A handful of designations does most of the work. H7/g6 is the close-running or sliding fit of guides and locating pilots; H8/f7 the running fit that leaves room for an oil film on a journal bearing; H7/k6 the transition fit that locates a part positively, as for a dowel or a gear on a shaft; and H7/p6 the light press fit that holds a bearing race or a hub without extra fastening. These few, read fluently, cover the great majority of fit call-outs a machinist sees.

Reading the fit into the machining

To the machinist, the grade in the call-out is a forecast of the operation that will hold it. A fine grade such as IT6 or IT7, with its tolerance in tens of micrometres, is the territory of reaming, boring, grinding and careful finishing, measured as it is made; IT8 and IT9 are routine CNC machining work held without drama; and the loose grades beyond are what drills and manual work deliver. The discipline is to cut the loosest grade that serves the function, because every step tighter costs time and care out of proportion to its size — and, equally, to read the fit honestly, since a call-out made on the machine must be met on the measuring instruments that check the limits, the same accuracy and repeatability this wiki treats in its own entry. Fits are, in the end, the classic meeting of drawing and machine: a code that says how two parts must relate, translated by the machinist into the sizes, operations and measurements that make them relate that way. Alongside GD&T, which governs geometry and position, ISO 286 governs the sizes of mating features — the two systems together carrying most of the tolerance language of engineering drawings.