§1Why limits exist
Interchangeable manufacture — any shaft from the bin fitting any hole from the bin — requires stating in advance how imperfect each may be. That statement is the tolerance.
Tolerance is also money: cost climbs steeply as tolerance tightens, because tighter work means slower feeds, better machines, more rejects and more inspection. The design skill is asymmetric — be exactly as tight as the function demands and ruthlessly loose everywhere else. A general-tolerance note on the drawing sweeps up the “everywhere else”.
Contents§2The vocabulary
| Term | Meaning |
|---|---|
| Nominal (basic) size | the shared reference — Ø25 for both members of the hero fit |
| Limits | the largest and smallest permitted actual sizes |
| Tolerance | the difference between the limits — the width of the zone |
| Deviation | a limit’s distance from the zero (nominal) line; the fundamental deviation locates the zone |
| Allowance | the intended clearance (or interference) between mating parts at their tightest condition |
| Maximum material condition | biggest shaft / smallest hole — the tightest legal assembly, the one a GO gauge must pass |
§3Three kinds of fit
Whether the zones clear, overlap or interfere sorts every fit into one of three families.
A clearance fit guarantees air between the parts across all legal combinations — bearings, slides, anything that moves. An interference fit guarantees metal overlap — pressed bushes, shrink-fitted rims, anything assembled once and expected to transmit load through grip. A transition fit may land either side — light keying and location work, where a tap of assembly is acceptable and precision of position is the point. Modern practice fixes the hole-basis convention: the hole’s zone starts at zero (letter H) and the shaft is shifted to create the fit — because holes are made by fixed-size tools (drills, reamers) while a shaft diameter is dialled at will on a lathe or grinder. One reamer, many fits.
Contents§4The ISO grade system
Two coordinates locate any tolerance zone: a number (IT grade — how wide) and a letter (fundamental deviation — where it sits). The grade widths are not arbitrary; they grow from one computed unit.
Ø25 sits in the 18–30 mm range; D = √(18 × 30) = 23.24 mm, so i = 0.45 × 2.854 + 0.023 = 1.31 µm. Hence IT6 = 13 µm, IT7 = 21 µm, IT8 = 33 µm, IT9 = 52 µm — computed here from the formula, and matching the published grade tables. Larger diameters get wider grades by the ∛D law: the same “quality of work” is a bigger absolute number on a bigger part.
The letter then parks the zone: capital letters for holes, lower-case for shafts; H starts the hole exactly at zero (the hero diagram); shafts a–g sit below zero (clearance), h touches it, j–k straddle (transition), and m onward sit above (interference). Full deviation values live in the standard’s tables — the system above is what makes those tables navigable.
Contents§5Reading a fit callout
Ø25 H7/g6 — nominal size, hole zone, shaft zone. Unpacked, it is four numbers and two guarantees.
The H7 hole: zero fundamental deviation, 21 µm grade → 25.000 – 25.021. The g6 shaft sits a little below zero with a 13 µm grade — representative limits 24.980 – 24.993. Worst-case tight: smallest hole on biggest shaft, clearance 25.000 − 24.993 = 0.007 mm. Worst-case loose: 25.021 − 24.980 = 0.041 mm. Every pair drawn from conforming bins runs, and none rattles beyond 41 µm — that sentence is the entire point of the system.
§6Choosing a fit
A handful of classic hole-basis pairings covers most machine design; choose by duty, then confirm against the tables for the size in hand.
| Duty | Typical callout | Character |
|---|---|---|
| Free running, generous oil film | H8/e8 – H9/d9 | loose clearance |
| Precision sliding / running | H7/g6 | close clearance — the hero fit |
| Location, assembles by hand | H7/h6 | line-to-line clearance |
| Location, light tap | H7/k6 | transition |
| Press fit, permanent | H7/p6 | interference |
| Interference fits carry real stresses — the Lamé arithmetic of the Plates, Shells and Cylinders page prices the grip; assembly force and torque capacity belong to the Machine Elements pages. | ||
§7GO / NO-GO gauging
Limit gauges answer the only production question — in or out — faster than any measurement, and Taylor’s principle says which gauge checks what.
Taylor’s principle: the GO gauge is made at the maximum-material limit and full-form — a full plug for a hole, a full ring for a shaft — so it simultaneously checks size and form: an oval or bent part that would jam the real assembly jams the gauge too. The NO-GO gauge is made at the minimum-material limit and deliberately checks point-to-point (a pin-ended or segmental contact), so a local undersize anywhere is caught. For the Ø25 H7 hole: GO plug at 25.000 must enter; NO-GO plug at 25.021 must not. Gauges themselves are toleranced — a common working discipline puts the gauge-maker’s tolerance at roughly a tenth of the work tolerance, inside the limits, so the gauge can only ever reject borderline-good work, never accept borderline-bad.
Contents§8Quick reference
The working core of the page on one card rack.
System
number = width (IT grade)
letter = position; H = hole at zero
Grade unit
i = 0.45∛D + 0.001D µm
IT7 = 16i (Ø25 → 21 µm)
Families
clearance · transition · interference
hole-basis by default
Classic fits
H7/g6 slide · H7/k6 locate
H7/p6 press
Gauging
GO: max material, full form
NO-GO: min material, point check