ISO fit tolerance basics: choosing H7/g6, H7/h6, H7/k6 or H7/p6 — with IT grade table

Reading a tolerance zone: what does Ø25 H7/g6 mean?

Ø25 H7/g6 describes a 25 mm hole and shaft fit: upper-case H7 is the hole's tolerance zone, lower-case g6 the shaft's. Each zone is a letter plus a number. The letter places the zone relative to the nominal size (the zero line) — the fundamental deviation; the number is the IT grade, setting the zone's width.

• An H hole has a lower deviation of 0 (it can only be larger than nominal); an h shaft has an upper deviation of 0 (only smaller).
• Shafts a–g are below nominal (clearance); k–zc are above (transition or interference); js is symmetric.
• A smaller number is tighter and more expensive: IT6 is narrower than IT7.

At Ø25, an H7 hole is 25.000–25.021 mm and a g6 shaft 24.980–24.993 mm. JIS B 0401 and Taiwan's CNS 4 are identical to ISO 286.

IT tolerance grade table (ISO 286-1)

Tolerance unit i = 0.45 · ∛D + 0.001 · D (µm)　IT6 = 10i　IT7 = 16i　IT8 = 25i　IT9 = 40i　IT11 = 100i

D is the geometric mean of the size range limits in mm. Standard tolerances for common ranges, in µm (1 µm = 0.001 mm):

• IT5–IT6: precision shafts, bearing seats, ground surfaces.
• IT7–IT8: precision holes (reamed, fine-bored), gear and pulley bores.
• IT9–IT11: general machining, loose fits, non-critical sizes.
• IT12–IT14: rough machining, sheet metal, castings and typical 3D printing.

Common hole-basis fits: H7/g6, H7/h6, H7/k6, H7/p6

The hole-basis system fixes the hole at H and varies the shaft zone to change the fit. It is the most common system because holes are made and checked with fixed reamers and plug gauges, while shafts are easy to turn or grind to any size. Values are minimum to maximum clearance (positive) or interference (negative), in µm:

The shaft-basis system fixes the shaft at h and varies the hole. It suits one plain shaft carrying several parts with different fits, cold-drawn bar (h9, h11) and standard dowel pins (ISO 8734 are m6). Linear bearings typically run on g6 or h6 shafts.

Judging clearance, transition and interference fits

Max. clearance Smax = ES − ei　　Min. clearance Smin = EI − es　(negative = interference)

ES and EI are the hole's upper and lower deviations, es and ei the shaft's. For Ø25 H7/k6: hole 25.000–25.021 mm, shaft 25.002–25.015 mm. Max clearance = 25.021 − 25.002 = 0.019 mm; min clearance = 25.000 − 25.015 = −0.015 mm, i.e. 0.015 mm max interference. Either can occur, so it is a transition fit.

• Smin ≥ 0: clearance fit — any in-tolerance pair assembles freely.
• Smax ≤ 0: interference fit — always needs pressing or shrink fitting.
• Otherwise transition — the result depends on the actual sizes; usually assembled with a mallet or light press.

Thermal expansion ΔD = α · D · ΔT (steel α ≈ 11.5 × 10⁻⁶ /°C)

TIPInterference fits can be assembled by heating the hub: a Ø25 steel hub heated by 150 °C grows about 0.043 mm — more than the 0.035 mm maximum interference of H7/p6, so it slips on. The ISO fit calculator returns deviations, clearances and fit type for any size.

Bearing fits: the 6204 example

Rolling bearing bores and outer diameters are not H or h — per ISO 492 they sit below the zero line. A normal-class 6204 (20 mm bore, 47 mm OD) has a bore deviation of 0 / −10 µm and an OD of 0 / −11 µm, so a k6 shaft is tighter in a bearing than in an H7 hole.

• Rotating inner ring, normal load: shaft k5 / k6. Ø20 k6 is +2 / +15 µm, giving 2–25 µm interference with the 0 / −10 µm bore — always tight.
• Heavy or shock loads: m6; light loads or easy removal: js6 / h6.
• Stationary outer ring: housing H7 (Ø47 H7 is 0 / +25 µm, 0–36 µm clearance) or J7.

TIPA printed bearing seat can't hold H7. Open the bore by 0.1–0.2 mm and print a test ring, or press in a metal bush. The bearing generator models 608, 6000 and 6200 series bearings for clearance checks.

3D printing vs CNC machining tolerances

• When a printed part needs an H7 hole, print it 0.2–0.3 mm undersize and ream it, or press in a metal bush or bearing.
• For printed fits use process clearances instead — slide about +0.2–0.3 mm, press about −0.05–0 mm on FDM (see the 3D-printing hole tolerance article).
• When ordering CNC parts, put H7, g6 etc. only on critical fits and use general tolerances ISO 2768-m elsewhere (±0.2 mm for 6–30 mm) to cut cost.