ISO Tolerances and Fits: H7, h6, Clearance and Interference

No part is ever made to its exact drawing size. A dimension of 20 mm really means “20 mm, within a stated margin”. That margin is the tolerance, and choosing it correctly is the difference between a part that assembles and one that does not — or one that is needlessly expensive.

The vocabulary

Figure 8.1 — Basic size, deviations, tolerance zone and the zero line

Basic size — the main figure written on the drawing (20).
Upper deviation — how far the actual size may go above the basic size.
Lower deviation — how far it may go below (or, if positive, the minimum amount above).
Maximum and minimum size — basic size plus each deviation.
Tolerance (T) — the difference between them; always a positive number.
Zero line — the line representing the basic size on a tolerance diagram.

A part is accepted if its measured size falls anywhere between the minimum and maximum size. It does not have to be “close to nominal” — anywhere inside the zone is equally good.

Reading a tolerance table

Values in ISO tolerance tables are in micrometres (µm); divide by 1000 for millimetres. If the table gives +25 / 0 for Ø32 H7, that is +0,025 / 0 mm: maximum size 32,025, minimum 32,000. For H holes the lower deviation is always zero, so the hole can never be smaller than its basic size. Tables are arranged in size ranges (e.g. 30–50 mm) and the upper limit belongs to the range.

Reading O32 H7 from a tolerance table: deviations +25 and 0 micrometres give maximum 32.025 and minimum 32.000 mm — Reading the table: Ø32 H7 → +25/0 µm → maximum size 32,025 mm, minimum size 32,000 mm (Turkish: En Büyük Ölçü = maximum size, En Küçük Ölçü = minimum size)

Table of nominal size, tolerance, minimum and maximum size for O32H7, O46h5, O30H8, O16h6, O12H7, O35h8 and O186h11 — The same method for other classes: H holes have a lower deviation of 0, h shafts an upper deviation of 0; limits are basic size plus each deviation

The ISO system: letters and numbers

ISO 286 describes a tolerance with a letter and a number, for example H7 or g6.

The letter gives the position of the tolerance zone relative to the zero line. Capital letters (A…ZC) are holes; lower case (a…zc) are shafts.
The number gives the width of the zone — the IT grade. IT01 is the tightest, IT18 the widest. The smaller the number, the tighter the tolerance.

Two rules worth memorizing. For H the lower deviation is always zero — an H hole is never smaller than its basic size. For h the upper deviation is always zero — an h shaft is never larger than its basic size. Everything else in the system is built around those two.

So Ø20 H7 means a hole 20,000 to 20,021 mm. Ø20 h6 means a shaft 19,987 to 20,000 mm. Same nominal size, opposite direction of tolerance.

The three types of fit

Figure 8.2 — Position of the tolerance zones in clearance, transition and interference fits

Fit	Condition	Typical pairs	Used for
Clearance	shaft always smaller than hole	H7/g6, H7/f7, H8/e8	sliding and rotating parts, guides
Transition	may be either, depending on actual sizes	H7/k6, H7/j6, H7/n6	accurate location, parts that must be dismantled
Interference	shaft always larger than hole	H7/p6, H7/s6, H7/u6	permanently joined parts, bushes, gear hubs

On a tolerance diagram the rule is easy to see: if the hole zone lies entirely above the shaft zone the fit is a clearance fit; entirely below, an interference fit; if they overlap, a transition fit.

Which fit is used where: fit character tables

Knowing that H7/g6 means a “just perceptible clearance” used for precision sliding bearings tells you what the part does. The two tables below summarise, for the hole basis system (H7 and H8 holes; H9 and H6 holes), the fit character given by the common shaft classes and where each is typically used. The tables are in Turkish; the column order is: fit type – hole – shaft – character – typical use.

Table of clearance, transition and interference fit characters and applications for H7 and H8 holes with shafts d9, e8, f7, g6, h6, js6, k6, m6, n6, p6, r6, s6 — H7/H8 hole-basis fits: d9–h6 clearance (Boşluklu), js6–n6 transition (Ara), p6–s6 interference (Sıkı), with a typical application for each

Table of clearance, transition and interference fit characters for H9 and H6 holes: H11, D10, E9, F8, G7, JS9, K7, N7, P9, P7 — H9/H6 fits: from coarse clearance to pressed interference, with character and example application

Assembling an interference fit by hammer, press and liquid-nitrogen cooling; minimum interference diagram — Interference fits are assembled by hammer, press or by cooling/heating. Minimum interference (EKS in the figure) = maximum hole size − minimum shaft size, a negative value

Hole basis and shaft basis

Figure 8.3 — How the fit types are formed in the hole basis and shaft basis systems

Hole basis. The hole is kept at H and the shaft letter is changed to obtain the fit: H7/g6, H7/k6, H7/p6.
Shaft basis. The shaft is kept at h and the hole letter is changed: G7/h6, K7/h6, P7/h6.

Both give identical fits. In practice the hole basis system is preferred, because a hole is made with a fixed-size tool — a drill, a reamer, a broach — while a shaft can be turned to any size. Changing the shaft is cheap; changing the hole means a new tool.

Shaft basis is used where a single long shaft or a standard drawn bar has to carry several different fits along its length — for example a transmission shaft with a bearing, a gear and a pulley on it.

How a fit is written on a drawing

On an assembly drawing: Ø20 H7/g6 — hole tolerance first, shaft tolerance second.
On a detail drawing: only that part’s own tolerance, e.g. Ø20 H7 or Ø20 g6.
Deviations may also be written directly as figures, e.g. 20 ^+0,021₀.
Dimensions with no tolerance are covered by the general tolerance note, usually ISO 2768-m.

Worked Example: Ø30 H7/g6 Tolerance Zones

Let us put real numbers on a fit. For Ø30, an H7 hole gets +21/0 µm and a g6 shaft −7/−20 µm. The hole zone sits above the zero line and the shaft zone just below it, so this is a clearance fit: at tightest 7 µm and at loosest 41 µm of clearance — the shaft always enters the hole, never jams:

Second example: Ø25 H7/h6 step by step

H7/h6 is the clearance fit at the “slides when lubricated” limit. For Ø25 the values are read from the 18–30 mm range: H7 hole +21/0 µm, h6 shaft 0/−13 µm. Hole 25,000–25,021, shaft 24,987–25,000. Minimum clearance 0 (smallest hole, largest shaft), maximum clearance 0,034 mm.

Step by step calculation of the O25 H7/h6 fit: hole +21/0, shaft 0/-13 micrometres, maximum and minimum sizes — Ø25 H7/h6: reading the deviations from the table, converting to millimetres and calculating the limit sizes (Turkish: Üst sapma = upper deviation, Alt sapma = lower deviation)

Test yourself

What does the capital letter in H7 tell you?
That it is a hole, and where its tolerance zone sits relative to the basic size.
What does the 7 in H7 tell you?
The IT grade — the width of the tolerance zone. Smaller number, tighter tolerance.
What is the lower deviation of any H hole?
Zero.
Ø30 H7/p6 — what kind of fit is that?
An interference fit; the shaft is always larger than the hole.
Why is hole basis preferred?
Because holes are made with fixed-size tools; it is far cheaper to vary the shaft.
A shaft measures 19,992 mm and is specified Ø20 h6 (19,987–20,000). Accept or reject?
Accept — it is inside the zone.
What governs a dimension with no tolerance written on it?
The general tolerance standard named in the title block, typically ISO 2768-m.

Next lesson. Size tolerance controls how big a feature is. It says nothing about whether a face is flat, an axis straight or two bores concentric. Lesson 9 covers geometrical tolerancing — the 14 symbols, the tolerance frame and datums.