GlossaryKind of term: Standard and conventionIT tolerance grade
What is an IT tolerance grade?
An IT grade is one of the standard tolerance grades of ISO 286, IT01 to IT18, and sets the width of a size tolerance according to nominal size.
IT stands for international tolerance. ISO 286-1 has twenty standard tolerance grades: IT01, IT0 and IT1 to IT18. The lower the number, the narrower the tolerance. The width depends on the nominal size: for a size over 18 up to and including 30 mm, IT7 is 21 µm and IT9 52 µm, and at a larger size the same grade is wider. An IT grade only says how wide the zone is, not where it sits. In a tolerance class such as H7, the letter gives the position and the number the IT grade.
What does the IT tolerance grade depend on?
The values follow a fixed scheme. For sizes up to 500 mm the values derive from a tolerance unit that grows roughly with the cube root of the size; the standard gives them as a table, and the table is what applies. From IT6 on, each grade is about 1.6 times as wide as the one before, so the width increases tenfold every five grades. Halving a tolerance therefore makes the zone about one and a half grades tighter. The system assumes that a grade represents roughly the same manufacturing difficulty at every size; that is a premise of the scheme, not a statement of what a given process can hold.
What does the standard say about the IT tolerance grade?
The grades and the tables are in ISO 286-1 and ISO 286-2, current edition 2010. A value is only unambiguous with the nominal size, because the boundaries of the size ranges are fixed in the standard; a number from memory is easily wrong near a range boundary. The values hold at the reference temperature of 20 °C. Limit deviations per tolerance class, that is grade and position together, are tabulated in ISO 286-2.
How does the IT tolerance grade work in practice?
The IT grade says a lot about the route. Handbooks give guide values per process, with drilling in the coarser grades and reaming, fine boring or grinding in the finer ones, but what a process holds depends on machine, tooling, size, material, setup and temperature. A tighter grade can stay within the same operation, with a slower feed, an extra finishing pass and more measuring time, or it can add an operation such as reaming or grinding, sometimes outsourced. The choice of gauge follows the grade too: the narrower the zone, the smaller the measurement uncertainty has to be.
What is the IT tolerance grade often confused with?
An IT grade is not a tolerance class: H7 is a tolerance class, made of a position (H) and an IT grade (7). Nor is it a fit: only two tolerance classes together, such as H7/g6, make a fit. And an IT grade is not a general tolerance: that covers dimensions without a tolerance of their own and appears with a class such as ISO 2768-m near the title block.
Frequently asked questions
How much more does halving a tolerance cost?
That is not fixed. Halving makes the zone about one and a half IT grades tighter. If the operation stays, it can cost a slower feed, an extra pass, more measuring or rejects; if reaming, boring or grinding is added, you price that operation.
Why is IT7 wider on a large diameter than on a small one?
Because ISO 286 lets the width grow with the nominal size, roughly with its cube root. The premise is that a larger size is harder to make and measure equally accurately. A grade stands for comparable effort, not the same number of micrometres.
How do you look up the value of an IT grade?
In the ISO 286-1 table, in the size range the nominal size falls into. For a size over 18 up to and including 30 mm, IT6 is 13 µm, IT7 21 µm and IT8 33 µm. Limit deviations per tolerance class are in ISO 286-2.