IT Grade

Key takeaways

IT Grade belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect IT Grade to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of IT Grade from memory before moving on to harder problems.

Reference excerpt

Note: in this context, IT does not mean Information Technology, but it is an Engineering term. An IT grade is an internationally accepted code system for tolerances on linear dimensions. Such code systems may be used to produce interchangeable parts. In engineering, the word tolerance refers to a range of allowable dimensions or values. Standard tolerance grades are a group of tolerances for linear sizes characterized by a common identifier. For SI measurements, a system of tolerance grades defined in ISO 286 is frequently used and identified by the letters IT followed by a number specifying how precise the requirements are, relative to the nominal size of a part. For example, IT14 refers to a group of tolerances used in manufacturing. For a part dimensioned at 10 mm, IT14 allows for up to 0.36 mm of variation in size. As the IT grade number increases, the tolerances increase; similarly, for larger nominal sizes, the standard tolerances increase. For a part dimensioned at 100 mm, IT14 allows for up to 0.87 mm of variation in size.

Definition The ISO 286 defines the international tolerances grades for nominal sizes up to 3.15 meters as follows:

From IT6 to IT18, the standard tolerances are multiplied by the factor 10 at each fifth step. This rule applies to all standard tolerances and may be used to extrapolate values for IT grades not given in Table 1. For example, the nominal size range 120 mm up to and including 180 mm, the value of IT20 is:

IT20 = IT15 × 10 = 1.6 mm × 10 = 16 mm

IT grades may be used with alternate prefixes which identify how the tolerance limits are distributed around a nominal value. When used with the IT prefix, IT grades do not specify how the tolerance limits are placed around the nominal value, alternate prefixes are used for this purpose. For example, if the tolerance limits are distributed symmetrically above and below the nominal value, the prefix "js" may be used. For example a part dimensioned (in millimeters) as 4 js7 is equivalent to 4 ± 0.006 (where 4 IT7 is 0.012.) Other standardized prefixes include the letters A, B, C, CD, D, E, EF, F, G, H, J, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC (for holes), and the lower-case equivalents (for shafts.) Prefix letters I, L, O, Q and W are not used to avoid confusion. Of these, the letter prefixes H and h are easiest to explain as the tolerance lies entirely on one side of the nominal size. A hole dimensioned at 4 H7 may range from 4.00 - 4.012 mm, and a shaft at 4 h7 may range from 3.988 - 4.00 mm.

Preferred tolerance classes and fits ISO 286 identifies a set of preferred tolerance classes for holes which include G7, H7, JS7, K7, N7, P7, R7, S7, F8, H8, E9, H9, D10, A11, B11, C11, and H11. The set of preferred tolerance classes for shafts includes g6, h6, js6, k6, n6, p6, r6, s6, f7, h7, e8, d9, h9, a11, b11, c11 and h11. To completely specify the fit between a hole and corresponding shaft, it is common to specify a pair of the above tolerance classes, for example H7/g6. As with all IT grades, the smaller numbers correspond to tighter tolerances. Under normal circumstances, only a small number of the possible fits are practically required, and ISO 286 identifies preferred fit combinations including these as most preferred:

Other uses An industrial process may have an IT grade associated with it, indicating how precise it is. When designing a part, an engineer will typically determine a critical dimension (D) and some tolerance (T) on that dimension. Using this formula, the engineer can determine what IT Grade is necessary to produce the part with those specifications. Thus, if injection molding is capable of IT13 and a part requires IT5, one cannot use injection molding to produce that part to those specifications. It is useful in determining the processes capable of producing parts to a specification.

See also Manufacturing Interference fit Process capability

References

External links ISO Hole and Shaft tolerances/limits, Roymech, UK. ISO Hole and Shaft Tolerances iPhone App, Trelleborg Sealing Solutions Germany. Hole & Shaft Tolerances Deviations And Fits – ISO 286

Worked examples

Example 1 — a first encounter with IT Grade

Start with the simplest possible case. Write down what IT Grade claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to IT Grade before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about IT Grade ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of IT Grade

In research: IT Grade appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses IT Grade in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: IT Grade is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mechanical standards, so understanding it makes those chapters shorter.
In everyday life: Look for IT Grade outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study IT Grade in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what IT Grade means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain IT Grade out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is IT Grade in simple terms?

Note: in this context, IT does not mean Information Technology, but it is an Engineering term. An IT grade is an internationally accepted code system for tolerances on linear dimensions.

Why does IT Grade matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study IT Grade?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on IT Grade.