IT Grade

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:

ISO 286 - Table 1

Nominal size				International tolerance grade
>		≤		IT01		IT0		IT1		IT2		IT3		IT4		IT5		IT6		IT7		IT8		IT9		IT10		IT11		IT12		IT13		IT14		IT15		IT16		IT17		IT18
mm				tolerance in μm																										tolerance in mm
0		3		0	.3	0	.5	0	.8	1	.2	2	.0	3		4		6		10		14		25		40		60		0	.10	0	.14	0	.25	0	.40	0	.60	1	.00	1	.40
3		6		0	.4	0	.6	1	.0	1	.5	2	.5	4		5		8		12		18		30		48		75		0	.12	0	.18	0	.30	0	.48	0	.75	1	.20	1	.80
6		10		0	.4	0	.6	1	.0	1	.5	2	.5	4		6		9		15		22		36		58		90		0	.15	0	.22	0	.36	0	.58	0	.90	1	.50	2	.20
10		18		0	.5	0	.8	1	.2	2	.0	3		5		8		11		18		27		43		70		110		0	.18	0	.27	0	.43	0	.70	1	.10	1	.80	2	.70
18		30		0	.6	1	.0	1	.5	2	.5	4		6		9		13		21		33		52		84		130		0	.21	0	.33	0	.52	0	.84	1	.30	2	.10	3	.30
30		50		0	.6	1	.0	1	.5	2	.5	4		7		11		16		25		39		62		100		160		0	.25	0	.39	0	.62	1	.00	1	.60	2	.50	3	.90
50		80		0	.8	1	.2	2	.0	3		5		8		13		19		30		46		74		120		190		0	.30	0	.46	0	.74	1	.20	1	.90	3	.00	4	.60
80		120		1	.0	1	.5	2	.5	4		6		10		15		22		35		54		87		140		220		0	.35	0	.54	0	.87	1	.40	2	.20	3	.50	5	.40
120		180		1	.2	2		3	.5	5		8		12		18		25		40		63		100		160		250		0	.40	0	.63	1	.00	1	.60	2	.50	4	.00	6	.30
180		250		2	.0	3		4	.5	7		10		14		20		29		46		72		115		185		290		0	.46	0	.72	1	.15	1	.85	2	.90	4	.60	7	.20
250		315		2	.5	4		6		8		12		16		23		32		52		81		130		210		320		0	.52	0	.81	1	.30	2	.10	3	.20	5	.20	8	.10
315		400		3		5		7		9		13		18		25		36		57		89		140		230		360		0	.57	0	.89	1	.40	2	.30	3	.60	5	.70	8	.90
400		500		4		6		8		10		15		20		27		40		63		97		155		250		400		0	.63	0	.97	1	.55	2	.50	4	.00	6	.30	9	.70
500		630						9		11		16		22		32		44		70		110		175		280		440		0	.70	1	.10	1	.75	2	.80	4	.40	7	.00	11	.00
630		800						10		13		18		25		36		50		80		125		200		320		500		0	.80	1	.25	2	.00	3	.20	5	.00	8	.00	12	.50
800		1,000						11		15		21		28		40		56		90		140		230		360		560		0	.90	1	.40	2	.30	3	.60	5	.60	9	.00	14	.00
1,000		1,250						13		18		24		33		47		66		105		165		260		420		660		1	.05	1	.65	2	.60	4	.20	6	.60	10	.50	16	.50
1,250		1,600						15		21		29		39		55		78		125		195		310		500		780		1	.25	1	.95	3	.10	5	.00	7	.80	12	.50	19	.50
1,600		2,000						18		25		35		46		65		92		150		230		370		600		920		1	.50	2	.30	3	.70	6	.00	9	.20	15	.00	23	.00
2,000		2,500						22		30		41		55		78		110		175		280		440		700		1,100		1	.75	2	.80	4	.40	7	.00	11	.00	17	.50	28	.00
2,500		3,150						26		36		50		68		96		135		210		330		540		860		1,350		2	.10	3	.30	5	.40	8	.60	13	.50	21	.00	33	.00

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:

• Hole-basis system of fits

  Basic hole	Tolerance classes for shafts
  	Clearance fits	Transition fits	Interference fits
  H7/	g6 h6	js6 k6 m6 n6	p6 r6 s6
  H8/	f7 h7 e8
  H9/	e8
  H11/	b11 c11

• Shaft-basis system of fits

  Basic shaft	Tolerance classes for holes
  	Clearance fits	Transition fits	Interference fits
  h6	G7 H7	JS7 K7 N7	P7 R7 S7
  h7	F8 H8
  h9	F8 H8 E9 H9 D10 B11

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

External links

• ISO Hole and Shaft tolerances/limits, Roymech, UK.
• ISO Hole and Shaft Tolerances iPhone App, Trelleborg Sealing Solutions Germany.