IT Grade

IT grade(s) refer to an internationally accepted code system for tolerances on linear sizes. The main intention of this code system is the fulfilment of the function fit, e.g. to allow interchangeable parts.

In engineering, a tolerance refers to a range of allowable dimensions or values, and is typically used to specify requirements for a mechanical part or an industrial process. 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 are frequently used, and idenfied by the letters IT followed by a number specifying how precise the requrements are, relative to the [[Real_versus_nominal_value|nominal size of a part.

For example, IT14 refers to a group of tolerances that might be 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.

The following table defines the international tolerances grades:

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	1		1		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	1		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 

An industrial process has an IT Grade associated with it, indicating how precise it is. When designing a part, an engineer will typically determine a key 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 has an IT Grade of 13 and a part needs an IT Grade of 5, one cannot injection mold that part to those specifications. It is useful in determining the processes capable of producing parts to the needed 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.