Selection and Application of Tolerance Grades

Tolerance Grade refers to the grade that determines the precision of dimensions. According to the national standard, there are 20 grades according to GB/T1184-1996, ranging from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance grade (processing precision), the greater the allowable range of dimension variation (tolerance value), and the lower the processing difficulty.

Application Scope and Examples

IT01	Used for exceptionally precise dimension transfer benchmarks, such as specially precise standard gauge blocks.
IT0	Used for exceptionally precise dimension transfer benchmarks and in aerospace for particularly important precision fit dimensions. Examples include specially precise standard gauge blocks, individual particularly important precision mechanical part dimensions, and calibration of gauges for checking IT6-level shafts.
IT1	Used for precision dimension transfer benchmarks, particularly important extremely precise measurement tools, and a few extremely precise fit dimensions. Examples include high-precision standard gauges, calibration of gauges for checking IT7 to IT9-level shafts, and individual particularly important precision mechanical part dimensions.
IT2	Used for highly precise measurement tools and particularly important precision fit dimensions. Examples include checking the size manufacturing tolerance of IT6 to IT7-level workpieces, calibration of gauges for checking IT8 to IT11-level shafts, and individual particularly important precision mechanical part dimensions.
IT3	Used for precision measurement tools, high-precision fits of small-sized parts, and shaft and housing fits with C-class roller bearings. Examples include checking the size manufacturing tolerance of IT8 to IT11-level workpieces and calibration of gauges for checking IT9 to IT13-level shafts, as well as precision fits of individual small-sized parts in special precision navigation instruments for the aerospace and maritime industries.
IT4	Used for precision measurement tools, high-precision fits, and fits with P4 and P5-level roller bearings for shafts and housings. Examples include checking the size manufacturing tolerance of IT9 to IT12-level workpieces and calibration of gauges for checking IT12 to IT14-level shafts, as well as fits with P4-level bearing holes (diameter > 100mm) and machine tool spindles matching P5-level bearing holes.
IT5	Used for applications with very small tolerance requirements and high shape tolerance requirements, providing stable fits. This is equivalent to the highest precision in the old national standard and is used for particularly important fit dimensions in machine tools, engines, and instruments where precision is critical. Examples include checking the size manufacturing tolerance of IT11 to IT14-level workpieces and calibration of gauges for checking IT14 to IT15-level shafts, as well as matching machine tool housing holes with P5-level roller bearings.
IT6	Used for surfaces with high uniformity requirements, ensuring relatively high fits. This is equivalent to the old national standard 2-level shaft and 1-level hole accuracy and is widely used in important fits in machinery. Examples include checking the size manufacturing tolerance of IT12 to IT15-level workpieces and calibration of gauges for checking IT15 to IT16-level shafts, as well as matching housing holes with E-class bearings and machine tool spindles matching roller bearings.
IT7	Widely used in general machinery, similar to IT6 but with slightly lower accuracy, equivalent to the old national standard intermediate accuracy shaft or 2-level hole tolerance. Examples include checking the size manufacturing tolerance of IT14 to IT16-level workpieces and calibration of gauges for checking IT16-level shafts, as well as matching flange hub holes with E-class bearings and machine tool spindles matching roller bearings.
IT8	Considered medium accuracy in mechanical manufacturing and falls within a higher precision range in instrument, instrument, and watch manufacturing due to smaller basic dimensions. Examples include checking the size manufacturing tolerance of IT16-level workpieces, fitting dimensions for bearing shell linings along the width direction, and general fits in radio instruments.
IT9	Similar application conditions to IT8 but used when lower precision than IT8 is required. Examples include fits between shaft sleeves and holes in machine tools, fits between operating parts and shafts, and fits between idler pulleys and shafts.
IT10	Similar application conditions to IT9 but used when lower precision than IT9 is required, equivalent to the old national standard 5-level accuracy tolerance. Examples include fits on brackets in electronic instruments, cutting machined parts and circular hole dimensions in the computer industry, and unspecified tolerance dimensions in metal processing.
IT11	Widely used in cases with relatively large clearances where significant variations do not pose a danger. Equivalent to the old national standard 6-level accuracy tolerance. Examples include flange hub stops and holes, sliders and sliding gears, and grooves on machine tools.
IT12	Used when very low fit precision is required, allowing for large clearances after assembly. Suitable for parts with essentially no fit requirements. Slightly smaller than the tolerance of the old national standard 7-level accuracy. Examples include non-fit dimensions and inter-process dimensions.
IT13	Similar application conditions to IT12 but slightly larger than the tolerance of the old national standard 7-level accuracy. Examples include non-fit dimensions and inter-process dimensions, cutting machined parts and round hole dimensions in computers and typewriters, and dimensions of holes with center distances in two-hole centers.
IT14	Used for non-fit dimensions not included in the dimension chain, equivalent to the old national standard 8-level accuracy. Examples include limit deviations of non-annotated dimensions in mechanical processing parts such as machine tools, automobiles, tractors, metallurgical machinery, mining machinery, petrochemicals, motors, electrical appliances, instruments, aerospace, maritime, medical equipment, watches, bicycles, sewing machines, papermaking, and textile machinery.
IT15	Used for non-fit dimensions not included in the dimension chain, equivalent to the old national standard 9-level accuracy. Examples include stamping parts, wooden mold castings, heavy-duty machine tool manufacturing, and limit deviations of non-annotated dimensions when the basic dimension is greater than 3,150 mm.
IT16	Used for non-fit dimensions, equivalent to the old national standard 10-level accuracy. Examples include dimensions of castings in typewriters, outline dimensions of enclosures in the radio manufacturing industry, general outline dimensions in surgical instruments, tolerance dimensions for bending and stretching, dimensions of wood parts in textile machinery, dimensions of plastic parts, and dimensions of molds and free forgings.
IT17 and IT18	Used for non-fit dimensions, equivalent to the old national standard 11th or 12th level accuracy tolerance. Used for plastic molding dimensions, general outline dimensions in surgical instruments, and tolerance dimensions for cold work and welding.

Last updated on Jan 17, 2024