What is Tolerance Grade of CNC Machining?

What is tolerance grade?

Tolerance grade refers to the grade that determines the degree of dimensional accuracy.

The national standard stipulates that it is divided into 20 grades, from IT01, IT0, IT1, IT2 to IT18.

The larger the number, the lower the tolerance grade in processing accuracy, the larger the allowable range of variation in tolerance value of the size, and the lower the processing difficulty.

Division of tolerance grades
Tolerance grades	Application range and examples of tolerance grades
IT01	Used for particularly precise dimension transfer benchmarks, such as particularly precise standard gauge blocks
IT0	Used for particularly precise dimension transfer benchmarks and particularly important precision matching dimensions in aerospace. For example, particularly precise standard gauge blocks, individual particularly important dimensions of precision mechanical parts, and calibration gauges for calibration and inspection of IT6-grade shaft gauges
IT1	Used for precise dimension transfer benchmarks, extremely individual precision matching dimensions that are particularly important for high-precision measuring tools. For example, high-precision standard gauges, calibration gauges for calibration and inspection of IT7 to IT9-grade shaft gauges, and individual particularly important dimensions of precision mechanical parts
IT2	Used for high-precision measuring tools and particularly important precision matching dimensions. For example, to check the manufacturing tolerance of the size of the gauges for IT6 to IT7 workpieces, to check the calibration gauges for IT8 to IT11 shaft gauges, and the size of some particularly important precision machinery parts
IT3	used for precision measuring tools, high-precision precision matching of small-sized parts, and shaft diameters and housing apertures that match C-grade rolling bearings. For example, to check the gauges for IT8 to IT11 workpieces and to check the calibration gauges for IT9 to IT13 shaft gauges, machine tool spindles that match the inner ring holes (up to 100mm in diameter) of particularly precise P4-grade rolling bearings, journals for precision machinery and high-speed machinery, housing apertures that match the outer rings of P4-grade radial ball bearings, and precision matching of some particularly precise small-sized parts on navigation instruments in the aviation and marine industries.
IT4	used for precision measuring tools, high-precision precision matching, and shaft diameters and housing apertures that match P4 and P5-grade rolling bearings. For example, gauges for checking IT9 to IT12 workpieces and calibration gauges for calibrating IT12 to IT14 shafts, machine tool spindles matching P4 bearing holes (aperture > 100mm) and P5 bearing holes, journals of precision machinery and high-speed machinery, machine tool housing holes matching P4 bearings, piston pins and piston pin seat apertures of diesel engines, reference holes or shaft diameters of high-precision (1 to 4) gears, special precision apertures of instruments used in aviation and marine industries
IT5	is used for regulations with very small fit tolerance requirements and very high shape tolerance requirements. This type of tolerance grade can make the fit properties relatively stable, equivalent to the highest accuracy in the old national standard, and is used for particularly important fit dimensions in machine tools, engines and instruments. It is less used in general machinery. For example, the gauges for checking IT11 to IT14 workpieces and the calibration gauges for calibrating IT14 to IT15 shafts, the machine tool housing holes matching P5 rolling bearings, the machine tool spindles matching E-level rolling bearing holes, the journals of precision machinery and high-speed machinery, the machine tool tailstock sleeves, the high-precision indexing plate journals, the indexing head spindles, the precision screw reference journals, the outer diameter of high-precision boring sleeves, etc.; the matching of precision holes in the spindle instrumentation in the engine, the holes of the 5-level precision gears and the reference shafts of the 5-level and 6-level precision gears
IT6	The mating surface has a high uniformity requirement, which can ensure a fairly high matching quality and is stable and reliable to use. It is equivalent to the old national standard 2-level shaft and 1-level precision hole. It is widely used in important matching in machinery, such as the inspection of IT12 to IT15-level workpiece gauges and the calibration gauges for IT15 to IT16-level shaft gauges; the housing holes matching E-level bearings and the machine tool spindle journals matching roller bearings, the assembled bronze worm gears and wheel housing outer diameters in machine tool manufacturing, the shafts of gears, worm gears, couplings, pulleys, and cams Neck; machine tool lead screw support journal, rectangular spline centering diameter, radial drilling machine column, etc.; machine tool fixture guide outer diameter, precision shaft in precision instruments, precision shaft in aviation and navigation instruments, automation instruments, post and telecommunications machinery, particularly important shaft in watches, cylinder liner outer diameter in engines, crankshaft main journal, piston pin, connecting rod bushing, connecting rod and bearing outer diameter; 6-grade precision gear reference hole and 7-grade, 8-grade precision gear reference journal, special precision such as 1-grade or 2-grade precision gear top circle diameter
IT7	widely used in general machinery, with similar application conditions to IT6, but with slightly lower precision, equivalent to the tolerance of the old national standard intermediate precision shaft or 2-grade precision hole. For example, the gauges for checking IT14 to IT16 workpieces and the calibration gauges for calibrating IT16 shafts; the aperture of the assembled bronze worm wheel rim in machine tools, the aperture of couplings, pulleys, cams, etc., the chuck seat hole of machine tools, the rocker hole of radial drilling machines, the bearing hole of lathe screws, the inner hole of machine tool chuck guides, the connecting rod hole and piston hole in engines, and the positioning hole of hinged studs; important parts in textile machinery, parts with higher requirements in printing and dyeing machinery, inner holes with precision matching in precision instruments, important inner holes in electronic computers, electronic instruments, and meters, important inner holes in automation instruments, reference holes for 7-level and 8-level precision gears, and reference shafts for 9-level and 10-level precision gears
IT8	is of medium precision in mechanical manufacturing, and of high precision in instrument, meter and clock manufacturing due to its small basic size. It is widely used in agricultural machinery, textile machinery, printing and dyeing machinery, bicycles, sewing machines, and medical equipment. For example, the gauge for inspecting IT16-level workpieces, the dimensional matching of bearing seat bushings along the width direction, the matching of the span gear shaft, the ratchet needle wheel and the splint in watches, and the general matching in radio instruments,
IT9	is used when the application conditions are similar to IT8, but the accuracy is lower than IT8, which is slightly larger than the old national standard 4-level precision tolerance value. For example, the matching of the outer diameter of the shaft sleeve and the hole, the operating part and the shaft, the idle pulley and the shaft, the shaft and the bearing of the operating system in the machine tool, the general matching parts in textile machinery and printing and dyeing machinery, the inner hole of the oil pump body in the engine, the inner hole of the valve guide, the matching of the flywheel and the flywheel sleeve, the general matching dimensions in automation instruments, the dimensions of the parts with higher requirements in watches that are not marked with tolerances, the matching dimensions of the key width in the single-key connection, and the matching dimensions of the moving parts in the typewriter
IT10	is used when the application conditions are similar to IT9, but the accuracy required is lower than IT9, which is equivalent to the old national standard 5-level precision tolerance. For example, the fit of brackets in electronic instruments and meters, the fit of insulating bushing holes and collector ring bushing shafts in navigation instruments, the fit dimensions of riveted parts in typewriters, the size of watches with general uninjected tolerances when the basic size is less than 18 mm, and the size of watches with higher uninjected tolerances when the basic size is greater than 18 mm, and the fit dimensions of the oil seal retaining ring hole and the crankshaft pulley hub in the engine
IT11	is widely used in places where the gap is large and significant changes will not cause danger. It can also be used for places with low fit accuracy and large gaps are allowed after assembly, which is equivalent to the 6th level of accuracy tolerance of the old national standard. For example, flange stop and hole, slider and sliding gear, groove, etc. on machine tools; agricultural machinery, locomotive carriage parts and stamping processing matching parts, unimportant parts in watchmaking, untolerated dimensions in tools and equipment for watchmaking, rougher active matching in textile machinery, lower matching dimensions in printing and dyeing machinery, threaded connection and rough dynamic connection in grinding machine manufacturing, gear top circle diameter tolerance not used as measurement reference, etc.
IT12	The matching accuracy requirements are very low, there is a large gap after assembly, suitable for parts with basically no matching requirements, requiring higher untolerated dimension limit deviation, which is slightly smaller than the old national standard 7-level precision tolerance. For example, non-matching dimensions and process dimensions, engine separation rods, untolerated dimensions of process equipment in watchmaking, limit deviations of untolerated dimensions of metal processing in the computer industry, connection of spanner holes and spanner seats in machine tool manufacturing, etc.
IT13	The application conditions are similar to IT12, but slightly larger than the old national standard 7-level precision tolerance value. For example, non-matching dimensions and process dimensions, cutting parts and disc holes in computers and typewriters, and unindicated tolerance dimensions of the center distance between two holes
IT14	is used for non-matching dimensions and dimensions not included in the dimension chain, which is equivalent to the 8th grade precision tolerance of the old national standard. For example, the limit deviation of unindicated tolerance dimensions in machine processing parts such as machine tools, automobiles, tractors, metallurgical machinery, mining machinery, petrochemicals, motors, electrical appliances, instruments and meters, aviation and navigation, medical equipment, clocks, bicycles, sewing machines, papermaking and textile machinery
IT15	is used for non-matching dimensions and dimensions not included in the dimension chain, which is equivalent to the 9th grade precision tolerance of the old national standard. For example, stamping parts, wooden mold casting parts, heavy machine tool manufacturing, when the basic size is greater than 3 150 mm, the limit deviation of the unindicated tolerance dimension
IT16	is used for non-matching dimensions, which is equivalent to the 10th grade precision tolerance of the old national standard. For example, the dimensions of castings in typewriters, the dimensions of boxes in radio manufacturing, the general dimensions of surgical instruments, the dimensions for bending and stretching, the dimensions of wooden parts in textile machinery, the dimensions of plastic parts, the dimensions of wooden mold manufacturing and free forging
IT17 IT18	is used for non-matching dimensions, equivalent to the tolerance of grade 11 or 12 accuracy of the old national standard, used for plastic molding dimensions, general dimensions of surgical instruments, tolerances for cold working and welding dimensions