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Tolerance level refers to the level that determines the degree of dimensional accuracy. The national standard stipulates that it is divided into 20 levels, from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance level (processing accuracy), the larger the allowable variation range (tolerance value) of the size, and the less difficult the processing. Basic definition of the concept of free tolerance What is free dimensional tolerance? In the old national standard (HG) 159-59, the accuracy level is divided into 12 levels in terms of reference part tolerance. The tolerances of two levels of precision reference parts, taken from the 8th and 9th levels, are called free dimensional tolerances. Divide the deviation into ； One-way (+) or -) and two-way (±) are indicated in the annotation of the free dimensional tolerance. ； ①Free dimensional tolerances apply only to machined surfaces. ②Free dimensional tolerances are not marked on working drawings. ③The one-way deviation is marked with (-) for the shaft, the hole, hole depth, groove width, groove depth and groove length are marked with (+), and the other two-way positive and negative deviations (±) are used. ④When there are free dimensions that cannot be incorporated into the above clear principles and there is a one-way deviation requirement, the designer should note it in the construction drawing, otherwise it will be manufactured according to two-way deviation. In the revised national standard (GB) 1800-79, the standard tolerance is divided into 20 levels. Right now ； IT01, IT0, IT1 to IT18. IT stands for standard tolerance, and the codes of tolerance levels are represented by Arabic numerals, decreasing in order from IT01 to IT18. And formulated (GB) 1804-79 the limit deviation of dimensions without tolerances, there are three regulations: ①The specified limit deviation applies to dimensions for metal cutting and non-cutting processing. ② Deviations of dimensions without tolerances noted on the drawings shall be specified in the corresponding technical documents according to the series specified in this standard. ③The tolerance levels for dimensions without tolerances are specified as IT12 to IT18. Use H (+) for general holes ； Use h (-) for the axis ； Use (±)?IT (i.e. Js or js) for the length. When necessary, ?IT (i.e. Js or js) can be used regardless of hole, axis or length. According to the international standard ISO2768, the following is a tolerance table for linear dimensions without tolerances. This unindicated tolerance applies to metal cutting dimensions as well as general stamping dimensions. Scope of application These limit deviations apply to: (1) Linear size: For example, external dimensions, internal dimensions, radius, distance, rounding radius and chamfering height ； (2) Angular size: Includes angular dimensions for which angle values are not usually marked, such as right angles (90°) ； Linear and angular dimensions of machined assemblies. These limit deviations do not apply in the following cases: (1) There are linear and angular dimensions specified by other general tolerance standards ； (2) Reference dimensions in brackets. Grade classification: Selection and application of tolerance grades. Application range and examples of tolerance grades. IT01 is used for particularly precise dimensional transfer standards, such as particularly precise standard gauge blocks. IT0 is used for particularly precise dimensional transfer bases and particularly important precision fitting dimensions in aerospace. For example, particularly precise standard gauge blocks, some particularly important dimensions of precision mechanical parts, calibration gauges IT1 for calibration and inspection of IT6 grade shaft gauges are used as precise dimensional transmission standards, and very few precision fitting 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 axis gauges, individual particularly important precision mechanical parts dimensions IT2 for high-precision measuring tools, particularly important precision fit dimensions. For example, the dimensional manufacturing tolerances of gauges for grade IT6 to IT7 workpieces, the calibration plug gauges of grade IT8 to IT11 shaft gauges, the size IT3 of individual particularly important precision mechanical parts are used for precision measurement tools, high-precision precision fit of small-size parts, and the shaft diameter and housing aperture of C-grade rolling bearings. For example, gauges for inspecting IT8 to IT11 grade workpieces and calibration gauges for calibrating and inspecting IT9 to IT13 grade shaft gauges, machine tool spindles that match the inner ring bore (diameter to 100mm) of particularly precise P4 grade rolling bearings, journals of precision machinery and high-speed machinery, shell apertures that match the outer ring of P4 grade radial ball bearings, and precision matching of individual small-sized parts with special precision on navigation instruments in the aviation and maritime industries. IT4 is used for shaft diameters and housing apertures of precision measuring tools, high-tight magpie wood fits, and P4 and P5 level rolling bearing fits. For example, gauges for inspecting IT9 to IT12 grade workpieces and calibration gauges for calibrating IT12 to IT14 grade shaft gauges, machine tool spindles matching P4 grade bearing holes (bore diameter >100mm) and P5 grade bearing holes, journals of precision machinery and high-speed machinery, machine tool shell holes matching P4 grade bearings, diesel engine piston pins and piston proper bore diameters, The reference hole or shaft diameter of high-precision (level 1 to level 4) gears, the special precision hole diameter of instruments used in the aviation and navigation industries, IT5, is used under regulations that require small fitting tolerances and high shape tolerances. This type of cylinder is equivalent to the highest precision in the old national standard, and is used for particularly important fitting dimensions in machine tools, engines and instruments. It is rarely used in general machinery. For example, gauges for inspecting IT11 to IT14 grade workpieces and calibration gauges for calibrating IT14 to IT15 grade shaft gauges, machine tool box holes matching P5 grade rolling bearings, machine tool spindles matching E grade rolling bearing holes, journals of precision support machines, machine tool tailstock sleeves, high-precision indexing plate journals, indexing head spindles, precision screw reference journals, outer diameters of high-precision boring sleeves, etc. ； The fit of precision holes in the main shaft instrument in the engine, the holes of grade 5 precision gears, and the IT6 mating surfaces of the reference shafts of grade 5 and 6 precision gears have high uniformity requirements, which can ensure very high matching properties and are stable and reliable in use. It is equivalent to the old national standard grade 2 shaft and grade 1 precision hole. It is widely used in important fits in machinery. For example, the gauge for inspecting IT12 to IT15 grade workpieces and the calibration gauge for calibrating IT15 to IT16 grade shaft gauges. ； Housing holes matching E-class bearings and machine tool spindle journals matching roller bearings, assembled bronze worm gears in machine tool manufacturing, journals for mounting gears, worm gears, couplings, pulleys, and cams on the outer diameter of the wheel shell ； Machine tool screw support journal, centering diameter of rectangular spline, column of radial drilling machine, etc. ； The outer diameter of guide parts of machine tool fixtures, precision shafts in precision instruments, precision shafts in aviation and navigation instruments, automated instruments, cutting machinery, particularly important shafts in watches, outer diameters of cylinder liners in engines, main journals of crankshafts, piston pins, connecting rod bushings, outer diameters of connecting rods and bearing bushes ； The reference hole of grade 6 precision gears and the reference journal of grade 7 and 8 precision gears are particularly precise, such as the top circle diameter of grade 1 or 2 precision gears IT7, which is widely used in general machinery. The application conditions are similar to IT6, but the accuracy is slightly lower, equivalent to the tolerance of the old national standard intermediate precision shaft or grade 2 precision hole. For example, gauges for inspecting IT14 to IT16 grade workpieces and calibration gauges for calibrating IT16 grade shaft gauges. ； Bore diameter of assembled bronze worm gear rim in machine tools, bore diameter of couplings, pulleys, cams, etc. 6~30>30~120>120~400>400~1000>1000~2000>2000 Precision f±0.05±0.05±0.1±0.15±0.2±0.3±0.5— Medium m±0.1±0.1±0.2±0.3±0.5±0.8±1.2±2.0 Coarse c±0.2±0.3±0.5±0.8±1.2±2.0±3.0±4.0 Coarse v—±0.5±1.0±1.5±2.5±4.0±6.0±8.0 Limit deviation of rounding radius and chamfering height dimension Tolerance level 0~33~6>6~30>30 Precision f±0.2±0.5±1.0±2.0 Medium m Rough c±0.4±1.0±2.0±4.0 Coarse v Note: For the meaning of rounding radius and chamfering height, please refer to GB/T6403.4 Limit deviation of angular dimensions Numerical tolerance level 0~10>10~50>50~120120~400>400 Precision f±1°±30′±20′±10′±5′ Medium m Rough c±1°30′±1°±30′±15′±10′ The roughest v±3°±2°±1°±30′±20′ Generally, the standard number and tolerance grade code are indicated near the title bar of the drawing or in the technical requirements and technical documents (such as enterprise standards) without indicating the tolerance. For example, when selecting the medium level, mark it as: Structures whose geometric tolerances are not marked in the GB/T 1804-m drawings must be marked according to the levels in GB/T1184-1996 "Tolerance Values Not Noted for Shape and Position Tolerances". Straightness and flatness have no tolerance values. Tolerance grade 0~10>10~30>30~100>100~300>300~1000>1000 H0.020.050.10.20.30.4 K0.050.10.20.40.60.8 L 0.10.20.40 .81.21.6 Verticality without tolerance value tolerance level 0~100>100~300>300~1000>1000 H 0.20.30.40 .5K 0.40.60.81 L0.611.52 Symmetry without tolerance value Tolerance grade 0~100>100~300>300~1000>1000 H0.5 K0.60.81 L0.611.52 Circular runout without tolerance value Tolerance grade Circular runout tolerance value H0.1 K0.2 L0.5 Tolerance grade selection principle The essence of selecting tolerance grade is to correctly resolve the contradiction between the use requirements of machine parts and the manufacturing process and cost. The principle of selecting a tolerance level is to select a lower tolerance level as much as possible while meeting the use requirements of the parts. Precision requirements should be harmonized with production possibilities, i.e. using reasonable machining techniques, assembly techniques and existing equipment. However, when necessary, methods should be adopted to improve equipment accuracy and improve processes to ensure product accuracy. Choosing the appropriate tolerance level for mating dimensions is extremely tricky. Because in many cases, it will determine the working performance, service life and reliability of the matching parts, as well as the manufacturing cost and production efficiency of the parts. (1) When selecting a tolerance level, you should first ensure the usage requirements. (2) When selecting the tolerance level, it is necessary to not only meet the design requirements, but also consider the possibility and economy of the process. Choosing the optimal temperature is a very complex technical and economic issue. It not only needs to consider the processing cost, but also the increased assembly cost due to the improvement of processing accuracy, as well as the impact of accuracy on product performance and economic indicators (reliability, life, fuel consumption, etc.). (3) In machinery manufacturing, tolerance levels are stipulated to ensure not only the accuracy of the machine and the interchangeability of parts, but also the economy of manufacturing the machine. That is to say, as long as low precision can ensure the function and accuracy of the machine, don't require excessive precision of parts. That will increase manufacturing costs. Specifically, the tolerance level should be determined based on the type of machine and the use of a certain part. Tolerances include dimensional tolerances and fit tolerances. When using, you can refer to the various zero tolerance levels listed in the mechanical design manual for flexible application.