Tolerance level refers to the level of accuracy of the size. The national standard is divided into 20 levels. From IT01, IT0, IT1, IT2~IT18, the higher the number, the lower the tolerance level (machining accuracy), the allowable range of tolerance (tolerance) The larger the value, the smaller the processing difficulty. Selection and application of tolerance levels: IT01: For special precision size transfer benchmarks, such as special precision gauge blocks. IT0: For special precision size transfer benchmarks and precision fit sizes that are especially important in aerospace. For example, the special precision gauge block, the size of the precision mechanical parts that are particularly important, and the proofreading gauge for the proofing of the IT6-axis axle gauge. IT1: Extremely precise fit sizes for precision dimensional transfer benchmarks and high precision measurement tools. For example, high-precision standard gauges, proofreading proofreading gauges for IT7 to IT9-level shaft gauges, and individually important precision mechanical part sizes. Titanium Machinery is the main product of the company with machine tool electric spindle, rotary thimble, screw screw, shaft machining, CNC lathe machining, shank shank and chuck post. Professional quality assurance! Because of professionalism, it is excellent! IT2: For precision tooling with high precision, especially for precision fit sizes. For example, to verify the dimensional manufacturing tolerances of IT6 to IT7 workpiece gauges, proofread the proof gauges for IT8 to IT11 shaft gauges, and the size of precision mechanical parts that are particularly important. IT3: Precision precision tools for precision measurement tools, small-size parts, and shaft diameters and housing bores for C-class rolling bearings. For example, the inspection of the IT8 to IT11 workpiece gauge and proofreading inspection of the IT9 to IT13 axis gauge gauge calibration gauge, with the special precision P4 grade rolling bearing inner ring hole (diameter to 100mm) matching machine spindle, precision machinery and high-speed mechanical The journal, the housing aperture matched with the outer ring of the P4 grade radial ball bearing, and the precision of the individual precision small individual parts of the navigation instrument in the aviation and marine industry. IT4: For precision measuring tools, high precision precision fit and P4 and P5 rolling bearings for shaft diameter and housing bore. For example, check the IT9 to IT12 workpiece gauge and the proofreading gauge for the IT12 to IT14 shaft gauge, the P4 grade bearing bore (aperture >100mm) and the P5 grade bearing hole for the machine tool spindle, precision machine and high speed mechanical Journal, housing hole for P4 bearing, diesel piston pin and piston pin hole, high precision (Class 1 to 4) gear reference hole or shaft diameter, special precision for instruments used in aviation and marine industry Aperture. IT5: Under the regulations with small tolerance requirements and high shape tolerance requirements, this kind of tolerance level can make the matching property more stable, which is equivalent to the highest precision in the old national standard, and is especially important for cooperation in machine tools, engines and instruments. Size, less application in general machinery. For example, check the IT11 to IT14 workpiece gauge and the proofreading gauge for the IT14 to IT15 shaft gauge, the machine tool housing hole matched with the P5 grade rolling bearing, the machine tool spindle matched with the E-class rolling bearing hole, precision machinery and high-speed mechanical Journal, machine tailstock sleeve, high-precision indexing disc journal, indexing head spindle, precision lead screw reference journal, outer diameter of high-precision bushing, etc.; precision hole matching in the spindle instrument of the engine, 5 The hole of the precision gear and the reference shaft of the 5th and 6th precision gears. IT6: With the requirement of high uniformity of the surface, it can guarantee a fairly high matching property. It is stable and reliable, and is equivalent to the old national standard 2nd axis and 1st precision hole. It is widely used in mechanical coordination. For example, inspection IT12 Calibration gauge for IT15 grade workpiece gauge and proofreading IT15 to IT16 shaft gauge; housing bore matched with E-class bearing and machine spindle journal matched with roller bearing, assembled bronze worm gear and wheel housing in machine tool manufacturing Diameter mounting gears, worm gears, couplings, pulleys, cam journals; machine tool screw support journals, centering diameter of rectangular splines, uprights of radial drilling machines, etc.; outer diameter dimensions of guides for machine tool fixtures, precision Precision shafts in instruments, precision shafts in aerospace and marine instruments, automated instrumentation, postal and telecommunications machinery, particularly important shafts in watches, outer diameters of cylinder liners in engines, crankshaft main journals, piston pins, connecting rod bushings, connecting rods and The outer diameter of the bearing bush; the reference hole of the 6-stage precision gear and the reference journal of the 7-stage and 8-level precision gears, especially the top diameter of the gear of the first-class or second-class precision. Titanium Machinery is the main product of the company with machine tool electric spindle, rotary thimble, screw screw, shaft machining, CNC lathe machining, shank shank and chuck post. Professional quality assurance! Because of professionalism, it is excellent! IT7: Widely used in general machinery, the application condition is similar to IT6, but the accuracy is slightly lower, which is equivalent to the tolerance of the old national standard intermediate precision axis or the second precision hole. For example, check the IT14 to IT16 workpiece gauge and proofread the IT16 grade gauge gauge; the assembled bronze worm wheel rim aperture in the machine tool, the aperture of the coupling, pulley, cam, etc., the machine chuck hole, the radial drilling machine Rocker arm hole, bearing hole of lathe screw, inner hole of machine tool chuck guide, connecting rod hole, piston hole and hinge stud locating hole in engine; important parts in textile machinery, high requirements in printing and dyeing machinery The parts, the precision matching inner hole in the precision instrument, the important inner hole in the electronic computer, electronic instrument and instrument, the important inner hole in the automatic instrument, the reference hole of the 7th and 8th precision gears and the precision gear of the 9th and 10th grade Reference axis. IT8: It is medium-precision in mechanical manufacturing. In the manufacture of instruments, meters and watches, it is a high-precision range due to its small basic size. It is used in agricultural machinery, textile machinery, printing and dyeing machinery, bicycles, sewing machines and medical equipment. Wide quantity. For example, the IT16-level workpiece gauge, the size of the bearing bushing in the width direction, the spanning of the crown in the watch, the pawl dial wheel, and the like in the radio meter of the splint are generally matched. IT9: The application conditions are similar to those of IT8, but the accuracy is lower than that of IT8, which is slightly larger than the old national standard 4 precision tolerance value. For example, the outer diameter and hole of the bushing in the machine tool, the control member and the shaft, the idle pulley and the shaft, the shaft and the bearing of the control system, the general matching parts in the textile machinery, printing and dyeing machinery, the inner hole of the oil pump in the engine, the valve guide The inner hole, the cooperation of the flywheel and the flywheel sleeve, the general fit size in the automatic instrument, the size of the undemanding tolerance of the higher part in the watch, the key width of the single key connection, and the matching size of the moving part in the typewriter. IT10: The application conditions are similar to those of IT9, but the accuracy is lower than that of IT9, which is equivalent to the 5-level precision tolerance of the old national standard. For example, the cooperation of the electronic instrument and the bracket in the instrument, the insulating bushing hole in the navigation instrument and the bushing shaft of the power ring, the matching size of the riveting part in the typewriter, the general unfilled tolerance is required when the basic size of the watch is less than 18 mm. Dimensions, and larger un-tolerance dimensions greater than 18 mm, the size of the oil seal collar hole in the engine to match the crankshaft hub. IT11: It is widely used in occasions where the gap is large and there is significant change and it will not cause danger. It can also be used for low precision. After assembly, a large gap is allowed, which is equivalent to the 6-level precision tolerance of the old national standard. For example, flanges and holes on the machine tool, slides and sliding gears, grooves, etc.; agricultural machinery, locomotive parts and parts for stamping, parts that are not important in the manufacture of watches, tools for the manufacture of watches and The size of the equipment without tolerances, the coarser movement of the textile machinery, the lower fitting size required in the printing and dyeing machinery, the threaded connection and the rough dynamic connection in the manufacture of the grinding machine, the tolerance of the top diameter of the gear for the measurement reference, etc. . IT12: The precision of the matching precision is very low. There is a large gap after assembly. It is suitable for the parts that have basically no matching requirements. The dimensional limit deviation of the high unfilled tolerance is required, which is slightly smaller than the 7-level precision tolerance of the old national standard. For example, non-matching dimensions and inter-process dimensions, engine splitter bars, unfilled tolerances for process equipment in watch manufacturing, limit deviations for unmachined tolerances in metalworking in the computer industry, and connection of wrench holes and wrench seats in machine tool manufacturing Wait. IT13: The application conditions are similar to those of IT12, but slightly larger than the old National Standard 7 accuracy tolerance value. For example, non-matching dimensions and inter-process dimensions, machined parts and wafer holes in computers, typewriters, and unfilled tolerances for center-to-center spacing. IT14: For non-fitted dimensions and dimensions not included in the dimension chain, equivalent to the 8-level precision tolerance of the old national standard. For example, in machine tools, automobiles, tractors, metallurgical machinery, mining machinery, petrochemicals, electrical machinery, electrical appliances, instrumentation, aviation and navigation, medical equipment, watches, bicycles, sewing machines, paper and textile machinery, etc. The limit deviation. IT15: For non-fit dimensions and dimensions not included in the dimension chain, equivalent to the 9-level precision tolerance of the old national standard. For example, stampings, wood-cast parts, heavy-duty machine tools, dimensional tolerances for unfilled tolerances when the basic dimensions are greater than 3 150 mm. IT16: For non-matching dimensions, equivalent to the 10-level precision tolerance of the old national standard. For example, the size of castings in typewriters, the dimensions of cabinets in the radio manufacturing industry, the general dimensions in surgical instruments, the dimensions for bending and bending, the size of wood in textile machinery, the size of plastic parts, the manufacture of wood molds and freedom. Forged dimensions. IT17, IT18: For non-fit size, equivalent to the tolerance of 11 or 12 precision of the old national standard, for plastic molding size, general dimensions in surgical instruments, tolerances for cold work and welding dimensions. The principle of selection of tolerance levels: The essence of selecting the tolerance level is to correctly resolve the contradiction between the use requirements of the machine parts and the manufacturing process and cost. The principle of selecting the tolerance class is to use a lower tolerance class as much as possible while meeting the requirements for the use of the part. Accuracy requirements should be harmonized with the production possibilities, ie with reasonable processing techniques, assembly processes and existing equipment. However, if necessary, measures to improve equipment accuracy and improve the process are required to ensure the accuracy of the product. It is extremely important to choose the appropriate tolerance level for the fit size. Because in many cases, it will determine the workability, service life and reliability of the part, while at the same time determining the manufacturing cost and production efficiency of the part. (1) Select the tolerance level first to ensure the use requirements. (2) When selecting the tolerance level, it is necessary to meet the design requirements as well as the possibility and economy of the process. Choosing the best machining accuracy is a very complex technical and economic issue. It not only considers the processing cost, but also considers the assembly cost increased due to the improvement of machining accuracy, and the influence of accuracy on product performance and economic indicators (reliability, life, fuel consumption, etc.). (3) In the mechanical manufacturing, the tolerance level is defined to ensure the accuracy of the machine and the interchangeability of the parts, and to ensure the economics of the manufacturing machine. That is to say, as long as the low precision can guarantee the function and accuracy of the machine, do not excessively demand the accuracy of the parts. This will increase the manufacturing cost. The tolerance level should be determined according to the type of the machine and the use of a certain part. The tolerances include dimensional tolerances and fit tolerances. Refer to the recommended tolerance levels of the various parts listed in the mechanical design manual for flexible application. In the old national standard (HG) 159-59, the accuracy level was divided into 12 levels on the reference part tolerance. Taken from the accuracy of the 8 and 9 precision reference parts, called the free size tolerance. Divide the deviation into one; one-way (+) or (-), two-way (±) two hints in the annotation of free dimensional tolerance; 1 free size tolerance only applies to machined surfaces. 2 Free size tolerances are not marked on the work drawing. 3 One-way deviation For the shaft (-) number, for the hole, hole depth, groove width, depth of the groove and the length of the groove (+), the other two-way positive and negative deviation (±). 4 Can not be included in the above-mentioned clear principle of free size, and when there is a one-way deviation requirement, the designer should note in the drawing, otherwise it will be manufactured according to the two-way deviation. In the revised national standard (GB) 1800-79, the standard tolerance is divided into 20 levels. That is; IT01, IT0, IT1 to IT18. IT stands for standard tolerances, and the code numbers for tolerance classes are expressed in Arabic numerals, decreasing in order from IT01 to IT18. And to develop (GB) 1804-79 unspecified tolerance size limit deviation, there are three provisions: The specified limit deviation is applicable to the size of metal cutting and also to the size of non-cutting. 2 The deviation of the un-tolerance dimension on the pattern shall be specified in the corresponding technical documents according to the series specified in this standard. 3 Tolerance dimensions for unfilled tolerances are specified as IT12 to IT18. The general hole uses H(+); the axis uses h(-); the length uses (±)IT (ie Js or js). If necessary, IT (ie Js or js) can be used regardless of the hole, axis or length. According to the international standard ISO2768, the following is a tolerance table for linear dimensions without tolerance. This unfilled tolerance is suitable for the size of metal cutting and is also suitable for general stamping dimensions. Scope of application, these limit deviations apply to: (1) Linear dimensions: eg outer dimensions, inner dimensions, step dimensions, diameters, radii, distances, rounding radii and chamfer heights; (2) Angle size: includes angular dimensions that do not normally indicate angular values, such as right angle (90°); linear and angular dimensions of machined assemblies. These limit deviations do not apply to the following situations: (1) Linear and angular dimensions as specified by other general tolerance standards; (2) Reference dimensions in parentheses.