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What is tolerance class

What is tolerance class?

Dealing with NC machining every day, I believe you must have a deep impression on machining accuracy, but how much do you know about “tolerance level”?
Tolerance level refers to the level to determine the degree of dimensional accuracy. According to the national standard, it is divided into 20 levels. From IT01, IT0, IT1, IT2 to IT18, the larger the number is, the lower the tolerance level (machining accuracy), the larger the allowable variation range (tolerance value), and the smaller the machining difficulty is.
According to the different functions of product parts, the machining accuracy to be achieved is different, and the machining form and process to be selected are also different. This paper introduces the machining accuracy of several common machining forms such as turning, milling, planing, grinding, drilling and boring.
It standard tolerance class table

Basic dimensions    mm Tolerance class
IT01 IT0 IT1 IT2 IT3 IT4 IT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16 IT17 IT18
Greater than To μm mm
3 0.3 0.5 0.8 1.2 2 3 4 6 10 14 25 40 60 0.10 0.14 0.25 0.40 0.60 1.0 1.4
3 6 0.4 0.6 1 1.5 2.5 4 5 8 12 18 30 48 75 0.12 0.18 0.30 0.48 0.75 1.2 1.8
6 10 0.4 0.6 1 1.5 2.5 4 6 9 15 22 36 58 90 0.15 0.22 0.36 0.58 0.90 1.5 2.2
10 18 0.5 0.8 1.2 2 3 5 8 11 18 27 43 70 110 0.18 0.27 0.43 0.70 1.10 1.8 2.7
18 30 0.6 1 1.5 2.5 4 6 9 13 21 33 52 84 130 0.21 0.33 0.52 0.84 1.30 2.1 3.3
30 50 0.6 1 1.5 2.5 4 7 11 16 25 39 62 100 160 0.25 0.39 0.62 1.00 1.60 2.5 3.9
50 80 0.8 1.2 2 3 5 8 13 19 30 46 74 120 190 0.30 0.46 0.74 1.20 1.90 3.0 4.6
80 120 1 1.5 2.5 4 6 10 15 22 35 54 87 140 220 0.35 0.54 0.87 1.40 2.20 3.5 5.4
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 4.0 6.3
180 250 2 3 4.5 7 10 14 20 29 46 72 115 185 290 0.46 0.72 1.15 1.85 2.90 4.6 7.2
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 5.2 8.1
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 5.7 8.9
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 6.3 9.7
500 630 4.5 6 9 11 16 22 30 44 70 110 175 280 440 0.70 1.10 1.75 2.8 4.4 7.0 11.0
630 800 5 7 10 13 18 25 35 50 80 125 200 320 500 0.80 1.25 2.00 3.2 5.0 8.0 12.5
800 1000 5.5 8 11 15 21 29 40 56 90 140 230 360 560 0.90 1.40 2.30 3.6 5.6 9.0 14.0
1000 1250 6.5 9 13 18 24 34 46 66 105 165 260 420 660 1.05 1.65 2.60 4.2 6.6 10.5 16.5
1250 1600 8 11 15 21 29 40 54 78 125 195 310 500 780 1.25 1.95 3.10 5.0 7.8 12.5 19.5
1600 2000 9 13 18 25 35 48 65 92 150 230 370 600 920 1.50 2.30 3.70 6.0 9.2 15.0 23.0
2000 2500 11 15 22 30 41 57 77 110 175 280 440 700 1100 1.75 2.80 4.40 7.0 11.0 17.5 28.0
2500 3150 13 18 26 36 50 69 93 135 210 330 540 860 1350 2.10 3.30 5.40 8.6 13.5 21.0 33.0

Note: when the basic size is less than 1 mm, there is no IT14 to it18

Classification of tolerance grade

Selection and application of tolerance grade
Tolerance class
Application scope and examples
IT01
Used for special precision dimension transfer reference, such as special precision standard gauge block.
IT0
It is used for special precision dimension transfer datum and special important precision fit dimension in aerospace. For example, the special precision standard gauge block, the size of some special important precision mechanical parts, the calibration gauge for checking IT6 shaft gauge.
IT1
It is used for precision dimension transfer reference and very important precision fit dimension of high precision measuring tools. For example, high precision standard gauges, calibration gauges for checking IT7 to IT9 shaft gauges, and dimensions of some particularly important precision mechanical parts.
IT2
It is used for high precision measuring tools, especially for precision fit size. For example, to check the manufacturing tolerance of IT6 to IT7 workpiece gauge, to check the plug gauge of IT8 to IT11 shaft gauge, and to check the dimensions of some particularly important precision mechanical parts.
IT3
It is used for precision measuring tools, high precision fit of small size parts, and shaft diameter and shell aperture fit with C-grade rolling bearing. For example, the checking gauges for checking IT8 to IT11 workpiece gauges and for checking it9 to IT13 shaft gauges, the machine tool spindle matching the inner ring hole (diameter to 100 mm) of P4 precision rolling bearing, the Journal of precision machinery and high speed machinery, and the shell bore matching the outer ring of P4 radial ball bearing, In aviation and navigation industry, the precision matching of special precision small size parts of navigation instruments.
IT4
Used for precision measuring tools, high precision precision fit and P4, P5 rolling bearing with the shaft diameter and shell aperture. For example, check the it9 to it12 workpiece gauge and the IT12 to IT14 shaft gauge, the P4 bearing hole (diameter > 100 mm) and P5 bearing hole matching machine spindle, precision machinery and high-speed machinery journal, P4 bearing matching machine shell hole, diesel engine piston pin and piston pin seat diameter, High precision (grade 1 to grade 4) gear reference hole or shaft diameter, special precision hole diameter of instruments used in aviation and navigation industry.
IT5
Under the regulations with small requirements for fit tolerance and high shape tolerance, such tolerance grade can make the fit property more stable, equivalent to the highest precision in the old national standard, and is used for the special important matching dimensions in machine tools, engines and instruments, and is rarely used in general machinery. For example, check the IT11-IT14 workpiece consumption gauge and the calibration gauge for the IT14-IT15 axis, the box hole of the machine tool matching the P5 rolling bearing, the machine spindle matching the e-level rolling bearing hole, the Journal of precision machinery and high-speed machinery, the machine tail frame sleeve, the high-precision indexing disk journal, indexing head spindle, precision screw reference journal, The outer diameter of the high precision boring sleeve, etc; The matching of precision holes in the main shaft instrument of engine, the holes of the 5-stage precision gears and the reference shafts of the 5-stage and 6-stage precision gears.
IT6
It is equivalent to grade 2 shaft and grade 1 precision hole in the old national standard. It is widely used in the important matching of machinery. For example, the checking gauge for checking IT12 to IT15 workpiece gauge and the checking gauge for checking IT15 to IT16 shaft gauge; Shell hole matched with E-class bearing and machine tool spindle journal matched with roller bearing, assembly type bronze worm gear in machine tool manufacturing, Journal of gear, worm gear, coupling, belt pulley and cam installed on outer diameter of wheel housing; Machine tool screw bearing journal, centering diameter of rectangular spline, column of radial drilling machine, etc; The outer diameter of the guide part of the machine tool fixture, the precision shaft in the precision instrument, the precision shaft in the aviation and navigation instrument, the automation instrument, the post and telecommunications machinery, the particularly important shaft in the watch, the outer diameter of the cylinder liner in the engine, the outer diameter of the crankshaft main journal, the piston pin, the connecting rod bushing, the connecting rod and the bearing bush; The reference hole of grade 6 precision gear and the reference Journal of Grade 7 and grade 8 precision gear are especially precise, such as the top circle diameter of grade 1 or grade 2 precision gear.
IT7
It is 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 grade 2 precision shaft or grade 2 precision hole in the old national standard. For example, check the gauge for IT14 to it16 workpiece and check the gauge for IT16 shaft; In the machine tool, the flange hole diameter of the assembled bronze worm gear, the hole diameter of the coupling, the belt pulley, the cam, etc., the chuck seat hole of the machine tool, the rocker hole of the radial drilling machine, the bearing hole of the lathe lead screw, the inner hole of the chuck guide part of the machine tool, the connecting rod hole, the piston hole of the engine, and the reaming stud positioning hole; It is an important part of textile machinery, a high demand part of printing and dyeing machinery, an inner hole in precision instrument, an important inner hole in computer, electronic instrument and instrument, an important inner hole in automatic instrument, a reference hole of Grade 7 and grade 8 precision gear and a reference axis of grade 9 and grade 10 precision gear.
IT8
In machinery manufacturing, it belongs to medium precision. In instrument, meter and clock manufacturing, it belongs to high precision range because of its small basic size. It is widely used in agricultural machinery, textile machinery, printing and dyeing machinery, bicycle, sewing machine and medical equipment. For example, inspection of IT16 workpiece gauge, size fit of bearing pedestal bushing along the width direction, fit of watch middle span gear shaft, pawl needle wheel and splint, and general fit of radio instrument.
IT9
The application conditions are similar to IT8, but when the precision is lower than IT8, it is slightly larger than the grade 4 precision tolerance of the old national standard. For example, the outer diameter and hole of the shaft sleeve in the machine tool, the matching between the control part and the shaft, the idling pulley and the shaft, the matching between the shaft and the bearing of the control system, the general matching parts in the textile machinery and printing and dyeing machinery, the inner hole of the oil pump in the engine, the inner hole of the valve guide, the matching between the flywheel and the flywheel sleeve, and the general matching dimensions in the automatic instrument, In the watch, the dimension without tolerance is required to be higher, in the single key connection, the fit dimension of the key width, and in the typewriter, the fit dimension of the moving parts.
IT10
The application conditions are similar to it9, but it is required to be adopted when the accuracy is lower than IT9, which is equivalent to the grade 5 accuracy tolerance of the old national standard. For example, the matching of the bracket in electronic instruments and instruments, the matching size of insulating bushing hole and collector ring liner sleeve shaft in navigation instrument, riveting parts in typewriter, the general dimension without tolerance, the dimension without tolerance higher than 18 mm, and the matching size of oil seal ring hole and crankshaft belt hub in engine.
IT11
It is widely used in the situation where the clearance is large and there is no danger due to significant change. It can also be used in the situation where the fitting accuracy is low and large clearance is allowed after assembly, which is equivalent to grade 6 accuracy tolerance of the old national standard. For example, on the machine tool flange stop and hole, slider and sliding gear, groove, etc; Agricultural machinery, locomotive body parts and matching parts for stamping processing, unimportant parts in clock and watch manufacturing, dimensions without tolerance indication in tools and equipment for watch manufacturing, rough movable matching in textile machinery, matching dimensions with lower requirements in printing and dyeing machinery, screw connection and rough dynamic connection in grinder manufacturing, Tolerance of top circle diameter of gear not used as measuring datum.
IT12
The requirement of fit accuracy is very low, and there is a large gap after assembly, which is suitable for parts without fit requirements basically. The limit deviation of dimension with higher requirements without tolerance indication is slightly smaller than the grade 7 accuracy tolerance of the old national standard. For example, non fit dimension and inter process dimension, engine separation rod, unmarked tolerance dimension of process equipment in watch manufacturing, limit deviation of unmarked tolerance dimension of metal processing in computer industry, connection of wrench hole and wrench seat in machine tool manufacturing, etc.
IT13
The application conditions are similar to IT12, but slightly larger than the grade 7 accuracy tolerance value of the old national standard. For example, non fit dimension and inter process dimension, machining parts and wafer hole in computer and typewriter, and the unmarked tolerance dimension of center distance between two holes.
IT14
It is used for non fit dimension and dimension not included in dimension chain, which is equivalent to grade 8 precision tolerance of old national standard. For example, in machine tools, automobiles, tractors, metallurgical machinery, mining machinery, petrochemical industry, motors, electrical appliances, instruments and meters, aviation and navigation, medical devices, clocks and watches, bicycles, sewing machines, paper making and textile machinery and other mechanical processing parts, the limit deviation of tolerance dimensions is not noted.
IT15
It is used for non matching dimensions and dimensions not included in dimension chain, which is equivalent to the precision tolerance of grade 9 of the old national standard. For example, stamping parts, wood mold casting parts, heavy machine tools manufacturing, dimensional limit deviation without tolerance noted when the basic dimension is greater than 3150 mm.
IT16
It is used for non fit dimension, equivalent to grade 10 precision tolerance of old national standard. For example, the dimensions of casting parts in typewriter, the dimensions of box body in radio manufacturing, the general dimensions in surgical instruments, the dimensions for bending and extension processing, the dimensions of wood parts in textile machinery, the dimensions of plastic parts, the dimensions of wood mold manufacturing and free forging.
IT17, IT18
It is used for the tolerance of non fit dimensions, which is equivalent to grade 11 or grade 12 accuracy of the old national standard. It is used for the tolerance of plastic molding dimensions, general external dimensions of surgical instruments, cold working and welding dimensions.

Selection principle of tolerance grade

The essence of selecting tolerance grade is to correctly solve the contradiction between the use requirements of machine parts and the manufacturing process and cost.
The principle of selecting tolerance grade is to select lower tolerance grade as far as possible on the premise of meeting the requirements of parts. The accuracy requirement should be consistent with the possibility of production, that is, reasonable processing technology, assembly technology and existing equipment should be adopted. However, if necessary, it is necessary to improve the accuracy of equipment and process to ensure the accuracy of products. It is very important to select the appropriate tolerance level for the fit size. 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) The selection of tolerance grade should first ensure the use requirements.
  • (2) When selecting tolerance grade, not only the design requirements should be met, but also the possibility and economy of technology should be considered. Choosing the best machining accuracy is a very complicated technical and economic problem. It should not only consider the processing cost, but also consider 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 the mechanical manufacturing, the tolerance level is based on the principle that it can not only ensure the accuracy of the machine and the interchangeability of parts, but also ensure the economy of the manufacturing machine. That is to say, as long as the low precision can guarantee the function and precision of the machine, do not require the precision of parts too high. That will increase the manufacturing cost. Specifically, the tolerance level should be determined according to the type of the machine and the usage of certain parts. The tolerance includes dimensional tolerance and fit tolerance. When using, refer to the recommended tolerance level of various parts listed in the mechanical design manual for flexible application.

The concept of free tolerance

Basic definition

What is free dimensional tolerance? In the old national standard (Hg) 159-59, the accuracy level was divided into 12 levels in the tolerance of datum parts. It is called free dimensional tolerance when it is taken from the datum tolerance of Grade 8 and grade 9. The deviation is divided into three parts: unidirectional (+) or (-), bidirectional( ±) Two are indicated in the notes of free dimension tolerance.

  • ① Free dimensional tolerances apply only to machined surfaces.
  • ② The free dimensional tolerance is not marked on the working drawing.
  • ③ One way deviation uses (-) sign for shaft, (+) sign for hole, hole depth, groove width, hole depth and groove length, and two-way positive and negative deviation for the rest( ±).
  • ④ If the free dimension can not be included in the above-mentioned clear principles and there is one-way deviation requirement, the designer shall indicate it in the working drawing, otherwise it shall be manufactured according to two-way deviation.

In the revised national standard (GB) 1800-79, the standard tolerance is divided into 20 levels. I.e; IT01, it0, it1 to it18. It stands for standard tolerance, the code of tolerance grade is indicated by Arabic numerals, and the grade decreases from IT01 to it18. The limit deviation of dimensions without tolerance indication in (GB) 1804-79 is formulated.

  • ① The specified limit deviation is applicable to both metal cutting and non cutting dimensions.
  • ② The deviation of dimensions without tolerance indication on the drawing shall be specified in the corresponding technical documents according to the series specified in this standard.
  • ③ The tolerance grades of dimensions without tolerance indication are it12 to it18. H (+) is used for general holes; H (-) for shaft; For length( ±)? It (i.e. JS or JS). If necessary, it can be used regardless of hole, shaft or length? It (i.e. JS or JS). According to the international standard ISO 2768, the following is the tolerance table of linear dimension without tolerance indication. This undeclared tolerance is applicable to the dimension of metal cutting and general stamping.

Scope of application

These limit deviations apply to:

  • (1) Linear dimension: such as outer dimension, inner dimension, step dimension, diameter, radius, distance, rounding radius and chamfering height;
  • (2) Angle dimension: including the angle dimension that is not marked with angle value, 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 specified in other general tolerance standards have been established;
  • (2) Reference dimensions in brackets.

Tolerance levels of various machining methods

Turning

Workpiece rotation, turning tool in the plane for straight line or curve movement of cutting. Turning is generally carried out on the lathe to process the internal and external cylindrical surface, end face, conical surface, forming surface and thread of the workpiece.
Generally, the turning accuracy is it8 ~ it7, and the surface roughness is 1.6 ~ 0.8 μm.

  • 1) Rough turning strives to improve turning efficiency by using large cutting depth and large feed rate without reducing cutting speed, but the machining accuracy can only reach it11 and the surface roughness is r α 20~10 μm.
  • 2) Semi finish turning and finish turning adopt high speed and small feed rate and cutting depth as far as possible, the machining accuracy can reach it10 ~ it7, and the surface roughness is r α 10~0.16 μm.
  • 3) High speed finish turning of nonferrous metal parts with diamond turning tools can make the machining accuracy reach it7 ~ it5 and the surface roughness R α 0.04~0.01 μm. This kind of turning is called “Mirror Turning”.

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Milling

Milling refers to the use of rotating multi edge tool cutting workpiece, is an efficient processing method. It is suitable for machining plane, groove, various forming surfaces (such as spline, gear and thread) and special surface of die. According to the same or opposite direction of the main movement speed and the workpiece feed direction during milling, it can be divided into forward milling and reverse milling.
Generally, the milling accuracy can reach it8 ~ it7, and the surface roughness is 6.3 ~ 1.6 μm.

  • 1) Rough milling machining accuracy it11 ~ it13, surface roughness 5 ~ 20 μm.
  • 2) The machining accuracy of semi finish milling is it8 ~ it11, and the surface roughness is 2.5 ~ 10 μm.
  • 3) The precision of finish milling is it16 ~ it8, and the surface roughness is 0.63 ~ 5 μm.

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Planing

Planing is a kind of machining method that uses planer to make horizontal relative linear reciprocating motion on workpiece, which is mainly used for shape machining of parts.
Generally, the planing accuracy can reach it9 ~ it7, and the surface roughness is ra6.3 ~ 1.6 μm.
1) The rough planing accuracy can reach it12 ~ it11, and the surface roughness is 25 ~ 12.5 μm.
2) The precision of semi finish planing can reach it10 ~ it9, and the surface roughness is 6.2 ~ 3.2 μm.

3) The precision of finish planing can reach it8 ~ it7, and the surface roughness is 3.2 ~ 1.6 μm.

Grinding

Grinding is a kind of machining method that uses abrasives and abrasives to cut off the surplus material on the workpiece. It belongs to finish machining and is widely used in the machinery manufacturing industry.
Grinding is usually used for semi finish machining and finish machining, the accuracy can reach it8 ~ it5 or even higher, and the surface roughness is generally 1.25 ~ 0.16 μm.

  • 1) The surface roughness of precision grinding is 0.16 ~ 0.04 μm.
  • 2) The surface roughness of ultra precision grinding is 0.04 ~ 0.01 μm.
  • 3) The surface roughness of mirror grinding can reach 0.01 μ It is less than 5 m.

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Drilling

Drilling is a basic method of hole processing. Drilling is often carried out on the drilling machine and lathe, or on the boring machine or milling machine.
The machining accuracy of drilling is low, generally it can only reach it10, and the surface roughness is generally 12.5 ~ 6.3 μm. After drilling, reaming and reaming are often used for semi finishing and finishing.
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Boring

Boring is a kind of inner diameter cutting technology which uses tools to enlarge holes or other circular profiles. Its application range is generally from semi rough machining to finish machining. The tools used are usually single edge boring tools (called boring bars).
1) The boring accuracy of steel materials can reach it9 ~ it7, and the surface roughness is 2.5 ~ 0.16 μm.

2) The machining accuracy of precision boring can reach it7 ~ IT6, and the surface roughness is 0.63 ~ 0.08 μm.

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Source: Network Arrangement – China Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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