IT Grade

Last updated

Note: in this context, IT does not mean Information Technology, but it is an Engineering term.

Contents

An IT grade is an internationally accepted code system for tolerances on linear dimensions. Such code systems may be used to produce interchangeable parts. In engineering, the word tolerance refers to a range of allowable dimensions or values. Standard tolerance grades are a group of tolerances for linear sizes characterized by a common identifier. For SI measurements, a system of tolerance grades defined in ISO 286 is frequently used and identified by the letters IT followed by a number specifying how precise the requirements are, relative to the nominal size of a part.

For example, IT14 refers to a group of tolerances used in manufacturing. For a part dimensioned at 10 mm, IT14 allows for up to 0.36 mm of variation in size. As the IT grade number increases, the tolerances increase; similarly, for larger nominal sizes, the standard tolerances increase. For a part dimensioned at 100 mm, IT14 allows for up to 0.87 mm of variation in size.

Definition

The ISO 286 defines the international tolerances grades for nominal sizes up to 3.15 meters as follows: [1]

ISO 286 - Table 1
Nominal sizeInternational tolerance grade
>IT01IT0IT1IT2IT3IT4IT5IT6IT7IT8IT9IT10IT11IT12IT13IT14IT15IT16IT17IT18
mmtolerance in μmtolerance in mm
030
.30
.50
.81
.22
.034610142540600
.100
.140
.250
.400
.601
.001
.40
360
.40
.61
.01
.52
.545812183048750
.120
.180
.300
.480
.751
.201
.80
6100
.40
.61
.01
.52
.546915223658900
.150
.220
.360
.580
.901
.502
.20
10180
.50
.81
.22
.035811182743701100
.180
.270
.430
.701
.101
.802
.70
18300
.61
.01
.52
.546913213352841300
.210
.330
.520
.841
.302
.103
.30
30500
.61
.01
.52
.54711162539621001600
.250
.390
.621
.001
.602
.503
.90
50800
.81
.22
.035813193046741201900
.300
.460
.741
.201
.903
.004
.60
801201
.01
.52
.5461015223554871402200
.350
.540
.871
.402
.203
.505
.40
1201801
.223
.55812182540631001602500
.400
.631
.001
.602
.504
.006
.30
1802502
.034
.571014202946721151852900
.460
.721
.151
.852
.904
.607
.20
2503152
.54681216233252811302103200
.520
.811
.302
.103
.205
.208
.10
31540035791318253657891402303600
.570
.891
.402
.303
.605
.708
.90
400500468101520274063971552504000
.630
.971
.552
.504
.006
.309
.70
50063091116223244701101752804400
.701
.101
.752
.804
.407
.0011
.00
630800101318253650801252003205000
.801
.252
.003
.205
.008
.0012
.50
8001,000111521284056901402303605600
.901
.402
.303
.605
.609
.0014
.00
1,0001,2501318243347661051652604206601
.051
.652
.604
.206
.6010
.5016
.50
1,2501,6001521293955781251953105007801
.251
.953
.105
.007
.8012
.5019
.50
1,6002,0001825354665921502303706009201
.502
.303
.706
.009
.2015
.0023
.00
2,0002,50022304155781101752804407001,1001
.752
.804
.407
.0011
.0017
.5028
.00
2,5003,15026365068961352103305408601,3502
.103
.305
.408
.6013
.5021
.0033
.00

From IT6 to IT18, the standard tolerances are multiplied by the factor 10 at each fifth step. This rule applies to all standard tolerances and may be used to extrapolate values for IT grades not given in Table 1. For example, the nominal size range 120 mm up to and including 180 mm, the value of IT20 is:

   IT20 = IT15 × 10 = 1.6 mm × 10 = 16 mm  [2] 

IT grades may be used with alternate prefixes which identify how the tolerance limits are distributed around a nominal value. When used with the IT prefix, IT grades do not specify how the tolerance limits are placed around the nominal value, alternate prefixes are used for this purpose. For example, if the tolerance limits are distributed symmetrically above and below the nominal value, the prefix "js" may be used. For example a part dimensioned (in millimeters) as 4 js7 is equivalent to 4 ± 0.006 (where 4 IT7 is 0.012.)

Other standardized prefixes include the letters A, B, C, CD, D, E, EF, F, G, H, J, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC (for holes), and the lower-case equivalents (for shafts.) Prefix letters I, L, O, Q and W are not used to avoid confusion. Of these, the letter prefixes H and h are easiest to explain as the tolerance lies entirely on one side of the nominal size. A hole dimensioned at 4 H7 may range from 4.00 - 4.012 mm, and a shaft at 4 h7 may range from 3.988 - 4.00 mm.

Preferred tolerance classes and fits

ISO 286 identifies a set of preferred tolerance classes for holes which include G7, H7, JS7, K7, N7, P7, R7, S7, F8, H8, E9, H9, D10, A11, B11, C11, and H11. The set of preferred tolerance classes for shafts includes g6, h6, js6, k6, n6, p6, r6, s6, f7, h7, e8, d9, h9, a11, b11, c11 and h11.

To completely specify the fit between a hole and corresponding shaft, it is common to specify a pair of the above tolerance classes, for example H7/g6. As with all IT grades, the smaller numbers correspond to tighter tolerances. Under normal circumstances, only a small number of the possible fits are practically required, and ISO 286 identifies preferred fit combinations including these as most preferred: [3]

Other uses



An industrial process may have an IT grade associated with it, indicating how precise it is. When designing a part, an engineer will typically determine a critical dimension (D) and some tolerance (T) on that dimension. Using this formula, the engineer can determine what IT Grade is necessary to produce the part with those specifications. Thus, if injection molding is capable of IT13 and a part requires IT5, one cannot use injection molding to produce that part to those specifications. It is useful in determining the processes capable of producing parts to a specification.

See also

Related Research Articles

<span class="mw-page-title-main">Paper size</span> Standard sizes of paper

Paper size standards govern the size of sheets of paper used as writing paper, stationery, cards, and for some printed documents.

<span class="mw-page-title-main">Engineering drawing</span> Type of technical drawing used to define requirements for engineered items

An engineering drawing is a type of technical drawing that is used to convey information about an object. A common use is to specify the geometry necessary for the construction of a component and is called a detail drawing. Usually, a number of drawings are necessary to completely specify even a simple component. These drawings are linked together by a "master drawing." This "master drawing" is more commonly known as an assembly drawing. The assembly drawing gives the drawing numbers of the subsequent detailed components, quantities required, construction materials and possibly 3D images that can be used to locate individual items. Although mostly consisting of pictographic representations, abbreviations and symbols are used for brevity and additional textual explanations may also be provided to convey the necessary information.

<span class="mw-page-title-main">Geometric dimensioning and tolerancing</span> System for defining and representing engineering tolerances

Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof. GD&T is used to define the nominal geometry of parts and assemblies, the allowable variation in size, form, orientation, and location of individual features, and how features may vary in relation to one another such that a component is considered satisfactory for its intended use. Dimensional specifications define the nominal, as-modeled or as-intended geometry, while tolerance specifications define the allowable physical variation of individual features of a part or assembly.

<span class="mw-page-title-main">Engineering tolerance</span> Permissible limit or limits of variation in engineering

Engineering tolerance is the permissible limit or limits of variation in:

  1. a physical dimension;
  2. a measured value or physical property of a material, manufactured object, system, or service;
  3. other measured values ;
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel ;
  5. in mechanical engineering, the space between a bolt and a nut or a hole, etc.
<span class="mw-page-title-main">Hole punch</span> Office tool for making uniform holes in paper

A hole punch, also known as hole puncher, or paper puncher, is an office tool that is used to create holes in sheets of paper, often for the purpose of collecting the sheets in a binder or folder. A hole punch can also refer to similar tools for other materials, such as leather, cloth, or plastic or metal sheets.

<span class="mw-page-title-main">Screw thread</span> Helical structure used to convert between rotational and linear movement or force

A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

<span class="mw-page-title-main">Go/no-go gauge</span> Inspection tool

A go/no-go gauge refers to an inspection tool used to check a workpiece against its allowed tolerances via a go/no-go test. Its name is derived from two tests: the check involves the workpiece having to pass one test (go) and fail the other (no-go).

An interference fit, also known as a pressed fit or friction fit, is a form of fastening between two tightfitting mating parts that produces a joint which is held together by friction after the parts are pushed together.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule. NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which also abbreviates as "NPS". The European and international designation equivalent to NPS is DN, in which sizes are measured in millimetres, see ISO 6708. The term NB is also frequently used interchangeably with DN.

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.

When two probability distributions overlap, statistical interference exists. Knowledge of the distributions can be used to determine the likelihood that one parameter exceeds another, and by how much.

Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined. Engineering fits are generally described as a "shaft and hole" pairing, but are not necessarily limited to just round components. ISO is the internationally accepted standard for defining engineering fits, but ANSI is often still used in North America.

In engineering and machining, an allowance is a planned deviation between an exact dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An allowance, which is a planned deviation from an ideal, is contrasted with a tolerance, which accounts for expected but unplanned deviations.

The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present.

Tolerance analysis is the general term for activities related to the study of accumulated variation in mechanical parts and assemblies. Its methods may be used on other types of systems subject to accumulated variation, such as mechanical and electrical systems. Engineers analyze tolerances for the purpose of evaluating geometric dimensioning and tolerancing (GD&T). Methods include 2D tolerance stacks, 3D Monte Carlo simulations, and datum conversions.

ISO 128 is an international standard (ISO), about the general principles of presentation in technical drawings, specifically the graphical representation of objects on technical drawings.

C11 is an informal name for ISO/IEC 9899:2011, a past standard for the C programming language. It replaced C99 and has been superseded by C17. C11 mainly standardizes features already supported by common contemporary compilers, and includes a detailed memory model to better support multiple threads of execution. Due to delayed availability of conforming C99 implementations, C11 makes certain features optional, to make it easier to comply with the core language standard.

<span class="mw-page-title-main">Graduated pipette</span> Pipette with its volume, in increments, marked along the tube

A graduated pipette is a pipette with its volume, in increments, marked along the tube. It is used to accurately measure and transfer a volume of liquid from one container to another. It is made from plastic or glass tubes and has a tapered tip. Along the body of the tube are graduation markings indicating volume from the tip to that point. A small pipette allows for more precise measurement of fluids; a larger pipette can be used to measure volumes when the accuracy of the measurement is less critical. Accordingly, pipettes vary in volume, with most measuring between 0 and 25.0 millilitres.

<span class="mw-page-title-main">Preferred metric sizes</span> Metricated industry standards

Preferred metric sizes are a set of international standards and de facto standards that are designed to make using the metric system easier and simpler, especially in engineering and construction practices. One of the methods used to arrive at these preferred sizes is the use of preferred numbers and convenient numbers, such as the Renard series and 1-2-5 series, to limit the number of different sizes of components needed.

Geometrical Product Specification and Verification (GPS&V) is a set of ISO standards developed by ISO Technical Committee 213. The aim of those standards is to develop a common language to specify macro geometry and micro-geometry of products or parts of products so that the language can be used consistently worldwide.

References

  1. International Organization for Standardization. ISO 286-1:2010 Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes, Part 1. p. 20.
  2. Ibid. p. 12
  3. Ibid. p. 28