Introduction

ISO 10303 is an International Standard for the computer-interpretable representation of product information and for the exchange of product data. The objective is to provide a neutral mechanism capable of describing products throughout their life cycle. This mechanism is suitable not only for neutral file exchange, but also as a basis for implementing and sharing product databases, and as a basis for retention and archiving.

Geometric tolerances are used in the specification of a feature of the shape of a product. A geometric tolerance defines a tolerance zone, which is a region in space in which the feature must lie. The shape of the zone depends upon the kind of tolerance and feature.

EXAMPLE 1   A flatness tolerance zone is bounded by two parallel planes.

The tolerance is a length measure that is a size dimension of the tolerance zone.

Geometric tolerances of form be used to control the form of an individual feature. Tolerances of form include flatness, straightness, cylindricity, and roundness also called circularity. The tolerance zone of a tolerance of form is not related to any other features and might be in any location or orientation. Tolerances of form might be applied to elements of a feature.

EXAMPLE 2   A flatness tolerance might require only that a feature is locally flat throughout. The tolerance applies to any restricted region of a certain size.

Geometric tolerances might control the location or orientation of a feature in relationship to others. In this case, the tolerance zone is disposed about the theoretically exact location of the feature it controls, relative to a datum system.

A datum system defines a Cartesian coordinate system that is sufficient to locate and orient the tolerance zone. The datum system is constructed from features, called datum features of the part. Datums are normally points, lines, and planes that are derived from datum features, or from restricted regions called targets. Datums might lie on the surface or on a centre of symmetry of a feature of size. Features of size are symmetrical about a point, line or plane and have a size dimension.

EXAMPLE 3   A hole is a feature of size with a centre line.

EXAMPLE 4   The two opposing surfaces of a slot form a feature of size with a centre plane.

A datum system need not be a complete coordinate system. It provides sufficient datums to uniquely locate the tolerance zone. The number of datums that is required, depends on the kind of datum features and toleranced features and their theoretically exact spatial relationship.

The theoretically exact location of the toleranced feature relative to the datum system is obtained either directly query of the geometric model of the shape, or from explicit instances of geometric dimensions. In either case, the dimensions must be distinguished as theoretically exact either by designation or by convention.

Tolerances of location include position, concentricity, coaxiality, and symmetry. Tolerances of orientation include angularity, and its special cases parallelism and perpendicularity.

Tolerances of position control the location of features of size. The tolerance of a Position tolerance might vary depending on whether the size of the feature departs from its greatest or least allowed size.

Position tolerances might be applied simultaneously to patterns, and composite position tolerances can be applied to further refine the relative locations among the features of the pattern.

The method of deriving the datum from the imperfect features of a workpiece might be deliberately left indeterminate, so as minimize the needless rejection of parts. In this case, any of a set of candidate datums might be used to locate the tolerance zone.

EXAMPLE 5   A hole is larger than its required minimum can float when used as a datum feature.

Tolerances of profile control the outline of a feature in a plane of projection or cross section. Profile tolerances might be applied with or without reference to a datum system. Profile tolerances might control form, orientation, and location depending on how they are applied. Profile tolerances include profile of a line, and profile of a surface.

Runout tolerances are composite tolerances that control a feature relative to a datum axis. Runout tolerances include circular runout and total runout.

Geometric tolerance and datum specifications are interpreted according to the algorithms of external standards, such as ISO 1101, which must be referenced for the tolerance to be well defined. The means of referencing the standards for a particular product definition is not provided in this part of ISO 10303.

Clause 1 defines the scope of the application module and summarizes the functionality and data covered. Clause 3 lists the words defined in this part of ISO 10303 and gives pointers to words defined elsewhere. The information requirements of the application are specified in Clause 4 using terminology appropriate to the application. A graphical representation of the information requirements, referred to as the application reference model, is given in Annex C. Resource constructs are interpreted to meet the information requirements. This interpretation produces the module interpreted model (MIM). This interpretation, given in 5.1, shows the correspondence between the information requirements and the MIM. The short listing of the MIM specifies the interface to the resources and is given in 5.2. A graphical representation of the short listing of the MIM is given in Annex D.

In ISO 10303, the same English language words can be used to refer to an object in the real world or concept, and as the name of an EXPRESS data type that represents this object or concept.

The following typographical convention is used to distinguish between these. If a word or phrase occurs in the same typeface as narrative text, the referent is the object or concept. If the word or phrase occurs in a bold typeface or as a hyperlink, the referent is the EXPRESS data type.

The name of an EXPRESS data type can be used to refer to the data type itself, or to an instance of the data type. The distinction between these uses is normally clear from the context. If there is a likelihood of ambiguity, either the phrase "entity data type" or "instance(s) of" is included in the text.

Double quotation marks " " denote quoted text. Single quotation marks ' ' denote particular text string values.

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