GD&T: Fundamental concepts
From WikiSTEP
A tolerance describes a constraint on the acceptable deviation of a manufactured object from the ideal design. Tolerances are applied to the geometric aspects or features of a part, such as edges, faces and holes.
The fundamental principles of geometric tolerances can be found in national and international standards such as ANSI Y14.5M-1994 or ISO 5459-1981.
There are several subtypes of the geometric_tolerance entity, which are not mutually exclusive. For example, tolerances that reference datums are of type geometric_tolerance_with_datum_reference. Tolerances that include a modifier such as maximum material condition are of type modified_geometric_tolerance. Many typical engineering tolerances combine these. In these cases, complex entities instances will occur in the Part 21 file.
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Dimension and Tolerance
Dimension is a term for a specification of the value of a parameter of some aspect of the shape of a mechanical part or assembly. A dimension can be implied by the geometric model, or it can be explicitly modeled, which is what this guide covers. The term dimension can also refer to the numerical value itself; however in this document the term value is used. Tolerance is a general term for permitted variations in the shape of manufactured parts. Tolerance treats the how the realized form or measurements of a real manufactured object compare to their design ideals.
Dimensions and dimensional tolerances
The dimensions and dimensional tolerances addressed here are:
- directional dimensions
- location dimensions such as angular, curved, or linear distances
- size dimensions such as angular, thickness, or other
- the association of dimensions with geometry
- the representation of dimensional tolerances including:
- plus-or-minus deviations
- maxima, minima, and nominal dimensions
- limits and fits
- significant digits
- the association of dimensional tolerances with dimensions
Geometric tolerances
The geometric tolerances addressed here are:
- Angularity
- Circular runout
- Circularity
- Coaxiality
- Concentricity
- Cylindricity
- Flatness
- Parallelism
- Perpendicularity
- Position
- Profile of a line
- Profile of a surface
- Roundness
- Straightness
- Symmetry
- Total runout
Tolerance modifiers (Maximum and minimum material condition, regardless of feature size and projected tolerance zone) are also addressed.
Feature Entities and Attributes
Dimensions and dimensional tolerances are applied to aspects of the product shape. The product and product shape are modeled as in other geometry and PDM applications.
GD&T Features in STEP are modeled as shape_aspects. The term feature in some dimensioning standards is reserved for definitional elements that lie in the surface of the part. In such cases, a term such as derived element, is used for derived geometry. In computing systems and in some dimensioning standards, either is called a feature, and these are distinguished as integral feature and derived feature. In this recommendation, all are modeled as shape aspect. For integral features, shape_aspect.product_definitional=�.TRUE.� For derived elements it is shape_aspect.product_definitional=�.FALSE.�
Identifying Features
The surface of a part can be partitioned into features, to which dimensions are applied. Normally a feature boundary corresponds to a locus of discontinuities of surface curvature, as when a straight side encounters a corner fillet. For the purposes of GDT, every point on the surface is either in the interior of one feature or on the boundary of two or more features. When a finished part is measured, each point of the surface belongs to exactly one feature.
It is sometimes convenient to treat as a single feature a union of these natural geometric features. Unions can be disjoint, for example:
- a pattern of holes
- a surface that is interrupted by a slot
- the two sides of a slot
Unions can be made of contiguous features for example:
- the all-around shape of an irregular hole.
Similarly its may be necessary to identify a restricted region of a feature, for example:
- Where a tighter tolerance is required
- to indicate how a finished piece is mounted on a inspection workbench.
Data elements of the representation of GDT features and derived elements
It is recommended that when available the advanced_face be used for representing a feature, since its topology is well-defined.
Derived center elements such as points, curves, and surfaces may be unbounded and can be represented by geometry primitives. In GD&T the derived elements are considered to be implicitly bounded where they intersect another feature of a part. Any geometric_representation_item or topological_representation_item could potentially be incorporated into a feature or derived element’s representation.
Datum Systems
Some types of tolerances refer to one or more datums in order to represent the requirements on the shape. Datum systems are related datums that provide a reference system for describing requirements on the product shape.
Datums
A datum is a theoretically exact geometric reference, such as an exact point, axis or plane, to which toleranced features are related. A datum is the origin from which the location or geometric characteristics of features of a part are established. A datum may be based on one or more datum features of a part.
Datum Features
Datum Features are tangible features of a part, for example a face that provides a reference system for measurements of the actual part. Datum Features must lie on the physical boundary of the shape. Consequentially, Datum Feature entities are related to topological entities that represent those boundaries in the solid model such as an advanced_face.
Datum Targets
A datum Target designates a specific point, line or area of contact on a part that is used in establishing a data reference frame (definition from ANSI Y14.5). It differs from a datum_feature in that it identifies a restricted region of a feature, i.e. a point, line or area of a surface rather than a topological feature. Typically, two or more datum target elements are used to define a datum.
