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 (size, form, orientation, location) and micro-geometry (surface texture) of products or parts of products so that the language can be used consistently worldwide.
Background GPS&V standards cover: Dimensional specifications Macrogeometrical specifications (form, orientation, location and run-out) Surface texture specifications Measuring equipment and calibration requirements Uncertainty management for measurement and specification acceptance
Other ISO technical committees are strongly related to ISO TC 213. ISO Technical Committee 10 is in charge of the standardization and coordination of technical product documentation (TPD).
The GPS&V standards describe the rules to define geometrical specifications which are further included in the technical product documentation. The technical product documentation is defined as the: "means of conveying all or part of a design definition or specification of a product". The technical product documentation can be either a conventional documentation made of two dimensional engineering drawings or a documentation based on computer-aided design (CAD) models with 3RD annotations. The ISO rules to write the documentation are mainly described in ISO 128 and ISO 129 series while the rules for 3RD annotations are described in ISO 16792.
ISO Technical Committee 184 develops standards that are closely related to GPS&V standards. In particular ISO TC 184/SC4 develops ISO 10303 standard known as STEP standard (see STEP-file). GPS&V shall not to be confused with the use of ASME Y.14.5 which is often referred to as geometric dimensioning and tolerancing (GD&T).
History and concepts History ISO TC 213 was born in 1996 by merging three previous committees: ISO Technical Committee 10 Sub-committee 5 (ISO/TC 10/SC5) Geometrical Tolerancing ISO Technical Committee 57 (ISO/TC 57) Surface Texture ISO Technical Committee 3 (ISO/TC 3) Limits and fits
Operation GPS&V standards are built on several basic operations defined in ISO 17450-1:2011: Skin model Partition Extraction Filtration Association Collection Construction Reconstruction Reduction
Those operations are supposed to completely describe the process of tolerancing from the point of view of the design and from the point of view of the measurement. They are presented in ISO 17450 standard series. Some of them are further described in other standards e.g ISO 16610 series for filtration. Those concepts are based on academic works.GEOSPELLING: A Common Language for Geometrical Product Specification and Verification to Express Method Uncertainty, Proceedings of the 8th CIRP Seminar on Computer Aided Tolerancing, Charlotte USA Mathieu, L., Ballu, A., 2003. The key idea is to start from the real part with its imperfect geometry (skin model) and then to apply a sequence of well defined operations to completely describe the tolerancing process. The operations are used in the GPS&V standards to define the meaning of dimensional, geometrical or surface texture specifications.
Skin model The skin model is a representation of the surface of the real part. The model in CAD systems describes the nominal geometry of the parts of a product. The nominal geometry is perfect. However, the geometrical tolerancing has to take into account the geometrical deviations that arise inevitably from the manufacturing process in order to limit them to what is considered as acceptable by the designer for the part and the complete product to be functional. This is why a representation of the real part with geometrical deviations (skin model) is introduced as the starting point in the tolerancing process.
Partition The skin model is a representation of a whole real part. However, the designer very often, if not always, needs to identify some specific geometrical features of the part to apply well-suited specifications. The process of identifying geometrical features from the skin model or the nominal model is called a partition. The standardization of this operation is a work in progress in ISO TC 213 (ISO 18183 series).
Several methods can be used to obtain a partition from a skin model as described in
Extraction The skin model and the partitioned geometrical features are usually considered as continuous, however it is often necessary when measuring the part to consider only points extracted from a line or a surface. The process of e.g. selecting the number of points, their distribution over the real geometrical feature and the way to obtain them is part of the extraction operation. This operation is described in ISO 14406:2011
Filtration Filtration is an operation that is useful to select features of interest from other features in the data. This operation is heavily used for surface texture specifications however, it is a general operation that can be applied to define other specifications. This operation is well known in signal processing where it can be used for example to isolate some specific wave length in a raw signal. The filtration is standardized in ISO 16610 series where a lot of different filters are described.
Association Association is useful when we need to fit an ideal (perfect) geometrical feature to a real geometrical feature e.g. to find a perfect cylinder that approximates a cloud of points that have been extracted from a real (imperfect) cylindrical geometrical feature. This can be viewed as a mathematical optimization process. A criterion for optimization has to be defined. This criterion can be the minimisation of a quantity such as the squares of the distances from the points to the ideal surface for example. Constraints can also be added such as a condition for the ideal geometrical feature to lie outside the material of the part or to have a specific orientation or location from an other geometrical feature. Different criteria and constraints are used as defaults throughout the GPS&V standards for different purposes such as geometrical specification on geometrical features or datum establishment for example. However, standardization of association as a whole is a work in progress in ISO TC 213.
Collection Collection is a grouping operation. The designer can define a group of geometrical features that are contributing to the same function. It could be used to group two or more holes because they constitute one datum used for the assembly of a part. It could also be used to group nominally planar geometrical features that are constrained to lie inside the same flatness tolerance zone. This operation is described throughout several GPS&V standards. It is heavily used in ISO 5458:2018 for grouping planar geometrical feature and cylindrical geometrical features (holes or pins). The collection operation can be viewed as applying constraints of orientation and or constraints of location among the geometrical features of the considered group.
Construction Construction is described as an operation used to build ideal geometrical features with perfect geometry from other geometrical features. An example, given in ISO 17450-1:2011 is the construction of a straight line resulting from the intersection of two perfect planes. No specific standard addresses this operation, however it is used and defined throughout a lot of standards in GPS&V system.
Reconstruction Reconstruction is an operation allowing the build of a continuous geometrical feature from a discrete geometrical feature. It is useful for example when there is a need to obtain a point between two extracted points as can be the case when identifying a dimension between two opposite points in a particular section in the process of obtaining a linear size of a cylinder. The reconstruction operation is not yet standardized in the GPS&V system however the operation has been described in academic papers
Reduction Reduction is an operation allowing to compute a new geometrical feature from an existing one. The new geometrical feature is a derived geometrical feature.
Dimensional specification Dimensional tolerances are dealt with in ISO 14405: ISO 14405-1:2016 Linear sizes ISO 14405-2:2018 Dimensions other than linear or angular sizes ISO 14405-3:2016 Angular sizes The linear size is indicated above a line ended with arrows and numerical values for the nominal size and the tolerance. The linear size of a geometrical feature of size is defined by default, as the distances between opposite points taken from the surface of the real part.This is a major discrepancy between GPS&V standards and GD&T (ASME Y14.5) where the dimensional specification include a limitation on the shape deviation more or less as is done when the envelope requirement Ⓔ is added to the specification in GPS&V standard. The process to build both the sections and the directions needed to identify the opposite points is defined in ISO 14405-1 standard. This process includes the definition of an associated perfect geometrical feature of the same type as the nominal geometrical feature. By default a least-squares criterion is used. This process is defined only for geometrical features where opposite points exist.
ISO 14405-2 illustrates cases where dimensional specification are often misused because opposite points don't exist. In these cases, the use of linear dimensions is considered as ambiguous (see example). The recommendation is to replace dimensional specifications with geometrical specifications to properly specify the location of a geometrical feature with respect to an other geometrical feature, the datum feature (see examples).
Angular sizes are useful for cones, wedges or opposite straight lines. They are defined in ISO 14405-3. The definition implies to associate perfect geometrical features e.g. planes for a wedge and to measure the angle between lines of those perfect geometrical features in different sections. The angular sizes are indicated with an arrow and numerical values for the nominal size and the tolerance. Angular size specification is different from angularity specification. Angularity specification controls the shape of the toleranced feature but it is not the case for angular size specification.
Size of a cylinder We consider here the specification of a size of a cylinder to illustrate the definition of a size according to ISO 14405-1. The nominal model is assumed to be a perfect cylinder with a dimensional specification of the diameter without any modifiers changing the default definition of size. According to ISO 14405-1:2016 annex D, the process to establish a dimension between two opposite points starting from the real surface of the manufactured part which is nominally a cylinder is as follows: Partition of the real surface to identify the portion of the whole surface of the part that is submitted to the specification Extract points from the partitioned surface Reconstruct the surface from extracted points if the number of extracted points is not infinite Filter the reconstructed surface Associate a perfect cylinder to the filtered surface using a least-squares criterion Identify the straight line which is the axis of the associated cylinder Built a plane perpendicular to the associated cylinder axis to identify a cross section Consider the section line which is the intersection of the plane perpendicular to the associated cylinder axis, with the filtered surface Associate a perfect circle to the section line using a least-squares criterion Consider a straight line in the cross section passing through the centre of the associated circle Two opposite points are defined as the intersection between the straight line and the section line
See example hereafter for an illustration.
Dimension with envelope requirement Ⓔ