[2] 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.
The GPS&V standards describe the rules to define geometrical specifications which are further included in the technical product documentation.
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).
ISO TC 213 was born in 1996 by merging three previous committees:[9] GPS&V standards are built on several basic operations defined in ISO 17450-1:2011:[10] 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.
[11] 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.
The model in CAD systems describes the nominal geometry of the parts of a product.
This is why a representation of the real part with geometrical deviations (skin model) is introduced as the starting point in the tolerancing process.
Several methods can be used to obtain a partition from a skin model as described in[12] 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 criterion can be the minimisation of a quantity such as the squares of the distances from the points to the ideal surface for example.
It could also be used to group nominally planar geometrical features that are constrained to lie inside the same flatness tolerance zone.
An example, given in ISO 17450-1:2011 is the construction of a straight line resulting from the intersection of two perfect planes.
[note 1] The process to build both the sections and the directions needed to identify the opposite points is defined in ISO 14405-1 standard.
ISO 14405-2 illustrates cases where dimensional specification are often misused because opposite points don't exist.
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: See example hereafter for an illustration.
The symbol Ⓔ modifies the definition of the dimensional specification in the following way (ISO 14405-1 3.8): The maximum inscribed dimension for a nominally cylindrical hole is defined as the maximum diameter of a perfect cylinder associated to the real surface with a constraint applied to the associated cylinder to stay outside the material of the part.
The minimum circumscribed dimension for a nominally cylindrical pin is defined as the minimum diameter of a perfect cylinder associated to the real surface with a constraint applied to the associated cylinder to stay outside the material of the part.
However, those standards are very useful for the user of GPS&V systems as they cover very common aspects of geometrical tolerancing namely groups of cylinders or planes and profile specifications (lines and surfaces).
It is a member of the following set: How to build the situation features and therefore the specified datum, is currently mainly defined through examples in ISO 5459:2011.
The invariance class graphic symbols are not defined in ISO standards but only used in literature as a useful reminder.
It is a volume when it is intended to contain a tolerance feature which is a surface It can often be described as a rigid body with the following attributes: Theoretical exact dimensions (TED) are identified on a nominal model by dimensions with a framed nominal value without any tolerance.
Those dimensions are not specification by themselves but are needed when applying constraints to build datum or to determine the orientation or location of the tolerance zone.
For each example we present: The deviations are enlarged compared to actual parts in order to show as clearly as possible the steps necessary to build the GPS&V operators.
This specification could be useful when one surface (datum plane in this case) has a higher priority in the assembly process.
For example a second part could be required to fit inside the slot being guided by the plane where the datum has been indicated.
For example a second part could be required to fit inside the slot being guided by the plane where the datum has been indicated.
The part is not conformant to the specification for this particular real part, as the toleranced feature (orange line segment) is not included in the tolerance zone (green) This specification could be useful when the two surfaces (plane in this case) have the same priority in the assembly process.
This specification could be useful when the holes is actually located from the edges of the plates in an assembly process and where the A surface has a higher priority over B.