ISO 25378-2011 pdf free download.Geometrical product specifications (GPS) — Characteristics and conditions — Definitions.
4 General presentation
4.1 General principles of the specifications
A GPS specification corresponds
a) to a requirement:
⎯ on all the workpieces taken individually: individual specification,
⎯ on the population of workpieces, taken collectively: population specification, or
b) to a compound requirement combining the two previous types of requirements.
In the case of a single specification, the condition shall be applied to any characteristic value.
This characteristic is a characteristic on one feature (for example, the form characteristic of a nominally planar surface) or between several features (for example, the orientation characteristic between two surfaces) (see ISO 22432).
NOTE This characteristic can be a global characteristic (for example, the diameter of a least-squares cylinder associated to a nominally cylindrical surface) or a function of a local characteristic (function of the two-point diameter of a nominally cylindrical surface).
In the case of population specification, the condition shall be applied on the population of characteristic values.
This population of characteristic values is obtained from a statistic (particular function) on the characteristic values observed on the population of workpieces. The individual characteristic observed on the workpiece shall be a global characteristic.
4.2 General principle of the characteristics
A geometrical characteristic corresponds to quantification or qualification of one or more constraints
⎯ on one ideal or non-ideal feature (individual characteristic), or
⎯ between one ideal or non-ideal feature and another ideal or non-ideal feature (relationship characteristic).
This constraint can be a size, surface texture, form, orientation or location constraint.
Since the characteristic is related to the real workpiece, all types of deviations from the nominal feature can exist.
The geometrical characteristic can be seen as the observation of one or more parameters of a feature:
⎯ surface texture, form, size related to the same nominal feature itself [single (individual) characteristic];
⎯ orientation, location of an ideal or non-ideal feature from one or more other ideal features (relationship characteristic).
The geometrical characteristic can permit the freedom parameters of a feature to be observed independently:
⎯ surface texture/form freedom (1 parameter of shape freedom);
⎯ size freedom (1 parameter of size freedom);
⎯ angular freedom (3 degrees of rotation freedom);ISO 25378 pdf download.