The skin model : a fundamental concept for product specification and verification

In modern production engineering, new complex products with controlled tolerances are being increasingly adopted to improve companies’ market position. Geometrical variations are inevitably generated during the manufacturing stage due to the limited accuracy of manufacturing technologies. Within the context of Product Lifecycle Management (PLM), information communication and sharing requires to manage the geometrical variations along the whole product lifecycle. The geometrical variations should be considered early in the tolerancing process in the design stage. Different concepts have been proposed to build coherent and complete tolerancing processes along the whole product lifecycle. Among them, GeoSpelling proposed by Ballu and Mathieu in 1995, as the basis of the actual Geometrical Product Specification (GPS) standards, enables a comprehensive modelling framework and an unambiguous language to describe geometrical variations covering the overall product life cycle thanks to a set of objects as features, operations and characteristics based on the fundamental concept of the “Skin Model”. The Skin Model concept is the basic concept of GeoSpelling. It is an abstract model of the physical interface between a workpiece and its environment. Different from the nominal model, which is deemed as an ideal feature, the Skin Model is a shape model to describe the non-perfect shapes. In GeoSpelling, a specification is as a condition on a characteristic defined from geometric features, which are created from a Skin Model by different operations (partition, extraction, filtration, association, collection and construction). To be able to understand the meaning of a specification based on this definition, a tool called GPS card has been created. This tool considers every type of specifications, for isolated parts and for assemblies. This tool also compensates the contradictions and lacks of current ISO GPS standards. It is a very useful tool for inspection and industrial measurement activities. It permits to define without ambiguity the characteristic to be measured. Geospelling is able to express clearly the measurand and the sequence of operations with the same language used for the specification. It is what ISO 213 calls the Duality principal. This approach helps inspection people to highlights the method uncertainty, the difference between the characteristic to be measured and the measurand. The concept of the Skin Model has been initially created for describing without ambiguity the quantity to be measured. After the choose by ISO, as starting point for creating new standards in the field of Specification and Verification and fifteen years later, the idea was to create digital Skin Models for simulation and visualisation. This idea was to operationalize GeoSpelling on both nominal model and Skin Model. However, this operationalization has not been successfully completed in a digital manner and few research studies have focused on the Skin Model representation and simulation. As a response, the concept of Skin Model Shapes has been developed as a novel approach for the consideration of product shape variability. It employs discrete geometry methods and computational techniques, such as point clouds, surface meshes and geometric processing, to model shape variability and to facilitate the communication of geometric product information throughout the product design, manufacturing, and inspection processes. Suitable representation methods should be carefully considered for different purposes. A shape representation scheme can be defined as a mapping from a computer structure to a well-defined mathematical model, which defines the notion of the physical object in terms of computable mathematical properties and is independent of any particular representation scheme. Skin Model Shapes, which are particular Skin Model representatives from a simulation perspective, are generated. In this regard, a Skin Model Shape is a specific outcome of the conceptual Skin Model and comprises deviations from manufacturing and assembly. The process for generating Skin Model Shapes is split into a prediction and an observation stages with respect to the available information and knowledge about expected geometric deviations. This talk highlights the foundations of the concept of Skin Model and its potentials for the representation of product geometry considering geometric variations along the product lifecycle, and illustrates main applications in the context of product and process development.