Parametric Feature Based Design in Manufacturing Systems

An example of CAD application where the feature- and constraint-based representation model is most appropriate is parametric feature-based manufacturing [47].

Parametric modeling was commonly used for the construction of complex models on which parameters were used to provide for subsequent customization. The parameters defined during the design and modeling process are relative to the individual geometric characteristics of the model or to the model as a whole. For example, the parameters can control characteristics such as length, height, width and hole radius.

On the other hand, feature-based modeling is a representation scheme based on the combination of individual feature components. In this context a feature is a unit that can be defined as a connected set of geometric elements (i.e. a subpart) associated with attributes that describe its shape and behavior, such as geometry, topology, functionality and connectivity with other features. In traditional approaches each feature is linked to a set of local parameters that control its attribute values. Here, the feature-based model is complemented through the use of local and global constraints. The constraints are applied locally, in reference to the parameter values or the geometric characteristics of the primitives of the features to impose design or user-defined specifics such as hole size, pocket depth, and globally, in reference to the connectivity and the inter-feature relations of the model.

Much work has been performed on the definition of features in relation to various CAD applications. Features are often perceived as 3D solid components that can be classified into feature libraries depending on their shape or geometry. This point of view is described for instance in [48], where the authors present a library of features fit for manufacturing applications, and in [49], where design features for machining are examined. In [50] features are defined as pierced voxels that are used to create traditional pierced jewellery. However, features can also be defined from surfaces, which is especially common in freeform design applications. For instance, [51] examines freeform surface features, whereas [52] presents a taxonomy of freeform features. Other work uses the notion of feature points and feature lines ([53], [54]) for applications usually related to data segmentation for reverse engineering, or shape deformation and manipulation. Since the definition of a feature is not strict, Hoffmann and Joan-Arinyo in [55] suggest the use of user-defined features in feature based modeling.

Parametric and feature based modeling is an essential component of current CAD design systems. In traditional CAD systems, CSG and Brep models are created by adding and subtracting parts in the model and by applying transformations and various design operations. Design intent was not a concern in these systems and therefore precise editing that involved structural and arbitrary topological modifications of parts of the model was almost impossible without rebuilding the model from scratch. Editing a part of the model is feasible if the design steps are undone until the model returns to the previous state, when the part was created. This of course is possible if the design history of the model is recorded and it is obvious that even though editing theoretically concerns a part of the model, ultimately the whole design process is affected. Feature based CAD systems overcome this limitation by capturing design intent. Since the models are constructed using parameters and features, local editing is possible without necessarily affecting the whole model. Changes are propagated through the model based on the parameters and constraints defined in the system and based on the attributes and connectivity of the features. Feature-based constraint-based modeling systems provide libraries of feature components to be used in the design process and some support user- defined features. Applications such as custom design are feasible since components of models can be combined or re-designed to satisfy user defined preferences or requirements.

Many commercial CAD modeling systems support parametric and/or feature-based modeling. Systems such as PRO/Engineer [56], AUTOCAD [57], IRONCAD [58], CATIA [59], Solidworks [60], SolidEdge [61] and Alibre design [62], which have been developed mainly for mechanical engineering, manufacturing and industrial design applications, have been integrated with parametric and/or feature based modeling capabilities. Architectural Desktop and AUTOCAD are CAD systems used in architectural applications that support parametric modeling. 3D Studio Max [63] and Maya [64] are parametric feature-based modeling systems used for artwork and animation. There are also systems that have been developed for specific CAD applications, such as jewelry, clothing and textile design, marine applications and furniture.

The above modeling systems are very efficient for manufacturing and production applications. However more freeform applications, such as aesthetic and custom design, are still challenging even with these systems. An interesting case is jewellery design. A large number of CAD systems for jewellery design are parametric feature-based. They provide graphical interfaces with excellent rendering capabilities. The majority of these systems provide built-in libraries of settings and cut gems and stones and advanced feature-based design tools. Some systems provide advanced functionality that provides the use of builders

for recording design steps and for defining parameter values for parts to be used in the process. Also, the majority of these systems have the capability of exporting models to rapid prototyping machines. However, in most CAD systems for jewellery, designing is performed manually using various tools and usually the design steps cannot be programmed to be executed automatically and accurately. This means that each different piece of jewellery has to be created basically from the beginning by hand, making custom design applications difficult and time-consuming. Also these systems require that the user has designing skills or knowledge of using CAD systems. In the following we will present an interesting example of jewellery application that is difficult to carry out with existing CAD systems: the construction of traditional pierced jewellery, .

Figure 1. Using a chisel to create carvings around a hole.

Figure 2. A structural element (feature).

In [50] ByzantineCAD, a feature-based CAD system suitable for the design of pierced Byzantine jewellery is presented. The system is automated and parametric meaning that the user-designer sets some parameter values and ByzantineCAD creates the jewellery model that corresponds to the specified values. This provides the designer with the ability to rapidly create custom-designed jewellery, based on the preferences of the customers such as including their initials on a ring. ByzantineCAD introduces a feature-based and voxel-based approach to designing jewellery, through the definition of elementary structural elements with

specific attributes and properties that are used as building blocks to construct complex pierced designs.

More specifically, pierced Byzantine jewellery are gold jewels with pierced designs that were made along the coastlines of the eastern Mediterranean Sea during the period 3rd –7th century A.D. Their originality is due to the particular processing technique that is used for their creation resulting in a special aesthetic effect. Pierced jewellery was created from thin sheets of gold. The designs were engraved on these sheets of gold with a thin sharp tool. After the outlining of the designs, holes following their shape were created and these were decorated with triangular carvings, using an iron chisel.

(a)

Figure 3. Pierced voxel elements such as (a) are used as features to create complex solid plaques representing designs, i.e. letters or words, that are sized and modified appropriately to construct custom-designed jewellery (i.e. ring).

In ByzantineCAD a feature library of carved, pierced voxel elements is defined in accordance to the craftsmanship used in traditional Byzantine jewellery. The design of pierced jewellery is made up of cylindrical holes that have carvings around them. Each hole with the corresponding carvings around it is considered for the purposes of reconstruction as a structural element (feature). Each feature is a solid made of a rectangular parallelepiped with a cylindrical hole and the corresponding carvings around the hole (figures 1,2) . According to the aesthetic rules that characterize traditional pierced jewellery, all structural elements have the same size but differ in the position of the hole and the carvings around it. The hole can be located either in the center of the parallelepiped or in the center of any of the four quarters.

Note that, in terms of computer aided design and manufacturing, the cylindrical hole can be positioned anywhere in the rectangular parallelepiped; the above restriction follows from careful interpretation of the traditional artistic patterns used. Attributes of these feature elements are characteristics such as the number of carvings around the cylindrical hole, the position of the hole in the parallelepiped, the directions of the carvings and more. A large number of different structural elements can be created by a hole and various carvings and, since not all of these feasible feature elements are valid for use in creating pierced designs, restrictions concerning the carving directions are defined based on aesthetic and artistic rules.

These feature elements are combined like 3D building blocks to create complex carved plaques representing pierced designs (Figure 3). The structural elements are placed side by side, either on top, bottom, right or left of each other, and unioned into a new object. The rules determining how the different features can be combined are defined by the designs to be recreated. The construction of these plaques is constrained by the parameter values defined by the user-designer in reference to characteristics of the plaque such as length and width. The plaques are then used to create jewellery such as rings and necklace pendants. By parameterizing the process of creating pierced jewellery, it is very easy to modify characteristics of the jewellery such as the size and the designs represented.