All of the above are important for operation, manufacturing, and control of products.
2.5 Feature‐Based Modelling with Design Intents
There are numerous ways to create computer models using a given geometry. However, geometry and shape of an object have their purpose in a product and it is very helpful for a designer to take design intent into account when creating computer representations of solids.
Design intent is a term used to describe how the model should be created and how it should behave when it is changed. Design intent should be built into the model according to how dimensions and relations are established, since changes to a model will yield a different result for each different design intent.
Design intent is not just about the size and shape of features, but it can be extended to cover tolerances, manufacturing processes, design constraints, and relationships of features and dimensions. The use of design intents is an effective approach to build a parametric model of a part that is fully constrained and easy for modification. For example, sketches can be dimensioned to reflect design intents in parametric modelling. If the design intents for the part have not been adequately considered, the model might be useless from a practical viewpoint (Rynne 2006).
With regards to design changes, geometric features created at the design phase are closely related to manufacturing activities at other phases of a product cycle. It is ideal that designers are able to model solids directly with expected features. The advancement of computer aided methods in Figure 2.34 shows the growing expectation of the functional requirements (FRs) on computer aided technologies, which is from two‐dimensional drafting and drawing to high‐level solid modelling with consideration of design intents.
Figure 2.34 From 2D drafting and drawing to interactive solid modelling with design intents.
Feature‐based modelling refers to the construction of object geometry as a combination of form features with design intents. The designer specifies the features in engineering terms, such as holes, slots, or bosses, rather than in geometric terms, such as circles or boxes. The concepts of features can also be extended to include non‐graphic information. This information can be used in activities such as drafting, numerical control (NC), finite‐element analysis, and kinematic analysis. Furthermore, feature‐based packages frequently record the geometric construction and modification sequences used in building the model.
Bunge (1983) gave the principles of feature‐based modelling as:
1 The physical world consists of things that are considered to be objects regardless of their contents. Objects can be characterized by their features, known or to be detected by scientific instruments. Features are quality and quantity characteristics, together with the correlations between them.
2 In terms of design, products and their various parts can be interpreted as objects, while features are characteristics associated with them. Relations between characteristics are described and regulated by correlations and restrictions.
3 As regards mechanical products, the geometric form is of primary importance in respect of material realization; therefore, it seems to be natural that the geometry of objects is derived from given features and their relations.
Features in an object can be defined by three aspects: (i) the characteristics of geometric shape, (ii) the characteristics of processes, and (iii) the ontological interpretation for the meanings of features. Figure 2.35 shows an example of a machined part with some geometric features with logical associations of points, edges, and surfaces of the part. Figure 2.36 shows an example of classified manufacturing features for prismatic parts by Šibalija et al. (2013), while Figure 2.37 shows a few exemplified parts in a chair model with their ontological features.
Figure 2.35 Example of geometric features.
Figure 2.36 Classified manufacturing features for prismatic parts (Šibalija et al. 2013).
Figure 2.37 Examples of ontological features in a chair model.
2.6 Interactive Feature‐Based Modelling Using CAD Tools
A SolidWorks modelling environment is used to illustrate the application of interactive feature‐based modelling. As shown in Figure 2.38, a part usually consists of a number of features. The corresponding solid of a feature can be added or removed from the volume of the part model by an Operator, which is associated with the feature type. The geometry and shape of a feature can be created by built‐in tools (e.g. Fillet or Chamber) or sketch tools (e.g. Extruded Boss or Cut). The parameters for dimensions and relations are defined when a feature is created. Each built‐in tool is equipped with a wizard to guide user inputs. The sketch tools provide the flexibility for the user to define the dimensions of a feature by hand. Finally, all sketches, paths, dimensions, and constraints are defined upon certain references such as points, axes, planes, and coordinate systems.
Figure 2.38 Types of features in feature‐based modelling.
When an original CAD model of a part is available, any entities (edges, axes, surfaces, planes, or features) in the model can be identified and reused as the references to modify or create new features. Note that the information of any entity in the model is included and can be accessed through the model tree interfaces. Figure 2.39 shows some major feature‐based modelling tools in SolidWorks. These tools are associated with the functional requirements to create and modify different types of features, as shown in Figure 2.38. Designers should be aware of these tools and know how to access them when they are needed.
Figure 2.39 Feature‐based modelling tools in SolidWorks.
Figure 2.40 shows the procedure of modifying or creating a feature in a solid model. It begins with determination of design intent. Design intent represents the strategies to create a certain feature. For example, a cylindrical shape can be created by Extruding, Revolving, Lofting, or Sweeping, but the amount of information in creating a feature and the effort in modifying the feature are different from one another. The design intent is your selection of modelling technique when a number of modelling options are available. Multiple design intents might