Computer Aided Design and Manufacturing. Zhuming Bi

such as Extrusion, Sweep, or Revolve, requires the user to create one or more sketches to define the geometry and/or paths, and the rest of the inputs are defined in the feature property window. Revising or adding a feature in a complex part model with many free‐form boundary surfaces is not a trivial task. Designers are usually required to try different modelling strategies (for example, selecting and creating a sketch plane) to create a feature successfully.

Figure 2.40 Creating or modifying a feature in feature‐based modelling.

Figures 2.41 and 2.42 show two examples where a built‐in feature and a sketched feature are created, respectively.

Figure 2.41 Example of creating a built‐in feature.

Figure 2.42 Example of creating a sketched feature. (a) Create a reference axis. (b) Use an axis and a flat face to create a reference axis. (c) Create a sketch on a new plane. (d) Create an extrude cut at the outlet.

2.7 Summary

A solid object consists of (i) geometric elements at different levels, from vertices to edges, faces, volumes, and features, and (ii) the topological relations of geometric elements. Geometric modelling is used to create computer representations of solid objects, while geometric modelling lays the foundation for using computer aided techniques. The commonly used modelling methodologies include wireframe modelling, surface modelling, space decomposition, constructive solid geometric modelling, and feature‐based modelling.

      The information of geometric representation of an object from different modelling methods may be different. A wireframe model consists entirely of points, lines, and curves; it does not have the information for face or volume, and no topological data are needed in modelling. A surface model stores the topological information in corresponding objects, but a surface model can still be ambiguous in some cases. Both a surface model and a solid model can be utilized to identify a shading area and a solid model has complete, valid, and unambiguous spatial addressability. In addition, a lower‐level model (i.e. a wireframe) can be extracted from a high‐level surface or solid model.

      The same geometry can be modelled in different ways. However, it is desirable to take into consideration the design intents in geometric modelling. Design intent is beyond the size and shape of features and can be extended to manufacturing features such as tolerances, manufacturing processes, and design constraints. The use of design intents is an effective approach to build a parametric model of a part that is fully constrained and easy to modify. By far, feature‐based modelling is the most advanced method, where a part model is modelled as a set of features as well as their topological relations. Modern computer aided software tools support feature‐based modelling methods.

2.8 Modelling Problems

      1 2.1 Create solid models for drawings with standard views (front, right, top, and isometric views) and annotated masses for the parts in Figure 2.43 (unit: inch per pound; thickness: 0.25 inch; materials: grey iron).Figure 2.43 Drawings for modelling Problem 2.1.

      2 2.2 Create solid models for drawings with standard views (front, right, top, and isometric views) and annotated masses for the parts in Figure 2.44 (unit: mm per gram; materials: grey iron).Figure 2.44 Drawings for modelling Problem 2.2.

      3 2.3 Create a pipe model with the dimensions specified in Figure 2.45. Have a few examples of design intents you incorporate in the modelling process and create a similar drawing to that in Figure 2.45 with the answers about design intents and total mass in grams.Figure 2.45 Drawing for modelling Problem 2.3.

References

      1 Bi, Z.M. and Kang, B. (2014). Sensing and responding to the changes of geometric surfaces in flexible manufacturing and assembly. Enterprise Information Systems 8 (2): 225–245.

      2 Bunge, M. (1983). Treatise on Basic Philosophy, Epistemology and Methodology I: Exploring the World, vol. 5. Dordrecht: D. Reidel Publishing Company.

      3 Gallier, J. (2008). Curves and surfaces in geometric modelling: theory and algorithms. https://www.cis.upenn.edu/∼jean/tabcont.pdf.

      4 Rynne, A. (2006). Design intent. https://www3.ul.ie/∼rynnet/designintent-solidworks.php.

      5 Šibalija, T.V., Majstorović, V.D., Erčević, B.M., and Erčević, M.M. (2013). Process planning for prismatic parts in digital manufacturing. The 7th International Working Conference on Total Quality Management – Advanced and Intelligent Approaches, Belgrade, Serbia (4–7 June 2013). https://pdfs.semanticscholar.org/8ed9/a00902e634e5e9a4dec39763d383119a12d0.pdf.

      6 TutorialsPoint (2018). 3D transformation. https://www.tutorialspoint.com/computer_graphics/3d_transformation.htm.