TABLE OF CONTENTS Solid faces in existing models can be used to create features, as we discussed in the previous chapter. However, in the practice of engineering design modeling, existing solid faces are often not sufficient. As a result, virtual datum references including datum planes and axes are necessary. These datum references can be used not only in assisting modeling, but dimensioning as well.
Because of the introduction of datum planes and axes, model references have fallen into two categories: solid and virtual. A soliddatum reference is a reference established on a solid entity and it is physical. In manufacturing and quality control engineering, solid references must be used so that they provide a basis for dimensions to be measurable, and a basis for measurements to be checked against. They are used throughout the entire production process. For example, all the axial dimensions of the shaft in Figure 7-1 are given based on its left end. When it is machined using a conventional lathe, the ends will be faced tothe desired length first. The large diameter in the middle portion will be turned next, and then the smaller diameter portion is turned to finish.
Virtual references, on the other hand, are often used during design modeling(see Figure 7-2
Upon completing this chapter, you will be able to:
Create simple datum planes
Create complicated datum planes and axes
Understand when and where datum planes and axes are used
