Chapter 13 Sec 13.5 - Dimensional Constraints Advanced Dimensions Since we have already discussed the basic idea of constraints, this introductory section will be shorter. The idea here is to now move from constraining pieces of geometry (and their positions relative to each other) to constraining the actual dimensions of the drawn design. So if two circles, representing drilled holes, absolutely need to be 3" apart, then constraining that dimension will ensure it won't change regardless of what other design work goes on around them. Dimensional constraints are also a major part of 3D parametric design, and can be easily transferred to 2D work as was done here in AutoCAD. Also because the geometry has to obey the constrained dimension, one can change the geometry by simply changing the dimension value, which is quite a departure from the normal order of business with AutoCAD. This ability to change geometry by updating dimensions is called "dimension driven design" and is really the only way to change the size of an object (usually called a "feature") when using the high- end 3D design software such as CATIA or SolidWorks. At this point you may start to see why all of the preceding is referred to as parametric design. We are designing by changing or assigning parameters to our geometry. The geometry itself is almost secondary to the relationships and data that drive its existence and creation. This type of philosophy and approach is one of several steps critical to assuring that what we have is a valid engineering design, not just a "pretty picture." The data behind the design has to make sense! Working with Dimensional Constraints Learning the basics of dimensional constraints is straightforward. You can either add them in right away to your design or convert existing regular dimensions to constraints. Dimensional constraints have their own toolbar (Figure 13.18), or they can be accessed through the Ribbon (Figure 13.19).