AUGIWorld Magazine - January/February 2008 - (Page 23) many dimensions are needed to fully constrain the sketch. See Figure 4. I continue adding dimensions or constraints as needed to fully constrain my sketch, and looking in the lower right corner, the number of dimensions continues to decrease to 0. Figure 4: Dimensions or constraints needed As you add a dimension, the number decreases by one. Adding constraints may decrease the number by more than one. As soon as I locked the point at the origin, the number went from 12 to 10. The first line I sketched was a vertical line from the origin. That line and the point at the origin turned black— those are shown as constrained. When a drawing is fully constrained, all the geometry is black. Before placing dimensions, I try to apply all my constraints first. If I need two lines that are in line with each other, I use the “colinear” constraint. If I need Figure 6: Fully constrained drawing two lines to be of the same length, I might use the “equal” constraint. An the “Show Constraints” icon. Select the alternative to the equals: when dimensioning “Show Constraints” icon, then select the the second line, you can select the dimension piece of geometry for which you need to from the first line. That will also make the two modify the constraint. A Constraint Bar pops up, with all the constraints that are lines equal. When placing points or circles in your associated to it. Right-click, select Done, sketch, you may need them to be vertical click on the constraint you need to delete, or horizontal to each other; I use those con- and then press the delete key. That piece straints. You could also use a vertical and of geometry should turn green, which horizontal dimension to locate each point, means it needs to be constrained. Change as well. I normally use constraints and to what you need, and it should turn black. use fewer dimensions, to keep my sketch Congratulations! Your drawing is now fully constrained again. less cluttered. If I need to adjust the features later, it is easy to remove or delete constraints, then Follow up Constraining geometry is adding relaadd a different dimension as needed. Dimensions are like constraints in that they tionships between the different parts of both determine the variance of one piece geometry in your sketch—similar to addof geometry to the other. If you try and ing dimensions. In some cases, you can replace a constraint that has already been duce the number of dimensions of needed, set, you get an error message, as shown in thus “uncluttering” your sketch while still maintaining design intention. Figure 5. all constraints. Also, in the lower right hand corner, the message is “Fully Constrained,” and all the geometry is black. Hopefully, you now have a good basic understanding of constraints. There will be a time when you need to delete one and change to a different constraint, or you need to add a dimension to redefine how that feature is needed. I prefer to use Show Constraint for a single piece of geometry. Refer to Figure 1—next to the “constraints” icon is Figure 5: Constraint error Just select “cancel” and continue. A fully constrained drawing might look like Figure 6. That has dimensions and I have shown Jan/Feb 2008 David Keener is a Design Engineer for SMT Industries in Sidney, Ohio, and also an Adjunct Professor at Edison Community College in Piqua, Ohio. He has been using AutoCAD for more than a decade and Autodesk Inventor for more than two years. He can be reached at d_keener@smtind.com. 23 http://www.bluebeam.com
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