
Please Pass the Parametrics
4 Mar, 2010 By: Lynn AllenCircles and Lines Tutorial: If you have an appetite for easier drawing and editing, AutoCAD 2010 serves up new geometric constraints.
Editor's note: This article was originally published in the Winter 2010 edition of Cadalyst magazine.
Autodesk's newer, more powerful programs such as Revit and Inventor have had the advantage of parametrics since the beginning, but AutoCAD always lagged behind. Finally, with version 2010, AutoCAD users have the advantage of powerful parametric tools that make it easier than ever to edit designs. I promise, once you've had a taste of this exciting technology, you'll never want to go back!
The world of parametrics in AutoCAD 2010 has two components: geometric constraints and dynamic constraints. I'll begin by discussing geometric constraints, and save dynamic constraints for a future column.
For starters, geometric constraints sound a little intimidating. Let me assure you, that couldn't be further from the truth. Geometric constraints are very easy to work with, and you can ease into them one toe at a time or jump in with both feet — it's entirely up to you.
Geometric constraints are very similar to object snaps, except that where object snaps are temporary, geometric constraints stick until you remove them from your geometry. With object snaps, you can create geometry that is perpendicular, but there is nothing to keep the geometry perpendicular after the fact. With geometric constraints, you tell your geometry how to behave — perpendicular, parallel, etc. — and AutoCAD will keep it that way. I like to think of geometric constraints as persistent object snaps, or rules that you've applied to your geometry. These handy tools can save you plenty of\ time during drawing and editing.
How to add geometric constraints. Geometric constraints can be found on the Parametric tab of the AutoCAD 2010 ribbon menu (figure 1). Constraint options include coincident, collinear, concentric, fix, parallel, perpendicular, horizontal, vertical, tangent, smooth, symmetric, and equal. You can apply geometric constraints to almost any object.
Let's start using an example of a randomly drawn foursided polyline (figure 2), which we will convert to a rectangle using our constraints. I selected the parallel constraint from the palette, then applied it to the two sides indicated in the figure. The second side of my object adapted to the first and is now parallel. Be aware that the object you select first wins! The second object will always conform to the first.
Next, add another parallel constraint, and you'll have a parallelogram (figure 3, left). Adding one perpendicular constraint will convert the object to a rectangle (figure 3, right).
Now your constrained objects are bound by the rules you have applied. Try using grips to break any of these rules — you'll see it's just impossible. Small images known as constraint bars appear to remind you of the applied geometric constraints. Moving your cursor over the constraint bars displays a tool tip and highlights the corresponding geometry (figure 4).
Let's try another simple example, this time using my favorite constraint, Equal (=). Draw three circles random in size, then apply the Equal constraint to all three. As mentioned, the first circle selected will dictate the size of the others. You'll also find that the Equal constraint has a nice multiple option, making it easy to add more than one object. Notice that if you change the size of one of the circles, the other two follow automatically — fantastic!
How to remove geometric constraints. What if you want to remove one of these constraints from your geometry? No problem. Simply rightclick on any constraint bar icon and select Delete from the rightclick menu.
Though the constraint bars don't print, you may decide you want to turn off their display. You can do so easily by selecting Hide All on the ribbon (refer back to figure 1) or from the rightclick menu of any constraint bar.
By applying geometric constraints, you can ensure that an arc is always tangent to a line or concentric to a circle, or that two lines are always perpendicular. You can line up objects using the collinear constraint or match up objects using Equal. You can force objects to be symmetrical so that if you edit one object, the other maintains its symmetry with the first. (Think of it as the Mirror command on steroids!) You can apply myriad behaviors to your geometry that will go a long way in making your designs easier to edit.
This "Circles and Lines" column has only covered the basics of working with geometric constraints — there's plenty more where this came from, and I'll cover more in subsequent columns. Give geometric constraints a try, and you'll find the world of editing will become faster and easier than ever!
Until next time, happy AutoCADing!

AutoCAD Tips!
Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD howto, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's free Tips & Tools Weekly enewsletter and we'll notify you every time a new video tip is published. All exclusively from Cadalyst! 
