So you’ve drawn your sketch, and at this point it’s probably looking a lot like you’ve drawn it on a napkin. How do we get it to be the right size and shape? To make sure your sketch has the right size we’ll add dimensions, and to ensure the sketch maintains the right shape, we’ll add geometric constraints. We’re used to working with dimensions in AutoCAD—but what exactly are constraints?

Rules to Live By
Think of constraints as geometric rules by which your sketch must abide. You can place a vertical constraint on a line that forces that line to be vertical regardless of what you do to the sketch. You may tell two circles that they are always to be concentric. You might force a line and an arc always to be tangent to one another. You’re essentially setting up a relationship between the various objects in your sketch. Constraints don’t stop with sketching—you’ll find that you can also place constraints in assemblies to control placement of components, direction of movement, and so on.

After drawing your sketch, you can easily add constraints by selecting them from the Sketch panel bar or by selecting Add Constraints from the right-click shortcut menu.

(A) You can see all existing constraints by selecting the Show Constraints tool from the panel bar. Rows of constraint glyphs (I just love that word!) appear. To delete a constraint, simply select the constraint, right-click to display the shortcut menu, and select the Delete option.

A fully constrained sketch is one that can't move at all (which means that at least one fixed constraint was added). You'll find that a fully constrained sketch turns a darker color. Some people insist on fully constraining their base sketch, some do not-the choice is yours.

Dimensioning (B)
Dimensioning, technically another form of a constraint, is used to control the size of your sketch. The dimensioning feature in Autodesk Inventor is one of my favorites because it’s so easy—you need to use only one dimensioning tool for all your dimensions (imagine that!). Autodesk Inventor is intelligent enough to choose the proper dimension type depending on what you select. Pick General Dimension from the panel bar and select the desired geometry. If for some reason the dimension that appears is not what you were after (for example, Diameter rather than Radius), simply right-click and switch to the desired type. After you place the dimension, the Edit Dimension dialog box appears and you can input the desired value. If the Edit Dimension dialog box doesn’t appear, simply double-click on the value to edit it.This is where the real power of Autodesk Inventor begins—you’ll find that you can change the size of your sketch at any time just by changing the dimensions.

Objects: A Smart Collection of Sketch Entities
A funny thing happens when you add dimensions to a fully constrained sketch. The addition of the dimensions and constraints turns the lines and arcs in the sketch into a smart object.Now when you make changes to any one entity in the sketch, all of the remaining entities know “exactly” how to update to maintain the implied shape and size. Autodesk Inventor technology is based on dynamic and associative relationships. A good understanding of constraints is definitely key to your success in Autodesk Inventor!

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