How to Use Dimensional Constraints in AutoCAD 2014

The objects you add to a drawing from the Dimensional panel in AutoCAD aren’t the same as the dimension objects you add from the Annotate tab. Dimensional constraints are driving dimensions — when you change the value of one of these dimensions, the geometry changes.

A lot is happening behind the scenes as you apply parametric constraints. You can get a sense of how these constraints work at keeping drawing objects in order by using the Stretch command on objects after you apply a constraint to them.

These steps present a simple example of dimensional constraints:

  1. Start a new drawing and make the Ribbon’s Parametric tab current.

  2. Turn on the appropriate precision drawing aids on the status bar, such as Snap, Ortho, and Osnap.

  3. Draw some reasonably precise geometry by applying a precision technique.

    The following example uses the RECtang and Circle commands to draw the geometry. The rectangle is ten units square, and the 2.5-unit-radius circle is deliberately drawn away from the middle of the square.

    image0.jpg
  4. On the Dimensional panel of the Parametric tab, click the top part of the Linear split button.

    A linear dimension icon appears beside the pickbox, and AutoCAD prompts you to either specify the first constraint point or pick an object.

    Like the DimLInear ommand, the Linear dimensional constraint tool is inferential — which way you drag the crosshairs controls whether you create a horizontal or vertical dimension. Also, as with DimLInear, you can press Enter at the command line and select an object to dimension.

    Hover the mouse over the Linear button to see (unlike its neighbor, Aligned) that the button is split into two parts. To force a linear dimensional constraint to be either horizontal or vertical (rather than dependent on the direction you drag the crosshairs), click the bottom part of the Linear button and choose from the drop-down menu.

  5. Press Enter at the command line to confirm that you want to select an object, and then select the bottom horizontal line segment.

    If you see red markers at the midpoint and endpoints of the bottom line, you didn’t press Enter — and AutoCAD is in Point Selection mode rather than Object Selection mode.

    AutoCAD generates a preview of a dimensional constraint and prompts you for a location.

  6. Click to locate the dimension position.

    AutoCAD draws a dimensional constraint with a highlighted text field displaying the dimension name (d1, in this example) and the value returned by AutoCAD.

  7. Press Enter to confirm the value and the dimension location.

    Best practice is usually to accept the default value for all dimensional constraints as they are applied and then edit them later to the exact values that are desired. This avoids problems such trying to make the width of a slot greater than the width of the piece that contains the slot, which can turn your drawing inside out.

    If the dimensional constraints disappear as soon as you place them, click the Show All button on the Parametric tab’s Dimensional panel.

    image1.jpg

    Because dimensional constraints aren’t regular dimension objects, you can’t plot them, so it doesn’t matter where you put them or what they look like.

  8. Repeat Steps 4–6 and add a dimensional constraint to the rightmost vertical edge of the rectangle.

    AutoCAD draws a second dimensional constraint, this one named d2.

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