Command of the Month: DimAssoc and DimReAssociate

by Ralph Grabowski

Associative dimensioning is possibly the subtlest new feature in AutoCAD 2002. It works without you noticing it. New users don’t even need to know associative dimensioning has changed; users upgrading from earlier releases of AutoCAD, however, need to change their dimensioning practices to accommodate the new method.

"But, wait!" I hear you cry. "Hasn’t there always been associative dimensioning in AutoCAD?"

True enough, at least since AutoCAD v2.5, which came out in 1986. An associative dimension updates its position and text to reflect changes to the associated object. What has changed over the years is where dimensions attach their extension lines:

* Before AutoCAD v2.5, dimensions didn’t attach their extension lines to anything.

* Between AutoCAD v2.5 and AutoCAD 2000i, dimensions attach their extension lines to "defpoints" (short for definition points). These point objects were created by AutoCAD, and were placed on the DefPoints layer. The drawback to defpoints became clear when the dimensioned object was edited: the selection set had to include the defpoints, otherwise the dimension failed to update.

* As of AutoCAD 2002, dimensions attach their extension lines to the object being dimensioned. Change the object, and the dimension changes as well.

This puts Autodesk in a bit of pickle: if they called it "associative" dimensioning before AutoCAD 2002, what do they call it now that dimensioning is even more associative? (Perhaps they could adopt detergent marketing, and call it "New & Improved!") As it turns out, Autodesk has decided to revise history, and call the old style of associative dimension "non-associative." But then Autodesk goes on to admit "if the definition point on the object moves, the dimension value is updated."

Not all dimensions, however, have become truly associative in AutoCAD 2002. Autodesk notes that dimensions created with the QDim command do not attach to objects.

A system variable new to AutoCAD 2002 switches between the three dimensioning styles I listed earlier. The DimAssoc variable accepts the following values:

0 Creates "exploded" dimensions (pre-AutoCAD v2.5 style).

1 Creates "non-associative" dimensions (AutoCAD v2.5 - 2000i style)

2 Creates "associative" dimensions (the default value).

Tips

The DimAssoc variable replaces the DimAso variable, which used to toggle between 0 (exploded dimensions) and 1 (associative dimensions).

When a 2002 drawing is opened in 2000i or earlier, the value of DimAssoc is stored, then restored when the drawing comes back to 2002.

When a 2000i (or earlier) drawing is opened in 2002, what happens to the dimensions? Nothing. They retain their exploded or "non-associative" nature, where the extension lines are attached to defpoints, not objects.

To convert the dimensions of a drawing to 2002-style associatively, AutoCAD includes the rather-difficult-to-type Dimreassociate command. It lets you interactively attach extension lines to objects. The process is as follows:

Step 1: Start the Dimreassociate command, and select all objects in the drawing. AutoCAD filters out non-dimension objects. (You can, if it works out better for you, select individual dimensions.)

Step 2: AutoCAD places an X symbol at the first extension line of the first dimension it finds. If the X has a box around it, the extension line is already attached to an object; if the X has no box, it is attached to a defpoint.

Step 3: You have three choices: pick a point (using object snap) on the object; have AutoCAD pick the points for you (by selecting the object); or skip this extension line and move on to the next one.

Step 4: Once you finish attaching extension lines to objects, you may need to edit the positions of the dimension and extension lines, because they sometimes jump to unwanted locations.

And, if you should ever want to remove associativity from dimensions, AutoCAD 2002 includes the Dimdisassociate command.

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Command: DimReAssociate

Purpose: Attaches extension lines to objects.

Notes: Meant for converting pre-AutoCAD 2002 dimensions to associative dimensions.

Aliases: dre

Shortcut: Alt+NN

Menu bar: Dimensions | Reassociate

Tablet menu: none

Status bar: none

Toolbar: Modify II

Related commands:

Dim... All dimension commands.

DimDisAssociate Removes associativity from a dimension.

Explode Reduces a dimension to individual objects.

Related system variables:

DimAssoc Determines the associativity of new dimensions.

Example command usage:

The subsequent prompts vary, depending on the type of dimension (linear, aligned, 3-point angular, 2-line angular, diameter, linear, ordinate, or radial). Sample prompt: