Parametrics Tip: CCONSTRAINTFORM

A few days ago I was preparing some training on the new Parametric Design feature in AutoCAD 2010 and was reminded of how much I enjoy this new functionality. The next few posts from me will focus on some of the lesser known aspects of working with geometric and dynamic constraints.

There are three types of dimensional constraints:

• The dynamic type constrains objects to a value and displays a dimension-like representation. Dynamic constraints do NOT plot.
• The annotational type is identical to a dynamic dimensional constraint except that it displays using the current dimension style. Annotational constraints DO plot.
• The reference type doesn't constrain an object at all but simply displays the objects value using a dimension-like representation. Reference constraints do NOT plot.

Annotational constraints can be very useful in situations where you want to contrain an object and you want that value to display just like a dimension. In fact, an annotational constraint uses the current dim style and has all the same editable properties as a standard dimension.

To create an annotation contraint, you typically start by creating a dynamic constraint, selecting that constraint and then editing the Constraint Form field in Properties to change it from Dynamic to Annotational.

This process can get tedious if you have a lot of dynamic constraints that need to be turned into annotational constraints. Fortunately, there is a quicker way.

If you know ahead of time that what you want to apply are annotational constraints, you can set the CCONSTRAINTFORM system variable to 1. CCONSTRAINTFORM controls whether constraints are applied to objects as annotational or dynamic. When set to 0 (the default), DIMCONSTRAINT automatically creates dynamic constraints. When set to 1, DIMCONSTRAINT automatically create annotational constraints.

Using the CCONSTRAINTFORM system variable can save you time by sparing you from having to convert between dynamic and annotational constraints. Note: this system variable is only used during the initial creation of dimensional contraints. Once a dimensional constraint has been created, you would need to use Properties to convert between the two types.