geometric constraint

Understanding Dimensional Constraint

December 6, 2009 in AutoCAD, CAD, Revit

Another parametric feature available is dimensional constraint. We discussed about geometric constraint, and this time dimensional constraint.

Dimensional Constraint Types

Basically we use dimensional constraint to maintain distance between points or objects. Usually dimensional constraint don’t work alone. We use geometric constraint along with dimensional constraint.
There are three types of dimensional constraint:

Dynamic Constraint
Annotational Dimension
Reference Constraint

Annotational Dimension

In a nut shell, annotational constraint behave like common dimension we know all these years. You can plot it, you can assign it on a layer, and the size behave like other annotation. The size is not affected when you zoom the drawing.

Dynamic Constraint

Dynamic constraint is a bit different. it’s only for display purpose. It will not be printed. If you remember how you use parameter in dynamic block, it also display at the same size when you zoom the drawing.
You can change the form, dynamic or annotational by changing the ‘form’ variable.

Command:
DIMCONSTRAINT
Current settings: Constraint form = Dynamic
Select associative dimension to convert or
[LInear/Horizontal/Vertical/Aligned/ANgular/Radial/Diameter/Form] <LInear>:F
Enter constraint form [Annotational/Dynamic] <Dynamic>: A

Reference constraint

Reference constraint is not created directly. You can create a dynamic or annotational dimension, and then convert it to reference dimension.

We use reference constraint only for showing the distance values. We can’t change the parameter like dynamic or annotational constraint. Let’s compare them.
Annotation constraint size will follow the object size when you zoom. Dynamic constraint will show the same size, no matter how you zoom it. The reference dimension is shown with brackets.

Using Dimensional Constraint

After you placed dimensional constraint, you will see the parameter name like d1, d2… dia1, dia2… by default. We can change it later. You can change the parameter by selecting the constraint, right click your mouse above dimension, and select edit constraint.
Or you can change the name and value in properties palette.

We can input calculation in expression field. For example, we want the base length is dependant to body length plus 20mm clearance. This workflow is used heavily in manufacturing design.
By using dimensional constraint, we don’t modify the object. We change the parameter value, and the object will be adjusted. To make the result predictable, we use it together with geometric constraint. We will create some objects using these parametric behavior later. But that’s the concept.
As I don’t write too much details in this parametric feature, you can also read them in my friend blog, Orhan Toker:

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Source: CAD Notes – Understanding Dimensional Constraint
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Tags: amp nbsp, Annotational, constraint, dimension, Dimensional, Dynamic, dynamic constraint, F Enter, geometric constraint, linear amp, parametric feature No Comments »

Understanding Geometric Constraint

December 6, 2009 in AutoCAD, CAD, Revit

Finally, Autodesk add parametric features in AutoCAD 2010. Why finally? This feature is already there in manufacturing industry for decades. If you have used any manufacturing application like Inventor, CATIA, SolidWorks, ProE, etc, then you should be already familiar with this feature.
There are 3 panels in parametric tab: geometric, dimensional, and manage. Let’s talk about geometric first.
Geometric constraint will maintain how your objects related to each other. Let’s see this example.
Let’s say we have a rectangle. We know that the sides have to be perpendicular to each other. But sometimes during the design, you may need to change it. If I stretch one of the rectangle vertex, then it would not be a rectangle anymore. AutoCAD doesn’t know that you want to keep it as a rectangle.
stretch vertex

We can prevent this by telling AutoCAD that we want them always perpendicular to each other. We can add perpendicular constraint. So I add a perpendicular constraint to the two sides. I try to stretch the vertex again.

As you can see, the two sides are kept to be perpendicular to each other. But the other edges don’t. We have to add all constraint to keep it a rectangular.

That’s the concept.
I don’t know if this gives many advantages to AEC industy or or. I know that MicroStation has this feature years ago, but it’s not become a popular feature. Maybe because Bentley Systems doesn’t have solutions in manufacturing industry.
There are some use that I can think of: in dynamic block. But I’m not sure if we can use it extensively in drafting. I saw the example in new features workshop, the sample is for manufacturing, not AEC. Do you have any idea, where we can use it in AEC industry? I will try to post some dynamic blocks using this parametric feature, and maybe you can suggest me how we can use it in drawings.