AutoCAD Tutorial: Use parametric constraints to constrain models

admin — Thu, 03 Dec 2009 17:56:08 +0000

AutoCAD 2010 introduced a great new feature, parametric constraints. You can constraint objects in two ways:

Geometrically: You can constrain objects to meet at a point, be horizontal, be vertical, be perpendicular, be concentric, and more
Dimensionally: You can constrain the dimensions of objects. For example, a you can constraint a line to be 4 units long, or two lines to be 4 units apart.

This tutorial, adapted from my book, AutoCAD 2010 & AutoCAD LT 2010 Bible, takes you through the process of constraining the simple model you see here. The goal is to constraint the objects so that all you can do is stretch the model, keeping the relationships the same. However, you want the circle’s diameter to remain unchanged, because it represents a hole for a fixed-size axle.

: Parametric constraints in AutoCAD

Starting at Point 1, draw a horizontal line to the left, 4 units long.
Continue to draw a line segment vertically down, 4 units long.
Continue to draw a line segment horizontally to the right, 4 units long. End the LINE command.
Start the ARC command. At the Specify start point of arc or [Center]: prompt, select the endpoint at Point 2.
At the Specify second point of arc or [Center/End]: prompt, choose the End option. Then choose the endpoint at Point 1.
At the Specify center point of arc or [Angle/Direction/Radius]: prompt, press Shift + right-click to open the OSNAP shortcut menu. Choose Mid Between 2 Points. then choose the endpoint at Point 1 and the endpoint at Point 2.
Start the CIRCLE command. At the prompt for the center, specify the center object snap of the arc. Set the radius to 0.75 units.
Click the Parametric tab in AutoCAD 2010. Choose AutoConstrain in the Geometric panel.

Notice that AutoCAD adds the following constraints:

Perpendicular
Parallel
Horizontal
Tangent
Concentric
Coincident

: Geometric constraints in AutoCAD

You can see which constraint is which by hovering your cursor over a constraint icon; a tooltip appears with the name of the constraint. Coincident constraints don’t display an icon at first, so put your cursor over the small blue square until the icon appears; then move your cursor over the icon to see the tooltip.

: Parametric constraints in AutoCAD

To constrain the three lines to be of equal length, click the Equal button in the Geometric panel of the Parametric tab. Then pick at Point 3 and Point 4 in the model above. Because of the other constraints, you don’t need to constraint the bottom horizontal line.
To fix the circle’s diameter, in the Dimensional panel, click the Diameter button. Select the circle and specify a location for the dimension line as prompted. Press Enter to end the command.
Start the SCALE command and select all the objects in the model. Using the lower-left corner endpoint as the basepoint and 1.5 as the scale factor. The model just moves!! That’s because you’ve constrained the size of the circle.
Undo the SCALE operation.
Repeat the SCALE command. Select all the objects in the model, but this time, press Shift and select the circle to deselect it. Now try the same scaling operation. This time it works.
Now start the STRETCH command. Specify a crossing window (from right to left) from Point 5 to Point 6 in the above model. and again use the lower-left corner as the base point. Drag to the right. Notice that you can’t elongate the model, because the 3 lines are constrained to be equal. You can only scale the model, which is just what you want. The circle stays the same size.

: Stretching the model with parametric constraints in AutoCAD 2010

AutoCAD Tutorial: Create a rectangle with an adjustable corner

admin — Mon, 05 Oct 2009 20:09:48 +0000

Have you noticed how grip-editing a viewport is different from grip-editing a rectangle?

When you grip-edit the upper-right corner of a viewport to stretch it, the viewport keeps its rectangular shape. You can freely choose your new corner location, so you’re not just scaling. You can change the proportion between the width and height, all without losing its rectangle-ness.

Stretching a viewport

But when you grip-edit the upper-right corner of a rectangle, the rectangle is distorted. The rectangle doesn’t know that it’s supposed to remain a rectangle. AutoCAD just thinks of it as a 4-sided, closed polyline. Indeed, select it and check in the Properties palette (Ctrl+1) and you’ll discover that it is indeed just a polyline.

Stretching a rectangle in AutoCAD

AutoCAD 2010’s geometric parameters can help you easily create a rectangle that knows it’s a rectangle. And, what is a rectangle? Well, “rect” means right, as in 90°. “Angle” is self-explanatory. So, we need to ensure that the angles will stay 90°.

Here’s how:

Draw a rectangle.
Go to Parametric tab> Geometric panel, and click the Perpendicular button.
At the Select first object: prompt, click the top horizontal line of the rectangle.
At the Select second object: prompt, click the left vertical line. You’ll see the Perpendicular icon near the upper-left corner of the rectangle.
Repeat the process for the two lines that meet at the lower-left corner and the lower-right corner.

A rectangle with 3 perpendicular geometric constraints

Note: If you try to do all 4 corners, you’ll get a message telling you that AutoCAD can’t apply the constraint. That’s because when you have 3 corners constrained to perpendicular, the last corner is superfluous, and is therefore an overconstraint.

You can't overconstrain an object in AutoCAD

Now, you can grip-edit that upper-right corner and it will behave just like a viewport!

The rectangle now stretches like a viewport!

Here’s a video of the same operation:

AutoCAD Tips Blog » geometric constraints

AutoCAD Tutorial: Use parametric constraints to constrain models

AutoCAD Tutorial: Create a rectangle with an adjustable corner