Lesson 1: Introduction to Parametric Drawing

Learning Objectives
On completing this lesson, you will be able to do the following:

■ Describe the purpose of parametric drawing.

■ Recognize geometric constraints, dimensional constraints, and the
constraint cursor icon.

■ Identify the objects that are constrained and what constraints apply to
each object.

■ Turn on and off the display of geometric and dimensional constraints, and
control the precision and unit style of dimensional constraints.

File Name: PD_Deck1.dwg

Command: CONSTRAINTBAR, DYNCONSTRAINTDISPLAY


Parametric drawing is a method for designing with constraints. Constraints are
rules that are applied to 2D geometry. By applying constraints, you can control
the location, size, distance, angle, and other parameters of objects in a drawing.
As a result, you can identify and maintain specifications in a project.

There are two types of constraints:


■ Geometric constraints control the relationships of objects with respect to
each other.

■ Dimensional constraints control the distance, length, angle, and radial values
of objects.


Let’s explore these constraints by taking a tour of a constrained drawing of a
backyard deck plan.

To view a constrained drawing

You take a tour of a constrained drawing of a simple backyard deck.

1 Start AutoCAD and then on the ribbon, click the Parametric tab.

2 On the Dimensional panel, click Show Dynamic Constraints to make
sure that they are turned off. On the Command line, you should see the
following:

Enter new value for DYNCONSTRAINTDISPLAY <1>: 0

3 Open the drawing file, PD_Deck1.dwg. It should be located in the C:\My
Documents\Tutorials folder.

This is the plan view of the deck. However, this drawing could represent
a mechanical part or a section of a wide gutter instead, depending on
your discipline.

4 Move your cursor over the objects in the drawing. Notice the small, blue
cursor icon. It indicates that each object has one or more constraints
applied to it.

To view geometric constraints

The display of all constraints has been turned off. Let’s turn on the display of

geometric constraints to see what they look like.

1 On the Geometric panel, click Show All.

2 Click and drag the constraint icons to relocate them as shown.

All the geometric constraints are now displayed. Later, you will learn how
to create and delete them.

3 Move your cursor over each constraint icon and notice the tooltip that
describes it.

4 Continue moving your cursor over each constraint icon and notice that
the associated objects are highlighted so you can determine which objects
are constrained.

The two vertical lines are constrained to be parallel and equal in length.
The left vertical line is constrained to be perpendicular to the horizontal
line.

5 Move your cursor over each object and notice that the geometric
constraints associated with it are highlighted.

6 Move your cursor over each blue square on the corners of the deck to
display the Coincident constraint icon.

constrain the endpoints of the lines and the arc to remain in contact.

7 On the Geometric panel, click Hide All to turn off the display of the
geometric constraints.

To view dynamic constraints

Let’s turn on the dimensional constraints to see what they look like.


1 On the Dimensional panel, click Show Dynamic Constraints.

Dynamic constraints are the most common type of dimensional constraints
used. The other type, called annotational constraints, are used for plotting
and you will learn about them later.

This deck was constructed using metric units (centimeters).

Dynamic constraints look like dimensions, but behave in the opposite
way. Dimensions are driven by objects—they report the lengths, angles,
and radial sizes of objects. In contrast, dynamic constraints drive
objects—they determine the lengths, angles, and radial sizes of objects.

The default format of dynamic constraints includes a name such as d1
or rad1, and a lock icon. Later, you will learn how to change the values
of these constraints to control the size and shape of a design.


2 Zoom in and out of the drawing. Notice that the dynamic constraints
always remain the same size.

You can change the numeric display style of dynamic constraints by
changing the units or the display precision.

3 At the Command prompt, enter units.

4 In the Drawing Units dialog box, under Length, change the linear
precision from 0.00 to 0 and click Ok.

5 If you use architectural units, open the drawing file, PD_deck1_imperial.dwg.
Then use the UNITS command to change the unit style to Architectural
with a precision of 0'-0".

To view the effects of constrained objects

1 Click Show All to turn on the display of geometric constraints.

2 Start the Move command and select the horizontal line. Move it to a
different location.
Because the deck is fully constrained, all the objects are dependent on each
other, and the shape and size of the deck does not change.

3 Close the drawing without saving it.

Summary



Parametric drawing has many applications for disciplines that require applying
and maintaining design specifications. Geometric and dimensional constraints
work together to include the design intent and requirements within a drawing.
In the following lessons, you learn how to apply and work with constraints.


Review Questions


1 How would you describe the purpose of parametric drawing and
constraints to someone?
2 What is the purpose of the blue icon that appears when you roll over
some objects?
3 How can you tell which geometric constraints are applied to a specific
object?
4 What is the purpose of dimensional constraints?
5 How can you change the precision displayed in dimensional constraints?