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Parametrics 102: Sizing Things Up
28 Jan, 2010 By: Bill FaneLearning Curve tutorial: With the magic of parametrics in AutoCAD 2010, you can start with a rough sketch of a basic concept and then refine and revise the dimensions as the design proceeds.
It was a warm and sunny Christmas day. Captain LearnCurve and his gorgeous wife were sitting outside in T-shirts and shorts having Christmas dinner with their daughter, son-in-law, and three grandchildren ...
Wait a minute! Doesn't the Captain live in Vancouver, host city of the 2010 Winter Olympics? Why were you wearing T-shirts and shorts at Christmas?
There are two answers. First, Vancouver is not as cold as many people think. Second, they weren't in Vancouver, they were in a condo on Maui. They had arrived a week before Christmas. The kids stayed for three weeks, but the Captain and his gorgeous wife were stuck there until early March. Life is tough.
Okay, I'm so relaxed here that I can't figure out a groaner of a segue ...
And the bad news is ... ?
... so let's just jump right into this month's topic.
My previous column introduced the parametric functionality in AutoCAD. We saw that there were two kinds of parametric constraints, and learned that the geometric ones work like "sticky" object snaps. We built three orthographic views of a part without using a miter line or any construction geometry, and saw how everything stayed aligned and connected when we moved or stretched anything.
Now let's look at the dimensional constraints. It could be argued that these are the "true" parametric constraints if we look at the origin of the word "parametric," which comes from two Greek words:
- "para" means to work with, or to work alongside. For example, ambulance crews are paramedics; they work with the medical people.
- "metros" means measure. The words "meter" and "metric" come from this same root.
Let's go back to our high school math. The general equation for a line is y=mx+b. It says that you can find the current value of the y coordinate of a point on the line by taking the current x value and multiplying it by m, the slope of the line, and then adding b, the offset value where the line crosses the y axis. In this example, m and b are parameters. You can define any line simply by varying these two values.
In AutoCAD, dimensions normally are associative. If you stretch the length of a line, then the dimension value will change.
On the other hand, a parametric dimension is the opposite: If you change the value of the dimension, then the line changes length. Meanwhile, all of the attached geometric constraints are still active, so everything updates. The value is not an after-the-fact measurement, it is the driving value.
Let's try this. Draw a horizontal and then a vertical line and apply geometric constraints to them. The easy way is to use Autoconstrain to apply the Horizontal, Perpendicular, and Coincident constraints, and then manually add a Fix constraint to the left end of the horizontal line.
Two constrained lines.
Next, make sure the system variable is still set to its default value of 0 (zero).
Now apply two dimensional constraints using the Linear option of the DimConstraint command. To do this, click on the Parametric ribbon tab and then click on the Linear button 
in the Dimensional panel.
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