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How to Use Parametrics in AutoCAD 2014

Parametric (rule-based) drawing is by far the best way to enforce design intent in 2D drafting. Design intent in AutoCAD (or in any other engineering software) means that when drawings are edited to make “this part” wider and “that hole” larger, all the attached or related objects behave in a predictable way that honors the designer’s intentions in creating the drawing.

Before AutoCAD 2010, there was simply no way to maintain the design concepts behind an AutoCAD drawing. You could use AutoCAD drawing and editing commands to draw accurate, precise plans, sections, and details, but they were, to AutoCAD, simply a bunch of lines and circles.

Suppose that an engineer has taken a second look at a baseplate and determined that its 1-1/2″ bolts should be changed to 1-3/4dp. To revise the drawing by using AutoCAD in the traditional way, you draw a new, larger circle for the bolt and erase the old one.

Now the nut is too small, and so is the hole in the plate. And you have four bolts to change — that’s a whole lot of editing to fix this drawing. Even so, the good news is that it would still be a lot easier than using paper and pencil.

In AutoCAD, you can add some intelligence to those lines and circles by applying parametric constraints to them.

Unfortunately, the following exercise doesn’t work in AutoCAD LT.

Follow these steps to use parametrics to constrain a line to a circle:

  1. Start a new drawing.

  2. Draw a circle and a line anywhere in the drawing.

    Specifying exact sizes and alignments isn’t critical. You’ll see why in a moment.

  3. Apply a tangent constraint between the circle and the line.

    Parametric functions are found on the Ribbon’s Parametric tab. (Nothing in AutoCAD should surprise you by now.) Click the Tangent button, and then select the circle and the line. (The sequence doesn’t matter.) Observe how they move until they’re tangent to each other and how a little gray icon (a constraint bar) appears to indicate the type of constraint. Note that tangent doesn’t necessarily mean that they touch.

  4. Grip-edit the line and the circle.

    You can move the circle or line, change the diameter of the circle, or move either end of the line — but no matter what you do, the other object obediently remains tangent.

  5. Apply a diameter dimension to the circle.

    Don’t use the normal DimDIameter command. Instead, click the Diameter button in the upper-right corner of the Dimensional panel of the Parametric tab on the Ribbon. When you select the circle as prompted, AutoCAD places a funny-looking gray dimension that reads something like this: dia1=5.3716.

    It has the same background color as the mText edit box; in fact, that’s exactly what it’s inviting you to do. When you enter the desired diameter (say, 7.5), the circle resizes itself to match the diameter value you entered, and the line adjusts itself to remain tangent. It’s the opposite of associative dimensions.

  6. Apply an aligned dimension to the line.

    Again, be sure to click the Aligned button from the Dimension panel on the Parametric tab, and select each end of the line in turn (or press Enter and then select the line). This time, when it places the dimension, enter =dia1*2 (including the equal sign and omitting spaces). The line automatically resizes itself to become twice as long as the diameter of the circle.

  7. Double-click the diameter dimensional constraint and type a new value, such as 5, and press Enter.

    Surprise! The circle size changes, and the line changes its length so that it’s still twice as long as the new diameter of the circle. (That is one smart drawing.) Imagine a similar effect on your productivity in creating and editing drawings.

    You can still grip-edit the position of the line and circle, but you can’t change the diameter of the circle or length of the line using grip editing, nor can you stretch the line length any more.

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