How to Use Geometric Constraints in AutoCAD 2014 - dummies

How to Use Geometric Constraints in AutoCAD 2014

By Bill Fane, David Byrnes

If you’re using AutoCAD 2014, you can constrain drawing objects geometrically by clicking buttons on the Geometric panel of the Parametric tab. Try the following steps to use geometric constraints:

1Start a new drawing. Make the Ribbon’s Parametric tab current, and turn off Snap, Ortho, Polar, Osnap, and Otrack on the status bar.

For production drawings, you will want to make sure you use precision drafting aids, but in this example, you gain a better sense of parametrics with an extremely imprecise drawing.

Two circles inside a trapezoid.

2Draw some linework by using the PLine command, and then add a couple of circles.

Every drawing is tackled differently, so look ahead and figure out the most efficient way to apply the constraints you need in order to maintain your design intent. In this example, you end up with two concentric circles in the middle of a square.

Applying constraints is easier if at least one point on the geometry is fixed in space. In many cases, consider applying the Fix constraint first — it constrains a point to a single location in the drawing area.

Clicking on a line to fix it in Autocad.

3Click Fix in the Geometric panel, and then click a point that you want to fix in space.

A blue padlock icon displays beside the pickbox, and the red pick point marker appears when you hover over a point that you can constrain. The padlock icon appears in the constraint bar, indicating that the endpoint of the polyline segment is fixed in place.

With at least one point on the geometry fixed in place, you can start constraining the geometry by closing the gap in the linework.

4Click Coincident in the Parametric tab’s Geometric panel.

You use a coincident constraint to make two points coincide. A blue coincident icon appears near the pickbox, and as you move the crosshairs over an object, a marker appears over relevant points. In the case of lines or polyline segments, they’re at the endpoints and the midpoint.

5Click an endpoint on the first polyline segment that you want to connect, and then click an endpoint on the second segment.

The endpoint of the second polyline segment jumps to the endpoint of the first line, and a small blue square appears at the intersection. If you don’t see the little blue square, click Show All in the Geometric panel.

6Apply some orthographic parameters so that the linework starts to look a little more like a rectangle. Click Horizontal in the Parametric tab’s Geometric panel, and then click a line or polyline segment that you want to align with the drawing’s X-axis.

The bottom segment of the polyline realigns itself horizontally from the endpoint nearest to the spot where you picked the line (unless a Fix constraint is added first), and a horizontal constraint marker (a constraint bar) appears near the object.

7Click Vertical to align a line or polyline segment with the drawing’s Y-axis.

The left vertical polyline segment realigns itself at 90 degrees to the horizontal segment, and a constraint bar showing a vertical constraint appears. With one line segment horizontally constrained and another vertically constrained, you’re halfway to a geometrically precise rectangle.

8Because rectangles have parallel and perpendicular sides, apply those constraints to your linework. Click Parallel in the Parametric tab’s Geometric panel, click the vertically constrained line segment, and then click the line segment opposite to it.

Because a vertical constraint is already applied to one segment, it doesn’t matter which line you pick first. If neither line had an existing constraint, the second line you picked would become parallel to the first line.

To make the final side of the almost-rectangle orthogonal with the other three, you could use another parallel constraint and click the bottom line. However, you don’t want to wear out the Parallel button, so use Perpendicular on the final segment.

Click Perpendicular button in Autocad to make a square.

9Click Perpendicular on the Geometric panel, click either vertical side, and then click the non-orthogonal side.

Again, because three of the four sides are already constrained to horizontal and vertical, it doesn’t matter which segment you pick first.

To delete a constraint, move the mouse pointer over the constraint marker to display the constraints bar, right-click, and choose Delete from the menu that appears.

From four non-orthogonal, not-even-closed line segments, applying a handful of constraints yields a perfect rectangle. However, you really want a square, which is where the Equal constraint comes in.

10Click Equal on the Geometric panel, click the bottom side, and then click either of the vertical sides.

It’s a perfect square! Now you have to constrain those circles. The design intent in this example is to make the circles concentric and to locate their centers in the exact center of the rectangle. First, the easy part: Make the circles concentric.

11Click Concentric in the Geometric panel, click one circle, and then click the other.

The two circles are concentrically constrained. A new constraint bar appears in their vicinity. Move one circle by clicking it, and watch the other tag along.

Because you can constrain only objects or points on objects, you have to add some construction geometry to maintain the design intent.

Drawing a line between opposite diagonal points.

12Draw a line between diagonally opposite corners using Endpoint object snaps.

Apply coincident constraints between the endpoints of this line and the corners of the rectangle.

A square with two concentric circles.

13Click Coincident in the Geometric panel. Click either circle so that the parametric marker appears in the center, move the pickbox over the construction line, and click when the parametric marker is over the midpoint of the line.

You’re done! You can test your design intent by using the Stretch command on the corner of the rectangle that doesn’t display a square, blue coincident icon. As you drag the corner, you should see the two circles moving, too, always maintaining their position in the middle of the rectangle.