Circles and Lines: Housework Made Easy

7 Oct, 2006 By: Lynn Allen

AutoCAD 2007's new 3D Sweep tool makes it easy to create tidy curves and other hard-to-design geometry.

For the past few months we've toured AutoCAD 2007's incredibly cool 3D features. Of course, going over all of AutoCAD's 3D features would fill up a book, so we're concentrating on those that you're most likely to run into as you begin to explore 3D.

The Sweep command is one that's critical to all 3D users. Though a primitive form of sweeping known as Tabsurf (tabulated surfaces) has existed in many releases, the rules behind the command were many and limiting, plus using it was cumbersome at best. The new updated Sweep command is much more flexible and has very cool new options.

Beginning Sweep
To create a new solid or surface, sweep a profile along an open or closed 2D or 3D path. If the profile is an open curve, you'll end up with a surface and a closed curve generates a solid. If you feel adventurous, you can sweep more than one object at a time. The only rule is that all the profile objects must lie along the same plane. Sweeps are fabulous for creating stair railings, pipes, hoses, architectural moldings, screws and the like. Normally I would stay away from any command called Sweep for fear there would be housework involved -- but you're going to find this command to be a true gem.

You can use the following objects as a profile: lines, arcs (including elliptical), 2D polylines and splines, circles, ellipses, planar 3D faces, regions and planar surfaces. There are also a few legacy objects you can use, such as 2D solids and traces, although it's rare that anyone uses those archaic commands. (I'm sure to get emails from that statement!)

As you can see, there are quite a few objects that qualify as profile-worthy.

Now let's take a look at the objects that qualify as valid path objects: lines, arcs (including elliptical), 2D polylines and splines, circles, ellipses, 3D splines and polylines, helices and edges of solids or surfaces. How do you select the edge of a solid or surface? Don't forget the handy CTRL key for selecting individual edges.

Use the Sweep command to sweep a profile along a path. This image shows the result if a circle is the profile and the helix is the path to generate a spring.

You find the Sweep command in the second row of the 3D Make control panel on the dashboard. When you start it, you see these prompts:

Command: SWEEP
Current wire frame density: ISOLINES=4
Select objects to sweep: 1 found
Select objects to sweep:
Select sweep path or [Alignment/Base point/Scale/Twist]:

At that point, you have these options:

Alignment. By default, AutoCAD aligns the profile perpendicular to the sweep path (if it isn't already). If you'd like the profile to retain its original direction throughout the sweeping process, then select Alignment and specify No when asked:

Align sweep object perpendicular to path before sweep [Yes/No]:

In the past you had to have the profile aligned to begin with or it didn't work, so this is a huge improvement!

Here you can see when the triangle is aligned (green) and when it is not (red).

Base Point. By default AutoCAD places the centroid (middle) of the profile along the path and then sweeps it. If you'd prefer the profile to be swept in another location, you can specify a different base point to work from. If you select a base point that does not lie on the same plane as the selected objects, it's projected onto the same plane and then swept.

Scale. If you'd like your sweep to scale up as you go, select an ending scale factor. The program begins with a scale factor of 1 and then uniformly applies the scale factor along the sweep until the end. For example, if you select 3 as your scale factor, the object starts the path with a scale factor of 1 and ends with a scale factor of 3, as shown below.

A scale factor gradually scales from the start to the end of the path.

Twist. A nice feature that has never been possible before in AutoCAD (not without a little black magic, anyway) is the new Twist option. Now you can specify a twist angle for the object being swept. This angle indicates the amount of rotation along the entire length of the sweep path from start to finish. The angle needs to be less than 360 degrees. You'll see this prompt:

Enter twist angle or allow banking for a non-planar sweep path [Bank] <n>:

Banking permits the curve being swept to bank naturally along the 3D path for 3D polylines, splines and helices. You might find that you prefer banking to the default sweeping procedure.

Here's a demonstration of a variety of different takes on the same path and profile. The first figure (green) has a twist angle of 180, so the crescent makes a half-turn while it is being swept. The second figure (yellow) has banking on. Notice how it makes a very nice transition around the turns. The third figure (red) is the result you'd get with no twisting at all (default).

You really need to play around with the various twist options to grasp the subtle difference between them.

By default, AutoCAD deletes the defining geometry that composed the Sweep. The system variable DELOBJ controls whether or not this geometry remains behind or is banished. Here are the settings for DELOBJ as it relates to the Sweep command.

0	All the original defining geometry is retained
1	The profile is deleted
2	The profile and the path are deleted
-1	You are asked if you want to delete the profile
-2	You are asked if you want to delete everything (path and profile)

You can also set DELOBJ in the 3D Modeling tab of the Options command as seen below.

Use the Options dialog box to control whether or not the defining geometry is deleted.

Give the good old Sweep command a try, and you'll find it opens doors to all types of geometry that would be nearly impossible otherwise. Until next month -- Happy AutoCADing!

About the Author: Lynn Allen

AutoCAD Tips!

Lynn Allen

Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD how-to, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's Tips & Tricks Tuesdays free e-newsletter and we'll notify you every time a new video tip is available. All exclusively from Cadalyst!
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