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AutoCAD

Circles and Lines: As the 3D World Turns

5 Sep, 2006 By: Lynn Allen

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Discover AutoCAD 2007's Presspull, Extrude and Revolve commands for 3D.

AutoCAD 2007 offers some fabulous (and easy-to-use) 3D tools that just might entice more users to step away from 2D and enter this new dimension. Last month, we made our way through 3D solid primitives, and this month we'll continue our journey through 3D as we tackle some other ways to create 3D objects. Once again, I suggest working from the new Dashboard, your one stop shop for all things 3D -- or 90% anyway.

Presspull Command
One of my absolute favorite additions to AutoCAD is the new Presspull command. You create a bounded (closed) area and Presspull lets you dynamically turn it into a 3D solid. The cool thing about Presspull is that all you have to do to start it is to hold down Ctrl+Alt, pick within your bounded area and the solid is dynamically created before your very eyes -- no need to key in anything! You can also pull it up or press it down. If you draw a bounded area on the side of an object and press it inward, it cuts through the object! Forget the days of Boolean operations, unions and subtract -- now you can use the much smarter Presspull command. AutoCAD's Help file does a beautiful job of demonstrating this with an AVI -- look for the demo under Concepts after searching Presspull. In fact, this figure was taken straight from the new friendlier Help file.

I'm guessing that many of you glossed right over that tip to go to the Help file and you only go there when you absolutely have no other choice, right? Give Help another chance and you'll find some great explanations for many of the new 3D commands. Let's face it, the Help file is free, so you might as well take advantage of it.

Presspull Boundary
Presspull (Ctrl+Alt) has a few restrictions regarding the boundary. The easiest way to figure out if your boundary will pass is to think of the Hatch command. If you can hatch the area, then you'll be able to use Presspull on it (that's hatching with a zero tolerance without any gaps). Notice here that I was able to use Presspull on an area that was bounded by another solid (the cylinder) with no problem at all. In addition, my new solid isn't attached to the cylinder, so I can move both at will.

When using Presspull on a bounded area that resides on an existing solid, pressing the area inward performs a subtract while pulling the area outward performs an extrude.

Incidentally, if the system variable IMPLIEDFACE is turned off, you won't be able to perform this very cool command, so make sure it's on. Also, the entire boundary must be displayed on the screen in order for Presspull to work.

Extruding
The Extrude command was updated to work better in the 3D world as well. It's a simple command, if you extrude a closed area, you get a 3D solid and an open area generates a 3D surface. You can extrude the following objects:

• Lines
• Arcs
• Elliptical arcs
• 2D polylines
• 2D splines
• Circles
• Ellipses
• Planar 3D faces
• 2D solids (Does anyone use these anymore?)
• Traces (Does anyone use these, either?)
• Regions
• Planar surfaces
• Planar faces on solids

Do you remember the good old ISOLINES system variable? ISOLINES controls the number of contour lines that display on your curvy solids. The next figure shows the effect a value of 4 would have on the display while in the 3D Wireframe Visual Style. You can bump this number up or down as you see fit. A quick view of the Extrude command follows:

Command: EXTRUDE
Current wire frame density: ISOLINES=4
Select objects to extrude:
Specify height of extrusion or [Direction/Path/Taper angle]:

Use Direction to specify two points that determine the height and the direction of the extrusion -- helpful if you don't intend to Extrude straight up. Path let's you select another object as the path to follow for extruding. The path object is moved to the centroid of the profile. Taper Angle lets you taper the extrusion -- a positive angle tapers it inward while a negative angle tapers it outward. If you select a large taper angle, it may not reach its full extrusion height. Incidentally, Extrude isn't as smart as Presspull in that it doesn't recognize crossing objects as valid objects to extrude (the area in the second figure would never work with Extrude).

Revolve Command
The Revolve command is popular for creating nice curved surfaces or solids. It's as simple as selecting a profile to revolve around an axis. A closed profile creates a solid while an open one creates a surface. The axis can be defined by another object or by indicating the X-, Y- or Z-axis. The axis basically needs to be a linear segment of some type -- which could be a line, straight pline segment or even an edge of a solid.

FYI, you can grab the edge, face or vertex of any solid by using the Ctrl key. This is extremely valuable if you intend to use a part of a solid in any of the before mentioned commands.

The Revolve command won't let you revolve blocks, so explode them first. You can choose to revolve your profile all the way around the path or partway, and it uses the right hand rule to determine the direction.

Command: REVOLVE
Current wire frame density: ISOLINES=4
Select objects to revolve: 1 found
Select objects to revolve:
Specify axis start point or define axis by [Object/X/Y/Z] <Object>: O
Select an object:
Specify angle of revolution or [STart angle] <360>:

Note that you can choose to keep the original objects used to create your surfaces and solids or delete them. When set to 1, the DELOBJ system variable deletes the profile curves in Extrude and Revolve (and Sweep and Loft). When set to 2, it deletes all the defining geometry, no questions asked. When set to -1 (yes?-1), it prompts you to delete the profile curves. When set to -2, it prompts you to delete all the defining geometry. I'm not sure what's up with the negative value settings -- it's not consistent with other AutoCAD system variables. Regardless, select the value that works best for you. You can also set these settings in the Options dialog box, under the 3D Modeling tab.

We're making our way through the basics of generating 3D geometry -- next month we'll tackle the updated and powerful Sweep command along with the new (and much needed) Loft command. Until then -- Happy AutoCADing!