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AutoCAD

Circles and Lines: The Lofty World of 3D

6 Nov, 2006 By: Lynn Allen

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The Loft command makes it easy to create freeform shapes.

Prior to AutoCAD 2007, the creation of free-form shapes was nearly impossible. The inability to loft, something that's existed in 3DS Max since its inception, has kept many of us from attempting complex objects. AutoCAD 2007 now brings us the ability to loft (finally!) and opens the doors to many possibilities. This month, we'll tour the newfound world of lofting in AutoCAD and what this new feature brings to the design table.

Despite the many solid primitives that exist in AutoCAD, there are many shapes that can't be generated with primitives alone. You can use the Loft command to create free-form models. Lofting is especially useful when you have a model that changes from one shape to another, or if the model needs to change its size and orientation as it travels through 3D space. Lofting is done by drawing a series of cross sections and lofting through them. These cross sections define the shape of the resulting solid or surface. If your cross sections form a continuous closed loop, you generate a solid, and if it forms an open loop (or a closed loop made up of separate objects), you generate a surface model. Cross sections are typically curves, lines or a circle. AutoCAD draws a solid or surface in between the spaces of the selected cross sections.

The process is fairly easy to follow, but practice makes perfect. You'll need to select a minimum of two cross section profiles, in the order you want your loft to transition. The following objects can be used as valid cross sections: lines, arcs, elliptical arcs, 2D polylines, 2D splines, circles, ellipses and points (for the first and last cross sections only). After selecting the cross section profiles, you can indicate a transition method in the Loft Settings dialog box as seen below.

Command: LOFT
Select cross sections in lofting order: 1 found
Select cross sections in lofting order: 1 found, 2 total
Select cross sections in lofting order: 1 found, 3 total
Select cross sections in lofting order:
Enter an option [Guides/Path/Cross sections only] <Cross sections only>:

There are basically four different options you can use when lofting with cross sections only: Ruled, Smooth Fit, Normal to and Draft Angles. Let's discuss the differences between them, as it can get a tad confusing!

Ruled: The transition from one cross section to another is a straight one and has sharp edges at each cross section. The figure below demonstrates the result of selecting a ruled transition with the three closed polylines as profiles. Because I used three closed objects to generate my loft, the resulting object is a solid.

Smooth Fit: Use this option to get a nice smooth transition between the cross sections. The start and end cross sections will still have sharp edges.

Normal to: Use this option when you'd like the transition to be perpendicular to the plane of the cross section. You specify which cross sections you want this to affect: Start, End, both Start and End, or All.

Draft Angles: Use this option to set the transition angle, as well as the percentage of the distance between the cross sections of the sides for the start and end profiles. The draft angle is the beginning direction of the surface where angle 0 is defined as being outward from the plane of the curve. This is a little tricky to understand until you try a few examples for yourself. The figure below demonstrates draft angles of 0, 90 and 180, respectively. The magnitude of the draft angles controls the distance on the surface from the start cross section (in the direction of the assigned draft angle) before the surface begins to bend back toward the next cross section.

In addition to just using cross sections, you can also use guides or a path. This gives you additional control over the resulting loft.

Guides
Guides are lines or curves that further help you define the shape and way the model transitions from one cross section to another. You'll find that guides help you avoid wrinkles in the resulting lofted object (and none of us like wrinkles!). The following objects can be used as guides: line, arc, elliptical arc, 2D and 3D splines, 2D and 3D polylines. There are some general rules that all guides must follow. They must start on the first cross section, end on the last cross section and intersect each cross section (rules, rules, rules!).

Paths
Paths are useful to indicate a specific route you'd like your loft to follow, as seen below. Incidentally, I borrowed this figure from the Help menu. Don't forget that the Help files in AutoCAD are much easier to follow now; you should use them. The only rule about path curves is that they need to intersect all the planes of the cross sections (they can be open or closed). The following objects can be used as a loft path: lines, arcs and elliptical arcs, splines, helices, circles, ellipses, 2D and 3D polylines.

You should keep track of the LOFTNORMALS system variable while lofting. The settings in the Loft Settings dialog box can also be controlled by this system variable (it's just as easy to do it from the dialog). Be sure it is set to 1 (for Smooth) prior to starting the Loft command when you want to use the Guides option. You'll be much happier with the results than with what we see in Figure 6. The following are the settings for LOFTNORMALS:

  0  Ruled
  1  Smooth
  2  First Normal
  3  Last Normal
  4  Ends Normal
  5  All Normal
  6  Use Draft Angle and Magnitude.

I think you'll find that the new Lofting capabilities in AutoCAD 2007 open the door to all types of design scenarios. Give it a try and you'll see for yourself just how easy it is to make a myriad of great shapes. Until next month, Happy AutoCADing!