Submittal for AU 2011 is reaching its deadline, and of course I have not sumbitted yet. However, I do have a plan around what I'll propose, so I know what I won't be talking about. This is why I wanted to share the document from a quite successful class I did on the AutoCAD 3D Power Track last year.
I'll post it in chapters. After finishing these posts, I'll add some more around enhancements on surface modeling in AutoCAD 2012.
So here goes part 1:
AutoCAD 2011 introduced Surface Modeling, bringing unprecedented power and flexibility into AutoCAD. Between meshes, solids and surfaces, you can have different approaches to modeling. And what is even better, there are workflows between them.
There are some aspects to keep in mind when talking about surface modeling in AutoCAD. We need to talk about the many object types, but the main guiding principles when we built the feature were: explicit surfaces, associative surfaces, and direct manipulation. Any surface created with the option NURBS Creation on, or converted into a NURBS surface will be defined only by the grid of UVs.
a. Explicit Surfaces
However, you may prefer to work with Explicit surfaces when the method of creation is clear, and there is no need to change it (Revolve, Extrude, Loft, Blend, Patch). These surfaces maintain the relationship with the method used for their creation. For example, a revolved surface will let you change the angle of revolution any time after its creation. If it was a NURBS surface, this would not be possible that easily.
b. Surface Associativity
But we also have Associative surfaces. An Explicit surface could have been done without the Surface Associativity option. In that case, a change in the geometry that originated the surface will not change the surface. An associative surface is controlled by the original geometry. This means that if you used two splines to generate a loft, the control for the edges of the loft will not belong to the surface, but to the splines used for its creation.
Summarizing, a surface can be both Associative and Explicit at the same time. It can also be Explicit only. A NURBS surface can’t be associative.
c. Direct Manipulation
Most of the interactions with surfaces and splines are done through the canvas, and not involving dialog boxes, which tend to disrupt the process. You can see direct manipulation in the multi-function grips on splines, on selection cycling, and in the controls for surfaces. All these will be covered in the following example.
d. Example of surface modeling
Even more important than learning the commands and different surface objects, is to know when to apply which surface. Let me explain the whole process by showing you the final result and identify the surface types.
The first surface we’ll do is a Network Surface using the three longitudinal curves and the two curves we did between them. A Network Surface needs curves on both directions, and they don’t even need to be intersecting. Of course, if they intersect and have coincident vertices, then the surface will look exactly like we want. If not, there will be some interpolation done by the computer, which we won’t exactly control.
Now we’ll complete the sides. We will use a Loft for this purpose, and we’ll consume the remaining spline, and the edge of the surface we just created. Why? Because the selection of a surface edge will allow us to control the continuity between the surfaces. In this case, we want a perfect curvature between these two surfaces, so we’ll need G2 continuity. If we had selected both splines for the loft, we would not have had the chance to define continuity. That’s also the reason why we needed the Network Surface first.
The next operation is a loft, but in this case we can use the splines (the top spline and the top splines from the sides). Why? The answer is simple. We just need to have coincident edges, but not continuity, since this top surface will be a second part of the cover. We can also use the surface edge with G0 continuity. If we do this, we’ll have associativity between the surfaces.
The last operation will consist in closing the gaps on both ends. This will be done with Patch Surface. Patch can operate with surface edges or curves, but in this case we’ll use surface edges, even though we’ll force the solution to be a G0 (with no continuity in curvature or tangency).
In part 2, we'll go through each section in more detail.
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