July 1, 2008
Columnist Gerald Davis provides the reader with a brief overview of some of the handy 3-D CAD tools he uses when doing basic modeling techniques.
CAD software of the 3-D variety can be a lot of fun to use. Perhaps it's because the software does a lot of work quickly.
Figure 1a shows an example of the power of 3-D CAD software. To create a sheet metal handle, all I had to do was sketch five lines and answer questions about thickness and bend radius. The software automatically completed modeling the 29 other surfaces for me.
As shown in Figure 1b, the 3-D CAD system can show me those surfaces in a realistic setting with reflective surfaces and pretty colors. In Figure 1c you can see that the software even will generate a flat pattern of the sheet metal part. In this example, the part isn't very complex, and calculating the flat pattern by hand wouldn't be very hard. However, it took me just a mouse click to make the flat.
This sort of automatic modeling does have a downside. Because the computer is automating a lot of the surface modeling, the part designer will discover some restrictions as to what the software can do. Sometimes it takes human intelligence to figure out how to model a design.
Figure 2 is a small variation on the sheet metal handle. I've added an embossed area to make the handle more comfortable to use. With the software I'm using, I can't automatically unfold this kind of curve. I've broken the "rules" by creating bends that intersect each other. If I needed a flat pattern of this part, I would have to purchase additional software. That's not the end of the world, but I wanted to be clear about the limitations of some systems.
Staying with my theme of isn't that cool, Figure 3a shows a sketch of a handle formed from rod stock. All I had to do was sketch the centerline of the bent rod and the rod's diameter. In Figure 3b you can see the finished part. This is nearly as easy as modeling the sheet metal part!
However, the software I'm using will not unbend this bent rod automatically. If I need to know what the raw stock flat length should be, I'm out of luck.
Take a look at Figure 4a. It looks a lot like the rod handle shown in Figure 3b, doesn't it? My tip of the month is to model the rod using sheet metal tools. In Figure 4b you can see how I started modeling the rod handle as sheet metal. This is another simple five-line sketch. I answered the questions about thickness and bend radius—in this example, the thickness is 0.25 inch, the flange width 0.25 in., and the bend radius 0.50 in. Now I have a square rod handle made out of sheet metal that will unfold.
In Figure 4c I rounded the edges of the square sheet metal to make it look like a rod. If you could see my computer screen up close, you'd notice that this model is not perfectly realistic because tiny flat areas are present instead of a smooth, round cross section.
However, as you can see in Figure 4d, I can get a flat pattern for the rod stock. If that is my primary goal, this trick of modeling a rod as sheet metal will serve. If I really needed a realistic rod model, then Figure 3b is a better modeling method.
As a side note, the 3-D CAD software I'm using has many overlapping tools. That is to say, there is usually more than one way to model anything. The examples I'm showing you in this article are not necessarily the best or only way to do things. At least you're getting some insight into how it might be done.
Figure 5a shows a handle with a bit of artistic style. Industrial designers like to dream up stuff like this. Old-time machine tool operators like me know that critters like this one are tough to prototype. This almost certainly will require casting or molding.
Here comes another side note. The term prismatic has been hijacked by some of us in the CAD world to refer to objects that are reasonably easy to fabricate using lathes, mills, or sheet metal processing equipment. My dictionary tells me that a traditional definition of prismatic is "of, relating to, or being a prism," so it is only a little bit of a stretch to consider things that are of, relating to, or being boxes and rods as "prismatic."
Even though Figure 5a would be a challenge to machine on a mill, it isn't that hard to model with the 3-D CAD software I'm using. In Figure 5b you can see the setup I used to take advantage of a modeling tool called a boundary surface.
I created a series of five cross-sectional profile sketches. For this quick example, I just sketched ellipses and didn't worry too much about specific sizes. Not shown in Figure 5b is the centerline sketch that I used to locate the ellipses, but it is the same idea as we've used in the previous examples.
Once the cross-sectional profile sketches were complete, I used the boundary surface tool to connect them together, as shown in Figure 5c. I could have used a surface loft, but the boundary surface is much easier to control and results in very smooth and attractive surfaces. I left the sketch profiles visible in Figure 5c for your reference.
You might notice that the surface in Figure 5c is hollow. This leads me to point out the difference between solid modeling and surface modeling. Solids represent volumes of space that are completely enclosed by surfaces; those surfaces also must knit together to form a watertight seal—no overlaps or gaps. If those requirements are not met, then the model is made up of surfaces instead of solids.
At your next cocktail party, you can dazzle your friends with a discussion of manifold and nonmanifold models. Manifold is the technical term for solid body.
In Figure 5d you can see the result of my next few modeling steps to make the curvy handle. I mirrored the boundary surface to create the other half of the handle. After capping the small ends at the pivot rods, I added a couple of small fillets to give the part a finished appearance.
This has been a quick run through some basic modeling techniques that demonstrate how handy 3-D CAD tools can be. However, the terminology and methods are not always intuitive or obvious. It just takes a bit of training and practice.
Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to firstname.lastname@example.org.
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