May 29, 2014
After defining the project in January, columnist Gerald Davis is now set to make a virtual prototype of the Shashlik Grill in 3-D CAD.
In Part I (Precision Matters, January 2010, p. 24), we defined the project. We are designing a small charcoal grill for shish-kabobs that we‘re calling the Shashlik Grill.
We are going to make a virtual prototype of it with 3-D CAD. Using a top-down modeling technique will give us maximum flexibility in revising the design, which includes both changing sizes and changing the names of components in the model.
If you’re not using the same 3-D CAD software as I am, then you’ll have to translate some of the terminology. However, the concept of parametric modeling is widely applicable. The idea is to tie features to each other so they remain functional as the model evolves. For example, we want holes for axles and the wheels to stay aligned and properly sized no matter how big they are.
Our goal is to model the Shashlik Grill so we can easily change the height, distance between shelves, grilling area, wheel size, and other parameters. To do this, we will:
During the process we’ll adjust the size of components to develop the ideal product. As it is convenient, we will finalize the names, part numbers, and revisions and launch into the documentation process—design review presentations, mechanical drawings, and illustrations.
Figure 1 illustrates the beginning of our setup of reference geometry. These reference planes are useful for sketching upon, mating parts to, or using as stopping surfaces when extruding 3-D solids.
I like to name the reference planes so I can remember what they’re for. For example, I renamed the default system plane called “Top” to “earth” so I can remember that the wheels will rest on the Earth Plane. The default system planes “Front” and “Right” seem like fine names to me in this project.
At an arbitrary distance—something like 23.125 inches—above the Earth Plane, I created a plane that I called “Leg Top Plane.” When we create the legs, we’ll model them using this plane. By changing the location of the Leg Top Plane, the length of the legs—and all other parametrically linked components—will change automatically.
In Figure 2 you’ll see planes for locating the wheel axles, lower tray, and upper tray. As starting points, I’m guessing the wheels will be about 3 in. in diameter, so I set the Axle Plane at 1.5 in. above the Earth Plane.
The Lower Tray Plane is about 8 in. above the axles. The Upper Tray Plane is about 10 in. above the Lower Tray Plane. As a designer, you have the power and responsibility to establish the relationships—distances—of the reference geometry. You could dimension everything relative to Earth. However, if the design intent is to ensure that the lower tray can accommodate a 9.5-in. bottle no matter how big the wheels are, then dimension the distances between the tray planes relative to each other at distances greater than 9.5 in.
The good news is that you can make changes to how the reference planes relate to each other at any time. To start with, you only need to sort of get it right.
Now that the reference planes are set up, we’re ready to start modeling components. Let’s begin with the upper tray. With the 3-D CAD software that I’m using, the procedure is to Insert a New Part and then select the Upper Tray Plane. That creates a new “virtual” part. I then sketch a rectangle as shown in Figure 3.
Because this is to be a sheet metal part, I use the sketch to create a base flange as shown in Figure 4.
In Figure 5 I’ve used the miter flange tool to add flanges around the perimeter of the Upper Tray.
For now I’ll rename the new part Upper Tray—it was created as Part1^Shashlik—and save my work. We may decide to use a different naming scheme in the future. Now is a good time to take a break.
In Part III of this series, we’ll continue to model components of the grill, such as the Lower Tray, Leg, and so forth. Figure 6 gives you a sneak preview of what that work looks like.
Keep in mind that this top-down modeling technique that we’re using is not the only way to do this work. It may not even be the best way in every situation. However, when it comes to virtual prototyping, it is efficient to use parametrically driven features. It takes some time to set up initially, but it will pay off later by speeding the revision process. The use of reference geometry to control parametric features reduces the complexity of figuring out what drives what.
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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