The next lesson in top-down design: A rack for skewers
October 26, 2010
Working on a 3-D design for the Shashlik Grill, columnist Gerald Davis continues working on the top-level assembly. This time he's modeling two skewer supports.
The purpose of our grill project is to design a mechanism to hold skewers over a bed of hot embers. So far we’ve modeled wheels, axles, legs, and shelves. All of our modeling has been constructed within the context of a master assembly—a top-down modeling technique. Our intent in following this 3-D CAD modeling method is to make experimenting with the overall size of the grill model as easy as possible. For example, if a section of the grill is enlarged, the component parts automatically stretch to maintain their function.
Part IX of this series ended with the intent to work on the parts for the skewer support. Figure 1 shows what our CAD model eventually will look like. Skewers will rest in notches in the side skirts. A charcoal pan is supported by flanges in the side skirts. By sliding the charcoal pan on those flanges, the cook can open or close the vents to adjust the cooking temperature. We are going to continue our theme of locating features in the context of our top-level assembly.
(The usual disclaimer: 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.)
In Figure 2 we see one of two identical skewer skirts. This sheet metal part has jog bends, an edge flange, and a few patterns of cutouts. The plan—or design constraint—of using identical parts to create a rack to support both the skewers’ ends reduces setup cost in manufacturing and possibly reduces confusion during assembly.
We’ll make the skewer skirt out of 18-gauge (0.048-inch) stainless steel.
In Figure 3 I have isolated the parts that I want to use to govern the features of the skewer skirt. To create a new model, I’m going to insert a new part into this assembly and then start constraining it to the model.
With the 3-D CAD software that I’m using, the previous sentence is almost literally the sequence of operations. For this example, I selected a leg face the skewer skirt will rest on and used it as the in-place anchor for the new part.
The first modeling step is to create a sheet metal base flange. Figure 4 shows the sketch and related dimensions that I used. I left 0.10 in. between the inside edges of the legs and the vertical edges of the skewer skirt to allow for the bend radius of the legs. The bottom edge is constrained to be collinear with the bottom edge of the end skirt. The top edge is constrained to be collinear with the top edge of the end skirt.
All of these dimensions are fairly arbitrary. The main idea is that the part is modeled in the context of the main assembly. If the relationship between the legs and the top shelf changes, then the skewer skirt should stretch to automatically fit.
You may recall from last month’s column how the jog bends were modeled in the short end skirts. We’re going to repeat that process for the jog bends in the longer skewer skirts. We could use a different technique—perhaps sketching the jogs in the base sketch. However, consistency in modeling technique is a blessing for those who edit the project in the future.
To create the offset jog bends, the CAD software I’m using needs a single line sketch that indicates where the jog is to be (see Figure 5a). I’ve dimensioned the jog bend line 0.125 in. above the top edge of the shelf. That should be enough to allow for the bend radius of the tooling.
In Figure 5b you see the PropertyManager dialogue for the Jog feature. Because I want the offset to match the material thickness (0.048 in.) of the shelf, I need to reduce the bend angle of the jog from the default 90 degrees to something less—45 degrees, for example. This allows for the tooling radius of the jog. You will certainly want to change all of these parameters to match the tooling in your manufacturing facility.
To add the second jog, I created another sketch. This time it was dimensioned 1.50 in. from the skirt’s top edge. Because it is just an aesthetic detail, it does not need to be dimensioned relative to other parts in the assembly.
To model the notches for the skewers, I created a single cut and then made a linear pattern of that cut to create the remainder of the notches (see Figure 6). The skewers are patterned on 3.5-in centers. This makes it easy to change the design for the cutouts; just edit a single sketch. Setting up a pattern is a lot faster than dimensioning individual sketches!
The vents can be created with a technique that is very similar to that used for the skewer rack. Make a cutout, and then make a pattern of that cutout. Figure 7 shows the dimensions that I used. The vents are on 2-in. centers in this example.
For safety, I added a radius to each of the corners along the top edge. That’s easily done with the Sheet Metal>Corner>Break Corner tool. Select a face, and all of the corners on that face get the treatment.
The flange that will support the charcoal pan is created in a two-step process:
In Figure 8b you can see that I dimensioned the sketch for the edge flange to create bend reliefs for the benefit of the press brake operators.
To finish up the skewer skirt, I cut holes for the bolts. These holes are constrained to be concentric with the holes that already exist in the legs. The completed end skirt is shown in Figure 2.
In Part XI of this series, we’ll work on the charcoal pan shown in Figure 9 . We’ll use a limit mate so that the charcoal pan will move in a realistic way when we drag it with the mouse.
Keep in mind that this top-down modeling technique 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.