Autodesk Mechanical Desktop: Suppress Your Best Features
By Bill Fane
My grandmother used to recite a little poem:
Yesterday upon the stair I met a man who wasn't there. He wasn't there again today. I sure do wish he'd go away! Feature suppression in Mechanical Desktop? software works exactly like this. You can make a feature go away, but it is still there. This is a good thing, because you can make a suppressed feature come back again.
More on that in a moment.
As you've seen in previous tutorials, the parametric-dimensioning capabilities of Mechanical Desktop are a powerful design tool. You can use them to play "what if" as you tweak and tune a part during the design phase, and you can easily develop a family of parts—all based on the initial design—that are similar in shape but vary in size, perhaps to suit different product models. Typically a family of parts share more design features than not; it's developing the differences, while keeping in mind the feature dependencies for the total design, that can get complicated. For example, a larger part in this family might need a reinforcing rib, or a larger part might need more holes for mounting bolts, or a larger tank might need an extra cleanout port. Do you have to do a redesign for each variant? Not if you use feature suppression. In this tutorial, we're going to develop some themes and variations on two simple designs, a bracket and a shifter fork, to show you how this capability works.
Figure 1: A simple bracket, with a reinforcing rib.
Figure 2: The shorter model of the bracket does not need the rib, so that feature is suppressed.
One Design, Many Alternatives Figure 1 shows a simple bracket that you want to make in several different sizes. Figure 2 shows a smaller size, which as you can see does not require a rib. To create Figure 1 based on Figure 2, you only have to do two operations: suppress the rib feature and change the dimension for the overall height. Suppressing a feature is a very simple operation:
1. Right-click on the rib feature in the browser. This calls up a shortcut menu.
2. Click the Suppress option. The rib feature is now highlighted.
Note: Clicking Suppress also highlights the filleting around the rib because the filleting is dependent on the rib.
3. At the prompt asking you if you want to suppress these features, click Yes. The features disappear and their browser entries are grayed out, indicating that they're suppressed in this part.
Figure 3: The rib can be unsuppressed, even in the small bracket.
Remember, the features are suppressed—not deleted. Just like a drawing layer that you've frozen, you can unsuppress these features at any time: 1. Right-click on the greyed-out rib feature in the browser, which calls up a shortcut menu with the options Unsuppress and Unsuppress +. 2. Choose the Unsuppress + option from this menu. The rib and its filleting reappear (see Figure 3).
Feature Dependencies Count Why did we choose the Unsuppress + option over Unsuppress? Because of the feature dependencies in the design. When a feature is suppressed, it is as though it had never existed. And when the parent object goes so do all its children, and their children, and so on. When you suppress a feature you also suppress all the other features tied to it. In our example, the filleting was applied to the rib so when the rib was suppressed the filleting disappeared too. Clicking Unsuppress + resurrects the parent object (the selected feature) and all its dependent children. Clicking Unsuppress just brings back the parent object and you have to resurrect the children objects one-by-one as needed.
So how you constrain the various design features becomes very important as you begin to create variants on the core design.
Figure 4: A shifter fork. When I suppress the counterbored hole...
What Happened? Figure 4 shows a shifter fork. To make a left-handed version, the counterbored hole must come in from the other side. "No problem," you say. You decide all you have to do is place the new counterbore on the other side, then suppress the existing one. But you wind up with the part shown in Figure 5, which is clearly not what you had in mind.
Figure 5: ...too much disappears.