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.
Figure 6: The problem in figure 5 is caused by feature dependencies between the work axis and the hole.
Figure 6 shows the heart of the problem. To produce the sketch for the peg, you needed the work plane and to place the work plane you needed the work axis. In this design, the work axis was attached to the counterbored hole, so the axis is dependent on the hole. When the hole is suppressed, Mechanical Desktop software suppresses the work axis and hence the work plane, and then the peg, and then the flats, hole, and chamfer on the peg.
The new counterbored hole from the other side was also suppressed because it was erroneously made concentric with the existing hole. Feature dependencies can arise whenever you dimension or constrain a later sketch profile back to an existing feature on the part. The later feature produced in the new sketch is dependent on the design of the existing feature.
To avoid problems, the general rule of thumb is to always dimension and constrain later features back to major features (in the part) rather than to minor details. In particular, avoid constraining anything to the tangency line where the filleting joins the rest of the part. In our example, the work axis should have been placed concentrically with the larger arc that defines the left end of the main body instead of being attached to the counterbored hole.
Thinking Ahead The preceding paragraph suggests another use for feature suppression. Consider our shifter fork example again. If the design of the original right-hand part was finished, complete with the fillet, you might conclude that you cannot properly constrain the left-hand counterbore to the larger arc. But you can: simply suppress the fillet, constrain the counterbore to the arc, then unsuppress the fillet.
As you would expect, feature suppression also affects 2D drawing views. When you suppress a part feature, the 2D drawing views update and remove the feature and its associated dimensions (see Figures 7A and 7B). If you unsuppress the feature then the dimensions reappear in the 2D drawing views.
Figure 7: Suppressing a part feature (top) also suppresses it (and its associated dimensions) in the 2D drawing views (bottom).
Feature suppression can also be used effectively while constraining assemblies, when it can sometimes be difficult to select the correct feature in a complex, detailed part because other features get in the way. In a nutshell, here's what you do:
1. Within the browser, double-click on the part to make it active and then expand its structure by clicking the plus sign (+) to the left of its name. 2. Right-click on the features that are getting in the way and suppress them. 3. Constrain the assembly as needed. 4. Unsuppress using Unsuppress +.
Note: When working with assemblies, keep in mind that suppressed features temporarily don't exist. If you attach an assembly constraint to a feature on a part and then suppress the original feature, the assembly constraint is suppressed. It does not go away, but it will not act to constrain the part until the feature is unsuppressed. Suppressing Features Automatically So far we have only discussed manual operations to suppress and unsuppress features, but there are two automatic methods available as well. Here's one: 1. Go to the top of the browser and right-click on the part name, which opens a shortcut menu. 2. Select the Feature Suppression option from this menu, then the Suppress by Type option. 3. Within the Suppress By Type dialog box, select any combination of any type of placed or sketched features. 4. Click OK. All features of the selected types will be suppressed.
Note: You can also do group unsuppressions by object type.
The other automatic method enables you to link part dimensions to an Excel spreadsheet. So you can flip back and forth between various suppressed and unsuppressed combinations with a single click in the browser. We'll discuss this method next time.
Conclusion Feature suppression is a powerful, versatile function that is extremely easy to use. It is especially valuable in creating a family of parts based on a core design and in constraining assemblies.