Although creating a part in AutoCAD Designer is, for the most part, pretty simple, it is not completely painless. For this review, I created an automotive connecting rod, a process that is depicted in the photo sequence that follows.
First, you create a simple sketch of the part's outline. The sketch doesn't have to be dimensionally accurate or even very neat. You then turn the sketch into a profile with the ADPROFILE command, and Designer automatically applies constraints that clean up your sketch (
photograph 1
). In the process, Designer makes some assumptions about the intended shape of the sketch. For example, it adjusts lines that almost
touch, so that they do touch; it makes lines that are nearly vertical or horizontal, exactly vertical or horizontal; and it makes arcs that are nearly tangential to lines, exactly tangential to those lines. In some cases, the assumptions that Designer makes are incorrect, but you can override them manually.
After you turn a sketch into a profile, you must apply some further constraints and dimensions to achieve the shape you really want. If a profile is underconstrained (i.e., if you do not give a profile all the constraints and dimensions that are required to achieve the desired shape), then there are no guarantees that Designer won't turn the profile into something unrecognizable the next time you change a dimension.
Fully constraining a profile can be a tedious job. One reason is that the dimensions and constraints that are necessary to accomplish the task are not obvious. In the connecting rod example, in addition to adding the required dimensions, I had to constrain the top and bottom arcs
so that they had the same X value, and I had to set the dimensions of the lines so that they had the same Y value (
photograph 2
).
Once you fully constrain the profile, you can turn it into a feature. In the rod example, this is a simple extrusion (
photograph 3
). At this point, the part is a solid model, although it is not yet complete; the rod still needs journal holes. You add the holes as sketches (
photograph 4
), turn them into profiles, constrain them to be concentric to the ends of the rod, and then subtract them as extrusions from the base feature (
photograph 5
).
Profiles must be constrained with respect to the base feature. This is where feature-based modeling departs from simple solid modelers, such as AME (the AutoCAD Modeling Extension). Although it might initially seem like a restriction, this requirement lets you build a part with little regard to what its final dimensions will be. In the case
of the rod, its overall length, width, and end diameters can be changed just by updating their dimensions (
photograph 6
). From a design perspective, this is an advantage, because it lets you design parts before you finalize their dimensions.
After you have established the basic features of a part, you can add fillets and chamfers (i.e., beveled edges). On the rod, I added chamfers in the journal holes and fillets at the base and top of the shank and around the outer edges (
photograph 7
). To show what the rod looks like, I asked Designer to apply a mesh to its surface and render it, which created the image shown with the article "Bringing 3-D Modeling to PCs."
Because Designer parts are solid models, the program is able to calculate their mass properties. You use the ADMASSPROP command in Designer to display a dialog box listing a part's mass, volume, center of gravity, principal axes, and moments of inertia. An entry field lets you specify the dens
ity of the material. All the entries in this dialog box have no default unit of measurement, so they are based on whatever units you used to create the part.
Once you have essentially completed a part, Designer has the capability to create a multiview drawing from it. The drawing can incorporate orthogonal and isometric views, as well as sections.
An important characteristic of Designer is that the part and the drawing are bidirectionally associative. This means that any change you make in one is automatically reflected in the other, making the drawing truly intelligent. After a designer or engineer creates a complex part, anyone who has a minimum of AutoCAD experience will be able to modify its dimensions. For example, the rod design could be modified to be any length, with any journal diameter, thickness, and beam width.
From the creation of one part, you can grow a whole family of parts. That, at least, is the theory behind Designer.
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A 3-D Feature Presentation: Anatomy of a Model
