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Driving equations between parts

You can also use an equation in one part to drive a dimension in another part within an assembly. To do this, edit a part in an assembly, open the Equations dialog box, add an equation, and write the equation, selecting a dimension from another part. When you do this, the syntax of the equation works, as shown in Figure 12.3.

FIGURE 12.3

Local dimensions can be driven by a dimension in another part.

Notice that the syntax makes no mention of the assembly at all, only of the remote part (equation block 1.sldprt). This means that as long as both parts are open, the equation can work. It does not require the assembly to also be open. Shared (linked) equations do not require both parts to be open, just that the file containing the equations be accessible.

Following best practices

While assembly equations are certainly a valid way to control part sizes, you should use assembly or part configurations, possibly with design tables, to accomplish something similar. Equations and configurations do not mix well because the two methods compete to control the dimensions. Configurations with design tables are better than equations.

Using Link Values and Global Variables in Assemblies

Link values and global variables work in assembly sketches, but they do not work between parts. Local assembly sketches can use these functions, and the parts can use them as long as they do not link dimensions in different documents, but they cannot cross any document barriers (links must remain within a single document). If you need to achieve something like this, an equation can serve the same purposes.

Working with Derived Sketches in Assemblies

Derived sketches are a very powerful, but surprisingly underused, feature. They enable you to use a parametric copy of a sketch in another feature. When you edit and change the original sketch, the Derived Sketch updates immediately. The derived sketch becomes like a sketch block in that you can't change it, but you can position, orient, and even mirror it as you need, on any plane in the part without regard to the derived sketch's relationship to the original sketch's orientation.

Derived sketches enable you to create any kind of feature you can create with normal sketches, so you should not have problems using them with extrudes, lofts, sweeps, and so on. Remember that if you are just trying to copy a sketch to use the copy in the exact location of the original, you may be able to simply reuse the sketch. Some features such as curves do not allow you to reuse their parent sketches, but others such as extrudes, revolves, and lofts do.

In addition to being used within a part, derived sketches can also be used in context, in an assembly. For example, you can use a parametric copy of a sketch from Part 1 in Part 2 within a given assembly.

To create a derived sketch, select a sketch from the FeatureManager, and then Ctrl+click a plane or planar face and select Insert⇒Derived Sketch. This command places you in a copy of the sketch where you cannot change dimensions, relations, or add or remove sketch entities.

The best tool for moving the derived sketch is the Modify Sketch tool. You can only add relations or dimensions to the derived sketch that will locate the sketch as if it were a static sketch block. Remember that the sketch can rotate as well as translate. If one corner of a derived sketch is locked down, the rest of the sketch can rotate around that point.

When you create a derived sketch, it appears in the FeatureManager, as shown in Figure 12.4.

FIGURE 12.4

Derived sketches are identified in the FeatureManager.

After you have created and oriented the derived sketch, if you want to break the link to the original sketch for any reason, you can easily do that. Right-click the derived sketch and select Underive from the available options. This removes all dimensions and relations from the copy of the sketch, such that it is fully underdefined.

You can also create derived sketches between parts in an assembly. The procedure is