Assembly equations in solidwoks tutorials

Assembly equations work mainly like part equations, but with some additional complications and considerations. For example, one of the additional features of assembly equations is the ability to drive the dimensions of one part from another part. The syntax is slightly different for this application, as shown in Figure 12.2. Overall, issues with equation order and using driven dimensions on the right side of the equation are the same between parts and assemblies.

assembly epuations       assembly epuations 1

FIGURE 12.1 Elements of an assembly


Equations are discussed in detail in Chapter 9.

assembly-epuations 4

FIGURE 12.2 An assembly equation driving one part from another

External references

Notice the “->” symbol after the Equations folder in the Assembly FeatureManager. This means that there is an external or in-context reference. An external reference means that an aspect of the part is dependent upon something outside of the part. This has file management implications because you must maintain the names of the files so that they always recognize the other file involved in the external relation. In-context means that one part has a relation to another part in positions determined by an assembly. So in this case, the in-context external reference can only be solved if the original part, the referenced part, and the assembly where the relationship was created are all open at the same time.


In-context references are discussed in depth in Chapter 16. 

When one part drives another part in this way, the assembly must also be open to drive the relationship. If just the two parts are open individually, then changing the driving part does not update the driven part; because the relationship was created in the context of the assembly, the assembly must also be open to facilitate the change.

Link values and global variables

Link values and global variables also work in assemblies, but they do not work between parts. Local assembly sketches can use these functions, and the parts can use them when edited in the context of the assembly, but they cannot cross any document barriers (links must remain within a single document).


Equations update with new part names regardless of how the part is renamed. Names of subassemblies also update when assembly files are renamed. This includes renaming a document using the Save As command, using SolidWorks Explorer, or using Windows Explorer. It also includes redirecting the assembly to the new part name, as well as renaming the assembly using each of these techniques. If the assembly can find the part and recognizes the part as the one that it is looking for, then the equation will work.

Some of the methods named previously for renaming parts are not recommended; for testing purposes I specifically tried to break the relationships in the equations by using them. SolidWorks Explorer and the Save As methods can be effective when used properly. References between files are a different issue altogether from an equation’s references to local file names.


While assembly equations are certainly a valid way to control part sizes, I would recommend using 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. I recommend configurations with design tables over equations.


Assembly configurations are discussed in Chapter 14. Design tables are discussed in Chapter 10.


You may have unexpected results if a single dimension is controlled from more than one location. For example, if you have a part-level equation and an assembly-level equation, then one of the equations will be automatically set to Read Only and will not be used.

Assembly layout sketch

SolidWorks has an assembly feature called Layout that uses a 3D sketch to lay out the major functions of an assembly, and even details of parts. The word layout also refers to a technique using 2D sketches in an assembly to do exactly the same thing. The distinction between the technique and the formal assembly feature is bound to be confusing. SolidWorks’ Layout feature only works in assemblies, but layout techniques have been used in parts as well as assemblies for many years. In this chapter I describe the technique, and leave the Layout feature for Chapter 16. When you look at the two functionalities, the feature is definitely intended to be used as an in-context tool, while the technique can be used most easily as a reference for controlling part position (through mating) rather than a way to directly control the sizes and shapes of the parts. So when I refer to a Layout (capital), I’m referring to the formal feature. When I refer to a layout or layout sketch (lower case), I’m referring to a technique where a sketch is used at the assembly level to control the assembly in some way.


The Layout feature is described in more detail in Chapter 16, while the technique using assembly sketches to lay out an assembly is described here. The material in this chapter is written as if the Layout feature does not exist, mainly to give you a straightforward view of how the technique works without worrying about two different functions at the same time.

The layout sketch is a very useful tool for laying out a mechanism in an assembly or even details on parts within the assembly. Sketches in the assembly have the same characteristics as they do in the part environment. In Figure 12.3, the assembly layout sketch is indicated with a heavy dashed line for emphasis.

When combined with in-context techniques, assembly layout sketches can help to determine the shape of parts, or the location, size, or shape of features within the parts. You can also use layout sketches to mate assembly components to far more robust and dependable mates, rather than mating part to part. The sketch shown in Figure 12.3 is used for both of these techniques. The shape of the frame and the major pivot points are established in the 2D sketch. The wheels are also mated to the sketch.

When you use an assembly layout sketch for either the in-context part building or simply part positioning, the main advantage that it offers is giving you a single driving sketch that enables you to change the size, shape, and position of the parts. You can use as many layout sketches as you want, and you can make them on different sketch planes. This enables you to control parts in all directions.

assembly epuations =

FIGURE 12.3 An assembly layout sketch


When using layout sketches, it is assumed that the relationships are created such that the sketch drives everything else. However, there is nothing preventing you from using other things in the assembly to drive the sketch. You should avoid this type of conflict, called a circular reference. It can create sketches that change with every rebuild and can seriously impact rebuild times. When using any type of in-context relations, you need to be careful to establish one or more driving entities, which are not in turn driven by other entities.

To take this a step further, it is best to avoid daisy chaining, where A drives B, B drives C, and so on. It is better practice to make A drive both B and C directly. This saves on rebuild times and troubleshooting. See the sidebar on using the skeleton or wide tree approach in Chapter 11 for more details on the benefits of this type of modeling and an example part. 

One of the drawbacks of this technique is that you give up dynamic assembly motion. To move the parts, you have to move the sketch and rebuild. The part does not move until the sketch is updated. If you need to combine layout functionality with dynamic assembly motion, see the Layout feature in Chapter 16. Virtual components Virtual components are covered in more depth in Chapter 16.

Virtual components

  Virtual components are parts that are saved so they are internal to the assembly. You can save them out so that they are external to the assembly and can be reused in other assemblies. You can also convert external components to virtual components. Virtual components, as the name suggests, can be either parts or subassemblies.

Best Practice

Using virtual components is a technique that is useful for concept work in assemblies, but you will not see them show up on any best practice list. The main limitation of this technique shows up in the form of data management and reuse. I recommend limiting your use of virtual components because the technique promotes what many users and administrators consider to be sloppy practice.

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