SolidWorks 2011 Assemblies Bible - Matt Lombard [31]
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 to give 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.
Caution
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 elements 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 (relations between items in an assembly), 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 a better practice to make A drive both B and C directly. This saves on rebuild times and troubleshooting and reduces future problems with lost references.
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, refer to the Layout feature in Chapter 6.
Working with 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.
You may consider using virtual components for certain types of parts that need to be in the assembly but might not require drawings, such as glue, paint, oil, and so on. You may even use virtual components to model purchased subassemblies that flex; for example, you could use a hinge, and use the hinge assembly as a part in a library. However you choose to use virtual components, make sure that they will not cause you any difficulties downstream in file management, data sharing, or other requirements.
Best Practice
Virtual components are useful for concept work in assemblies, but using them is not considered a best practice. Their main limitation is in the form of data management and reuse. You should limit your use of virtual components because the technique promotes what many users and administrators consider to be sloppy practice. Of course your specific situation will dictate best practice for your company.
Creating assembly reference geometry
Planes and axes are frequently created within assemblies to drive symmetry or placement of parts. You can use assembly layout sketches to create the reference geometry entities. When you create reference geometry within the assembly in this way, be aware that the normal history-based parent/child relationships are still followed. The familiar icons for reference geometry entities are also used in the assembly tree.
Comparing history-based and non-history-based portions of the assembly tree
Because features such as sketches and reference geometry are history based and found in the assembly tree, at least a portion of the assembly FeatureManager is history based. However, not all of it is. For example, the list of parts and subassemblies is not history based.
Sketches and reference geometry may appear before or after the list of parts, subassemblies, and mates. All