SolidWorks 2011 Assemblies Bible - Matt Lombard [37]
Cross-Reference
SolidWorks BOM functionality is discussed in depth in Chapter 17.
Businesses often represent assemblies and subassemblies in various ways by using Manufacturing Resource Planning (MRP) or Enterprise Resource Planning (ERP) software. The methods that accountants and manufacturers use to organize assemblies are not always the same as those that engineers or designers might choose, but some companies require the BOM on the drawing to match the MRP or ERP Bill of Materials.
Best Practice
When you are forced into modeling something in an unnatural way to satisfy an outside demand such as special BOM requirements, it might be best to detach the unnatural part and model normally. In the situation mentioned here — where MRP is forcing how the assembly is put together by requiring the BOM to match MRP — you should separate the BOM from the assembly structure rather than building an assembly that makes other SolidWorks functions difficult. This ensures that the BOM becomes a manually maintained document. Alternatives to this approach would be to make configurations or entirely new assembly documents to drive the BOM.
Grouping subassemblies by relative motion
A more natural way to group subassemblies is by considering relative motion. In the bicycle example, each wheel is a separate subassembly because it moves as a unit relative to the rest of the assembly. Figure 4.8 illustrates where relative motion might be on the bicycle.
FIGURE 4.8
Grouping subassemblies by relative motion
Grouping subassemblies by relative motion is great for assembly modeling, but it does not usually reflect product reality very well. Using this method, you often end up with parts in the subassembly that will have to be disassembled in order to actually put the physical parts together. However, if your only consideration is ease of modeling, then you should probably use this method.
Organizing groups of purchased components
If you are modeling a product that is created from a shopping list of purchased components, then it may make the most sense to organize your subassemblies into groups of parts that are purchased together. In fact, purchased subassemblies are often modeled as single parts, except when relative motion is required in the purchased assembly.
For example, in the bicycle assembly, the sprockets on the rear wheel are purchased as a separate unit, and yet the part that mounts onto the wheel moves relative to the sprockets that are driven by the chain. This is an example of a purchased part that would be modeled as a subassembly to show relative motion. The bicycle chain, another purchased subassembly, has not yet been added to this assembly, and is a more complex model. The desire to show all of the individual links moving through the path may override both the complexity of assembling it and the performance considerations of exercising all of the mates.
Although the BOM method of organizing assemblies sometimes leads to unnatural solutions, you should not discard it altogether. If you can devise concessions in order to make the BOM work automatically, then you should do this.
Depicting an assembly process
Manufacturing and assembly processes need to be documented as well as individual part design. You often need to create exploded-view assembly instructions for manufacturing or service documentation at each step of a multistep assembly process. Figure 4.9 shows an example of this type of process documentation.
This is certainly a task that is different from the initial design or modeling of the assembly, and it may require an entirely separate assembly model. Generally, you can perform the different steps by using a separate configuration for each process step, with exploded views for each configuration.
Influencing item numbering
Balloons number the parts