Assemblies Vs. Multi-Bodies Part 1

It is frequently asked when someone should use multi-bodies as opposed to assemblies. It mostly depends on what it is that you are trying to design, because every project is unique and has its own circumstances and individual requirements. The most helpful rule of thumb on when to use multi-bodies and when to use assemblies will come up when thinking down the line in the design process to the bill of materials.  How do you want the bill of materials to be displayed? Do you want multiple components shown as a single part? In this sense it would show on your bill of materials like a single part that was purchased completed from an outside vendor. Or, do you want each individual component to show as a separate part? This may apply if your project is either being designed or assembled in house as individual components. These are the primary differences that should be used to decide your workflow.

When you want it to appear as a single item pre-assembled from an outside vendor this is the perfect time to use multibodies. When you want all of the components to be listed as individual parts in the bill of materials because it will be assembled or manufactured in house, then this is the perfect situation to design the components separately and use assemblies.  This is the primary way to decide when to use multibodies or assemblies, but this does not cover every situation.  The next best way to decide between the two would be to figure out if you need to have mechanical motion between the parts. Now if you answered this as yes, the parts definitely need to be moving separately of each other, then you would need to design these as individual parts and use assemblies.  If there is no motion needed between the parts, because they are for example like press fitted, then that would be a situation where multibodies could be used very effectively.

Where as the model visually is no different, you can see that the feature manager design tree is significantly different between the assembly (top) and the multi-body part (bottom). Also the perspective bills of materials are significantly different.

The primary ways you can decide which method to use are listed above; however, there are some minor tradeoffs between the two design methods that should be considered.  Before getting into the tradeoffs, it is good to know the similarities between the two methods.  The first similarity is exploded views.  You can take advantage of the exploded view tool in both assemblies and in multi-bodies.  Also, applying material properties is something important that should be mentioned in the two methods.  Of course inside assemblies every part can have its own material applied, so can it in multi-bodies?  The answer is yes, in multi-bodies every separate solid body can actually have its own material properties applied.  So both of assemblies and multi-bodies can have separate materials to represent all of the individual components involved.  The problem when using different materials inside multi-bodies will come when using that multi-body part inside an assembly.  If I put a bill of materials into the assembly with the multi-body part, then the multiple materials inside the multi-body part will not all be listed.  If the part was ordered from a manufacturer preassembled, then the individual component materials may not be necessary to have listed, something to consider in your own personal workflow.

This is just part one of a two part blog article.  If this article has piqued your interest you can continue on to part two, and keep an eye out for that article. Keep an eye out for more great SOLIDWORKS content on the blog and subscribe to our YouTube channel.