Written by Alek Cook, Technical Support Engineer
Starting a design in SOLIDWORKS is the best place to question whether you are going to design your product as a multibody part or an assembly of parts. Both design methods hold their own values and overlap in some areas.
I am going to discuss what you should consider when starting your design that may alter your decision on making your it a multibody part or an assembly. I will also discuss similarities between the two and how you can improve performance with your design
The first thing to note is your Bill of Material (BOM) when starting your design. In a multibody part you are going to have one line item for the part. If you plan on manufacturing your design, and want to have a line item for each part in your design, this is not the approach you want to take.
NOTE: There is a catch to that, though: if you are welding together a structure and you are familiar with weldments, this is the approach you want to take. Using weldments to design a structure offers a cut list that can be put into a drawing view that will produce a list based on quantity, length and description.
If you are bolting together structures that you create using weldments, then you might want to consider using assemblies to generate a better list. For instance, prefab buildings that are bolted together are going to need a BOM. Another slight disadvantage is not being able to use interference detection in multibody parts in SOLIDWORKS 2018. You can still technically us interference detection, but you must import your multibody part into an assembly and then check include multibody part interferences when using interference detection. If you are using SOLIDWORKS 2019 you can now use interference detection in multibody parts.
If you are designing a majority of the parts that make your end product, assemblies is going to be the route you want go down. This becomes clear when you go to produce a BOM. You will have a line item for every part in your assembly. Often, designers will use components from outside vendors that are more economical to purchase than produce in house for their final product. Designing the part from the vendor as a multibody part is ideal so you can import the part into your assembly to capture that item in your BOM. This is one situation where both multibody part and assembly is best for your design.
Another thing to consider when bringing in vendor parts into your assembly is, do you want it to be a single part or a subassembly in your top-level assembly? Performance is what could be affected in this situation. If the subassembly is done well, this will out-perform an imported part from a vendor. When you import parts from your vendor they will come in as a “dumb solid” which can kill your performance in your top-level assembly.
There are similarities between multibody parts and assemblies. You can create exploded views of both designing methods. You can assign material to separate parts in your multibody part. One catch to that is if you bring that multibody part into an assembly, the material you set for each part (in the multibody part) will not be recognized.
The last similarity is choosing a configuration before you import the part. New to 2019, when you insert a part into a part you can select what configuration you want it to be. Also mentioned above in 2019 you can use interference detection.