Modeling with multiple bodies can be extremely useful in both simple and complex part design. In this article, we will discuss multibody parts in SOLIDWORKS, including what multibody parts are, how they differ from assemblies in SOLIDWORKS, and how they can be used.

What is a Multibody Part?

In SOLIDWORKS, a multibody part is exactly as it sounds: a part with multiple solid bodies. A very simple example is in the picture below, which shows a part with two bodies that are not connected. You can see that in the Feature Tree, it says Solid Bodies(2). This means there are two discrete bodies inside this single part file.

A multibody part in its simplest form.

While this may look like an assembly, it is actually quite different.

Multibody Parts vs. Assemblies

Usually when modeling something with multiple components in SOLIDWORKS, we tackle it piece by piece, or part by part. Then, when we have all the separate parts designed, we combine them into an assembly, and use Mates to ensure that they are correctly put together. Assemblies are a basic tool that most users are very familiar with, and something that is taught in most introductory SOLIDWORKS courses. 

While extremely useful in their own way, assemblies do have their drawbacks. One of these drawbacks comes when the parts being assembled are not completely finalized in their design. It is somewhat difficult (but not impossible) to make design changes to individual parts in the context of the assembly. To avoid issues, it is sometimes necessary to constantly switch between the assembly and the parts to make changes. You could also use Top-Down or In-Context modeling (if you don’t know what these are, I encourage you to learn about them), but these tend to lead to external references that can get messy. 

Those nasty X’s and arrows denote a broken external reference.

An easier way to design a simple assembly is by using multibody part design. This way, all the design work can be done in one file, and there are no messy references between the part and the assembly. 

However, multibody parts are not always a good replacement for an assembly. While some assemblies require only simple, static mates, some require more complex mates that simulate motion, such as limit distance or slot mates. In this case, a multibody part would not work since the discrete bodies can only be modeled in place and they cannot easily be moved.

How to Make a Multibody Part

Now that we know the difference between a multibody part and an assembly, you may be wondering how to create a multibody part. Let’s look at three different methods.

1. Don’t merge result

The simplest way to make a multibody part is when creating a Boss Extrude feature. Let’s look at a simple example:

We want the cylinder to be a separate body.

In this example, we want to create a cylinder extruded from one of the faces of this box. Since the two extrusions would be touching one another, SOLIDWORKS will by default want to merge them. However, we need them to be separate bodies. To achieve this, all we need to do is uncheck the “Merge Results” box in the Boss Extrude FeatureManager. 

Uncheck the “Merge result” box to create a separate body.

Now once we hit the green checkmark, we will have two Boss Extrude features and two Solid Bodies.

Two unmerged extrusions.

2. Use Cut Extrude

We can also create a multibody part by cutting an existing solid body. In this example, we need to create separate cubes from this existing rectangle.  

You can cut a single body to create multiple bodies.

Let’s create a sketch on one of the faces of the rectangle and open the Cut Extrude feature. Select Through All as the end condition and hit the green checkmark.

If a cut separates bodies, you must select which bodies to keep.

You’ll notice that a new dialog box pops up when you hit the green checkmark, called Bodies to Keep. Since we want to keep the two cubes that are a result of our Cut Extrude, we will keep the All bodies option selected and hit OK. We now have two separate cubes inside of a multibody part.

3. Use the Split Tool

The Split Tool is very useful in creating a multibody part from existing geometry. Let’s look at another example and use the Split Tool.

A cube that we will split using the Right Plane.

Here is a simple cube that we need to make into a multibody part. Let’s open the Split Tool by selecting it in the Features toolbar, or from Insert > Features > Split. 

Split tool FeatureManager.

We now see the Split tool dialog box. In the Trim Tools box, we can select either a sketch, a plane or a surface to split the body. In our case, we will use the Right Plane as our split tool. Select that plane and hit Cut Part. 

The resulting bodies from the split, selected and shown in the graphics window.

We can now select the bodies we want to separate in the Resulting Bodies section, and we will see them change color in the graphics window. Once we hit the green checkmark, we will be left with two separate bodies.

A multibody cube made using the Split tool.

What Else Can Multibody Parts Do?

Aside from the benefits talked about above when compared to assemblies, multibody parts can also be useful in other circumstances. One last benefit I would like to highlight is that features can be applied to discrete bodies individually. 

Let’s take another look at the example we just did, with the cube split using the Split tool. We now want to cut a hole in one half of the cube, but not the other. Or in other words, one body and not the other. 

Let’s make the sketch and open the Cut Extrude tool. Select Through All for the end condition. Normally, this would cut the hole through the entire part. But since this is a multibody part, we now have the option to select which bodies to apply the feature to. We do this by selecting the desired body in the Feature Scope section. 

A Cut Extrude feature applied to only one body.

Select the body on the right and hit the green checkmark. We now have a hole in one body, but not the other, as shown in the section view below. This allows us to make changes to the overall part without the Cut Extrude feature breaking.

A section view showing the Cut Extrude we just made, applied to the right body.

Final Thoughts

The ability to make multibody parts in SOLIDWORKS is extremely useful in many different circumstances. I have outlined a few different ways to use them in this article, but this is only scratching the surface. I encourage you to get creative with multibody parts and see how they can improve your own designs.