Assembling with Standard Mates

1. The Assembly Environment and the Onshape Mating Philosophy

An assembly is a collection of parts positioned relative to one another. In a professional workflow, this is where you verify fit, check for interference, and define mechanical motion. Onshape handles this process with a powerful and efficient system based on Mates.

Unlike some traditional CAD systems where you might apply multiple constraints (e.g., align two faces, make two edges concentric, set a distance) to position one part, Onshape typically uses a single Mate to define the relationship between two parts. This is much faster and easier to manage.

To understand how this works, let's look at the two core concepts: Degrees of Freedom and Mate Connectors.

Degrees of Freedom (DOF)

As you'll know from your mechanical engineering background, any unconstrained object in 3D space has six degrees of freedom:

• Translation along the X, Y, and Z axes.
• Rotation about the X, Y, and Z axes.

Mating is simply the process of removing these degrees of freedom to achieve the desired motion. A Fastened mate removes all six DOFs, rigidly locking two parts together. A Revolute mate (like for a hinge) removes five, leaving only one rotational DOF.

Mate Connectors

Instead of mating geometric entities like faces or edges directly, Onshape uses a special entity called a Mate Connector. A Mate Connector is essentially a local coordinate system (with an origin and X, Y, Z axes) that you can place on your part. A Mate then defines the relationship between the coordinate systems of two Mate Connectors.

This video provides an excellent explanation of this fundamental concept.

This approach is one of Onshape's key strengths. To get a quick sense of why it's so efficient, take a few minutes to read the first two sections of this article from Onshape.

2. A Tour of the Standard Mates

Onshape provides a set of standard mates that cover the vast majority of mechanical design scenarios. Each one allows for a specific type of motion. We'll focus on the four most common ones.

As you watch the following demonstrations, notice how each mate is defined by selecting a Mate Connector on each of the two parts you want to connect. Onshape automatically creates these "implicit" mate connectors for you as you hover over geometry.

While these four mates will handle many of your needs, Onshape provides others for more specific situations (Planar, Pin Slot, Ball, etc.). You can find a complete list with descriptions in the Onshape Help documentation, which is an excellent resource to bookmark for future reference.

3. Practical Application: Building Your First Assembly

Now, let's apply these concepts. The general workflow for creating an assembly is:

1. Create a new Assembly tab.
2. Insert the parts you modeled in your Part Studio.
3. Fix one base component to the origin. This provides a static reference for the rest of the assembly.
4. Use mates to connect the remaining parts to the fixed part or to each other.

First, let's understand why fixing a component is so important.

Your Turn: Assemble a Simple Hinge

Open your "Project 1" Onshape document.

1. Create a new Part Studio.
2. Quickly model two simple parts:
   • Part 1 (Base): A small rectangular block (e.g., 50x50x10 mm). Add a through-hole near one edge (e.g., 10 mm diameter).
   • Part 2 (Lever): Another rectangular block (e.g., 100x20x10 mm). Add a matching through-hole at one end.
   • Tip: Model these side-by-side in the same Part Studio. This ensures the holes are the same size.
3. Create a new Assembly tab (click the '+' icon at the bottom left and select 'Create Assembly').
4. Click the Insert button on the toolbar. In the dialog, select the two parts you just created from the Part Studio tab and place them in the graphics area. Click the green check to confirm.
5. Right-click on the Base part in the graphics area or in the Instances list on the left and select Fix. A small icon will appear next to its name, indicating it's locked in place.
6. Select the Revolute Mate from the toolbar.
7. Hover your mouse over the circular edge of the hole on the Base. A number of mate connectors will appear. Click the one at the center of the hole's opening.
8. Now, hover over the circular edge of the hole on the Lever and click the mate connector at its center.
9. The parts will snap together. You can use the Flip and Reorient buttons in the dialog if needed. Click the green check to accept the mate.
10. Click and drag the Lever part. It should now pivot around the hole, just as a hinge would.

Congratulations, you've created your first assembly!

Conclusion

In this lesson, you moved from individual parts to functional assemblies. You learned Onshape's unique and efficient approach to mating, which is fundamental to designing any mechanical system.

Here are the key takeaways:

• Assemblies define relationships: The Assembly tab is where you define how parts fit and move together.
• Mates control Degrees of Freedom (DOF): Mates work by constraining the 6 DOFs between two parts to achieve desired motion.
• Onshape uses Mate Connectors: Instead of mating geometry directly, Onshape mates are defined between local coordinate systems called Mate Connectors.
• Start with a Fixed Part: Every stable assembly should begin with one component that is Fixed to provide an anchor.
• Core Mates (Fastened, Revolute, Slider, Cylindrical): These four mates can define a wide range of common mechanical movements.

You now have the skills to not only model parts but also to assemble them into mechanisms. In our next lesson, "Analyze the motion of an assembly using mates and relations," we will explore how to drive and examine the motion you've just enabled, allowing you to validate your design's kinematic behavior.