Modeling Transmission Components in Onshape

1. Sourcing vs. Modeling: The Engineer's Workflow

As a design engineer, one of your constant considerations is "make vs. buy." In the world of CAD, this translates to "model vs. source." For common, off-the-shelf components like fasteners, bearings, pulleys, and motors, it is standard practice to source existing 3D models rather than spend time modeling them from scratch. For custom components, like a specialized bracket or a gear with non-standard parameters, you will model them yourself.

Today, we'll practice both. We will source our standard timing pulleys and model our gears and belt using powerful automation tools in OnShape.

2. Sourcing Standard Components

Sourcing is the process of finding a pre-made CAD model of a commercial off-the-shelf (COTS) part and importing it into your project. There are two main ways to do this:

A. Supplier Websites (e.g., McMaster-Carr, MISUMI)

Most major industrial suppliers provide 3D models of their products, typically in neutral formats like STEP or Parasolid (.x_t). The workflow is:

1. Navigate to the supplier's website and find the exact part number you need.
2. Download the associated 3D CAD model.
3. Import the file into your Onshape document.

B. Integrated Part Libraries (e.g., MKCad)

A more streamlined approach involves using part libraries integrated with OnShape. MKCad is a popular, community-maintained library for robotics and machine design components. It contains a vast collection of COTS parts that can be inserted directly into your assembly without leaving OnShape.

Let's learn how to access and use this powerful resource.

Once you have labeled the MKCad libraries, you can go to the "Insert" dialog in an Assembly tab, click on "Other documents," and use your "-MKCad" label to filter and search for parts. For our project, you would search for something like "S8M pulley 24 tooth" and "S8M pulley 72 tooth" to find the components we specified in the last lesson.

3. Modeling with FeatureScripts

For components that aren't standard or have geometry defined by mathematical relationships (like gears and belts), Onshape's FeatureScripts are incredibly powerful. A FeatureScript is a small program that automates the creation of complex features. You input the key parameters, and the script generates the geometry for you.

A. Modeling Spur Gears

Let's imagine our design required a custom gearset. In the last lesson, we calculated a potential 30:1 ratio, which would need a compound gear train. Let's model a single 5:1 stage.

The following video provides an excellent, concise walkthrough of using the built-in Spur Gear FeatureScript.

Let's recap the key steps from the video for creating a 5:1 gear ratio (e.g., a 20-tooth pinion and a 100-tooth gear):

1. Add the FeatureScript: Click the + (Add custom features) icon -> FeatureScript samples -> Filter for "gear" -> Select FeatureScript Spur Gear.
2. Create the Pinion (Small Gear):
   • Start the Spur Gear feature.
   • Set Number of teeth to 20.
   • Set a Pitch circle diameter (e.g., 2 inches).
   • Note the Module that Onshape calculates. This value is critical. It defines the size of the teeth.
   • Set the Depth (thickness) and add a Center bore.
3. Create the Gear (Large Gear):
   • Start a new Spur Gear feature.
   • Set Number of teeth to 100.
   • Crucially, instead of setting the diameter, copy the exact Module value from the first gear. This guarantees the teeth will mesh. Onshape will calculate the correct pitch circle diameter for you.
   • To position the second gear correctly, you'd create a sketch line representing the center distance before creating the gear, then use the Move origin option in the gear feature, as shown in the video. The center distance is the sum of the two pitch radii: $(d_1/2)+(d_2/2)$.

B. Modeling a Timing Belt

Just like with gears, there is a specialized FeatureScript for creating belts. This tool is a perfect example of design automation, saving you from the tedious process of sketching a complex belt path manually.

The workflow for using the Belt feature is typically:

1. Place your pulleys (which you sourced from MKCad) in an Assembly at the correct center distance.
2. Start the Belt feature.
3. Select the circular faces or mate connectors of the pulleys.
4. The tool generates the belt part, often automatically calculating the nearest standard belt length.

Your Task for Project 2

Now it's time to apply these workflows to our rotary indexing mechanism.

1. In your Onshape project document, create a new Part Studio. Name it "Transmission Components".
2. Using the methods described, either from MKCad or a supplier's website, source and insert the two pulleys we specified:
   • One S8M pulley with 24 teeth.
   • One S8M pulley with 72 teeth.
   • Ensure they both are specified for a 30mm wide belt.
3. Add the Belt FeatureScript to your toolbar. You may need to create a temporary assembly with your two pulleys to generate the belt part, which you can then use in your main assembly later. Try generating the S8M belt that connects them.

Conclusion

In this lesson, you've learned the essential, practical skills of populating a CAD model with both standard and custom transmission components. This is a direct simulation of a real-world engineering workflow.

Key Takeaways:

• Source, don't model: For standard COTS parts like pulleys, always look for a model from a supplier or a library like MKCad first.
• Embrace automation: Use Onshape's FeatureScripts to rapidly generate complex, parametric geometry like gears and belts. This saves time and reduces errors.
• Connect theory to practice: The parameters in the FeatureScripts (like module or pitch) directly correspond to the engineering principles we used for sizing in the previous lesson.

Preview of the Next Lesson:
With our primary transmission components now modeled, our next step is to design the parts that will hold them. In the next lesson, "Design and model the main shafts based on the transmission layout," we will begin designing the shafts for our pulleys, considering factors like diameter, length, and features for mounting.