# How to Define 3D PMI in a Gear Box

In the transition from 2D drawings to model-based definition (MBD), many engineers feel that defining dimensions and tolerances, along with other product and manufacturing information (PMI), in 3D isn’t necessarily faster than 2D drawings. This may be true, especially if you are not well-versed in MBD tools yet. As explained in another article, “Top 5 Reasons to Use MBD,” the benefits of MBD primarily come from the downstream consumption procedures such as machining, inspection, tolerance stack-up analysis and technical communication. However, in this article, let’s take a look at the challenge of the up-front 3D PMI authoring procedures using a gear box assembly. We’ll explore several handy but lesser-known tips and tricks in SOLIDWORKS MBD that can help speed up the 3D PMI definitions.

For example, Figure 1 shows a typical worm gear box with a motor mount (the circular mounting plate introducing the torque), an input shaft (the worm), a worm gear, an output shaft and an outer casting, along with standard components such as bearings, seals, caps and rings. Obviously, among all these components, the distance and angle between the input and output shafts are at the heart of this design because they determine the output torque stability as well as the gear box life expectancy.

Figure 1. The distance and angle between the input and output shafts.

Then how do you define their distance and angle? It’s actually quite easy. Just use the location dimension command and pick the input shaft cylinder and the output shaft cylinder. Then please notice the Linear Dimension on the in-context command bar as shown in Figure 2. This option will give you the perpendicular distance between these two shafts, or, to be more precise, between their center axes.

Figure 2. The Linear Dimension option to define the distance between the input and output shafts.

By the way, you can adjust the custom dimension text positions for this distance callout using the three options shown in Figure 3 (from left to right):

1. Solid Leader, Aligned Text applied in Figure 2
2. Broken Leader, Horizontal Text
3. Broken Leader, Aligned Text

Personally, I like the aligned text options (numbers 1 and 3) better because they can save the 3D viewport space, reduce 3D PMI overlapping and organize a large amount of PMI more efficiently as illustrated in a previous article regarding hole callouts.

Figure 3. Three custom dimension text positions.

Similar to Figure 2, you can call out the angle between their axes using the Angular Dimension option as shown in Figure 4. It might be a bit inconvenient to pick the input shaft, which is hidden behind the housing and the motor mount. Instead of rotating the assembly to select the input shaft from the graphics area, you can easily pick it up from the DimXpert tree since it has been created as a boss feature at the step shown in Figure 2.

Figure 4. The Angular Dimension option to define the angle between the input and output shafts.

These two options, Linear Dimension and Angular Dimension, may not be easy to discover because they show up only when two cylindrical faces are selected in a location dimension command, but they can very useful. For instance, they can also help define the cylindrical cooling paths in an injection mold design or tilted lubricant holes.

Let’s continue with several other handy techniques. In an assembly like this gear box, the overall size dimensions are always important for the assembly and packaging steps. In Figure 5, the overall height is called out from the bottom of the outer casting to the top of the motor mount.

Figure 5. The overall height from the bottom of the outer casting to the top of the motor mount.

You can use the location dimension to define the distance between the bottom planes and the motor mount outer cylinder. But there are several key options to pick from in order to obtain this 166.500 ± 0.25 mm height.

1. By default, the location dimension distance would be from the center axis of the outer cylinder to the bottom planes shown in Figure 6, which would be only 84 ± 0.25 mm. Apparently, that doesn’t provide the full picture in the context of assembly or packaging requirements because you need to prepare for the overall maximum size. The MaxArc Condition as shown on the left side of Figure 5 solves this problem. As you can see, the dimension now reaches the top of the cylinder, rather than the axis, to call out the maximum height. This option was also shared in a previous article,“Design for Manufacturing: How to Define Features Directly,” when we were defining a slot feature.

Figure 6. The default distance from a cylinder to the bottom planes.

2. Please notice that although the bottom four faces are coplanar, they are disconnected. In order to select them all to represent the actual manufacturing feature and enable the appropriate PMI cross-highlighting behavior, you need to create a compound plane as shown in Figure 7. After picking one face, on the in-context command bar, click the “Create Compound Plane” button. Then a list box will be expanded for you to pick other faces. It’s worth noting that this command only works when multiple faces are coplanar. So if any of the faces is not coplanar, it won’t be selectable from the list box.

Figure 7. Creating a compound plane for the bottom four faces.

Now as you can see in Figure 5, the height dimension highlighted all the bottom faces. Once they are created as one compound plane, you can reuse this plane going forward as one feature, so you don’t have to recreate it anymore.

In this gear box assembly, we shared several handy but lesser-known tips and tricks as summarized in Table 1.

Table 1. MBD tips and tricks along with their use cases.

 Tips and Tricks Use Cases Linear and Angular Dimensions Define the distance and angle between cylindrical faces such as worm gear shafts, injection mold cooling paths and lubricant holes Custom Text Position options Organize 3D PMI more efficiently to reduce overlapping Reuse DimXpert features from its tree nodes Reduce graphics area view manipulation and maintain dimensional feature consistency Max Arc Condition Define the maximum height of cylindrical faces Create Compound Plane Define manufacturing features with appropriate 3D PMI cross-highlighting behaviors

Oboe Wu is a SOLIDWORKS MBD product manager with 20 years of experience in engineering and software. He is an advocate of model-based enterprise (MBE) and smart manufacturing.