In this Question Line Video, Jason discusses datum preferences for hollow cylindrical parts, where both the ID and OD of the part are mating features.

We received the following question from one of our students: “For hollow cylindrical parts with an OD and ID, would there be a preference as to which one should be called out as a datum? Note that the mating parts slide on both OD and ID.”

When we are designing a part, it is imperative that we set up the datum reference frame to mimic the part’s functionality. This means that we consider the final assembly when choosing datum features, so we choose features that mate with other parts in the final assembly.

In the scenario that our student presented, both the ID and OD of the part are mating features. An example of this would be a collar that is keyed to a shaft, with a gear press-fit onto the OD of the collar. So how do we decide whether the OD or ID of the collar should be our datum feature?

Figure 1: Collar and Final Assembly

We are concerned about the coaxial error between the shaft, collar, and the outside of the gear. We must ensure that we have enough information to understand the worst-case tolerance stackup in our assembly regardless of whether we use the inner or outer diameter of the collar as our datum feature.

## Outside Diameter as Datum Feature

If we select the outside diameter of the collar as the datum feature, the inside diameter will be located coaxially to the datum axis created by the outside diameter. This datum axis is located at the center of the part, and the tolerance zone for the position of the inside diameter is centered on that datum axis. The inside diameter of the part is allowed to deviate diametrically only by the amount shown in the feature control frame.

Figure 2: Outer Diameter of Collar as Datum Feature

We can calculate the total tolerance stackup of this assembly using datum structures and the values found in the feature control frame, so selecting the OD of the collar as a datum feature is valid.

## Inner Diameter as Datum Feature

If we select the inside diameter of the collar as the datum feature, the outside diameter would be controlled by position with respect to the datum axis created by the inside diameter, as indicated in the feature control frame. As in the previous example, this datum axis would also be located at the center of the part. This outside diameter of the collar is allowed to deviate diametrically from the datum axis only by the amount shown in the feature control frame.

Figure 3: Inner Diameter of Collar as Datum Feature

In this scenario as well, we are able calculate the amount of coaxial error between the shaft, collar, and gear using datum structures and values in the feature control frame, so selecting the ID of the collar as a datum feature is also valid.

## Summary

We are able to calculate the stack-up error for the assembly regardless of whether the inner diameter or outer diameter of the collar is chosen as the datum feature. Therefore, either of these are valid choices for the datum feature. For more details on the tolerance stackup in both scenarios, reference the video above.

One thing to note when selecting datum features: It is often easier to simulate an external datum than an internal datum. It would be beneficial to take into consideration the tools available to your inspection department when choosing between these as datum features.

### Overwhelmed by the Complexity of GD&T?

Learn GD&T at your own pace and apply it with confidence in the real world.