This question line video is in response to a student’s question regarding locating datum features. This student wanted to know why a feature of size that is indicated as a datum feature does not need to be located.
In the example below, we see a hole feature that is indicated as Datum Feature B. The feature control frame for this feature of size controls the perpendicularity of the hole’s axis with respect to datum plane A. Our student questions why the location of this feature isn’t being controlled.
Figure 1: Internal Feature of Size as a Datum Feature
The simple answer to this question is that the location of this feature isn’t being controlled because this feature is a datum feature. As a datum feature, it is the locating feature for all other features on this part. It is setting the origin that the other features are located from.
This can be difficult to understand because it is an internal feature. Looking at another example can help us to better understand this concept. In the below drawing, our datum features are external surfaces of the part.
Figure 2: External Surfaces as Datum Features
In this drawing, our datum features result in planar datums which intersect at the bottom left corner of the part – making the origin this bottom left corner. We do not locate this corner – everything else exists in location to this corner.
Datum features do not have to be located. They merely exist, and everything else exists in location to them. Our primary, secondary, and tertiary datum features are setting up the reference point that everything else is located to.
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