OK — time to dive into another item that is new in the world of geometric tolerancing. The standard that was released earlier this year expanded the definition of a “feature of size.” This has an impact in that some GD&T symbols can only be applied to “features of size,” most notably position.
The 1994 standard defined a feature of size as a single entity: “One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces….” This is just a fancy way of describing things like a hole, pin, a part thickness, or other feature that can be measured directly for size.
A traditional “feature of size”
The 2009 standard now breaks “features of size” into two categories: regular and irregular features of size. The regular feature of size keeps the definition given above. The irregular feature of size is new: it is defined as “a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope….”
Thus, a grouping of objects can now form a “feature of size.” This allows a geometric tolerance to be applied to the group as if it were one feature. In the example below, I can position the imaginary circle with GD&T, and even allow bonus tolerance as the three pins get closer together (because the external circle created around the three pins will get smaller). This is different from positioning the three pins themselves.
Example of an “irregular feature of size”
For folks who have been around GD&T for a while, this might take some getting used to! But it can make sense for many applications such as in the example shown above.