Out there in the GD&T world, there is often confusion about parts that have irregular shape. We are told that the theory of GD&T requires datums to be 90º to one another. Sure, that’s great in a textbook where the examples are nice, rectangular, flat plates! But what about those other shapes?
It’s actually very easy. The confusion is that people mistake the term “datum” for “datum feature. The standard defines a datum as a theoretically exact point, axis, or plane. But a datum feature is defined as a physical portion of the actual part from which the datum is derived.
Think about those two terms, and you’ll see that irregularly shaped parts pose no problem. Even something shaped like a blob or a potato chip has a physical surface. It may require using datum targets, but a theoretical plane can still be constructed from those targets.
So again, it’s true that the theoretical datums mentioned in a feature control frame are perpendicular to each other. But those theoretical datums can be derived from any crazy-shaped surface.
If you have the new 2009 ASME standard, see pages 81-90 for some neat examples. (If you have the 1994 edition, see pages 54 and 78-79.) Stay tuned for future entries on the recent changes in the standard!
