Figure 4-3 in Y14.5-2009 provides great help in clarifying the difference between datum features and datums, but I do wish some different terms were used... For 4-3g the datum really should be identified as "line, point, & plane", rather than "axis, point, and center plane". Some complex features (compound curved surfaces) may have a center plane, but many, such as the one shown in figure 4-28 have a reference plane that cannot be called a "center plane". Using the same reasoning, the datum includes a reference line, but not an "axis".
So, for a compound curved datum feature, or for a "complex" datum feature as shown in figure 4-3g the datum is a point on a line on a plane... In other words, the datum feature simulator can be defined with basic dimensions from a point on a line on a plane (if "BSC" is included as in figure 4-28 then those basic dimensions are the same as those used to define the datum feature, but if (M) or (L) are referenced then the datum feature simulator will be constructed at MMB or LMB respectively... Then we get to talk about how the datum feature can fit with its simulator which is not necessarily a simple discussion).
Jimmerz - Make sure you push all thoughts that those three mutually perpendicular planes are "datum plane A", "datum plane B", and "datum plane C" out of your mind... In my opinion it is much better to think of a datum reference frame as a right hand rectangular coordinate system which only has planes "XY, "YZ", and "ZX". There are many cases, such as any time a cylindrical datum feature is used, for which the "planes of the datum reference frame" include no "datum planes". What do we have if three spherical features are identified as primary, secondary, and tertiary datum features? We have three datum center points and no datum planes, then we constrain a coordinate system to those points. Now with Y14.5-2009 available, we can add X, Y, and Z axes to a drawing which will tend to make more clear that the only planes for that case are XY, YZ, and ZX.
Dean
