Hi All,
FWIW, I support Dean's terminology policing. He and I both know from bitter experience (and the teachings of Bill T.) that strict attention to the details of terminology is essential where GD&T is concerned. We've been in many committee discussions where the source of a misunderstanding turned out to be terminology. I would go as far as to say that many of the difficult aspects of GD&T are at least partly caused by the confusing/counterintuitive/inconsistent/conflicting use of terminology in the standard itself. Don't get me started on the whole "datum plane" thing. ;^)
Regarding the term "datum feature shift" versus "datum shift", I don't have a big preference either way. The only thing that matters is that there can be relative movement between the datum feature (and thus the rest of the actual part) and the TGC (and thus the datums and the DRF). It doesn't matter if you think of it as the part moving relative to the gage, or as the gage moving relative to the part. You can find examples of both of these descriptions in Y14.5.
I have much more of an issue with the confusing way that Y14.5 defines and describes the term "datum feature shift/displacement":
"MMB or LMB modifiers applied to the datum feature reference allow the datum feature to shift/displace from the boundary established by the true geometric counterpart in an amount that is equal to the difference between the applicable (unrelated or related) AME for MMB, actual minimum material envelope for LMB, or surface of the feature and the true geometric counterpart."
I agree that this definition implies that the datum feature starts out at the location it would be at if the reference was RMB, and then shifts/displaces from there. But then it goes on to say that the amount of shift is the difference between the AME and MMB - shouldn't this be half of the difference? I would say that trying to quantify the amount of shift is ill-advised in the first place, because it really only works for the special case of a single datum feature such as datum feature B in chez311's sketch. In almost every real case, datum feature shift is a complicated mixture of rotational shift and translational shift. The "amount" varies depending on the direction.
The figure that illustrates datum feature shift/displacement (Fig. 4-26 in 2009, Fig. 7-18 in 2018) is one of my "pet peeve" figures. It has a square 4-hole pattern referenced as a secondary datum feature at MMB.
-it shows "Datum Axis B". According to Y14.5's own theory, the datum for this type of datum feature (linear extruded shape) would be an axis and center plane, not an axis. This is just so incredibly misleading, because a datum axis would leave one rotational degree of freedom open.
-the figure shows what looks like an "actual" coordinate system with planes and an origin, that was derived from the as-produced datum feature. It is not generally possible to uniquely define this actual coordinate system, it is only possible because the as-produced datum feature in the figure has equally sized holes at true position relative to each other.
-the figure shows "one possible displacement of the hole pattern with respect to the datum reference frame" and points to an X direction gap between the actual coodinate plane and the plane of the datum reference frame. The displacement is actually a mixture of X translation, Y translation, and rotation.
Evan Janeshewski
Axymetrix Quality Engineering Inc.