I agree that a standard should fully explain itself without additional materials unless it refers to materials that are themselves fully explained. I am simply asking anybody that has this Standard, if they understand it to please, if they could Maybee explain it in a clearer manner, to give me and others a better understanding of what they are saying. The lengthy definitions I understand.
Here is what James Meadows had to say: "This is the result of a long drawn out fight between individuals and committees. It's known as the "tertiary datum problem". Some of us said (as it currently says in the Y14.43 standard) that since the tertiary datum feature of size is only to give angular orientation to measurements taken from the secondary location datum axis (of a hole or shaft) that the datum feature simulator (real gage or fixture element) and the TGC (imaginary datum feature simulator) should translate (move) toward or away from the secondary simulator. Others said that both simulators should be represented as stationary, separated by their basic dimension, as is currently done for pattern datums. The stationary folks won the vote.
Then some of us pointed out that the result of this vote is that we aren't really measuring distance from the secondary hole or shaft, but are really measuring distance from both the secondary and tertiary datum features of size. What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement? They came up with the translation modifier. It says that the tertiary (or sometimes secondary) datum feature it is attached to can be represented by a simulator that moves toward or away from the secondary (or sometimes primary) datum feature that precedes it in the feature control frame. That way the tertiary datum feature of size only gives orientation to the measurements.
It's a mess created to solve a problem that rarely existed. But the faction got what it wanted and that was a default rule that all datum feature simulators (real) or true geometric counterparts (imaginary) are stationary and separated by their basic dimensions (unless the translation modifier is used).
Since you didn't mention the planar datum features shown with a profile tolerance that can be referenced at MMC and represented at their virtual condition distances/sizes or the RFS ones that can grow from their basic dimensions until they contact the part, I won't either.
Things have gotten out of control, but there is no stopping this runaway train."
Thanks Jim
