True Position of a Feature to One Planar Datum
True Position of a Feature to One Planar Datum
(OP)
Looking for a sanity check here. I see this a lot and I always tell people it's wrong until today when I stumbled onto it in ASME-Y14.5M-2009. Figure 4-39 shows datum B as a feature of size with a TP back to A, which is a perpendicular plane. There's no way this can be right. Am I crazy or should the TP be perpendicularity?





RE: True Position of a Feature to One Planar Datum
The position tolerance in Figure 4-39 is correct. Position is used here because B is a pattern of 2 holes and the tolerance needs to control their relative location (spacing) as well as their orientation to A. So it needs to be a position tolerance and not perpendicularity. If B was a single hole, then a perpendicularity tolerance would be appropriate.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: True Position of a Feature to One Planar Datum
Edited to add: Evan beat me to it, by one minute!
RE: True Position of a Feature to One Planar Datum
But seriously, there is no mistake in this figure. The position tolerance is there because the callout in addition to perpendicularity relationship to A controls spacing between the holes in the datum pattern.
If there was only one hole for B then perpendicularity tolerance would be the way to go
RE: True Position of a Feature to One Planar Datum
RE: True Position of a Feature to One Planar Datum
Certified Sr. GD&T Professional
RE: True Position of a Feature to One Planar Datum
RE: True Position of a Feature to One Planar Datum
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: True Position of a Feature to One Planar Datum
To refine perpendicularity of the holes perpendicularity callout should be used. Composite position may only refine spacing in this very case.
RE: True Position of a Feature to One Planar Datum
Certified Sr. GD&T Professional
RE: True Position of a Feature to One Planar Datum
RE: True Position of a Feature to One Planar Datum
Certified Sr. GD&T Professional
RE: True Position of a Feature to One Planar Datum
Why "Composite position may only refine spacing in this very case"
In other words, if we add a FRTZF on the two holes 2x Ø8.0 - 8.1,
PLTZF: pos Ø0.3 (M) to A (primary)
FRTZF: Ø0.1 to A (primary)
then why perpendicularity is not controlled within Ø0.1 (as a refinament)?
Spacing between the holes will also be within Ø0.1, but why the perpendicularity is NOT also within Ø 0.1?
Thank you for your input
RE: True Position of a Feature to One Planar Datum
RE: True Position of a Feature to One Planar Datum
In your example the upper segment would be meaningless as it would be simply overriden by the lower segment.
RE: True Position of a Feature to One Planar Datum
Certified Sr. GD&T Professional
RE: True Position of a Feature to One Planar Datum
Agreed. Thank you for the clarification and your input.
What confused me was the sentence :""Composite position may only refine spacing in this very case""
Now is clear. (at least until next time a similar issue shows up, with no guarantee that I will remember the outcome). I need to stay on top of this language, otherwise beats me every time.
RE: True Position of a Feature to One Planar Datum
Just remember not to make a rule out of what I said. There are certain types of geometries where your proposal (reference to A in both segments) would make sense.