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# 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

LargeNCharge86,

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

I would rebut that it's OK, since it's a pattern of 2 holes. They're located relative to each other, not just oriented relative to A.

Edited to add: Evan beat me to it, by one minute!

### RE: True Position of a Feature to One Planar Datum

How could you think even for a moment that the standard is wrong? (irony)

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

(OP)
Thanks for the responses. What's throwing me off is the orientation with a position tolerance. What is actually being checked for orientation here? The axis of each hole back to datum A?

### RE: True Position of a Feature to One Planar Datum

In this figure the position tolerance controls BOTH orientation - perpendicular to A - and simultaneously the 75 BSC position between the holes. The axis of the UAME for each hole must fall within the .3 at MMC tolerance zone (at each hole). So the zone controls both perpendicularity and position. You can use composite position to refine the perpendicularity if the design requires.

Certified Sr. GD&T Professional

### RE: True Position of a Feature to One Planar Datum

(OP)
Interesting. Thanks for the info on this. I learned something.

### RE: True Position of a Feature to One Planar Datum

Figure 7-8 in Y14.5-2009 has a good illustration of how positional tolerance zones can control orientation as well as location.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

### RE: True Position of a Feature to One Planar Datum

mkcski,
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

pmarc: Thanks. I stand corrected. Yes, with a single datum the interpretation is different and composite position is not allowed.

Certified Sr. GD&T Professional

### RE: True Position of a Feature to One Planar Datum

Composite position is allowed, but it will not work as perpendicularity requirement.

### RE: True Position of a Feature to One Planar Datum

pmarc: I don't follow. Why would you refine the position with composite when only position is being controlled (to a single datum)? Are you thinking of a lower frame with no datum reference, like fig 7-45 with coaxial hole patterns?

Certified Sr. GD&T Professional

### RE: True Position of a Feature to One Planar Datum

pmarc,

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?

### RE: True Position of a Feature to One Planar Datum

Yes. That's what I am thinking of. Composite position with A in the upper segment and with no datum feature references in the lower segment would control perpendicularity to A within the upper limit tolerance and spacing between axes within the lower limit tolerances.

### RE: True Position of a Feature to One Planar Datum

greenimi,
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

greenimi: I agree with pmarc. This is why I originally thought composite was not allowed (but it is without a datum ref in the lower frame).

Certified Sr. GD&T Professional

### RE: True Position of a Feature to One Planar Datum

pmarc,

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

You are welcome, greenimi.

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.

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