opposing face datum holes

[mfg11eng11guy]

I have a part that has opposing holes (nominally co-linear axis). holes are same size, separated by a large void. Pins get pressed into these holes and allow the part to pivot and I am only concerned about a projected tolerance zone. This is very similar to the example 7-44 and 7-45 given in ASME Y14.5-2009.

The shared axis of the holes acts as the primary datum, but I am unsure how to call this out.
Can I use position to do this using no datum structure? Can I only use form (Cylindricity) when establishing primary datum? If so, can I project a cylindricity zone?
Please advise.

 

[greenimi]

First Hole: A
Second hole: B
Position first hole to A-B primary
Position second hole to A-B primary
It is NOT self-referencing.

 

[Belanger]

Though the standard doesn't show it, I would say you can use a single position callout with no datum references to establish their coaxiality. Other things would then be positioned from that datum axis. (Greenimi's suggestion works too, but I'm not so adamant about them "NOT being self-referencing.")
As you say, it's kind of like Fig. 7-45, but without the first two layers of the feature control frame (since you are not attempting to position those holes to anything else).

If they're the same size then you don't even need to use separate datum identifying letters. Just have "2X" in front of everything and a single datum letter can identify the combined axis.

A side note, however -- think about whether the "M" modifier will be used in the positioning of those two holes, and also in the subsequent tolerances that reference this datum. My answer would be fine-tuned if that were known.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

See fig. 4-24 in Y14.5-2009.

 

[greenimi]

John-Paul,
Re:” but I'm not so adamant about them "NOT being self-referencing”
In my initial statement about A-B compound I based my argument on Fig 4.25/ 2009. Granted, the standard shows total runout, but I don’t see why I cannot apply it to position too (runout will ensure coaxiality, but also will report the form, am I right? )
Therefore, are you suggesting that 4-25 is “kind” of self-referencing?
Just asking. Don’t even think I am arguing with you. I know I am very defensive these days

 

[Belanger]

Sure, position can be used in place of runout there. But the idea of using different letters (A and B) is best when the datum features are a different size. If they are the same size then it's preferred to use the same letter, such as the two ends of the part in Fig. 4-24.

But I do agree that your suggestion is not self-referencing, because the datum axis created from the hyphenated letters is not the same as the axis being controlled by position.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Woosang]

John-Paul,
Re:"Though the standard doesn't show it, I would say you can use a single position callout with no datum references to establish their coaxiality."

I guess Y14.5-2009, 7.6.2.3 or Fig.7-59 shows this case.

 

[Belanger]

Yes -- I fired off my answer too quickly.
Pmarc also gave an example: Fig. 4-24.

If I recall, the 1994 standard didn't show an example (other than the lower portion of a composite FCF). That's what I was probably thinking of.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems