TRUE POSITION TOLERANCE ON A DATUM

[DetroitDesigner]

I am looking for something to back up what someone has told me. I had a drawing where a bushing centerline is defined as Datum A and it has a true position feature control frame with no reference to any other datum. Is this legal? I've uploaded a jpg of it.

 

[Belanger]

What is it positioned to? That should give you ammo for why it's illegal.

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

 

[JNieman]

Even if you were given a 3d model with the ambiguous "CAD MODEL IS BASIC" blanket note, it's not a meaningful callout.

1.3.64 True Position
true position: the theoretically exact location of a feature
of size, as established by basic dimensions.

.....

7.2 POSITIONAL TOLERANCING
Position is the location of one or more features of size
relative to one another or to one or more datums. A positional
tolerance defines either of the following:

There is no true position without some manner of relation to another thing. It has to have a defined exact location established by basic dimensions and/or datums.

 

[DetroitDesigner]

I know that's what I said! They are trying to tell me its a tolerance of the location of the bushing center line so in effect the Datum can move within the true position tolerance zone. I've never seen any true position that is not referenced back to a datum. Can anyone send me a link or document to show they are wrong. They are telling me its legal because its a primary datum or Datum A . . cant remember exactly what their logic was. They have this called out the same way on the other end also. help me get this straight whether I am right or wrong . . . Ive attached their second feature frame with no datum reference Ive mentioned.

 

[JNieman]

What exactly is it they think the MMC/MMB modifier is... modifying... in your first image, and in the lower frame of your second image?

 

[DetroitDesigner]

I know. There is nothing to reference the MMC to correct? So this is illegal right?

 

[DetroitDesigner]

Ok their definition is this. Checks the straightness of the full bushing ID cylinder position to itself at the math data nominal position.

 

[DetroitDesigner]

I found an example in the ASME Y14.5-2009 standard Fig- 7-59 page 153. I am not sure this applies since this is not a coaxial feature. Can someone please chime in.

 

[Nescius]

Fig. 7-59 sure does show a similar concept, but since they're trying to control the straightness of an axis...why not use a straightness tolerance? Could the weird MMB actually be a misplaced MMC?

 

[pmarc]

DetroitDesigner,
The callouts shown on your first two pictures are illegal per the standard because they both show single feature controlled (no 'nX' prefix in front of the diameter values). Notice that in the Fig. 7-59 there are two features controlled with the datumless position callout.

Additionally, on the second picture, if there is indeed just one feature to control, the composite position callout is not correct choice because composite positional tolerancing can't be applied to a single feature.

 

[DetroitDesigner]

So everyone seems to agree these are illegal call outs as I first thought. They are the customer so I need to tread lightly and present my case in a logical manner and back it up with proof from several books I have including the ASME standard which only shows a no datum reference used in a coaxial condition.

Thanks all!

 

[pmarc]

FYI, this does not have to be a coaxial features application (for instance, see link below - Subject: GD&T for Pipes, Hoses or Tubing), but it definitely has to be an application with two or more features controlled with that kind of callout.

http://www.geotolmeadows.com/newsletters/2013/sep2013.htm

 

[DetroitDesigner]

I guess my argument for this can also be why are you making this much more complicated than it needs to be. The two original jpgs I posted are separate views on the same drawing control true position. Why would you complicate it like this?

 

[3DDave]

Not illegal, just meaningless. The tolerance applies to the deviation from the true position location of the feature, but there is no location given. While a positional tolerance can have the side effect of controlling straightness, flatness, parallelism, et al, it is in reference to a true position, which requires a datum reference frame and therefore datums. No datums, no tolerance.

Just humor them and say all the parts pass. Eventually someone will either wise up or they will not know the difference. Also add a note to your documents that gives exactly the explanation they gave you and ask they sign it as an proof you know how the inspection should be handled.

Like the old joke - the name is spelled R-x-x-q-y, and pronounced "Smith."

 

[axym]

DetroitDesigner,

I agree with everything pmarc said.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

One thing that the Y14.5 series does not make clear is that in addition to explicit datums, which are identified with datum symbols and by their names in the feature control frames, there are implicit datums, distinct from the former practice of implied datum interpretation.

For nominally coaxial features the implicit datum is the mutually shared axis. Much like there is a datum plane that is used as a basis to measure flatness that is never explicitly identified, or the cylindrical datum that is implicit in the cylindricity callout. These are mentioned in the mathematical explanation section 14.5.1; I believe the term is 'spine.'

The implicit datums float relative to the part (or vice versa) as required to find an acceptable solution (if possible.)

On a single hole an implicit datum is always able to float to a location such that the related feature has 0.000... location error, so it is a meaningless callout.

One thing that is interesting is that two position callouts with no explicit datum share an identical datum callout, and therefore are subject to simultaneous requirements, though I think the OP example doesn't qualify as presented so far.

 

[dtmbiz]

DetroitDesigner

"Basic" training might help

It appears that you are wasting your time because the problem is that you are working with ASME Y14.5 and they are working with A Y13 1/2.

I have seen this before. Its called "willful ignorance". No known cure short of a lobotomy.

In all sincerity, if the people involved that are trying to rationalize the indefensible and don't want to consider what is actually in the standard; then what is the point?

If you get sound advice and affirmation of your understanding for the sake of sanity then not much more to be gained IMO.