TRUE POSITION TOLERANCE ON A DATUM
TRUE POSITION TOLERANCE ON A DATUM
(OP)
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.





RE: TRUE POSITION TOLERANCE ON A DATUM
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: TRUE POSITION TOLERANCE ON A DATUM
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.
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
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.
RE: TRUE POSITION TOLERANCE ON A DATUM
Thanks all!
RE: TRUE POSITION TOLERANCE ON A DATUM
http://www.geotolmeadows.com/newsletters/2013/sep2...
RE: TRUE POSITION TOLERANCE ON A DATUM
RE: TRUE POSITION TOLERANCE ON A DATUM
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."
RE: TRUE POSITION TOLERANCE ON A DATUM
I agree with everything pmarc said.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: TRUE POSITION TOLERANCE ON A DATUM
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.
RE: TRUE POSITION TOLERANCE ON A DATUM
"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.