The usual questions. 1) which GD&T standard? 2) is the dimension basic or directly toleranced? 3) if directly toleranced, is there a dimension origin symbol on the zero-position?
The dimension is directly toleranced, based on # of decimal places (2 is +/- 0.01", 3 is +/- 0.005" and 4 is +/-0.0005"), and is the basis of the question. For instance, we typically design tooling with stacked plates with common holes. As a method of checking the prints, we will use the same origin for all the plates. That origin may or may not coincide with a feature. Does the # of decimal places (ergo the tolerance) apply to the origin, or is it absolute? Thanks!
the problem is that there is nothing in Y14.5 indicating that the ordinate-zero is the origin of measurement, so you can measure from the dimensioned feature to the zero-feature.
While you don't say what drawing standard you use, I can't point you a section of ASME Y14.5M-1994 that I know of which would explicitly answer this. 1.9.2 doesn't go into that much detail.
However, the '0' is analogous to a datum in many respects.
Also just logically, if it has a tolerance, then using that dimension system seems almost unfeasible.
As long as there is no origin symbol shown instead of one arrow of typical coordinate dimension with +/- tolerance, such dimension should not be tied to anything. It is a size requirement not constrained to any other external features.
I do not think the answer for this question depends on GD&T standard used.
Just as a matter of straightforward common sense, it seems to me that the origin is the starting point. It should not be a matter of an origin measuring a certain distance from a feature but rather a feature measuring a distance from the origin, regardless of which plate you are checking. Without overcomplicating the issue, the origin should always be the 0,0 point from which to measure the other features.
Powerhound, GDTP T-0419
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I agree with Powerhound. If I say that I am 200 miles outside of Chicago, give or take a few miles, wouldn't you think of those few extra miles as the "tolerance" on my house, and not where Chicago is?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
I am with powerhound, the origin can be chosen for convienence all the easier with GD&T. Auto companies, ship and aircraft companies like to choose origins that are not even on the part but may reference a global assembly origin. With GD&T you can establish features on the part that are important mating features and then reference back with basic dimensions to the origin and dimension other features basic from there. The FCF does all the "heavy lifting" relating the features to their respective datum framework. since the dimensioning scheme leaves little clue.
Frank
Wait, CheckerHater... we are talking about ordinate dimensioning, which has a clearly delineated zero point (Chicago in my analogy).
Your question turns it around, so you are changing the zero point to be my house. So it's not the same in terms of how the tolerance is envisioned. The numerical distance is the same (duh), but the phrasing of the question has implications!
I think that using ordinate dimensioning is seen as equivalent to the dimension origin symbol. Admittedly, that's not spelled out in the standard (and I'm usually the hardliner about following the letter of the law!).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
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