Ha ha -- I just picked Chicago randomly; I didn't know they have an actual datum marker!
I am east of Chicago. Don't try to track me down, guys. I won't be home.
And I certainly agree that this whole thread is an illstration of why GD&T should be used. I don't want to appear to defend a drawing practice that might be confusing.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
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!).
It is spelled out in the standard. Ordinate dimension isn't called "ordinate" in Y14.5. It's called "Coordinate Dimensioning without dimension line". The 0 is only the baseline for the oridnate set. It is treated the same as baseline dimensioning or any other coordinate dimensioning scheme. It is not the same as the dimension origin symbol. Dimension origin symbol is used to specify that a tolerance is taken from on feature and not the other.
ISO used the same symbol with "running" dimensions, but it makes no statement (at least that I can find) that states the tolerance shall be taken from baseline only.
The tolerance for an "origin" in an ordinance scheme is dictated by the tolerance of the overall length and width of the part in relation to whatever features are between.
A hole 6" off the origin ±.005" is no different from an origin point off the hole ±.005". If you want to consider it as the origin has a tolerance in relation to the hole (as opposed to the other way around) that's very strange, but go for it. Just watch out for stacking tolerances.
Thanks, Matt. I didn't have the standard in front of me at the time. This is all well and good, but the OP's question was about where the tolerance resides. The standard doesn't spell that part of it out. So I guess we do have to treat it as if it were traditional dimension lines (arrowhead dimensioning).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
Tick is right. The only way to give a tolerance to the "starting point" of an ordinate dim is to have a higher level coordinate system with its own "untoleranced" starting point. Everything is relative. I think Eistein said that.
The Y14.5 is very clear about this. Ordinate dims are nothing more than coordinate dimensions that don't have dimension lines on the drawing. They are treated the same as any other coordinate dimensions. The tolerance is applied to the dimension itself; the feature at either end can vary within that tolerance.
Even the Origin Symbol doesn't tolerance one end or the other. It simply applies the tolerance variation to the far end only. The baseline is treated as sort of a datum (datum rules from GD&T don't apply to it, but it is still the reference from which the dim originates, rather than having no preference if the symbol isn't used).
