Unequal tolerance for a negative nominal
Unequal tolerance for a negative nominal
(OP)
Here's an off-the-wall question that came up this past week, and since Y14.5 doesn't directly address it, I'd like some opinions...
Suppose a zero/zero baseline is called out in each direction from a tooling hole, and positive and negative locations go out from that 0,0 position. If a designer then adds a +.000/-.010 to one of the negatively designated locations, how is that interpreted?
I guess a picture would help, but I hope you get the gist: Suppose another hole is dimensioned to the left of the 0,0 hole, and has a nominal distance of -2.250. Do you think the above tolerance would yield:
-2.250 max
-2.240 min
or
-2.260 max
-2.250 min
My first response would be that it should be avoided, because of the confusion! But if pressed for a specific answer, I would probably say that the negative sign is merely for direction, so the tolerance is meant to apply to the absolute value, resulting in the first max/min set given above.
Or better yet, tell the designer to add a note "all dimensions are treated as absolute values, whether positive or negative," but that sounds goofy. Thoughts?
Suppose a zero/zero baseline is called out in each direction from a tooling hole, and positive and negative locations go out from that 0,0 position. If a designer then adds a +.000/-.010 to one of the negatively designated locations, how is that interpreted?
I guess a picture would help, but I hope you get the gist: Suppose another hole is dimensioned to the left of the 0,0 hole, and has a nominal distance of -2.250. Do you think the above tolerance would yield:
-2.250 max
-2.240 min
or
-2.260 max
-2.250 min
My first response would be that it should be avoided, because of the confusion! But if pressed for a specific answer, I would probably say that the negative sign is merely for direction, so the tolerance is meant to apply to the absolute value, resulting in the first max/min set given above.
Or better yet, tell the designer to add a note "all dimensions are treated as absolute values, whether positive or negative," but that sounds goofy. Thoughts?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems





RE: Unequal tolerance for a negative nominal
To make it clearer, just place a nominal dimension with a bi-lateral tolerance so, instead of the first option, have a requirement of -2.245 +/-.005.
Dave D.
www.qmsi.ca
RE: Unequal tolerance for a negative nominal
not labeled positive or negative", but I don't have a copy of 14.5 with me right now.
Will greatly appreciate if somebody could prove or disprove the statement by referencig actual book.
RE: Unequal tolerance for a negative nominal
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Unequal tolerance for a negative nominal
I agree with Dave's response, but my point is that the standard doesn't actually say that. So someone could push the idea that from a purely mathematical stance, a unilateral negative tolerance on a negative nominal dimension pushes the feature farther to the left!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Unequal tolerance for a negative nominal
I don't think there is an unambiguous interpretation of the tolerancing you describe. Whoever drew up this print should have specified the tolerance explicitly (and, if still available should be made to revise it). Failing that, I believe the only recourse of anyone using such a print is to call up whomever they got it from and ask for clarification.
If I HAD to interpret such a drawing without recourse to the responsible party, I would assume negative means 'towards origin'.
RE: Unequal tolerance for a negative nominal
I think we decided no '-' sign because of the potential confusion.
However, I'm not sure of an explicit directive in Y14.5M-1994
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Unequal tolerance for a negative nominal
The only statement that supports this in ASME Y14.5-2009 is 2.2(a). The problem with 2.2(a) is that is only applies to Limit Dimensioning, and not Plus and Minus Tolerancing. So, ambiguity is still present. THEREFORE...
Either convert your dimensions to limits or add a statement on the drawing as to how to interpret tolerances on the negative dimensions.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Unequal tolerance for a negative nominal
If I was checking the drawing, it would be rejected in a second.
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Unequal tolerance for a negative nominal
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Unequal tolerance for a negative nominal
I agree with not using the minus sign on your dimensions. It is obvious from the drawing which way the dimension goes.
If there was any ambiguity about a +.000/-.010 tolerance, I would switch to the limit tolerances you provide in your example.