Positional tolerancing and implied symmetry
Positional tolerancing and implied symmetry
(OP)
Often I have patterns of holes or other features symmetrically located on a part. My question is if the symmetry is implied according figure B in the attached picture or is is neccessary to dimension each of the holes separately to the symmetry plane as in figure A?





RE: Positional tolerancing and implied symmetry
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Positional tolerancing and implied symmetry
Dave D.
www.qmsi.ca
RE: Positional tolerancing and implied symmetry
J-P, method presented on figure B is actually shown in Y14.5-2009 at least on 2 figs.: 7-4 and 7-18. Maybe there are more examples but I only did very quick search through the standard.
RE: Positional tolerancing and implied symmetry
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Positional tolerancing and implied symmetry
I actually prefer figure_B most of the time. There is less clutter on the drawing, and the design intent is clear.
Much of time, then I do a drawing like this, I show a double FCF indicating an accurate pattern and a sloppy overall location. The 60mm is critical. The 15mm dimension and the centreing, often much less so.
RE: Positional tolerancing and implied symmetry
RE: Positional tolerancing and implied symmetry
I agree that Figure A is more clear and that the theoretical exact dimensions 30 are consistent with datum B. However, Figure B is more clean in my view and I wish it was more clearly stated in the standard that symmetry is implied in this case.
A related case is shown in the attached figure. The situation is much the same but now in polar coordinates. Here it is more natural for me to use Figure B and I am almost sure that symmetry is implied. However, I want to be 100% sure.
RE: Positional tolerancing and implied symmetry
It helps all diciplines in reviewing the data.
Paul
RE: Positional tolerancing and implied symmetry
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Positional tolerancing and implied symmetry
Figure B is exactly same as Fig 7-18 Y14.5-2009 except the MMC and MMB.
SeasonLee
RE: Positional tolerancing and implied symmetry
A word of caution. This may sound "picky" however, there is a distinct difference between "symmetry" and "positional tolerancing for symmetrical relationships."
What you are discussing here is "positional tolerancing for symmetrical relationship." Using the true position symbol to establish a relationship of features referencing the feature's datum axis or datum plane.
Symmetry using the symmetry symbol is referencing "median points" of the feature's surfaces.
Ref 1994 5.13 and 5.14_Fig 5-60 and 5-61; 2009 7.7 and 7.72_Fig 7-65 and Fig 7-66
There is enough confusion surrounding the standard and IMO using the correct terminology is helpful to prevent some of it.
RE: Positional tolerancing and implied symmetry
Peter Truitt
RE: Positional tolerancing and implied symmetry
I am pretty certain we are discussing implied symmetry as the nominal position. If a symmetry symbol is on the drawing, the symmetry is not implied.
I cannot imagine a situation in which I would specify symmetry.
RE: Positional tolerancing and implied symmetry
Just being technical in a technical enviroment.
I do believe it is important to know exactly what is being talked about to avoid "implying" as much as possible.
According to the standard there is a distinct difference.
IMO I think this site is a "learning / teaching" resource as good as you will find online. Considering the knowledge range of those who participate, I believe it is helpful especially for new or less experienced users to know what "the standard states".
FYI; for an example,
An variable curvature profile symmetric about a center plane would be an example that I have run into.
RE: Positional tolerancing and implied symmetry
You are right, I should have been more precise.
(Actually I use the ISO-standard not ASME. In ISO tolerances of position, symmetry and coaxiality all apply to the extracted axis or median face of the actual feature.)
RE: Positional tolerancing and implied symmetry
However, I'm convinced B is ok, an that it makes a nicer more easy to read drawing most of the time in the real world for the reasons drawoh gives.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerancing and implied symmetry
B is OK. There is nothing wrong with it. I just found A clearer.
Dave D.
www.qmsi.ca
RE: Positional tolerancing and implied symmetry
I have a drawing with a variety of straight-line and curved geometry. If I create a vertical plane down the middle of the part, I have identical features with identical dimensions originating from that same center plane. The only features not originating from that plane are hole diameters. It seems pointless to dimension both halves and wondered how I can use a symmetry symbol. Back in the day, I'd just place a "CL SYM" at the bottom of the center line and the machinist knew exactly what to do with no ambiguity. I'd like to do more to follow a national standard and I'm not so sure that note is legal anymore.
Thanks.
Mike
RE: Positional tolerancing and implied symmetry
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Positional tolerancing and implied symmetry
Implied symmetry means that the nominal state of the part is symmetric about some feature. The geometry of the part is controlled by ± tolerances, positional FCFs and profile FCFs. Symmetry is not explicitly controlled. A significant asymmetric state would violate one or more of the tolerances noted above.
You may apply sloppy ± tolerances, positional FCFs and profile FCFs, and then apply an accurate symmetry FCF. Now, your otherwise sloppy part is specified to be symmetric about the datum feature(s).
I have no idea of why you would want to do this. If I wanted a rotating component balanced, I would specify the maximum radial force, and the rotation speed.
RE: Positional tolerancing and implied symmetry
If I understand you correctly, fig. 1-33 from Y14.5M-1994 or fig. 1-35 from Y14.5-2009 may help you.
However, if any features violate part's nominal symmetry, I wouldn't recommend this method.
RE: Positional tolerancing and implied symmetry
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerancing and implied symmetry
But I do like the idea of using symbology over notes, so I'll change my recommendation to that given in 1-35.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems