Positional tolerancing and implied symmetry 1

[PobbeS]

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?

 

[Belanger]

Although paragraph 1.4(k) of ASME Y14.5-2009 makes it sound like Figure B might be allowable, I would always use Figure A. In fact, the Y14.5 standard always shows it as Figure A in their subsequent pictures-- go with that.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[dingy2]

I agree with J-P, figure A is clearer.

Dave D.

http://www.qmsi.ca

 

[pmarc]

Both methods are valid however I agree figure A is much clearer and I would also recommend it.

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.

 

[Belanger]

Oops -- good call, pmarc. I guess it really is allowed. My personal biases stood out, I guess. We still all prefer Figure A.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[drawoh]

pmarc,

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.

JHG

 

[pmarc]

Since both methods are legal per Y14.5, I think it is a matter of personal preferences which one to choose. I can imagine applications when one method has some advantages over the other and vice versa. IMO it really does not make sense to deliberate about it more.

 

[PobbeS]

Thank's for your valuable comments.

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.

 

[PaulJackson]

I prefer A... but for another reason. It is good practice to layout the basic dimensions directly to the DRF origin orthogonal to the feature being controlled for inspection reporting purposes... that way the measured displacements relate directly to the specified basics.

It helps all diciplines in reviewing the data.

Paul

 

[Belanger]

Yes, like the example in your opening post, both figures here are also legal and mean the same thing.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[SeasonLee]

Figure B is exactly same as Fig 7-18 Y14.5-2009 except the MMC and MMB.

SeasonLee

 

[dtmbiz]

PobbeS,

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.

 

[ptruitt]

Wouldn't folks driving CMMs prefer basic ordinate dimensioning, usually originating at datums, and not want to see any implied anything? I must admit that my mind likes to see and think symmetry. What do the CMM drivers say?

Peter Truitt

 

[drawoh]

dtmbiz,

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.

JHG

 

[dtmbiz]

drawoh,

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.

 

[PobbeS]

dtmbiz,

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.)

 

[KENAT]

This comes up now and then, pretty sure at one point most of the 'experts' on here kept claiming only A was allowed.

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[dingy2]

Kenat:

B is OK. There is nothing wrong with it. I just found A clearer.

Dave D.

http://www.qmsi.ca

 

[redlerm]

Hi everyone.

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

 

[Belanger]

Redlerm -- be aware that the general term "symmetry" as used by most people takes on a very specific meaning when we then discuss the GD&T symbol called "symmetry." So I see what you are trying to do, but don't use the GD&T symbol symmetry. I would actually recommend continuing to use the CL SYM note.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems