GD&T - keeping features on center line of part

[badger2011]

I'm having some issues keeping a feature on center. Can you please look at the attached image?

I need a good GD&T call-out to keep these features on center. First move is to add a center line to this print and I’m assuming adding a datum. I would like the .563" thickness to stay on center with the .783" thickness to within .005". I'm used to working with round parts, so a run-out symbol came to mind, but those can only be used with round parts.


Thanks for any help you can offer here. I couldn't find anything well explained in my "Fundamentals of GD&T" book.

 

[drawoh]

However, symmetry seems to be out of political favor and generally frowned upon for reasons that escape me.

I wouldn't really say it's "political." Symmetry -- in the GD&T language, not Webster's dictionary -- is based on very precise definitions. It's just that ASME and ISO have slightly different definitions and the ASME one makes it much more tedious to inspect.

I have not checked the ASME definition of symmetry carefully. I think I would use symmetry in conjunction with sloppy profile tolerances. I don't care what the outlines look like, as long as they are symmetric. I do not know why I would do this. If I want something balanced, I will explicitly specify the balancing test, eg. 4000rpm, 1oz centrifugal force maximum.

--
JHG

 

[Belanger]

The definition of concentricity (ASME) says that the median points of each diametrically opposed element shall be within the specified tolerance zone. Sounds great on paper, but imagine how tedious it is to find each median point -- something that can't be touched, but rather is purely based on mathematical derivation. Then there's the issue of how many of these "median points" to check; that's a subjective thing that's usually left up to the inspector.

So I guess your comments are about right: a sloppy profile tolerance could allow an egg-shaped part, while still being concentric in the ASME world. But if balance is what you're after, I guess there are other direct ways to say that.

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

http://www.gdtseminars.com

 

[fsincox]

I understand, it may be looked at that the retention of symmetry and concentricity was just as much of a political of a decision, but since they do not accept the same definitions it seems a failed attempt at best.
See, I remember the talk, after 1982, when they were all going to harmonize for the next version. They did end up adopting the ISO datum look, which I have mentioned before was a silly idea if they didn’t go for the rest. Typical for today’s society though, “make it look like” but “not actually be like”.

 

[MechNorth]

I like the ISO datum symbol FSincox because it's easier for me to pick out, but then that's the only symbol that I've worked with too. The "unification" of the standards is a long and ongoing story of good intentions and bad delivery.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Looking to the newest editions of some important ISO GPS standards (8015:2011, 1101:2012, 5459:2011 or 14405-1:2010) I am afraid we can forget about ISO/ASME harmonization once and for all.

 

[CheckerHater]

There is always an easy way.

 

[MechNorth]

I have suspected as much, pmarc. When they swung onto the tangency of a metrology-based standard, there was little hope of reconciling into a single cohesive standard. There have been genuine efforts and concessions by ASME, but the response from ISO seems to be to push for more rather than migrate to common ground.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com