Symmetry Help

[[wi...]]

Good Afternoon,

I am measuring 4 flats in the shape of a square. It has a symmetry of these planes to a Cylindrical Datum. The top-bottom and left to right flats are being recalled with a symmetry plane and then using this plane we are using GDT Symmetry characteristic to evaluate. This seems pretty straight forward.

However I am having a hard time realizing and proving why the symmetry is not in tolerance. From those planes I've added in some perpendicularity callouts to see if we are truly grinding square.

I also added in form characteristics for each plane. Flatness characteristics all look within reason. And parallelism looks great also. Since none of these characteristics are showing me what's going on. I also added two straightness 2d lines across each flat to see if somehow there was taper on these faces.

I also added Cartesian distance points from each flat to centerline so we can see how much stock is left on each face back to the required Flat width.

The symmetry callout is .015mm, I am reporting one set of flats within tolerance and the other set at .02mm.

All other characteristics are under .002mm with only one perpendicularity being at .005mm.

I'm at a loss of what to tell the operator to adjust. IS there something I'm missing or is this strategy just not the best practice?

Any help would be much appreciated.

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[[Kl...]]

Please zip the inspection with actuals and put it into the Forum.

Sometimes only outliers creates bad results, because the min and max Points are used inside calculation.
If position angle are OK, results are according to Gaussian element calculation ("average")

[[Ro...]]

V block & a height indicator cant get you close?

If you're having trouble using planes to establish your symmetry, use lines.
2 lines in X, 2 lines in Y. Turn 2 symmetry lines into an intersect point that should be right at Zero

[[wi...]]

I ended up figuring it out. Height Stand and Indicator were not accurate enough. After doing some math and realizing the issue was more in the distances calculated then actual geometry being out. Splitting microns is never fun.

[[Aa...]]

The definition of a symmetry requirement in the ASME standard is often misunderstood.

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[[An...]]

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Misunderstanding - Sounds like an euphemism.
In my opinion this is pure rubbish.
I beg your pardon!

[[Aa...]]

Yeah, I know... any time someone mentions Y14.5, you have to chime in that you think the standard's rubbish, or worse...

[[Aa...]]

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Would you like it better if it was worded like this?

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That's from ISO1101:2012, and means the same thing.

[[An...]]

Still searching for the meaning of "congruence".
See attached.

Contribution_20_09_2019.pdf

[[Aa...]]

I assume you mean as it's used in Y14.5. My understanding is that it means the points lie on the datum axis or plane.

And that's one way that symmetry and coaxiality FCFs are different from those of position, profile, and angularity in Y14.5. The tolerance zone for the median points is fixed on a datum, not in a DRF. Symmetry and Coaxiality FCFs should only reference a single datum feature, and it's that feature alone that determines the axis/plane. (I don't believe this distinction holds in ISO).

Although I haven't seen it, I've been told that the latest Y14.5 completely dropped symmetry and coaxiality types.

[[An...]]

Some further thoughts about DRF's and congruency.
See attached.

Contribution_24_09_2019__.pdf

[[Aa...]]

Andreas, if I were in your shoes, I would find the ASME standard very hard to delve into. I would probably feel the same way about ISO1101, etc. that I think you do about the ASME standards if they were only printed in German. (Although your grasp of English is obviously kilometers ahead of the five or six German words I know, with kilometer being one of them 😜 .)

I appreciate your exploration of this concept, but I think you're getting too hung up on the word "congruent", and a definition of the word that I do not think was intended in context. Really, the tolerance can be understood from the standard without the sentences that use congruent. It points back to 7.6.4.1, which says, "The median points of all correspondingly located elements of the feature(s) being controlled, regardless of feature size, must lie within the...tolerance zone." For symmetry, that tolerance zone is with centered on the datum plane (whereas for concentricity, it's a cylinder (or sphere) centered on the datum axis (or datum point).

[[An...]]

Alright!
We cancel the term "CONGRUENT"
And we also do not regard the cylinder as a datum feature of symmetry.