Flatness tolerance zone is parallel to Parallelism tolerance zone? 3

Woosang · Nov 5, 2015

Hi, gentlemen

This is my first posting.
I'm a design engineer of commercial aircraft structures for Korean aerospace company.
I took ASME GDTP Senior level exam last month and got a letter from ASME a few days ago which says I passed the assessment.
I probably have some knowledge in theory, but it doesn't mean I am also the master in real life.
These days, I am reading carefully the threads in this forum and got many knowledge in practical applications.
I would like to thank you all guys for that.

Now, I would like to ask one thing that I do not understand while reading previous thread Link.

2015-11-05_14_18_50-Perfect_Form_Not_Required_on_Flatness_with_Parallelism_-_Drafting_Standards_GD_fnolxu.png

See the above image.
I guess there was some agreement that the parallelism tolerance zone and the flatness tolerance zone are not parallel to each other, which I do not understand why.

Here's my thoughts.
Datum feature A contacts the datum feature simulator and perfect counterpart of the simulator is simulated datum, which acts as datum plane A.
Datum plane A is one end of the flatness zone and a plane which is parallel to and .007 apart from datum A is the other end of the flatness tolerance zone.
Parallelism tolerance zone is two planes that are .007 away from each other and they are obvisouly parallel to datum plane A.
Now all the planes that establish two tolerance zones are parallel to each other, therefore those two tolerance zones are parallel to each other.
But, maybe there is some point that I am missing or wrong.
Would you tell me what I am missing here?

Thanks,
Woosang

greenimi · Nov 5, 2015

Question for the OP?
What is datum A?
(not datum feature A)

greenimi · Nov 5, 2015

A little help: datum A is a plane derived from the highest points of datum feature A. Am I right?

axym · Nov 5, 2015

Woosang,

The problem is in the statement "Datum plane A is one end of the flatness zone". This may not necessarily be true.

The reason why is that the flatness zone is independent of the datum plane.

The "flatness zone" is the closest two parallel planes that completely envelop the datum feature surface. The datum plane is a single plane through the "high points" of the datum feature surface. There are certain cases of surface variation in which the datum plane does not coincide with one of the planes of the flatness zone. In most cases they will, but not in every case.

This is easier to explain with a figure - I'll see if I can find one.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

greenimi · Nov 5, 2015

Took a while for me to understand the mechanics of the tolerance zone (degrees of freedom of the TZ), what is the TZ allowed to do and what is not.
Even now, I am not very sure I understand “the whole story” completely, but this particular case I do.
Thank you Evan for your input.

Woosang · Nov 5, 2015

There is a time difference definitely. I just turned on my computer at the office, maybe you are in bed in Europe or in the afternoon at America.

Thank you for your reply.

greenimi,

Yes, datum A is like you stated. But referring to Y14.5-1994, 4.2.1
"As measurements cannot be made from a true geometric counterpart that is theoretical, a datum is assumed to exist in and be simulated by the associated processing equipment. For example, machine tables and surface plates, though not true planes, are of such quality that the planes derived from them are of such quality that the planes derived from them are used to simulate the datums from which measurements are taken and dimensions verified."

I thought that parallelism tolerance zone would be parallel to simulated datum plane A, which is also the one plane of the flatness tol zone (maybe I'm wrong with the later one)

axim,

Yes, flatness is independent of datum. But in this case the toleranced feature is datum feature, so I thought that the one end of tolerance zone is the datum.
There would be some deviation between datum plane A and simulated datum plane A to which the parallelism is referenced. Is that the reason why the two zones are not parallel?
Would be more clear to me if you can put some figures. Thanks for that.

axym · Nov 6, 2015

Woosang,

Here is a figure, that I adapted from a different thread. The lower surface is the datum feature with the flatness tolerance, and the upper surface is the considered feature with the parallelism tolerance:

Datum_Plane_and_Parallelism_Zone_for_Forum_Post_zonpdr.png

This is a case in which the boundaries of the flatness zone do not coincide with the datum plane.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

Woosang · Nov 9, 2015

Thanks Axim,

My misunderstanding was apparently due to that I thought that the one end of flatness tolerance zone is a plane which contact to a surface at its highest points, which is not true.
But Y14.5 seems do not have clear description for that.

axym · Nov 9, 2015

Woosang,

This is a common misunderstanding. The figure in Y14.5-2009 shows the tolerance zone contacting the surface at its highest points, which is misleading because it doesn't need to. There are also GD&T textbooks that show an inspection method for flatness that involves laying the part on a surface plate to establish one plane of the tolerance zone. This is even more misleading, and could result in a false nonconformance on the part that I sketched.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

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