Parallelism on per unit basis

[powerhound]

Hey folks,

Another question has come up. I found another thread on this subject but didn't find a definitive answer in it. I don't find direct support for parallelism on a per unit basis. This seems to be a legitimate extension of principle but I'd like to get others opinions or maybe point me to the place in the 2009 standard where it exists. In reality we'd like to add a tangent plane modifier to it but that shouldn't make a difference in the answer.

Thanks,

Powerhound, GDTP S-0731
Engineering Technician
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[MechNorth]

Hey Powerhound,
No, there isn't direct support for it in the standards, however it is a logical extension of principles. A friend of mine uses it in the armaments industry and advocated for its inclusion in the '09 release of Y14.5. Unfortunately there wasn't enough support for it to go forward at that point. There was no valid argument posed against it either, and many of us recognized the value.

Jim Sykes, P.Eng, GDTP-S
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[powerhound]

Thanks Jim,

I was really beginning to think that the question was so stupid that no one wanted to dignify it with an answer...lol.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

I've had a few of those too ... then all of a sudden my inbox gets filled. Glad it helped.

Jim Sykes, P.Eng, GDTP-S
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[CheckerHater]

Flatness is a refinement of Parallelism.
Flatness per unit is a refinement of flatness.
If your Parallelism does not specify surface smooth enough, you add Flatness.
If Flatness still not smooth enough you apply flatness per unit.
Why not use existing controls before creating your own by “extension of principles”.
To me the reason ASME is not in the hurry to extend the principles is that there is no need to re-invent the wheel;
So I am intrigued: What exactly ‘Parallelism per unit” will achieve that Flatness cannot do?

 

[pmarc]

So I am intrigued: What exactly ‘Parallelism per unit” will achieve that Flatness cannot do?

Technically, Flatness or Flatness per unit will create a tolerance zone that can be at any angle to datum plane (within "global" Parallelism tolerance zone, of course), which is not the case when Parallelism per unit is used. So from purely geometrical point of view there is a difference.

Notice also that powerhound actually wants to apply tangent plane modifier to those callouts, so to me it seems that he is not interested in controlling Flatness at all.

 

[powerhound]

CH,

What pmarc said is correct. It's the tangent plane that is important in this case. Thanks for the lesson though.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

Yup, what pmarc said.

Jim Sykes, P.Eng, GDTP-S
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[CheckerHater]

How do you guys apply tangency on "per unit" basis?

 

[pmarc]

It is not a check of the same tangent plane as verified for "global" parallelism. Each "per unit" area has its own tangent plane that has to be within smaller tolerance zone.

 

[CheckerHater]

This is exactly my question:

How you establish TANGENT plane if your single unit spot falls within CONCAVE area?

Is your mathematic somehow different from my mathematic?

 

[Belanger]

CH, just imagine laying a flat plate onto that local area. The plate must be the exact dimensions of the area.
Now check the parallelism of the flat plate. Done!

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

http://www.gdtseminars.com

 

[Belanger]

(Well, not done -- you'd have to scoot the plate around and check many of those local areas.)

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

http://www.gdtseminars.com

 

[pmarc]

CH,
Even a concave area has got the highest points at the boundaries of the per unit area. They could be used to establish the tangent plane.
I would be rather worried if the local area was convex, so the tangent plane could freely rotate around the peak.

 

[MechNorth]

Also worth noting for casual readers, the tangent plane must be oriented wrt the datum reference frame.

Jim Sykes, P.Eng, GDTP-S
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[pmarc]

Jim,
Could you elaborate on your last comment? I am somehow confused by what you said - of course assuming that I understood it correct.

 

[CheckerHater]

Pmarc, John-Paul,
Sorry, but I don’t buy it.
I suggest opening book on geometry (please, no Wikipedia) and reading definition of tangency from there.
I am out of this.

 

[MechNorth]

Tangency in Y14.5 is a plane that contacts the high point(s). But how is that plane oriented? For form, it's not clear. For orientation, it would have to be wrt the datums referenced.

Jim Sykes, P.Eng, GDTP-S
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[pmarc]

CH,
I would not want to go into maths with that. Maybe you are right, maybe the definition of tangency is different to how I and J-P see it. However Y14.5 clearly says it is "a plane that contacts high points of the specified feature surface" (para. 1.3.45). In the light of that I do not think there is anything illogical in our earlier statements.

Jim,
The tangent plane, as a verified entity, is not related to any datum referenced, the parallelism tolerance zone is - at least this is how I see it.

 

[powerhound]

CH

I suggest opening the ASME standard and reading the definition, as well as studying the illustration, of tangent plane from there since that's what we're actually talking about. We aren't talking about geometry.

The tangent plane of a surface, as far as GD&T is concerned, is formed by the three highest points of that surface. This is where a plane is tangent to all three of them simultaneously. That is the plane that has to be parallel to the datum


Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II