Flatness and parallelism

[Koether]

Looking some of our old drawings I keep seeing surfaces with a parellel requirement along with a flatnessness requirement of the same magnitude.

I was under the impression that parallelism already requires flatness to within the parallelism requirement, majking an additional flatness requirement (equal or greater to the parallelism) completely meaningless.

I did search for this but I didn't find anything (that I agreed with) that adresses this exact question.


from y14.5 Sec 6.3.3:
"Parallelism is the condition of a surface or feature’s
center plane, equidistant at all points from a datum plane;
or a feature’s axis, equidistant along its length from one
or more datum planes or datum axis."

A datum plane is perectly flat so I don't see how this doesn't require flatness.

 

[dgallup]

If the flatness control is not a refinement of other geometric tolerances that control the flatness of the surface then it is an "illegal" specification.

Perpendicularity, Parallelism, Angularity, Total Runout and Profile of a Surface are all controls which can control the flatness of a surface so if one of those has the same effect as the flatness then the flatness is redundant and should be removed.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[CheckerHater]

Out of curiosity - how old are "old" drawings?

Also, are there any additional symbols / requirements ("free state", "per unit"), etc.?

 

[MechNorth]

Also the standard question, what standard is invoked on the drawing? ISO treats things differently than ASME does.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

I think both ISO and ASME see parallelism as controlling flatness. (The relationship between size and flatness would be different in ISO.)


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

 

[ewh]

... old, however can be a relative term.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[CheckerHater]

I am completely making it up, but imagine this hypothetical situation:
Flexible part; Flatness is called "per unit" to make sure surface is smooth. However Parallelism can only be achieved in Restrained state, so in a sense Flatness IS refinement.

Also from "never say "never"" department.
Per Y14.5-2009 Parallelism may be called with "tangent" requirement, which allows for surface to "cave-in". In this case Parallelism DOES NOT control Flatness.
Since somebody asked for this, could it be the 70-s or 60-s version of the standard had similar clause?

 

[Koether]

Well the original drawings are from the 70s, they were updated at some point but no one really put any thought into the GD&T. These specifically call out Y14.7 (97) and the flatness I am referring to is total.

Thanks for the feedback, it seems I came to the right place.

 

[Koether]

where in Y14.5 is the discussion of the tangent modifier?

 

[Belanger]

It's paragraph 6.6.1.3 of the 1994 edition, or paragraph 6.5 of the 2009 edition.

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