Flatness and parallelism
Flatness and parallelism
(OP)
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.
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.





RE: Flatness and parallelism
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.
RE: Flatness and parallelism
Out of curiosity - how old are "old" drawings?
Also, are there any additional symbols / requirements ("free state", "per unit"), etc.?
RE: Flatness and parallelism
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Flatness and parallelism
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Flatness and parallelism
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Flatness and parallelism
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?
RE: Flatness and parallelism
Thanks for the feedback, it seems I came to the right place.
RE: Flatness and parallelism
RE: Flatness and parallelism
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems