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Coplanar Datum Not Quite Coplanar

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RichardMc

RichardMc

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May 18, 2005
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I have a customer-generated drawing in which the product has a datum that is really key to the functionality of the product, so virtually every other dimension comes off of this datum. The datum is formed by 4 different surfaces that are located of 3 different sides of the part, but are obviously interrupted. We have some parts that are failing to fit up with the mating part. The problem is that, of the 4 datum surfaces, there are 2 small ones that are not planar with the other 2 surfaces. There is no profile callout for these datum surfaces. Is there some sort of implicit profile tolerance? Would a profile tolerance be the long-term solution to control this unacceptable condition?

Richard McInteer
 
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Normally for that kind of applications, when multiple interrupted nominally coplanar surfaces are used as datum features to derive a single datum, profile of surface tolerance is the only choice for controlling the actual amount of error in their coplanarity. If there is no such on the print, nothing limits the error, so a part can end up as yours and there is no chance to force any correction on a manufacturer .
 
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Jim,
I am just wondering... If you use Continuous Feature modifier instead of profile of surface, how much coplanarity error will be available?
In my opinion the notification <CF> itself is not enough, there still has to be a tolerance controlling the coplanarity.

Side question: assuming <CF> was used, like shown in fig. 7-45, could flatness control be used instead of profile of surface since 2 surfaces would be treated as a single datum feature?
 
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Based on the 2009 standard 2.7.5, CF is used to identify a group of two or more features of size where there is a requirement that they be treated geometrically as a single FOS. For the OP case, a coplanar surfaces is not a FOS, a profile control is a better way to control the coplanarity. See Figs. 2-8 through 2-10. and Figs. 8-14 through 8-16.

SeasonLee
 
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Right -- the issue is that CF on a feature of size is fine because Rule #1 comes along for the ride, thus controlling the alignment. But to apply CF on a surface doesn't have any Rule #1, so pmarc is pointing out that there is still no quantitative control on the potential offset.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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The OP states that the 4 datum features are on 3 different sides of the part. I'm imagining a cube with datum features on 3 sides of it forming a single datum. How can this be a datum at all without the use of datum targets? Maybe I'm misunderstanding the question.

Powerhound, GDTP S-0731
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I guess the OP stated that the surfaces were "interrupted," and that two of them were not "coplanar," so we should just assume it's something that has four legs or pads.
Also -- the flatness tolerance with the <CF> would take care of the coplanarity. Would that be an extension of principles? :)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Assuming to US stds I'd play safe and go with the profile.

Powerhound, I think you're misunderstanding. Simplistically I was imagining maybe a flat, square surface with a 'cross' shape relieved in the middles leaving the 4 corners to create the effective surface.

Posting guidelines faq731-376 (probably not aimed specifically at you)
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Yeah, that makes a lot more sense. It was the whole "three different sides" statement that got me all cankajawed. If I hadn't ever seen something nearly as crazy as that, I may have just fallen in line from the start.

Powerhound, GDTP S-0731
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