Parallel coincident faces

[andymat]

I am trying to define a geometrical tolerance scheme to control two separate faces to be in the same plane.

I can define one face as a datum and control the other parallel to within a tolerance band. And if the faces were offset from one another, I could place a basic dimension which would control the distance. But since I want them coincident, I cannot put a basic dimension of '0'.

Do I need to add an additional position tolerance? Or can anyone suggest a better way to achieve a tolerance between 2 parallel and coincident faces?

Many thanks.

 

[KENAT]

Perhap use profile of a surface control. It can be used to give the equivalent of flatness for multiple nominally coplanar surfaces. See ASME Y14.5-1994 section 6.5.6.1 or equivalent.

It's been discussed here before, maybe try finding it.

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[KENAT]

Hmm, did a quick look and the threads that turned up are more about using multiple surfaces as a datum, related but separate issue. If you search harder you may find something though if my initial post isn't enough.

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[dgallup]

I use flatness pointing to both surfaces with the text "COMMON ZONE" beneath the feature control frame. In some cases we use this on 3 surfaces.

 

[drawoh]

andymat,

One advantage of GD&T feature control frames is that you do not always need to apply a dimension to have a tolerance.

If it is not obvious that two surfaces are coincident, you can connect them with a phantom line. In general, if they look coincident, they are coincident. This is the same problem as when you have a feature located on a centre line.

JHG

 

[KENAT]

dgallup, what you put is not correct per ASME Y14.5M-1994, flatness only applies to a single surface (section 6.4.2)..

I've attatched an approximation of figure 6.5.6.1 from this standard showing how to use profile of a surface for multiple coplaner surfaces.

http://files.engineering.com/getfil...61f-a10d262e4cfb&file=COPLANARITY-6-5-6-1.tif

Andymat, are you actually using ASME stds, if not then there may be other options. If in the UK or somewhere and using ISO there may be different ways of doing it, I've got a vague recollection of some ISO standard allowing you to use postion for this, there may even have been a previous post about it, but I can't recal for sure.

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[Belanger]

What dgallup describes is the ISO way to do it. ASME says to use profile of a surface, and either say "2X" or use two leader lines or some other method to indicate that it covers both surfaces.

It is implied that the two surfaces are 0 mm (or inches) apart, and since a profile will be imposed across it, it is understood to be a basic dimension of 0.



John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com

 

[andymat]

Thanks for everyone's input. Yes, I am in the UK and working to BS ISO standard although my reference is a little out of date.

Thinking about it, a connecting line will imply that they are co-planer and with a parallel tolerance from one to the other this is probably all that is required.

I did think about using a profile tolerance but that does seem a little away from it's intended use.

I think I am making things more complicated for my self, although it's good to have a discussion.

Many thanks every one.

 

[dingy2]

The ISO standard allows flatness to be used for both surfaces with a chain line between the tops of the surfaces. Both surfaces are now assumed one surface.

ASME does not allow this.

Dave D.

http://www.qmsi.ca

 

[ModulusCT]

I agree with Kenat's image, however, I don't think any verbiage is necessary when connecting two surfaces with a phantom line like you see in the example. That alone indicates both surfaces are coplanar and no further explanation should be necessary.

 

[dingy2]

My mistake. It is a phantom line rather than a chain line.


Dave D.

http://www.qmsi.ca

 

[fcsuper]

"I did think about using a profile tolerance but that does seem a little away from it's intended use."

In ASME, using the profile in this way is intentional. I don't know about ISO rules, but I suspect both ASME and ISO will use the same method one day (as ASME Org is now a contributing member of the development of ISO).

With CF functionality in ASME Y14.5-2009, the flatness way of calling out the two surfaces may be allowed now.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fsincox]

To me a flatness callout with a note like "#X COMMON ZONE" says exactly what it means and should be understood by reasonable people. While I agree that profile is now the traditional ASME prefered method, and the method I would use. I think it does a diservice to general engineering communication to say we can not accept this reasonable alternative as an option. I was taught when all else fails you can always use notes to clarify intent, that is all that is really happening here. The fact it is also an accepted method by ISO is most likely the reason some are against it, I can not support that kind of narrowness.
Particularly after we steal their idea to make "CONTINUOUS FEATURE".
Frank

 

[KENAT]

fsincox, any particular reason you're digging up an old thread apparantly just to start another ISO V ASME debate?

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[fsincox]

Ken,
I am only interested in providing some support to people who work with the ISO standard when they do appear here, I have a selfish intrest, I want to learn more about it.
Frank