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ISO and form control

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greenimi

greenimi

Mechanical
Nov 30, 2011
2,392
I read some previous discussions about ISO and how ISO position is defined differently than ASME position:
I would like in this new thread to clarify what aspects/ characteristics of form, position in ISO is controlling? In other words, I would like to understand a little bit in more details position in the European system ISO.


Per previous discussion:
ISO - position tolerance controls location of extracted median line, extracted median surface or extracted (actual) center of the sphere. Since by definition extracted median line and extracted median surface are imperfect, position tolerance has control over form of these derived features, thus over certain aspects of form of the feature of size.
 
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I found this: “ if the drawing is created as per ISO-1101, then straightness value should be refinement of positional tolerance value. Because in ISO not the axis should be within the tolerance zone, it is the median line should be within the tolerance zone.”


Is it true?

 
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Kedu,
I think your above statement is true. I am also searching for an acceptable answer to my own posted question and I found (here on eng-tips) another good reference


in this discussing I found the following account (posted by pmarc) which, I guess, is in line with yours

"I agree that if one wants to separate form and location errors, position tolerance is a good choice. Thing to remember, however - it is only true in ASME, because in ISO position (or coaxiality) tolerance indirectly controls form (straightness) of what they call extracted median line of a cylinder."

I am still not clear what about the circularity and cylindricity in relationship to the position in ISO? Do they need to be refinement also or not ?
Anyone can help me with this follow-up question?
 
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According to an ISO book I have -Geometrical Dimensioning and Tolerancing for Design......- I would say yes , it does. Locational tolerances also limit the orientational deviations and form deviations of the toleranced feature.

 
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"In short, no, the two tolerances cannot "see" each other, so the envelope is always Ø16, independently of the straightness tolerance and the straightness tolerance is always 0,04 regardless of the size. This means that there are some combinations that are impossible, so if the two point size is exactly 16 everywhere, the median line has to be perfectly straight. Not because of the straightness tolerance, but because of the envelope requirement. The straightness tolerance only becomes an additional constraint, as the size gets smaller. So if it is a perfect, bent cylinder, it has to be Ø15.96 or less to take full advantage of the 0,04 straightness allowance."


I found the above statement on another discussion here on eng.-tips and based on ISO GPS default independency principle, I would think that the circularity and/or cylindricity and position “CANNOT SEE” each other, so, consequently do not have to be refinement of the position in ISO GPS.

But, and here is my dilemma, why straightness “CAN SEE” position and ISO position tolerance controls form (in order to be refinement)? In other words, why straightness has to be refinement of position and circularity/cylndricity doesn’t? Or does it?
 
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I am still struggling to get a clear (if all possible) answer on my own questions. Anyone could help?
 
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ISO_qlpwax.png


Picture from an ISO book.
 
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Is ISO concentricity same as position?
 
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greenimi said:
Is ISO concentricity same as position?
Click to expand...
Not sure, but I think if ACS - any cross section--is used then YES.
ISO_2_bf9vjj.png


Nonetheless, here is another example where form error is bigger than location.
 
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Does the cylindricity in Kedu's first picture looks kosher?
Size tolerance is smaller than cylindricity.
Also has E envelope symbol shown.
 
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