Tolerance combinations in ISO BS8888

[docgeoffyjones]

All

In ISO 5458 they show an example on page 7 of a tolerance combination. the symbol used is identical to Composite tolerancing as defined in ASME Y14.5M 1994 (chapter 5.4). My senior engineer has told me they are the same because "ISO and ASME are basically the same standard".

I do not believe the definition for tolerance combinations given in ISO 5458 is sufficent to draw the conclusion that it is idential to Composite tolerance defined in ASME Y14.5M 1994 chapter 5.4.

In the example in 5458 it is seems unclear to me if adding datum reference B to the lower segment of the tolerance control box would be a refinement of the patterns orientaion to datum B, or a refinement of the patterns postion and orientation to B.

Am I way off the mark here? is there something in the standard that i have missed that indicates they are the same thing?

 

[fsincox]

ISO and ASME are not the same. I think I can safely start there. What revision date of ISO 5458 are you looking at? Mine is 1987 and sounds different.
Frank

 

[pmarc]

docgeoffyjones,

I have a version of ISO 5458 from 1998 and (no matter what) the example you refer to looks correct, but I agree with you it does not say that: ''adding datum reference B to the lower segment of the tolerance control box would be a refinement of the patterns orientaion to datum B, or a refinement of the patterns postion and orientation to B''. Probably they idea that stands behind this example is the same as in Y14.5, but it is not precisely said.

I think you have noticed one of many places in ISO standards which are unclear, ambiguous and leave a place for different interpretation. It is one of the biggest advantages of Y14.5 over ISO that much more examples are shown and clearly depicted and we, as users, can be much more confident that what we do is correct or not.

Finally I would be very careful saying that both standards are basically the same. There are significant differences and plenty of books and articles have been written on this topic. If someone says they are the same, I am afraid he has no idea what he is talking about.

 

[fsincox]

I now have seen a copy of the 1989 version of ISO 5458. I would say it’s definition of composites stops at about the same point the 1982 ASME standard did. I have heard that battles in sued about the intent of secondary datums restated in a composite position tolerance. I believe this is one reason why it is covered so extensively in the 94 standard.
I am collecting references on the ISO GD&T or GPS standards as I have been interested in understanding the differences for a long time. The reference materials I have state this is where the commonality ends. The lower line in a composite is not exempted from location and for orientation only. The ISO does not hold the simultaneous requirements on any position tolerance unless it is stated specifically when datums referenced involve a feature of size, I have seen CZ (common zone)may be used for this. The ISO does not limit position application to features of size.
Those are a few differences I have read, based on books about the “bible” not from the actual standards as they seem mush more generalized and you need a lot of them. I would love to know more.
Frank

 

[fsincox]

I should clarify, some position tolerance differences only.
Frank

 

[docgeoffyjones]

Thanks for your responses im am using ISO 5458 1999 for position.

I'm glad it is not just me who finds ISO vague. when ever i suggest at work that ISO and ASME are different I quickly get shut down.