Composite Position Equivalent in ISO or BS

[G_Arbuckle]

I find the ASME composite tolerance really useful for tolerancing hole patterns like this, where you care about the patterns internal tolerance but it doesn't matter as much where the pattern is relative to the rest of the part.

However, I use BS8888 for drawing. Is there anything like this in the BS or ISO standards that performs the same function.
I've not managed to find it in the BS8888 document.

 

[pmarc]

BS standards generally follow ISO. In ISO, there is no composite tolerancing, however there is a way to define equivalent requirements with the use of two single segment position tolerances.

Assuming A is a flat bottom surface of a block and B is one of its sides, the corresponding callouts in ISO should look as follows:

|POS|dia. 0.8 (M)|A|B|C|
|POS|dia. 0.2 CZ (M)|A|B ><|

Where:
- CZ stands for Combined Zone, which is a modifier that ties the N position tolerance zones together in terms of their mutual basic (theoretically exact) spacing. This modifier, per today's ISO standards, does not have to be stated in the upper segment of the callout because in that segment the tolerance zones are fully constrained relative to the A|B|C datum system (i.e., they can't move - translate and rotate). See ISO 5458:2018.

- >< indicates that datum B reference constrains only rotational degrees of freedom of the tolerance zones. See ISO 5459:2024.

 

[greenimi]

|POS|dia. 0.8 (M)|A|B|C|
|POS|dia. 0.2 CZ (M)|A|B ><|

Pmarc,
Is it legal in ISO to modify B at MMC/ MMB/ MMR in the lower agement? I am asking about the legality not about functionality.
I suspect it is legal, since the segments are considered independent from each other ("they don't see each other")
(B in this case should be feature of size)

I kind of know that in ASME it is illegal to do so (FRTZF - lower segment - should follow/should be a subset of PLTZF-upper segment in the OP's callout)

 

[pmarc]

greenimi,
It wouldn't be legal in my example, where A and B are surfaces. But if B was a feature of size, then (M) modifier after B would technically be legal in the lower segment in ISO.

 

[greenimi]

It wouldn't be legal in my example, where A and B are surfaces. But if B was a feature of size, then (M) modifier after B would technically be legal in the lower segment in ISO

pmarc,

Does ISO have the MMB concept?
I am asking, for example, if B (surface) secondary has a LOCATION relationship with A primary, then, I think, in ASME we can legally use B(M) (surface, hence not a feature of size) at MMB in the FCF.
Is this MMB concept valid in ISO too?
Or maybe ISO is using a different approach for the same concept?

I hope you do understand what I am asking.
Thank you pmarc

 

[pmarc]

They do not allow for MMB/MMR on planar datum features even if the feature has location relationship to the higher order precedence datum.

I would say it's one of few areas where ASME has more tools in the toolset than ISO.