Common Datum System of Rectangular Pattern of Holes

[spudgunblunder]

Hi all,
Does the common datum system A (of 4 centres) control the slot to be symmetrically within a tolerance zone of 0.5mm along the TED line between the holes?
Ref section 6.6.4 of BS 8888:2020 (not sure if I can upload screen shot?)

 

[pmarc]

Since the datum pattern A constrains all non-redundant degrees of freedom of the position tolerance zones for the slots and each 0.5 wide position tolerance zone is basically* oriented and located relative to the datum established from the pattern of holes, all for zones are also basically* oriented and located relative to each other.

*basically = through implicit or explicit TEDs.

 

[greenimi]

Pmarc,

If symmetry is replaced with position, would the mathematical definition be identical or there would be subtle differences?
Just trying to extend a little the original question.
Thank you so much

 

[spudgunblunder]

Thanks pmarc,
So the 4mm slot IS constrained to the theoretically exact line between the two 10mm holes?
I think I correct to say this is confirmed by section 5.4.2 of BS EN ISO 5458:2018?

 

[pmarc]

greenimi,
Unlike in ASME, symmetry tolerance in ISO GPS is just a special case of position tolerance intended for features designed to be centered about a datum, and so the interpretaion of the two tolerances is identical.

Having said that, I am not sure I would use symmetry tolerance for the slots in the case being discussed as the individual slots are not symmetrical relative to the datum established from the pattern of 4 holes. Position would be my choice.

spudgunblunder,
Yes, you are correct that the tolerance zones for the slots are constrained to the theoretically exact lines between the holes.

The figure you posted support this statement, however it is worth to remember that if each of the holes in that figure (on the left) was controlled to a different datum system, the implicit TEDs you highlighted wouldn't necessary apply.