Combined Zone in a material condition modified datum system

[Burunduk]

ISO 5458:2018(E) seems to say that the explicit pattern specifications such as the CZ modifier are for cases where some non-redundant degrees of freedom are unconstrained. That is also reinforced by the fact that that there seem to be no examples with datum systems that constrain all DOF in that standard. Also, in ISO 1101:2017 there's an example of a pattern referencing A,B,C (all planar datum features) using no CZ. But what if there are suffecient datums to constrain all DOF, however a locating datum is a feature of size modified to apply at MMVC? Does that require CZ for the tolerance to invoke a pattern behavior and override ISO's default independency and not allow a separate datum shift for each feature?

 

[pmarc]

If the datum feature is a feature of size referenced at MMVC, then this means that not all DOFs may be constrained. If the unconstrained DOFs are non-redundant, the CZ modifier shall be specified to create a pattern of tolerance zones.

 

[Burunduk]

pmarc,
Are you saying that ISO consideres datum displacement as unconstrained degree(s) of freedom? I think at least per ASME Y14.5, even when there is some possible shift the degrees of freedom limited by the MMB datum feature simulator are considered constrained:
"3.9 CONSTRAINT
constraint: a limit to one or more degrees of freedom."

 

[pmarc]

They don't explicitly call it that way, but this is how they treat it, hence CZ must be used to override the default independency principle.

For ASME, I know there are people who call this situation a "partial constraint". The math standard, Y14.5.1-2019, treats such scenarios as the ones where multiple candidate datum sets or multiple candidate datum reference sets are possible. However, since by default all features controlled with a single location tolerance shall be treated as a pattern, they all must be evaluated in the same candidate datum or datum reference set, and so the independency doesn't apply.

 

[greenimi]

...CZ modifier are for cases where some non-redundant...

The thing that confuse me the most is the wording "redundant" and "non-redundant" used by ISO.

Pmarc and Burunduk,
Could you, please explain it in laymen terms what is the difference between those terms and "why" ASME is not using them (at least not to my knowledge)?

 

[Burunduk]

The thing that confuse me the most is the wording "redundant" and "non-redundant" used by ISO.

Pmarc and Burunduk,
Could you, please explain it in laymen terms what is the difference between those terms and "why" ASME is not using them (at least not to my knowledge)?

greenimi,
Consider a part with 3 mutually perpendicular planar surfaces used as datum features, and a hole in one of the surfaces. If the hole is controlled by position with reference to A primary, B secondary, C tertiary, and A is the surface the hole was drilled in, then the 2 rotational degrees of freedom A stops are non-redundant as it sets the orientation of the tolerance zone. But, you can say that the translational degree of freedom normal to datum A is redundant because it doesn't affect the location control for the hole. If A is changed to be tertiary it will only constrain that redundant translation, so, the datum reference (A tertiary) is not needed. One may say that redundant degrees of freedom, or more precisely the datum references that invoke them, are still useful for simultaneous requirements - but ISO doesn't have those as a default.

pmarc, thanks for the explanation.