Primary datum feature oriented to what?

[thedesigner75]

Hello All,

i can not get this concept: if i have a not so rigid part like an injected molded part...this part has a feature, sort of planar small surface, that is the datum feature that represents the primary datum so how can i qualify this primary datum in term of orientation? i can put a profile tolerance to check that the surface is not so rough, for flatness checking, and this is clear to me, but if this surface is bent? I need to put some orientation tolerance but to what datum as that is the primary datum ? Thank you for your time

Ciao

 

[greenimi]

...how can i qualify this primary datum in term of orientation?

Orientation to what? If this datum feature is primary (and it is acting like primary in the assembly) then other features are to be oriented and located to it. Not the other way around.

 

[thedesigner75]

Thank you Greenmini for your prompt replay!
I understand that…but this means that when i put the part on the primary datum feature simulator of the gage i will have the datum feature (bent) aligned to the datum and all the part out of tolerance….is there any way to check the orientation of a primary datum feature?

 

[greenimi]

looks like your primary datum feature is not acting like primary. So either chose another datum that is capable of stopping the degrees of freedom either use maybe datum targets points or lines for this task.
And you primary datum feature should be qualified for flatness or datumless profile (if multiple features are used)
Or maybe I am not fully understanding your issue.
This sentance is still not very clear to me".....i put the part on the primary datum feature simulator of the gage i will have the datum feature (bent) aligned to the datum and all the part out of tolerance...."

 

[thedesigner75]

Maybe also my english is not so good sorry for that…i need to sketch up something to be better understood…normally working with flexible plastic parts we have complex parts and small features (like the bad picture i attached above) working as multiple datum features, and these multiple datum features are normally small flat surfaces not coplanar and most of the time not even parallel one to other…so i’m little bit confused how to control them…

 

[greenimi]

If that's the case maybe multiple datum features should be used as a primary datum feature. Maybe defined with profile and F symbol (free state variation) could be used.
Fig 7-49 /ASME Y14.5-2018 could be used as reference.

 

[thedesigner75]

Sorry for long delay in the answer but it is busy time with kick-off of the tooling...
Continuing the discussion: yes we normally have multiple datum features as primary datum and all these datum features have sort of Destaco clamping features for restraining the parts...so if i put the profile tolerance with F modifier they have to measure those surfaces opening the clamping features but leaving the part on the gage (free state means not restrained but we need always to remove degree of freedom), is it correct? Thank you for your time!