【转帖】profile of a surface vs irregular shape as a datum

yang686526

profile of a surface vs irregular shape as a datum
i wonder if it is possible to refer profile of a surface to a datum which is an irregular shape. please take a look on attached example. let say that drawing is acc. to asme (but in this case i'm not sure that there is a difference between iso and asme).
many thanks for any remarks.
michal 77
i'm not the expert here but i'd say no.  what is the functional relationship between the datum and the feature? it would probably make more sense if the radius was the datum.  curious what other may have to say.
it is allowed, provided that the irregular surface datum is itself related back to the three primary datum.  i don't think you can use an irregular surface as a primary datum.  see asme y14.5m-1994 ? 4.5.10.1 and ? 4.6.7.
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

even if you could, good luck measuring that.
if you must use that surface, use datum targets to define datum "a".
that would only define datum a as a plane, not the irregular surface itself.
read the standard; it is technically allowable to use an irregular surface as a datum, though i don't think it is very applicable to this situation, and qa is another matter entirely.   
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

you can only do this provided you use datum targets. see 4.6.7.
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not necessarily.  you can use other types of datums as your three primary. ? 4.6.7 refers to establishing a datum from a complex surface, but ? 4.5.10.1 is more to the point of the op as i understand it, using an irregular surface (not a plane derived from three points on the surface) as a datum.   
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

maybe this is a stupid question, but here goes...
what in the world would possess someone to use that as a datum?
  
v
in our case, we make lofted composite parts, and datums are required to locate other feaures, such as holes and cutouts, as well as the overall profile of the part.  we will use the lofted surface as a datum, but only when it is referenced to datum points defining the three primary datums.
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

thanks a lot for your support.
i would like to show you, why i need an irregular shape as a datum. on attached file you can see simplification of a glass shape which is completely irregular (don't take into consideration the borders, they are not important). you can measure only openings for bolts (r10). blue area (r20) it's a contact area with other stuff, so it could be used as a datum.
in my opinion there are only to ways to control the real part. first is to apply profile of a surface to the entire body without datum (then measured body will be a datum to itself). i suppose that it can be done but i'm not sure about measuring in 'real life'. second way is to measure only a red area (honestly it's only important for me) in reference to mounting areas (blue circles). in my opinion second way is much better but then i have to establish datums on irregular shape.
do you thing it's possible to establish datums around the openings in my case? and if yes, using irregular shapes or datum targets?
ps - ewh you stated: we will use the lofted surface as a datum, but only when is referenced to datum points defining the three primary datums - i assume it means for you that area of irregular shape which can be a datum, must be correctly established by basic dims in reference to other (planes, bolts etc) datums. am i right?
michal77
now should be fine....