【转帖】surface profiles and datumn

yang686526

surface profiles and datumn
suppose you have a cone and a surface profile on the side surface relative to datum a (diameter of the base) and datum b (plane of the base). why do we need these datum? what do they mean?
thanks,
ali
in general, profile of a surface would require reference to a datum to establish proper orientation.
could you please expand a little on that? how does datum a provide orientatation control? if we know the basic location of the side surface then the profile will provide us with the tolerance for the side surface in the direction normal to the basic surface, then why do we need reference to datum a?
thanks.
my first thought was to insure that the surface of the cone is centered on the base circle (within the profile tolerance), as opposed as being offset.
but thinking on it more, if the side angle and height dimensions are basic, there should be no need to refer back to the base circle, as it will be a result of the other dimensions.
what dimensions are defining the cone?  are they basic?
taking this out of the gd & t spec: a profile is the outline of an object in a given plane (2d figure).  profiles are formed by projecting a 3d figure onto a plane or by taking a cross section through the figure.  
but in your case profile of a conical shape - the spec guides us by saying dimension the base diameter, make the angle basic and apply a profile to the concial surface
yes they are basic.
if the base dimension is not basic and the profile tolerance is smaller that the tolerance on the base, then i would think that the base should be a datum and the profile should refer to it.  otherwise, the top of the cone could be offset more than intended.
if the base diameter is basic, i see no good reason to label it as a datum.
if you have dimensions for the angle, height and base diameter, the part is over-dimensioned.
if you are using surface profile with regard to a model, all you need is the tolerance of the profile and the datums.
you need to datums to orientate the part.
does anybody know if there is a way to put an illustration (a simple graph) for the topics discussed on this forum to make discussions more clear?