【转帖】points to define a datum

yang686526

points to define a datum
i know that we need two point to define a line, so two points can be used to define a datum that is a line. i also know that we need three point to define a surface, therefore, three points can be used to define a surface that is a datum. but how about cylinders or spheres? are three points still enough?
i see that my question is misleading, to use a cylinder as a datum feature results in a datum that is a line, but supose we have a surface on the part which is part of the cylinder and it makes a lot of sense, from assembly stand point, to use this surface as a feature. how do we do that?
ali,
if i understand your problem correctly, the surface could be used s a secondary or tertiary datum feature along with the clyindriical surface.
no, we have a surface which is cylindrical (part of the cylinder) and we want to use this surface as a (pimary) datum.
the surface of the cylinder would be the datum feature. the centerline would not be a datum, but the axis of the datum feature.
a three point datum is a plane defined by the points.
ewh, i am not sure if i understand what you said. i agree that the cylindrical surface (part of the cylinder) is the datum feature. that will probably make the axis of cylinder as the datum (so in would be a datum). but my question is how many points do i need to define this cylindrical surface? think of it as datum target and how we use three point to define a plane, what happens if we have a cylindrical surface rather than a plane?
you cannot define a datum as a centerline.  the feature is the datum, in this case the cylinder.  the centerline is the axis of the datum.
i guess i don't quite understand the question.  the cylinder would be defined by two points to establish the axis and length, and one point to establish the radius.
ewh, i think the correct vocabulary is that the cylinder is the datum feature and it's axis is the datum. the datum feature is different than datum. i cannot use three point to define a surface as you mentioned. because the datum feature, that is the cylindrical surface would not seat on this three points as planes do on three point or line on two points. my personal thought is that i need four points to define this cylindrical datum feature. i need to think about it a little bit more to be able to explain it right, mean while please let me know what you think. thanks
per amse y14.5-1994 "the datum feature symbol identifies physical features and shall not be applied to center lines, center planes. or axes..." except in the situation of equalizing datums or those of a complex or irregular surface.  i realize that it is semantics, but the center line is not a datum (at least it can not be defined as such on a drawing).
i still don't know enough about your situation to be of much help, but will await any further clarification.
ali,
what you seem to have stated does not seem to clarify.  are you wanting to make an element of the cylinder a datum feature?  you have referred to a surface of the cylinder, or are you referring to the ends of the clyinder?