Help with datum

Attaching GD&T datum flags to planes and axes is not realy valid. Datum flags should be attached to faces and edges.

As "70's" mentioned - a datum 'flag' is really a datum feature symbol that specifies the FEATURE(s) of a part from which functional relationships are established. A better question for me to ask, is what kind of geometrical tolerance are you trying to establish on your roll of TP? Runout? concentricity?

Scott

BTW, to answer your question on how to do/fudge it even though it's not really correct - Is that while a datum feature symbol won't work with a center mark, it will attach itself to a centerline. So in place of a centermark, if you create two centerlines, constrain their mid-points to the origin (or appropriate planes), to look like a centermark and then you can attach a datum symbol to it.....

Scott

I am not terribly familiar with GD&T, so I was copying from the customer's origninal ACAD drawing. It had -A- and -B- datum marked as the front and top planes (when looking from the side). The GD&T frame called out the position and diameter as referenced from the centerline intersection of those two planes. After reviewing my tech drawing book it looks like this is not a valid method of applying GD&T. What they probably should have done was mark the inner diameter as a datum and refer to it when dimensioning/ tolerancing the outer diameter. I'll have to pick up a book on GD&T and brush up on it.

Gotcha - erg! Your right - that's a horrible use of GD&T - It's a great tool when used right, nasty when used wrong - It depends on the final application on whether the ID or OD is used as the datum, (again depending on the application....eg: If it's rotating (ie a shaft) runout or concentricity should be used on the other diameter to control it in regards to the datum)....Anyway, a great reference GD&T book that I keep around is the workbook by Al Neumann...If your interested -