huangyhg

setting up datums on a part shaped as a parallelogram
i have a part that is shaped as a parallelogram. clearly it makes sense to have the flat surface (plan view of parallelogram) to be datum a. one edge is datum b.
can i make a 3rd edge (which is not perpendicular to datum b due to parallelogram) the tertiary?

yep! datum planes (theoretical) are always mutually perpendicular, but datum features (real) may not always be mutually perpendicular.
see asme y14.5m-1994, paragraph 4.4.1.1, "parts with inclined datum features" for instruction on how it's done.
gdt_guy
use a datum target to specify where contact for tertiary datum should be. i highly recommend you do not rely on corner contact.
gdtguy nailed it. most people fail to distinguish between datum feature & datum ... the first is the physical feature on the part, whereas the second is a theoretical (perfect & infinite) plane generated by the datum simulator.
there's nothing wrong with using the entire third surface as the tertiary datum feature, depending on the functionality. the corner of the prallelogram will make contact with the tertiary datum simulator when the surface is inclined at an angle less than specified, otherwise it will make contact with some other point on the surface.
using a datum target makes it more sensitive to actual surface irregularities (less repeatable) than using the first point of contact with the datum simulator unless you use a movable datum target ... a small deviation in the angle of the actual surface can make a huge difference in the actual contact point on that surface (particularly if it's a datum target point).
jim sykes, p.eng, gdtp-s

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]