【转帖】help in defining a surface

yang686526

help in defining a surface
i am designing a cylindrical fixture with a flat cut into part of it. there is a hole sunk into this flat (not a through hole).
i have the cylindrical diameter as datum a and the bottom flat as datum b. the bottom of the hole is the other important surface in this fixture, but i am confused as to how to describe it.
i thought to use a basic dimension from the centerline to the bottom of the hole, and call a profile tolerance on the surface at the bottom of the hole. however, i wasn't sure if a planar feature could be dimensioned from an axis in this manner. all points at the bottom of the hole are not equidistant from the axis.
instead, i'm currently calling perpendicularity to datum b with a +/- size dimension ... though if i hold that dimension tight enough, does perpendicularity get me anything?
thanks for the help yet again,
matt
eng-tips forums is member supported.
if you read the drawing literally as shown, the .3250 +/-.0005 dimension comes from an axis at the center of the part, but it doesn't really say it's the axis from datum feature a.  additionally, the perpendicularity reference back to datum b is not appropriate since it is a secondary datum feature and restricts only a single degree of freedom in the datum reference frame.  the tolerance application applied to datum feature c needs to relate to primary datum a and secondary datum b.
based on the numbers used in the example, if you changed the .3250 dimension to a basic dimension and changed the perpendicularity tolerance to a profile of .001 in relation to primary datum a (rfs) and secondary datum b, the end result will be a tolerance zone that both locates and orients the datum c surface to the first two datums within a tolerance zone of .001.
i think that this callout would meet your design intent and clearly convey to downstream functions on how the requirement needs to be applied.
gdt_guy
something i'm not clear on, is there a small hole through the c'bore? is this for a screw/bolt or is the c'bore part of a feature that sits on the external flat surface?
chris
solidworks 06 5.1/pdmworks 06
autocad 06
chris, the answer is neither. the "hole" is really an external feature that sits on the external flat surface. sorry for the confusion due to the bad angle shown there. i removed the other views and any identifying information. i should have removed that feature as well, since it's not pertinent to this discussion.
i think gdt_guy's method is what i was trying to convey.
-matt
thanks. it's hard for me to give answers unless i understand the design intent. it makes a diff where the gd&t stuff goes.
gdtguy has a good answer.
chris
solidworks 06 5.1/pdmworks 06
autocad 06