yang686526

can a cylindrical feature with draft be a datum?
have a cylindrical cast feature with draft that i want to use as a primary datum axis.
my question is how to determine the axis..equalizing datum points at a basic location along the feature?
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a "cylindrical feature with draft" is a cone. a cone has an axis, and thus can be used as a datum. your datum flag still should be attached to the feature face.
the cone can be use as a datum but at least 2 cross sections will have to defined down the length of the cone to create the cones axis.
ja500,
it is more complicated than that, especially if the intended tolerances are tight compared to the accuracy of the casting.
fixturing can be easy if don0000 specifies the cone datum at mmc. the part is allowed to wobble a bit if the cone is undersize. a cone can be off dimension, out of round and off-angle.
fixturing can also be easy if don0000 calls up three datum targets on the cone. that way, you, me, and thetick can each build our own inspection fixture and attach to the same points.
i would have to think hard about two cross sections. if the inspector is allowed to pick them, the fixturing is ambiguous. you can call a round section up as a datum target, but you are assuming that the section is round. on a casting, it might not be.
jhg
yes with the datum symbol not attached to the surface though. there may be confusion over whether the axis is the datum or the bottom of the surface.
there is usually a draft allowance in the notes and the size on the drawing showing the feature. i would suggest that the diameter dimension for the size should have the datum symbol on the opposite side of the arrow heads from the dimension. now we truly know that it is the axis.
dave d.