【转帖】datum call-out on tangency of two radii

yang686526

datum call-out on tangency of two radii...
imagine a number eight lying sideways.  i want to establish the two points of contact as the datum.  i have a part that has this contour on one side and i would like to establish those two points of contact as the horizontal axis.
how or what is the best way to establish this datum?
thanks,
  
asme y14.5m-94 page 78 reflects the use of datum targets to create the datum when when one does not a datum feature but point(s) on a feature. this does not happen too often but sometimes the points are more appropriate such as in your situation.
make sure that the datum targets noted can be located or set up by production and quality. the example in the asme standard reflects a 45 degree v or linear distance between the points that can be located on coordinate measuring equipment.
hope this helps.
ded
how do you know where the two points of contacts are? do you lay the part on a flat surface to determine the two point of contact with the plane?