【转帖】how to locate a part relative to the center of another par

yang686526

how to locate a part relative to the "center" of another par
i have a requirement from our engineers that a vertical rail, in an assembled machine frame, be located a certain distance from the center or mid-point of a horizontal rail regardless of the horizontal rail's true manufactured length.
how would you dimension/tolerance this per asme y14.5m-1994? i may be overlooking something simple but it is friday and i can't seem to wrap my mind around this today.
thanks for your help.   
david
eng-tips forums is member supported.
off the top of my head (after a huge friday lunch) i'd add a datum to the length dimension of the horizontal rail in line with the dimension callout.  control the vertical rail relative to that datum with a true position callout.
i'm sure the others here will let us know if this is a good approach.
believe it if you need it or leave it if you dare.fff"> - robert hunter

ewhfff">'s approach seems technically correct but is likely going to confuse the under-schooled.
yes, i realize that... it was the first thing to come to mind, and would effectively control coaxiality through the size tolerance, so i went with it.
what would you suggest, tick?
believe it if you need it or leave it if you dare.fff"> - robert hunter

i got nothing better.  i suggest schooling for the confused.
i meant position, not coaxiality... that food is making me groggy.
believe it if you need it or leave it if you dare.fff"> - robert hunter

ewh,
would i then place a centerline on the horizontal rail (datum a) and dimension from that? i think it would be because you have defined the centerplane by making that surface datum a.
thanks ewh and the tick for the help. if this doesn't seem kosher to you please let us know.
david
see section 3.3.2b and figure 3-4c for my reasoning.
david
i think that would be correct, using a basic dimension.  you may want to label the centerline as "cl datum a" or some such.
believe it if you need it or leave it if you dare.fff"> - robert hunter

assuming that the horizontal rail is longer than it is cross sectionally wide... i would make the functional surface(s) that establish horizontal - primary, then i would do as ewh suggested and make the size of the overall length (or its functional hard points) secondary.
i am just commenting that the overall length probably will not serve as a stable primary datum feature if i am envisioning your frame correctly.
paul   
paul,
assume that the rest of the frame has been controlled and that this vertical rail is being added to a fully dimensioned frame but needs to be located from the true centerplane of a horizontal rail. the horizontal rail will become a quaternary(?) datum for the sole purpose of locating the vertical rail.
david