【转帖】how best to refine a hole position tolerance

yang686526

how best to refine a hole position tolerance
hopeflly someone can help, because i'm stumped.  i want to allow s hole's location to be held within a generous tolerance, but need it's orientation to be held to a tighter tolerance.  i'm typically used to seeing this with a pattern of holes where the top fcf controls the position and the lower fcf is held to a tighter position tolerance relative to the face in which the hole is drilled, effectively giving a perpendicularity refinement.
however, in this case, the primary datum is a perpendicular face and the secondary and tertiary datums are holes in the primary datum face.  i suppose i could use a parallelism refinement to a and b-c, but that give a square tolerance zone.  i'd like to use a composite tolerance, holding both upper and lower frames to the same a, b,& c datums like i would for a pattern of holes, except in this case there is only one hole in the pattern.  for a typical pattern, the upper frame would control location and orientation to a,b,& c and the bottom would control orientation to a,b,& c. the following picture should help illustrate what i'm talking about.  any ideas?
here the picture:
zubblwump,
   i think you need to replace one hole datum with an edge.  this gives you something to place a parallel or perpendicular specification to.  
                        jhg
if you do as drawoh recommends but make both edges datums then you can use a composite tolerance fcf and get what i think you're after. leave datum a as it is, make your longer edge datum b and the shorter edge datum c.
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thanks for the advice, but the problem i have with this is the edges have nothing to do with the functionalality of the part.  this is a cut and weld part, and the edges do not need to be held very accurately, so i don't want to relate this functionally critical hole to the edges, only to the functional mounting face and mounting holes.  the use of a, b & c fully contrains the parts movement, so i'd like to use these if possible.  would using a and the compound datum b-c be better?  that would at least give two perpendicular planes.  can someone please explain why a,b,& c won't work in a composite fcf?  in the attached picture, option a is the one i've been floating around here, and option b is one i don't like as much, but may also work. (would a composite positional tolerance to a and b-c work?)

leave your datum the way they are and use a positional tolerance of .01 mmc to a, b mmc and c mmc. positional tolerance defines the entire axis of the hole not just a center point so it would be redundant to use both positional tolerances.
think of it this way, if you were to check the part using a fixture that located the part using a, b and c and checks the hole location with a .06 positional tolerance. then you use a seperate pattern check fixture to check the part using the exact same datums and checks the exact same hole but with a tighter tolerance, would be crazy because if the part passes using the tighter tolerance it's going to pass the larger tolerance.
typically the upper feature control frame is used to locate two holes in a pattern of holes to your primary datum. then the lower control frame is used to control the position of additional holes in the pattern to your original two holes which are labeled as secondary datum along with a face plane. it is done this way because the location of all the holes to the primary datum is not critical but the location of the pattern of holes to themselves is.
the link below might be helpful for you. look at the types, positional tolerance is a location type of tolerance not orientation.
zubblwump,
   you could always use the .25 hole as datum_b and one of the flange holes as datum_c.  
   another possibilty would be to apply a +/- tolerance from the 0.25 hole and a +/- tolerance between the two flange holes.  apply a sloppy gd&t location tolerance with respect to datum_a only.  
   if the rotation has to be good and everything else sloppy, you could slot one of the flange holes.  one of the benefits of producing properly toleranced drawing is that you spot design issues.
                    jhg