【转帖】positional tolerancing of symmetrical features

yang686526

positional tolerancing of symmetrical features
folks-
i do a lot of sheetmetal design for electronics and often have a need to apply tolerances to symmetrical features.  one such example is the panel cutout for a 25-pin "d" connector.  i've seen examples where slots are controlled with a bilateral geometric positional tolerance feature control frame applied to the length dimension (tangent-tangent) and where a second bilateral fcf is applied to the width dimension.  but what about the panel cutout for the "d" connector?  we'd like to functionally link the two mounting holes with the cutout.  can a central width datum be created by the tangent-to-tangent dimension of the cutout?  technically speaking, the two tangents are not parallel opposing surfaces which seems to be a requirement for a width datum.  tia for your help!
tunalover
yes it could be.  however i would tend to question the need for positional tolerancing of a d hole on a panel.
ringman-
thanks for the input but i wasn't talking about a d-hole.  i was talking about a panel detail for a d connector.  these are two distinctly different things.
   
tunalover
i think ringman meant that. i also agree with him.
chris
sr. mechanical designer, cad
solidworks 05 sp2.0 / pdmworks 05
ringman and ctopher,
   a d hole consists of the d shaped hole, plus two more holes for the jack screws.  
   when i do a d hole, i dimension to one of the jackscrew holes on the main view and i do a detail.  the detail shows the d hole and the jackscrews dimensioned symmetrically.  i just use the +/- tolerances, since these are adequate, and since asme y14.5m-1994 states that if the holes are shown symmetric, they are symmetric.
   the big hole does not have to be accurate.  since the jackscrew is two screws, a +/-.005" tolerance is adequate assuming a .125" hole and 4-40unc jackscrews (.112"dia).
                        jhg