【转帖】implied centerlinessymmetry

yang686526

implied centerlines/symmetry
this topic has come up before most recently in
oh no.  don't get me started on this!
try section 4.5.3 (a) through (d) and see if that helps.  one quote is "the simulated datum is the center plane of the true geometric counterpart of the datum feature".
i'm not sure if this is the issue you're referring to or not.  you might be referring to an "implied basic zero", in which features that look like they're centered or symmetric are assumed to be nominally centered or symmetric unless there are dimensions showing otherwise.  it's along the same lines as implied 90 degree angles.  i don't know of any specific reference to implied basic zero in the standard though.
evan janeshewski
axymetrix quality engineering inc.
whee!
again, i got bruised when implied symmetry was brought up in different context in an earlier post/question.  my interpretation these days is that if you "imply" things that are important, you guarantee they will be missed.
since you can't "dual-dimension" (i.e. create ambiguity) when using basic dimensions (unless you screw up and have the numbers not add up), go ahead and add the basic dimensions that clarify the location of the centerlines.
kenat,
   we are moving into a gray area where we have to determine how obvious it is that the part is symmetrical.  a rectangular plate with four holes in it can obviously be symmetrical.  if we add one more hole that is not symmetrical, then i would be suspicous of the symmetry.
   as a checker, you ought to be able to criticize drawings for format and clarity, regardless of what the rules say.  this is no different than people who place the diameter of a hole in one view, and the depth in a section view on the next page.   
               jhg
drawoh - you place more esteem in my position as 'checker' than my colleagues or management do (my directs ok but the rest wouldnt' know a good drawing if it jumped up &  smothered their face).
if it's clear it's symmetrical, and is using appropriate gd&t then to me it's often clearer, or at least cleaner without a 'centering' dimension.  this is shown in the standard in a few places but i was hoping there was something a bit more concrete.
kenat,
page 107 'appendix a' has a little information:
were you thinking section a or c chris?
kenat,
section a
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
unfortunately y14.5m-1994 doesn't explicitly describe the implied basic zero condition.  it does show the idea in several graphics (fig. 5-4 for example), however it's not immediately clear that this absence of a centering dimension isn't an "omission for clarity".  also, it has been an accepted drafting technique pretty much forever, however common practice doesn't typically stand up in court.  
apparently they have rectified this shortcoming in the '09 release, section 1.4(k).
jim sykes, p.eng, gdtp-s
this is a gray area.  i think that if it isn't stated in the standard or stated on the drawing, then the drawing is incomplete.  in the example from that previous posting, the standard is specific to that particular case for the tooling holes, but the 2nd pattern that is driving from its datum may need at least a reference.  i think at least a reference is enough so that it is no longer implied.  but as you suggested, it should be ok to make the dim to the centerplane a basic.
matt lorono
cad engineer/ecn analyst
silicon valley, ca