【转帖】olerance of positoin - feature positioned to itself

yang686526

tolerance of positoin - feature positioned to itself ???
please review attached drawing for comment on the tolerance of position callouts circled.
1. i do not understand how a fcf can reference the feature positioned to itself in a datum reference block. (e.g. datum a is positioned with reference to itself in a-b)
2.  i believe the location tolerance and the basic dimension  relavent to it should have the same number of decimal places. (not shown on the drawing) (e.g. if tolerance in fcf is .xx then its basic dimensions should be xxx.xx not xxx) is this in the standard?
designbiz
"quality is in the details"

in your example, the drawing is establishing an a-b axis through the center of the tube, using batums a and b as the datum features for establishing the axis. you're not calling the datum to itself, as much as you are making it part of the a-b axis.
looking at the bends of this part, i would not use this axial dimensioning method. i would use profile-of-a-surface instead, to datums a and b, using the same a and b datum features.
regarding the number of places, since this is metric and unilateral dimensioning, it is iaw paragragh 2.3.1(a) of  y14.5-1994.
1. i've never been a fan of fcf's that reference a datum feature that includes the considered feature.  this method is not described in y14.5.  i have seen it in gd&t textbooks, but only for coaxial features and with mmc references on everything.
the presumed intent of the positional tolerances on features a and b is to control their locations and orientations "relative to each other" without making either one the datum feature.  which is fine.  this will allow them to function well together as multiple datum feature a-b.
to me, controlling a and b relative to each other doesn't require referencing the a-b datum feature.  if the datum reference were left off, then the rule of simultaneous requirements would tie a and b to each other.
this is all i have time for today.  more tomorrow.
2. this one is in the standard.  the same number of decimal places are not required for millimeter tolerances, only for inch tolerances.  see 2.3.1 (d) and 2.3.2 (d).
evan janeshewski
axymetrix quality engineering inc.
an example of this situation is on page 193 of the standard in fig. 6-51 with the exception that circular and total runout are used rather than positional. we have the example but no confirming verbiage.
unfortunately, this is one of the areas in the standard that just does not make any sense. how could one reference a datum that is not already established??
using the example that i mentioned, one could extrapolate that positional could also be used as shown in your example. i don't like it personally but it does look somewhat legal.
  
dave d.
dingy2,
   the example on fig 6-51 makes sense to me, and i can see how i would fixture it.  in the absence of a long, round feature, two short, concentric, widely separated features make a good centre axis.  i have done this on complex shafts.
   the op's drawing makes no sense to me.  i cannot see how i would fixture and inspect it.  if only one end had a positional tolerance, it would start to work for me.
   i observe that datums a and b locate the part completely.  i do not see a purpose for datum c, unless it controls the part's flexibility somehow.  i would call up datums a and b separately in the fcf, and define the datum b feature wrt datum a.
               jhg
i do appreciate the direction to the section for the decimal place question and to pg 193 by dingy2.
i would like to make clear this is not my idea of dimensioning an exhaust tube using the standard.
it is the companies standard template for dimensioning tubes.  i dont like it at all personally.
like dingy2 i had the same question. how could one reference a datum that is not already established?
seems like a paradox to me.
i can appreciate the logic ron and evan apply to make sense of the callout, however this is not a rotating
shaft that would make sense to create a single datum using both end diameters or 2 bearing diameters.  this is
an exhaust tube and could be done more more simply uses 3 datum references in a more conventional drf.
thanks for all of the commentaries !
  
designbiz
"quality is in the details"

have you checked out
ewh,
the thread you refer to is actually one i started. it is even more bizarre to me when the 2 features that are used to create a single datum are at compound angles. our tube ends are fairly often at compound angles to each other.
we beat that horse, til about dead.
thanks for your input.   
designbiz
"quality is in the details"

sorry about that, didn't check the details.  i just knew that a similar issue had been addressed.
yes, that one was beat to death.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

quite honestly, i don't see a problem with the op drawing.  per the other thread, once you've got your datums established, you can locate the position of any feature of size.  essentially, you know where in space the feature of size is supposed to be, and you just verify it as you would normally.  if the original question was really how do you verify position of datum-a back to itself, you aren't.  you are verifying the position of datum-feature-a to datum a-b.  two entirely separate things.  review the definitions of datum and datum feature.
jim sykes, p.eng, gdtp-s
well said! jim,