【转帖】unequal bilateral positioning

yang686526

unequal bilateral positioning
i have a hole that is called out with vertical positioning +-.005 and horizontal positioning +.006,-.001.  is there a way to use a positional tolerance (or any gd&t method) to allow more movement in one horizontal direction than the other? or does the hole need to be centered in the tolerance zone?
simply put, the hole can move left .001 and right .006. how do i show that with gd&t?
maybe bilateral wasn't the right word in the subject, bidirectional maybe.
jlang,
what version of the standard is being applied to your documentation?  what is the purpose of the hole and what is the interface?  it might be a case where positional tolerancing is of little or no benefit.
you know, a better question would be, does y14.5-1994 allow me to leave it as a plus/minus dimension without applying an fcf?
i'd look myself, but i don't have a copy because my company decided to wait a little bit and get the 2009 version.
there is no requirement in the standard that mandates the use of gd&t.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

quote (asme y14.5m-1994 ):
2.1.1.1 positional tolerancing method.  fff">preferably, tolerances on dimensions that locate features of size are specified by the positional tolerancing method described in section 5.  in certain cases, such as locating irregular-shaped features, the profile tolerancing method described in section 6 may be used.
some might interpret the above as a mandating use of either position or profile but many would argue otherwise.
was the +.006 -.001 calculated from first priniciples/function or are you just trying to translate an existing +- tol of unknown validity into gd&t?
kenat,
in terms of your actual initial question, i wonder if you could use position on the whole without the dia symbol.  essentially apply fcf like you would for a slot, one in 'x' and a separate one in 'y'.  i believe you'd have to nominally center the hole in both axis rather it being off set, however from a pass fail criteria this makes no difference.  
from a design intent point of view some might argue the unequal +- better captures intent, implying the hole is preferably in the left of the tol zone.  however, inspection wise, and hence presumably function wise the scheme would be equivalent except with position you could take advantage of the mmc principle to gain some tolerance.
kenat,
to me, this is like the "typ" issue; there is a preferred method, but the preferred method is not mandated as a requirement.  the give-away is the use of "preferably" in that quote.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

in response more directly to the initial question.  the hole does not have to be 'centered in the tolerance zone'.
it may fall anywhere within the zone after it has been properly defined, i do believe.
sorry to keep vering off topic, but i so rarely disagree with kenat, that i feel i have to explain myself further...
positional tolerancing is not a requirement of asme y14.5-1994.  there are requirements pertaining to the use of positional tolerances, but direct tolerancing is allowed.
quote:
2.1.1(a) as direct limits or as tolerance values applied directly to a dimension.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

ewh, while i do still use +- sometimes, i believe the section could be interpreted as the 'preferably' meaning to preferably use positional instead of profile but that one of those 2 should be used.
kenat,