【转帖】2 single segment fcf to make rectangular tolerance zone

yang686526

2 single segment fcf to make rectangular tolerance zone?
i have seen a number of drawings where a hole ends up with a larger tolerance east-west than north-south, so rather than expand the diametrical tolerance to cover both, i see two reference linear dimensions with separate control frames. (see top example) i was wondering if possible to combine these into two single segment control feature controlf frames on the diameter dimension. (see bottom example). is this allowed and is it clear?
to my interpretation, the first example has no interpretation possible under asme or iso.  the second example actually is a reference to a composite pos tol, which precludes its use in this manner.
to use positional tolerancing with an elliptical zone, you'll have to completely describe your pos tolerance zone within the content of the drawing.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
the top one doesn't explicitly match any example in 14.5m-1994 that i'm familiar with but i'm inclined to say the intent is clear.  you've dimensioned it like you would a slot or similar.  it's probably how i would have attempted to dimension the case you state, i may have left the dia symbol on the ref dims, not sure.
i think the second example is not composite positional tol but (incorectly applied) "multiple single-segment' fcf, or at least looks like it so is definitely wrong for your intent.  (see 5.4.1.8 of asme y14.5m-1994 for disambiguation from composite).
so in summary, i'd say your second option is wrong and unclear.
similar came up in a recent thread though i don't recall much detail being given.  there are a few very knowledgable   
the first drawing is called a "bi-directional" positional tolerances since the tolerances are not equal in both the x and y axis. this is used a lot on slots where the length of the slot usually has more tolerance than the widthand is covered by asme y14.5m-94 section 5.9 page 135.
the second example reflect 2 single segment feature control frames on the same feature. they relate different tolerances to different datums. there are two (2) requirements here.
dave d.
well done dingy,
jasonsp, figure 5-41 page 139 referenced from within the reference dave gave shows what you are trying to achieve, it doesn't have the reference dimensions shown but is otherwise similar to what you have.
kenat,
i was just looking through the book again and found fig5-41 on page 139. so it's clear that the top example is both by the book and clear. and the bottom example is just incorrect. thanks all!
my comment was presuming an elliptical zone.  sorry for the misinformation.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i believe the first example to be acceptable and proper.  however a close examination indicates that 'physical identification' of the datum features might just be in order.
it would provide maximun latitude for locating the hole while cproviding a control for the edge distance.
the top example is good but the bottom example is not. to use single segment fcf you still have to abide by the same rule as in composite, the top segment must have a larger value than the bottom. the top segment should read "position within a diameter of 0.5 to a, b, and c - the bottom segment should read "position within a diameter of 0.2 to a and b." this will give you more tolerance in one direction than the other but the full zone won't be rectangular like the top example. it will be more like a small cylindrical zone positioned within a larger cylindrical zone and the small zone can move within the larger zone from left to right only, not up and down. depending on your application, you can use the top example on your drawing, or change the bottom example to what i've recommended and you will have a legal callout.
here are 2 links on the subject that really helped me when i was struggling with the concept.
ringster,
  what do you mean? i see all the relative datums identified and the feature being controlled is shown with basic dimensions from their respective datums.
powerhound, gdtp t-0419
production manager
inventor 2009
mastercam x3
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
powerhound,
the standard contains only one sentence as i recall with regards to physical ident of datum features.  given a symmetrical part, it becomes necessary to mark the part physically by 'some means' to assure that everyone is using the same datum reference frame.
par 4.2 of the 1988  standard.
too often overlooked in my opinion.