【转帖】positioning irregular shaped feature

yang686526

positioning irregular shaped feature
i have an irregular shaped hole that is basically located and sized with tolerances (similar to the shape of this "0" if the left and right sides were flattened). i want to say it can be positioned with a dia. tolerance of .001 at mmc, but my book says i can only apply a positonal tolerance with cylinder, width or spherical features. a profile tolerance doesn't make sense because as the hole departs from mmc, it would break the tolerance zone.
i think i could use a positional tolerance attached to the height and one attached to the width, both with the note "boundary", but if they're the same tolerance it seems there's a better way.
any help?
do you mean you have essentially have a round end slot?
i think you are on the right lines, look at fig 5-47 in asme y14.5m-1994 and paragraph 5.10.
5.10 explicitly says "the diameter symbol is omitted from the feature control frame."
5.10.1c says"...where the same positional tolerance can be allowed for both {refering to length & width} only one fcf is necessary, directed to the feature by a leader and separate from the size dimensions.
if you need a sample diagram post again and i'll try and find time a lunch.
  
kenat,
why wait till lunch when this is more interesting that checking a half a$$ed assy drawing.
link is to my understanding of what the standard means at 5.10.1c .
(assume asme y14.5m-1994 1.1.4 regarding figures is invoked)
kenat,
ok, i missed of boundary below the fcf but hopefully you get the point.
kenat,
thanks, that's what i needed. i had actually shown it like that, but i just wasn't sure.
looks good, kenat.  my preference is to use a composite profile of a surface to control the size of the feature (if possible) and then the location.  that way it controlls the length, width & radii too.  i find too many (engineers, machinists & inspectors) don't understand the use of boundary.
jim sykes, p.eng, gdtp-s
mechnorth, we've had issues with them not understanding gd&t full stop.  i believe they'd be more confused by composite profile of a surface than by 'boundary', but i've been wrong before.
kenat,
quite understand about working with people with limited understanding of gd&t.  when i train, i emphasize surface profile & composite surface profile.  i've had 2 students now that were using boundary without understanding what it meant (reference prints had it, and they were told to just use it), and they confirmed that they and the shop were both understanding it to mean the center plane in both directions.  afterwards, one converted to composite profile and the other stayed with boundary, though he at least understood it now and was ready to use it appropriately.
one of the most interesting things about gd&t per y14.5 is that it often gives multiple ways to achieve essentially the same effect.  of course that's good and bad.
jim sykes, p.eng, gdtp-s
can you explain how a composite profile of surface works? i don't quite understand how to use it to control the size, orientation and location.
can size tolerances simply be converted to a profile tolerance (size +-.005 = profile tol. of .01)?
is the size controlled in the upper frame or lower? my understanding is the lower frame does not control location...correct?
i also thought a composite profile of surface was only used with patterned features.

simplisticaly (and i hope i get this right) the top line of the fcf controls gross location of the feature wrt the datums.  the bottom line is a refiniment in size, form & orientation.
look at 6.5.9 in asme y14.5m-1994 page 172 & related figures.
also +-.005 translates to an equalateral surface profile of .01, more or less.
composite profile of surface is not only used with patterns, in fact the example in the standard is a single feature.
kenat,
at the moment, composite position tolerances are exclusively used with patterns (don't know if anything is changing in 2009).  is that where the confusion comes in, jlang?  
at first i couldn't recall using a profile control on a pattern of features, but on reflection i have used a note underneath the fcf to the effect of "3 locations".  the composite profile control only controls individual features though, and does not control the inter-feature relationship.  the features are part of a group as established by the principle of simultaneous requirements and the note "3 locations".
jim sykes, p.eng, gdtp-s