【转帖】pattern locating tolerance zone shape

yang686526

pattern locating tolerance zone shape
has any consideration been given (by forum readers)to
applyihg a rectangular or square tolerance zone to the pltz when using composiite tolerancing on  a part which is essentially rectangular or straight edges as opposed to a diametral configuration?
if it were used,and the size were equal to the dia of the circle commonly prescribed, the resultant would provide a 'bonus tolerance' and additionally a more precise control wrt the edge boundaries of the part itsself.
i was advised by an authorative person that it is permissible, but no examples in the standard.  i don't quite understand that one.  
fig 5-16 of the standard is an example of where it would be beneficial, and appropriate, imho.
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ringman,
   are you talking about replacing the diamter symbol in the positional tolerance with a square or rectangle?  i would assume this is legal, but i do not know why you would do it.
   most tolerance zones have nothing to do with the hole pattern.  their requirement is that a screw or pin pass through them.  the tolerance zone that provides a maximum allowance and still works, is circular.
   the pitch circle on figure 5-16 guarantees 5.9dia clearance for screws or pins located exactly at nominal position.  the rectangular pattern allows 7.5dia clearance.
                       jhg
sure it is legal. just do not place anything inside the fcf in the tolerance section where the diametrical tolerance zone was previously placed. it turns into an x & y tolerance.
now, i don't know why one would use this on round holes though. if the features of size were not round, then i would use it but not if they were round.
dave d.
if your goal is to control the location of the tolerance zone more tightly in one axis than another, you can use single segment positional tolerancing. is that what you're trying to achieve or is it really a square tolerance zone?
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i agree with powerhound; you could use two single-segment positional tolerances if the two segments referenced different datums. this could create a square or rectangular tolerance zone. neither segment would have a diameter symbol.
if the datum references need to be the same for both directions, i believe you'd need to use bidirectional positional tolerancing. i don't have the standard right in front of me, but i recall seeing several examples of this (it should be in chapter 5). two seperate feature control frames are used, each associated with its own set of dimension and extension lines. neither fcf uses a diameter symbol; both fcfs can use the same datum references. square, rectangular, and "ring-shaped" tolerance zones are possible.
now a question for ringman: can you describe an application where this would be useful? i haven't been able to think of any yet.
sincerely,
josh church
gdtp s-0543
quality manager
vanderhorst brothers, inc.
a square or rectangle shaped tolerance zone is useful for round communication orifices, where the location in one axis needs to be controlled tighter to keep from breaking into a certain area or hitting another feature than the other axis.
david strole
engineering systems administrator
gdtp s-0132
the pltz that i mad mentioned is the 'footprint' for the actual hole relating pattern, frtz.  it does not directly reflect the geometry of the features being located.  
i had supposed that more on this forum would be familiar with composite tolerancing ang its applications.  i know that it was allowed and covered by the standard at one time.  why it got dropped out is still a mystery to me.
perhaps the closest example that is in the  current std is pg 105 fig 5-20.  imagine it with a square in place of the 0.8 tolerance zone.  if the square were 0.8 also this would result in a 'bonus' tolerance and at the same time take into account the more liberal allowance for the edge distance consideration on the part.
does this clarify my original question? i hope so.
i was originally going to dismiss this as complete rubbish, but then i took off the y14.5 blinders and i think i see the idea now.  if there was a round hole near the corner of a rectangular part, the diagonal direction is more forgiving with regards to edge distance conservation.  so a square zone would allow the most tolerance, for that functional requirement.  i have never seen this or thought of it before, but i don't think it's wrong.
there were a couple of other things that i'm not sure about though.  it was mentioned that the pltz doesn't directly reflect the geometry of the features being located.  this is not correct - the features each have their own tolerance zone and there is no overall pattern characteristic (like a center point) that is controlled.  the y14.5 term "pltzf" is very misleading in this respect.  the upper segment of a composite fcf locates all of the features in the pattern, it doesn't locate the "pattern" itself.  don't get me started on that one.
also, what's giving the impression that composite tolerancing got dropped out of the standard?  my copy of the '94 standard and the 200x draft include it.
  
evan janeshewski
axymetrix quality engineering inc.
starting with the last.  my statement was intended to convey that the square for composite  was allowed at one time and not that composite is dropped.  sorry about the misunderstanding there.
as to the pltz reflecting the geometry of the features being located (hole size or shape)i don' think it does.  but rather is a fence restricting the location of the pattern (2 holes, 4 holes or whatever).
perhaps there is overuse of the y14.5 blinders.   
quote:
perhaps the closest example that is in the  current std is pg 105 fig 5-20.  imagine it with a square in place of the 0.8 tolerance zone.  if the square were 0.8 also this would result in a 'bonus' tolerance and at the same time take into account the more liberal allowance for the edge distance consideration on the part.
   i use this type of composite tolerancing fairly often.  i have always used the diameter symbol on both tolerances.  this depends on what it is you are trying to do.  
   the 0.25 tolerance ensures that screws and bolts pass through clearance holes.  
   the 0.8 tolerance positions whatever is being clamped.  it allows the clamped part to be 0.4mm out of nominal position.  it allows rotation as well has a linear shift in any direction.  most lectures on gd&t point out that the round tolerance zone allows a maximum of error without losing functionality.  
   in your case, you might be willing to allow a bolted part to shift a maximum distance in both x and y, in which case, a square tolerance zone is valid.  i still have a hard time seeing why i would do this.  
                         jhg
if i understand your post correctly, you are talking about the tolerance zone being square rather than diametral, where typically the cylindrical tolerance zone is converted to a greater value to be circumscribed about the allowable square coordinate tolerance zone.
however, you are proposing if the diametral value is to remain the same and the diameter symbol is dropped off in the fcf then you would have converted a typical cylindrical tolerance zone to now be inscribed in the square coordinate tolerance zone hence giving you oppourtunity for bounus wrt  the now induced corners.  this tolerance zone oriented to wrt the datum feature planes which i presume are square/rectangular in nature.
i suppose that there is some potential here for functional value,for orientation purposes or for even maintaining a minimum material thickness to the edge, of course use of lmc would be in order then.  in general, i would say that there is also a definite communication risk here as well.  i would venture to guess that most would think that the diameter symbol was left off by accident or is a typo.  they would most likely treat it as cylindrical anyways without clarifying.
there are multiple ways to achieve this and thus reducing the risk.  there have been multiple replies to the op with suggestions of alternative methods to achieve close to what you are proposing in regards to orientation, however without the potential bonus and minimum material thickness being considered.  

in any case, if you truly want to accomplish this requirement one way is to include a note stating a square coordinate tolerance zone to be used on the upper segment as is allowed per para 3.2 to avoid any mis-interpretation.   
also, i agree with axym and do not understand your comment on composite being dropped.
i hope i have interpreted you correctly and that this has been helpful to you and the others in some way.