【转帖】composite two-single feature frame position tolerance

yang686526

composite & two-single feature frame position tolerance
group,
anyone have a simple way of explaining the difference between composite & two-single feature frame position tolerance?...arrg.
i know the basics - top frame is for the pattern, bottom frame is for the feature, but not sure when to apply one rather than the other.
ref pages 130 - 133 asme y124.5m-1994
thanks in advance.
jim weed
check out our whitepaper library.
on a composite feature control frame, the bottom section relates inside the pattern and its orientation to datum a (perpendicular) as and example. if the bottom section is referenced to datums a & b and b is a side, it means perpendicular to datum a and parallel to datum b.
on a single segment feature control frame the bottom section if it related to datums a & b as above, it would mean perpendicular to datum a (same as composite) and parallel but also dimensioned from datum b.
that's it.
most often a composite is more applicable than a single segment feature control frame.
dave d.
single segment fcfs are capable of controlling all six degrees of freedom (translations in xy&z) and (pitch, yaw and roll) to the degree that the datum features are capable of constraining them.
the top segment of a composite works identical to the single segments but the bottom segment can only control orientation of the pattern (pitch, yaw and roll) to the degree that the datum features are capable of constraining them.
the pattern features themselves are always constrained for all six degrees of freedom within the pattern regardless of the datum features specified. since the lower segment is  typically a refinement of the upper the pattern itself would be constrained by that smaller tolerance.
use multiple single segment controls when both location and orientation need to be refined in the lower segment.
use composite controls when location is permitted to vary according to the upper segment but orientation needs to be refined in the lower.
all identically referenced and modified fcfs become one pattern according to the simultaneous requirements rule unless it is stated otherwise but the lower segments of composite controls are exempt from that rule.
paul   
sorry for the redundant response dave and i were typing in unison. paul
  check this tip out on the tec-ease website. it has visual aids to help those of us who are more visual learners.
thanks everyone...i always learn something from this group.  composite is what i will use...the two single fcfs almost seem redundant, but i'm sure there's a case when it applies.  
jim weed
a simple way to think of composite tolerance (not two single-segment) callouts is that the top frame pltz controls the hole pattern to the datums (often the edges of a part) and the lower frame (frtz) controls hole-to-hole (and perpendicular to a).   
also, it's interesting to note that so many people think that .014 is an approximate conversion to +/-.005 linear, but it's not quite true for hole patterns.  .014 is approximately +/-.005 at each location/hole.  so a .014/.007 composite tol would be the correct way to control a hole pattern similar to a +/-.005 from edge and hole-to-hole linear dimensioning scheme.  only using one feature control frame back to abc is equal to baseline dimensioning scheme - that's why composite exists.
a point perhaps oftimes overlooked is that he pltz is more adequately or properly stated as rectangular, rather that diametric, dependant on the boundary of the part.
jamesdweed,
not stated, but presumably you are using y14.5-1994.  if not, i think there are some other issues.
"dependant on the boundary of the part" or possibly the shape of the bolt pattern/item mounted by the bolt pattern.
i've seen nominally 'round' connectors mounted by a square hole pattern.  in this case a round pattern locating tolerance zone should probably be round, shouldn't it?
kenat, probably the least qualified checker you'll ever meet...
a good way to understand the difference between composite and two single-segment position control is to look at a pipe flange with a six hole pattern perpendicular to the face of the flange datum 鈥淎鈥?and located or oriented to a center datum axis hole, datum 鈥淏鈥?
composite position - the upper larger tolerance zone (pltzf) needs to be perpendicular to datum plane 鈥淎 鈥渁nd located from datum axis 鈥淏鈥?with basic dimensions.
two single-segment 鈥?the upper larger tolerance zone (pltzf) will be the same as composite position as stated above. the lower tolerance zones also position the holes to each other.
composite position - the lower smaller tolerance zone (frtzf) needs to be perpendicular to datum plane 鈥淎鈥?and oriented to datum axis 鈥淏鈥?br />
two single-segment 鈥?the lower smaller tolerance zone (frtzf) needs to be perpendicular to datum 鈥淎鈥?and located on datum axis 鈥淏鈥? the lower tolerance zones also position the holes to each other.
bottom line is, composite position controls 鈥淥rientation only to the datums, not location.
if the design requires the six hole pattern to be on the center of the flange hole, use two single-segment.
[color red] only using one feature control frame back to abc is equal to baseline dimensioning scheme [/ color]
namwob,
there isn鈥檛 and never was an equivalent conversion from 鈥渂aseline鈥?dimensioning to a 鈥済eometric position callout with a diameter symbol.鈥?one is restricted with baseline dimensioning to a linear tolerance in the direction of the leader line from one feature to another and with the position callout to a diameter or cylindrical tolerance zone oriented and located 鈥渁s able鈥?from the specified drf.
you are correct however that many think that there is a conversion. the whole debate 鈥淚 think鈥?was precipitated 鈥渁t the onset鈥?by those teaching gd&t that there is 63% greater area of tolerance using diameter tolerance zone rather than a square tolerance zone with round features and further confused by those seeking to reverse the modern geometric diameter tolerance zones to individual coordinate limitations for coordinate scrutiny.
thoughts aside鈥?there is an ongoing debate about whether a geometric position tolerance callout 鈥渁pplied to a round feature鈥?that is not preceded by a diameter symbol invokes a 鈥渟quare鈥?shaped tolerance zone 鈥�or鈥?whether the zone remains a diameter value simply because it is applied to a round feature. i grew weary of following the debate because of its length and diversity but i think that simple baseline dimensioning does not infer that the tolerance zone is necessarily uniform 鈥渨ith right angles鈥?simply because it is drawn so鈥�since no drf has been established. if indeed a drf is established by a general note or by a geometric callout and the appropriate 鈥渄egrees-of-freedom鈥?constrain the zone鈥檚 parameters relative to the drf then it can be determined square, rectangle, circular 鈥渋f preceded by a diameter symbol鈥? a circular segment 鈥渁s illustrated in figure 5-42鈥? conical 鈥渁s illustrated in figure 5-40鈥? or whatever shape. whether it defaults to a circular because it is applied to a round feature will continue to be argued until the purists and practitioners are assuaged by a statement in the standard.
at first glance i could see some logic in your comments with respect to composite tolerances. since the lower segment of a composite constrains only orientations 鈥渘ot translations鈥?relative to the drf. one could imagine that a features axis or pattern of features axes, center-plane(s), or point(s) 鈥漣f spheres鈥濃