【转帖】irregular closed feature

yang686526

irregular closed feature
received a drawing for a flexible plastic part stating gdt on irregular closed features.
i have no experience with this callout.
could someone explain it to me.  i have attached a picture of the callout.
hc1autoeng,
   if datums a, b and c are orthogonal, i have problems with the positional tolerance.  it ought to have a diameter symbol in front of the 2.  i am not sure about the word "boundary", either.
   if i assume that the 2 is really ?2, the irregular form should be located inside a 2mm circle centred on the nominal position.  the irregular form outlines must be within of their correct position, allowing for positional error.  the error can be increased at lmc of the irregular features.
   you could print the maximum and minimum outlines of the irregular features on clear film.  sit the film on the face such that the feature outline is contained within the piece of film.  you must be able to do this.  each feature on the film must be sitting within 1mm of its nominal position.  otherwise, you reject the part.
                          jhg
boundary is in asme y14.5m for slots & the like to support use of mmc/lmc, see 1.3.1 & 1.3.2 & 6.5.5.1 & figure 6-19.
figure 6-19 has a similar callout, no dia symbol for the position, however it is 2 separate fcf but attatched to the same leader, perhaps because the all around can't apply to the position.
take a look at 6.5.5.1 & fig 6-19 that explain it.
kenat,
i thought position was only for features of size (holes and slots), not for irregular features.
i think a composite profile would be more appropriate.
boundary is used on a non cylindrical feature of size such as a rectangular hole, etc. where the boundary is the virtual condition size, shape, location and orientation of this feature. it must be confirmed with a checking pin though. unfortunately, most designers do not use the term "boudary" when applying positional tolerances to slots but it is a better call out.
i do not see a problem with reference datums a,b, & c - primary, secondary and tertiary. datum a is for perpendicularity to the mounting surface while datum b & c will give the location of the centre of the irregular shape hole.
the leader line with a circle around the elbow shown with the profile of a surface simply means "all around".
i don't see anything wrong with the call out.
  
dave d.
i don't see anything wrong with it either. this type of callout can be used to control any feature's odd or simple virtual condition boundary of position relative to the drf.
paul  
paul:
i cannot believe that you are agreeing with me???
there is just one exception i have though to your statement. the term "boundary" is used on noncylindrical features of size rather than any feature as you stated. a round feature of size at mmc really has the virtual condtion boundary superceding the centres as we have discussed here many times before.
see -
6.5.5.1 boundary control for noncylindrical feature - page 168
dave d.
quote:
datum a is for perpendicularity to the mounting surface while datum b & c will give the location of the centre of the irregular shape hole.
how is this center determined?
can someone with a standard give a ruling on the "legality" of using position for an irregular shape?  the references i have seem to indicate it is not proper.  it just doesn't sit right with me.
i quoted above the section in the asme y14.5m-94 standard that covers irregular shaped features of size using "boundary". the centres should be shown with basic dimensions from datums b & c on the drawing but one does not confirm centre location but the virtual condition boundary about the true position.
  
dave d.
thetick,
quote:
i thought position was only for features of size (holes and slots), not for irregular features.
   the profile tolerance of the irregular feature is very much more accurate that the positional tolerance.  the specification makes sense to me, although it is worth looking at the composite profile tolerance in figure 6-25 in asme y14.5m-1994.  
   if i were inspecting, i would require each feature of the irregular hole to be located within the positional tolerance.

                             jhg
if one uses a profile of a surface for an irregular shaped hole, one would use a coordinate measuring machine to sweep the surfaces of the hole. this is really costly to perform and it probably wouldn't be done on a regular basis.
if one used a positional tolerance at mmc on an irregular shaped hole using boundary, it could be checked with a checking pin and fixture simulating the virtual condition boundary, location and orientation. it could be checked on a regular basis (in the control plan) and once the fixture is made, the cost on the shop floor would be minimal.   
dave d.