【转帖】rue position and datum requirements

yang686526

true position and datum requirements
i need some justification on this one...
our design group has released a drawing with a hole pattern on a sheet of honeycomb core.  the requirement of the holes is tp of .250 lmc relative to datum 'a' only.  datum 'a' is the planar surface of the honeycomb.  my question is...
in controlling the position of these holes is it possible to control to only 1 datum feature?  since this is concerning position with basic dimensions, doesn't there need to be more controlling features, ie.  datum 'b' and/or 'c'?
joe jackson
goodrich corporation
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a planar surface is not enough as it ensures the part is flat but does not control rotation.
thanks,
i thought so, but when addressed the design group claimed that the pattern itself is "floating", but...
doesn't that contradict the purpose of positional requirements?
also, 2 of these locations are for an insert with a drilled hole.  they did the same for these 2 holes, tp relative to only datum 'a', then labeled these 2 holes as datum 'b' and 'c' respectively.  they then use these datum features (holes) to control surface profile of the exterior features!  
my point is...if the hole pattern itself is not contained as you stated, then how are these 2 features gonna control any other features of size?
btw...  is there any justification of this principal in the asme 14.5, i am not finding any direct explanation of this?
joe jackson
i could understand using datums b & c to control the other holes, but i am not sure what you mean by "exterior features".  it could mean that the perpendicularity of b & c are controlled relative to a, then the edges are cut relative to b & c.  this does seem backwards, but perhaps in the fabrication process it is simpler to trim the piece after drilling, in which case this would work.
joe,
if you want to control the position of the feature pattern as well as the features within themselves then use a composite true positional callout.  in asme y14.5m it spells out all the typical uses of tp.  i suggest getting the spec and validate this foryourself.  is this honeycomb core being machined in its solid state or stretched out?  if you're designing a panel the features for potted inserts are usually completed after the panel has its face sheets assembled.
more detail...
the honeycomb is being fabricated as a sandwich with inserts for drilling.  the drilling of the 2 "datum holes" are for post fixturing on a fixture plate for cnc routing.  the drawing is for the build of the sandwich with 2 holes drilled after cure for location purposes.
the problem is that the design is trying to control the location of the plugs and the 2 holes in relation to the plug centers with the tp callout relative to only 1 planar datum 'a'.
my problem is that i am trying to convince or justify, if you will, to the designers that this is a problem in manufacturing.  i need justification from the asme standard or likewise stating that it is theoretically impossible to control the position of the plugs and holes relative to 1 planar datum!  any help with that is greatly appreciated.  thanks.
joe jackson  
after thinking about what you're doing it sounds logical.  the two holes will become datum holes for the rest of the features on the honeycomb panel, correct?  so basically, these two holes are not critical in the x & y axis (which is why the tp .250 lmc) just the z axis which is datum a all this with respect to the machine milling the holes.
the rest of the features will be referenced from these tooling holes?  unless i'm missing something this sounds okay to me.
joe,
i agree with heckler and ewh, it is somewhat unusual but not incorrect.  it mayfff"> be incomplete, which seems to be the problem you have with the single datum reference.
if i understand your example correctly, datum a is one planer surface of the panel.  the two holes (datums b & c) are drilled into this surface.  the existing callout controls perpendicularity to the surface (a) but doesn't control their location relative to the plugs for instance.
if the other dimensions and tolerances are such that the holes are located "close enough" to the center of the plugs to work and the holes are perpendicular to datum a, then all subsequent features can be relatively precisely located using all three datums a, b & c.
regards,
redpen
sr. designer/lead checker
joe,
   the asme standard explicitly calls out a procedure where you have two positional tolerances for the hole pattern.  one pattern is from datums a, b and c, and allows a tp tolerance of something like 1.5mm dia.  the other is from datum a, and allows 0.25mm dia.  the indication is that you are not concerned about the location of your pattern, but you are concerned about the location of the holes with respect to each other.  
   your pattern cannot just float.  there has to be a tolerance for how far it can float.
                              jhg
after reading all the inputs sofar and thinking on it,  it appears that the drawing is attempting to combine detail and assembly information on a single drawing.
it would seem that the holes can be related using lmc with relation to the od of the insert.  that would of course require a datum/s relationship for that purpose. a separate detail would best serve that purpose.
at the assembly level, the  two holes could then be used to establish a datum reference frame for the location of other holes or the perimeter of the honeycomb panel.
form-critical printed circuit boards or ceramic substrates for hybrid circuits are sometimes dimensioned such that the outline of the part is controlled relative to a single datum hole.  the dimensions from the hole to the boundary are basic and the boundary is controlled with a profile tolerance.  the primary (perpendicular) datum would be 'a' for example.  the secondary datum is assigned to the centerline of the hole, call it 'b'.  in this case only two datums are required to adequately define the boundary and the datum hole.  in this example, the two mutually perpendicular datum planes passing through the hole's axis are free to rotate and a third "clocking" datum is not required.  a second hole can be located relative to the first also using datums a and b in this way where a third clocking datum is not required.
i hope this helps!
tunalover