【转帖】angular 9timing0 orientation of 2 different patterns

yang686526

angular (timing) orientation of 2 different patterns?
i have a casting that has two different hole patterns.  the first pattern (5 holes), i locate with three datums... one which sets the angular (timing) positioning of the pattern.  if the second pattern uses the first pattern as a secondary datum, do i still need to add the third datum to set the angular positioning?  or is the angular position relative to the first patterns angular position.  
i thought by using the first pattern as a datum, it only referenced the center point and not a timing relationship.  a coworker informed me otherwise.
any input would be greatly appreciated.
thanks!
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runz,
   gd&t requires you to completely constrain your part.  generally, this means having three datums.  controlling rotation is a valid way to do this, but there are other ways.
   usually, you ought to have only one set of datums.  whatever it is you used to locate your first set of holes should be sufficient to locate the second pattern.
   if you used one or two holes on a pitch circle as datums, the datums are the holes.  the centre of your pitch circle is an abstraction.  it is not possible to attach a machining or inspection fixture to the centre of a pitch circle.  
   your co-worker is correct.  
                      jhg
so if i use datums a,b & c to locate my first set of holes on the casting and that set of holes is made datum d, i should only have to use datums a & d to locate the second set of holes?  both sets of holes are on the same surface of the part and because i use the pattern as a datum, i do not need the timing datum c?
thanks,
runz,
   a datum is a feature of your part that you can fixture to.  your datums_a, b and_c are valid.  your datum_d is not.   
   use datums_a, b and_c to locate your second set of holes.
   having located your part with the existing datums, you may define additional datums if you have fixturing problems.  for example, you might have an angled surface or a flexible part.  it does not sound like you have any of these problems, so your original three datums get the job done.
                      jhg
runz,
   one other thought...
   the only problem i can see with datums_a, b and_c is that you might not care about the rotation of your second pitch circle.  the extreme solution for this would be to put a note on the drawing stating that that pitch circle number two may be rotated any way.
   i do not see how a manufacturer could take advantage of this to save money.  it could be awkward for the inspector too.  my experience is that if you make your drawings look complicated, the fabricators jack their prices up.
   datums_a, b and_c keep things simple.
                         jhg
what the heck is a pitch circle?
pitch circle = bc ("bolt circle" or "between centers")
thank you. bolt circle i understand. isn't it amazing that no matter how many names you know for something, there always seems to be one more name for it?
it threw me the first time i heard it.  i was wondering "is there a thread there, or maybe some gears?"
   oops!
   the acronym bcd threw me the first time i ever saw it, so i am a little prejudiced against it.  i am glad none of this terminology appears on asme y14.5m drawings anymore.
   i am assuming that runz is using a bolt/pitch circle of some sort.  he did not actually say he is using one.
                      jhg