【转帖】using the virtual condition of slot as datum

yang686526

using the virtual condition of slot as datum
drawing is attached.  in question is the size of the datum pin to simulate datum -c-.  my customer is saying that the size should be the vc of the slot (6.15 x 9.15) and that a separate gage to check the fcf at that slot is not needed.  should i be using a slot pin that only takes the mmc of the width of the slot (9.9) and use a separate gage to check the vc of the slot as it pertains to the a and b data?
one is required to confirm the positional tolerance on the slot using a gauge with a pin the virtual condition size, shape and orientation of the slot. once confirmed, you now have datum c.
when using datum c as a tertiary datum, one needs a pin the vc size and not the mmc size.
dave d.
dingy2,
   would it not be better to use the width of the slot as datum c?  
   if this were my drawing, there would be separate positional tolerances for the width and length of the slot.  i cannot see how the length could be part of a tertiary datum.  
                 jhg
for the purpose of checking the slot on the far right, would i define the mmc of datum -c- with a 6.9 wide slot pin, relieving the ends of the slot?
to check these fcfs there would need to be two gages employed.  
csydesign,
no, your customer is correct. if you wanted to use a 6.9 x 9.15 slot type pin or diamond pin your print should look like the one attached. i added the basic angle to help clarify the orientation of datum feature -c-.
paul

drawoh:
when one confirms that the slot is in position relative to datums a and also b, the gauge pin in this fixture is the virtual condition size, shape and orientation (that explains the term "boundary"). if the gauge pin can be placed in the slot without due force, then the slot becomes datum c.
the tertiary datum c in our case is for orientation only. datum b (secondary) is the intersecting point of 2 perpendicular planes and all dimensions come from it. yes, i would only use the width of datum c rather than the complete boundary but the width would be its virtual condition size.
the reason i wouldn't use the complete boundary for datum c is the fact that there could be a conflict with datum b in one direction. one must make sure that the pin in datum b is able to float in any direction without a constriction from datum c. again, datum c is only used for orientation.
hope this helps.
  
dave d.
unless the print is different than what was given then the customer is correct and a virtual condition elongated slot type pin 6.15 x 9.15 has to be used to constrain datum feature -c-. likewise a 6.15 x 11.15 pin located and oriented relative to |a|b(m)|c(m)| would qualify the other slot.

oooooppppps - i am wrong. the tertiary datum c is the full virtual condition size, shape and orientation as per drawing for confirming the other slot rather than the width of the datum c slot. to check the other slot relative to datums a, b and c, we need a stationary pin of 6.4 od into datum b, 6.15 x 9.15 pin in datum c slot and then a 6.15 x 11.15 pin in the other slot. this could be done on 1 fixture.
if i were the designer, i probably would have had the width of the datum c slot as datum c rather than the full slot. there could be a conflict between datum b and c the way it is drawing right now.   
dave d.
although not part of the original question, i do believe there are other problems with the definition.  should there not be three areas for the determination of plane a?  i would also question the dimensioning for c which is related only to a and b.  a being a plane and b an axis there is not direction provided for the legs as given.