Datum callout on FCF or Dimension

[jmarkus]

Please see attached image. Is there a difference between stacking the datum on the FCF versus on the dimension?

Thanks,
Jeff

 

[Tarator]

Yes, it is.

 

[Tunalover]

I see no difference. Attaching the datum to the FCF for the slot width effectively assigns that datum to the width dimension.

Tunalover

 

[SeasonLee]

Yes, no difference. There are 3 ways to specify a center plane, your post is same as what I mentioned on the 2nd and 3rd way. And you may delete "boundary" if you are using 2009 standard.

Season

 

[jmarkus]

I had been taught (perhaps incorrectly) that in the strict sense, if the datum is not on the FCF then it is the datum regardless of meeting the tolerance called out within it, but if the datum is on the FCF then it is only considered the datum once the tolerance in the FCF is met. This meant that if you were to check the part and the FCF was on the datum, then if the datum didn't meet the tolerance in the FCF then there was no reason to check any further callouts called back to that datum, since the datum didn't meet the requirement. If the datum was not on the FCF then you would continue to check the whole part and just report the relevant "out-of-tolerance" values.

Any validity to that?

Thanks,
Jeff

 

[powerhound]

No. That is not correct. Where the datum identifier is placed--in the case you have presented--has no bearing on how it is interpreted or treated.

You were taught the wrong thing. An unfortunately all too common occurence.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[jmarkus]

Thanks. I wonder what else I've been led astray on...

Jeff

 

[greenimi]

Okay, lets expand a little bit the thread (for my own clarification)
If would be 2 or more features with the same shape as the one in the original post (2 or more slots with radii at each end) how the interpretation would be different (if it would)?
In other words, in section view shown, the right section is exactly the same as the left one and we have 2x for the size (8 +0.1/-0) how the placement of the datum C will have any effect on the interpretation of the drawing (see Season Lee’s 3 scenarios/cases)
Case #1 “The datum feature symbol can be in line with a dimension line and touching the extension line on the opposite side of the dimension line arrowhead of a FOS”

Is datum C the pattern or ONLY the left ( “middle plane). Lets assume we are talking about slots and not threads (as shown in the OP)

Probably you would say: pattern. And the following up question is how I say then if I need only the left slot to be datum feature C


Case #2” The datum feature symbol can replace one side of the dimension line and arrowhead of a FOS dimension.”

Same as #1


Case#3 “The datum feature symbol can touch a FCF which is applied to a FOS.”
Here also I would say pattern.