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Datum confusion

Datum confusion

Datum confusion

(OP)
Hi,

I have a drawing that uses geometric tolerancing.  In many of the GD&T blocks there is what I guess might be referred to as a "compound datum" - datum "C-D."  C and D are parallel planes.  Particulary confusing to me is a datum point that is called "Intersection of B and C-D at D."  Datum B is a plane that is perpedicular to both C and D.  What does "C-D" mean when those 2 datums are parallel to eachother and never intersect?

Thanks,
Bill

P.S. I apologize if this is not the appropriate forum.

RE: Datum confusion

billthebutcher,

   This question is better posted to the Drafting Standards, GD&T & Tolerance Analysis forum, with an example of your drawing.

   The datum specification C-D in your feature control frame indicates that the datum is defined by two features.  In the ASME Y14.5M-1994 standard, look for figure 4-20.  

               JHG

RE: Datum confusion

Assuming ASME Y14.5-1994 (the only standard I have in front ofme atm), the C-D means simulating a single datum plane by simultaneous contact of the high points of C and D (para 4.5.7.1 & figure 4-20).

I'm not sure about the second point.

RE: Datum confusion

I believe C-D is the mid-plane between the two parallel datums C & D.  It's the same thing as when you align a datum symbol with a feature of size dimension.  In that case the datum is the middle of the feature.

"Intersection of B and C-D at D" sounds to me to be the same as "Intersection of B and D" since C-D is parallel to D.

RE: Datum confusion

Quote (dgallup):


I believe C-D is the mid-plane between the two parallel datums C & D.  It's the same thing as when you align a datum symbol with a feature of size dimension.  In that case the datum is the middle of the feature.

   No it is not.  See the standards noted above.
 

               JHG

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