Datum confusion 1

[billthebutcher]

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.

 

[IRstuff]

The correct forum: forum1103

TTFN

FAQ731-376

 

[drawoh]

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

 

[KENAT]

If you re-post you may want to state what dimensioning standard the print claims to follow.

Sounds off to me though, I'd suggest going back to whoever did the drawing and ask for clarification of what they actually want.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SteveMartin]

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.

 

[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.

"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.

 

[drawoh]

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