clarification about ASME y14.5 2009 figure 4-48 position : is datum C futile? 1

[supergee]

Hello,

I apologize in advance for English is my third language and I fear it is very difficult for me to translate my toughs on this one.

On figure 4-48 of asme y14.5 2009, looking at the 12.32-13.08 hole, there is a position box?? that says there is a .3 tolerance at LMC of the hole per primary datum A, secondary datum B and third Datum C.

I interpret it that my datum simulator for the holes axis must be parallel to the datum A. I then create a datum perpendicular to A passing though datum B. Datum C should then help orient my third datum simulator.

since the hole is a circle, hence has no feature that needs to be oriented in the C direction, why do they call it in the said box? couldn't they have omitted it? I don't get how the C in this case helps me inspect this hole.

thanks

Gerald

 

[pmarc]

The 12.32-13.08 hole is located from C (basic 40.5 dimension). Without reference to C the hole could lie anywhere along a line located by basic 10 dimension from datum plane A.

 

[supergee]

wait... I think I got it... It helps me figure out the distance of 40.5... I spent the last hour trying to get it... I guess it's a case of 3:00 : I need coffee now.

Gerald

 

[Vimalmechs]

Hi,

I got a doubt in Fig 4-48 (Y14.5- 2009).
1) In FCF, Secondary datum is DATUM B. There are two DATUM B planes deriving from datum target lines. Then how DATUM B works ?

Regards
Vimal VV

 

[Belanger]

Datum B can be thought of as a single plane, but with an intentional break and offset of 11.4 mm (from the 7.6 and 3.8 basic dimensions).

Or it can be thought of as two planes -- but again it's really only one datum because of that 11.4 designed-in offset.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Vimalmechs]

Hi JP,

PFA.

I am just making clear that whether my underatanding is correct. As per the conversation with you, the datum B can call out in the frame from two locations.because both are related by basic dimension.

In the fig, DATUM B is attached with the edge surface of pocket. And it is related with SURF X by basic dimension. In FCF i am calling DATUM B.

Based on my understanding i can dimension the hole from SURF X.Because SURF X is related with DATUM B. right ?

Regards
Vimal VV

 

[Belanger]

The general answer to your question is yes. In your sketch datum B is just the plane formed from the inner left-hand wall of that pocket. But because basic dimensions have no tolerance, they can be arranged in any manner, as long as there is a way to chain them from the hole back to datum feature B.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Vimalmechs]

Thanks JP.