A Pin as a Tertiary Datum Feature

[cmmguy75]

Hi,
I have another question regarding Tertiary Datum Feature again.
Sometimes, I see a drawing having 2 pins. Most of drawings pointing at one(1) Pin with label |C| and other pin will have True Position...
(Please see attached drawing).
My questions are:
1/ If a design is used one(1) pin for its tertiary datum feature for locking rotation (as shown in picture), then to establish this "locking line/axis" is to connect center of |B| to center of |C| which is pin@30deg. Not center of pin@210deg to pin@30deg. is that right?
I've seen some inspectors connecting center of 2 pins. To me it's not correct because other pin is asked to be checked with using |A|B|C|.
If the locking line/axis is connected from center of 2 pins and if pin@210deg is NOT located at center of a boldcircle 3.5" then other holes would be out of location.
2/ Why does the design use 2 pins. Does it cost more $$$?
Thanks,
Brandon

 

[JNieman]

1- If they aren't using the outside diameter as the secondary datum, but are using any other feature, they're wrong.

2- No clue. I don't know what that part is, does, or anything. With the inspector hat on, mine is not to wonder why... to poorly paraphrase.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[drawoh]

cmmguy75,

I can interpret the drawing as per ASME Y14.5-2009. The primary datum is the top face, controlling the plane. The secondary datum is the outside diameter, controlling X and[ ]Y. The tertiary datum is the one dowel pin, controlling rotation. Inspection is straightforward.

The scheme, while legal, does not make a lot of sense to me, unless the dowels are providing merely a crude location reference. If I am using a feature of size as a datum, I would prefer an accurate feature of size, like a dowel pin. Using one dowel as datum[ ]B, and the other as datum[ ]C is legal, easy to fixture, and it probably represents how you will assemble this thing.

If you are the inspector, JNieman is right, although the accurate quote is "Theirs not to reason why...".

--
JHG

 

[3DDave]

Pin C has no location tolerance to B, so it can be anywhere relative to B.
B has no orientation tolerance relative to A, so it can be tilted.

 

[powerhound]

Your drawing does have problems but if all the features and datum features were correctly controlled and located, it would be incorrect to orient the part using the two pins. It would have to be oriented using the axis of B and the axis of C.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II