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





RE: A Pin as a Tertiary Datum Feature
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.
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NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: A Pin as a Tertiary Datum Feature
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...".
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JHG
RE: A Pin as a Tertiary Datum Feature
B has no orientation tolerance relative to A, so it can be tilted.
RE: A Pin as a Tertiary Datum Feature
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II