What is the standard for defining datum planes?

[jammin7800]

Hopefully this is a simple question.

Do Datum planes need to be UNIQUELY defined?

I've got a customer blueprint in front of me. Note 5 says Plane X is defined using Datum Target Points X1, X2 and X3. Plane Y is perpendicular to -X-, through Target Pts Y1 and Y2. Plane Z is perpendicular to both X and Y, through Datum Target Point Z1.

So far, so good. No problems there.

But then, along comes BP Note 26.

Note 26 attempts to define a different datum system, using different target points. Problem is, the names of the Planes in Note 26 are still X, Y and Z. The target points are physically in different locations. They also have different names --- B1, B2, B3, J1, J2, and V1. But the planes they define are still X, Y, and Z.

It should be noted that the two sets of Planes are in the same location. That is, on a nominal part, they coincide.

But for clarity, I asked that the Planes in Note 26 be given different names (Planes B, J and V, for instance), and the note be amended to say that they do coincide on a perfect part. But the customer says that is not necessary.

What does GD&T say about this?

 

[drawoh]

Jammin7800,

Datum targets X1, X2 and X2 define datum_X. This is by definition as per the standard. Note_5 sounds redundant. Datums B1, B2 and B3 define datum_B, again by definition as per the standard. A note claiming that this is another datum_X sounds confused.

When you say they are nominally the same, does that mean that, as fabricated, they may be different?

JHG

 

[jammin7800]

As fabricated the parts would seat differently, depending on which datum setup they were in. For instance, if I was required to inspect a point on the root surface, I wouldn't be surprised to get different readings on that point in the different setups. Depending on how badly the part was deformed from nominal, the readings could be significantly different.

If I were to build fixtures to seat the castings, the target points on the fixtures would maintain the relationship defined on the BP. Put a perfect part in either fixture and all readings would come out the same. Put an actual, real-life, non-perfect part in the fixtures, and readings could differ, sometmes drastically.

 

[PeterStock]

Is there anything obviously refering to the note 26 datum set? If so I would tell the customer that you will build the part such the everything refering to X,Y and Z will be refering to the note 5 datum set, except the features you decide are "obvious". If they want you to use them interchangably, I would just use the note 5 set.

One thing, is the a cast, forged or welded part with machined features? If so is the note 5 set refering to cast/forged/welded features and the note 26 set to machined features? If that is the case then I would the cast datum set for cast features and for the initial machining. Then I would use the machined datum set for the machined features. In any case your customer should not be resuing X,Y and Z for both definitions.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[drawoh]

jammin7800,

If the parts seat differently, then the Note_26 datums should have new, unique ID. If your customer keeps referring to datums_X, Y and_Z, you do not know which_X, Y and_Z you are to use.

Are they trying to distinguish between the cast part and the machined part?

JHG

 

[jammin7800]

Thanks for all your replies.

The "second" XYZ datum set will be used in machining the castings.

Unfortunately, the print doesn't reference the ASME spec. Some of this customer's prints do, but bot this one. It ref's their own internal spec, which really doesn't cover this. So, technically, they can't violate a spec they don't adhere to.

Still, as a best practice, and to make things less confusng, I don't theink they ought to be "re-using" datums. I just think they don't want to have to re-draw the print.

 

[dgallup]

You can not have 2 datums with the same name. That is fundamental engineering, I don't care what standards you use. They could call the second set X', Y' & Z' if they want to, it doesn't matter, but they have to be different names.