This is only one example, there are many others like it. Even the diagram on the front cover of Y14.5 is misleading - the entity labeled "datum point" isn't a datum point - it's a point at the origin of the datum reference frame. There are many common misunderstandings relating to this, most of them (IMHO) caused by misleading terminology, definitions and diagrams in Y14.5.
#ASME Y14.5 DRAFTING HOW TO#
It also doesn't explain exactly how to get from a set of datum features to a set of datums to a datum reference frame. Part of the problem is that Y14.5 doesn't explain this. This datum would constrain all six degrees of freedom of a datum reference frame. If the two compound-angled cylinders were referenced together as a primary datum feature A-B, the datum would be a "point on a line in a plane". For example, the datum for a conical datum feature is a point on a line, and constrains 5 degrees of freedom if referenced as primary. A datum can be, and often is, some combination of a plane, a line, and a point. However, they don't need to - the definition of "datum" in Y14.5 is incomplete.
![asme y14.5 drafting asme y14.5 drafting](https://images-na.ssl-images-amazon.com/images/I/613a7C0FcaL.jpg)
I agree that the two cylinder axes cannot produce a single plane, axis or point.
![asme y14.5 drafting asme y14.5 drafting](https://demo.vdocuments.mx/img/378x509/reader019/reader/2020040207/5b01e1ae7f8b9ad85d8ece30/r-2.jpg)
It is legal to reference two cylinders at compound angles to each other as a compound datum feature. I don't see an attachment or link in the original post, but I'm going to comment on the compound datum issue anyway.
![asme y14.5 drafting asme y14.5 drafting](https://www.tec-ease.com/tipimages/january981a.png)
I have tried to be open minded about this call out but believe I can prove that 2 axis at compound angles (this is the case for many of our tube runs) to each other can NOT produce a single datum (plane, axis or exact point according to Y14.5m standard defintion).ΔΆ5+ yrs experience RE: ASME Y14.5m application and general drafting stds The in-house amendment in this case is virtually the same as the base Y14.5m standard but allows for a material condition to be added to the A-B. However I will tell you that the amendments do not address the fact that to use two part features to generate a single datum according to the standard (Y14.5m), can only be 2 co-planar or 2 coaxial features. This was my 1st posting and I didnt realize the ppt image could be manipulated. In my opinion it is not interpretable according to ASME Y14.5M 1994 and violates use of defining datums. I think you are gracious to judge that this drawing uses "poor practices".