Datums on thin part

[MattEdwards]

Let's say I have a thin plate with a hole in it:

I'd like to control the position of the hole relative to the midplane of the part. Is a single datum as shown sufficient, or should I also have a datum on the large surface perpendicular to the hole? If so, is it necessary to qualify Datum A with a perpendicularity control, even though it's so short as to be nearly 2D?

I work to ISO but also interested to hear the ASME answer if it differs.

 

[Burunduk]

Datum feature selection should be related to function. You are specifying a position tolerance because the location of the hole has some significance in some use application. So how does the part interface with the mating components? It is unlikely that only the two side faces are both locating and orienting it and the rest hangs in the air.

 

[Belanger]

Most likely, a single datum isn't sufficient. As Burunduk suggested, you'd probably want to identify one of the large faces as the primary datum feature, and then yes, the current datum feature A (perhaps rename it as B) should be qualified to the large face with a perpendicularity control. Even though the part is thin and perp might not seem necessary, it's more of a philosophical thing to have the secondary datum be linked in some way back to the primary datum (and it would also guide how a gage would be made.)

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

 

[Burunduk]

I would add to John-Paul's comment that with one of the large surfaces as the primary datum feature, there should be some means to differentiate between them - you would need to break the symmetry. For example adding a small but distinguishable chamfer (larger than the edge-break) near the face opposing the datum feature.

 

[drawoh]

MattEdwards,

Your primary datum really ought to be your top or bottom face.

Consider specifying your hole as a datum, then specifying an all around profile tolerance for the outline.

--
JHG