Defining Datum at mid-plane between none parallel surfaces (GD&T)

[Searc]

Hey, I am trying to finalise a machining drawing of a part shown in it's finish state here:

https://postimg.cc/DWDPQcYm.

The part is made via an unusual casting process - the part is cast between two plates, which once removed only leaves the middle profile surface (labelled A) and end surface (labelled B) requiring machining. After casting, a mandrel is pressed out of the hole and the hole itself only requires a small amount of machining to finishing sizes, unlike surfaces A and B. The rest of the surfaces are left as-cast.

The part is for a compression test, so I want to control the two functional features, that is the hole and end surface (labelled B) keeping them in align with the as-cast surfaces. To this end, using GD&T I want to keep the hole circular and perpendicular to as-cast surface next to the hole. I also want to keep the end surface (labelled B) parallel to the hole axis and perpendicular to the datum discussed below.

My question is about GD&T datums. I think I can use the hole as a axis datum, as after the mandrel is pressed out it is an as-cast surface before it is machined. I can also use the flat surface perpendicular to the hole axis. Using these two datums, the part is only unconstrained in one degree of freedom - it can spin around the hole. So, my question is how do I constrain this last degree of freedom, considering all other surfaces are not perpendicular or parallel. I think the practical answer are fixtures which will be machined to fit between the two symmetrical ridges to control the rotation around the hole - but how would you define this datum on a drawing? Using datum targets on opposite inside surfaces of the ridges to create a middle surface datum at the plane of symmetry of the part? Using common datums?

Also I'm open to any other tips or suggestions of a better method to controlling these features.

Thanks!

UK - Using BS8888

 

[Burunduk]

Datum targets for locking the rotation would be my direction of thinking.

 

[drawoh]

Searc,

How accurate do you need to be?

You need three datum features to restrain your part. Features of size are not accurate on castings, so they don't make good datums. If you can add some little nubs to your casting, you can use them as datum targets, as noted by Burunduk.

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JHG