One could easily argue that the pitch diameter or the outside diameter of the boss or the OD of the flange or even the bolt circle itself is datum B. A center line as a datum is extremely ambiguous. Talk to design and get it in writing what they want.
i think the intent is to keep the shaft true to the id and bolt pattern. the center line would be the spline. but for clarity talking to the design engineer could give you the correct answer your looking for.
That is the most likely feature to use but I've been surprised before. I guess my main point is that if design keeps sending ambiguous drawings your way without any pushback then the drawings will never improve. One of these days you'll guess wrong and it won't be pretty once the dust settles. So, talk to design and get it writing.
Thank you all. I will use the major of the spline bore which is what aligns the spline shaft. But I need to confirm before I get a print like this again. Unfortunately its a customer design and I'm the pesky engineer who approved it - LOL
