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You need the proper machinery
All cuts are easy once you have the cutter and the workpiece securely held in the correct relationship with one another 🙂
The trick with boring the guts out of a BB shell is getting the frame solidly mounted down to the mill table with the BB axis parallel to the spindle. Frame builders have a head start because they are already 95% of the way to having the right jigs.
gbj_tester
The bit isn't the most important part. You need the proper machinery so the drill is 100% perpendicular to the bracket.
I had to pay the guy 10 € for the job so maybe it's worth asking a frame builder near you?