I do everything the machinist way. I took a piece of round bar and faced it the same length as the inside of the cup. I tuned one end down to fit the inside of the bottom of the cup. I drilled and tapped tapped the other end 3/8.
I then chucked the piece up and spotted it with a 3/8 drill. I the bolted the piece to one of our small drill press tables, adjusting the table until a 3/8 drill was dead center of the spot.
Put the cup on, hold it down firm, and it drills a nice clean 3/8 hole dead center.
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