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I am having trouble with the joinery of a two-piece, turned wooden lamp.

The lamp consists of a stem and a base. Because of tight tolerances I cannot use simple dowel joinery, instead I must turn a round tenon on the bottom of the stem piece and glue it into a round mortise in the base (the resulting joint looks like a butt-joint). Once joined, the two pieces are shaped together on the lathe.

The problem is that the shoulder of the stem must be exactly 90 degrees to the tenon and perfectly flat because it will form the glue-line between the base and stem.

How can I make this joint?

Additional information:

– I am using cherry.
– The finished lamp is 14″ tall and 5″ wide at the base.
– The stem is pre-drilled down the center with a 1/4 inch hole (for the lamp cord).
– The finished stem is very narrow: It tapers from 7/8″ to 1/2″ over the first 1 1/2″ from the glue-line. Because the stem is hollow, a dowel would have to be a minimum of 1/2″ and this could penetrate the stem by only 1/2″ before it would begin to compromise the strength of the stem.
– The base is made from 6/4 flat stock, the stem from 2×2 square stock