Actually I shoulda said that this is not a tenon turner--it's just a way to chuck up a molded stem in a lathe so that the tenon can be turned and the stem faced.
What I do for stems is use delrin as the tenon. I face the pre-formed tenon off flat till the stem is the length that I want. I then drill out the mortise for the delrin tenon and epoxy it in.
I use a good forstner bit to face the stem before adding the tenon.