Truing up the base and adding a tenon insert today. First step is to mount it between centers, clean up an area for the steady rest to ride on, flatten the base and true up the hole. Having to take this step is one of the downsides of hollowing from both ends. There's no set way to mount it up but I typically use a cone on the head stock end for the neck opening and bring up the tail stock center to the tenon on the base.
You will likely have to make multiple adjustments to where you place your center on the tenon. You look for a spot that minimizes the amount of material you have to remove to true up the surface. The warping can vary all along the length of the piece so turn it slowly and watch the surface, adjust a bit, turn and look again. Keep an eye on what your final tenon size will end up being. If you get too far off the original center to balance out the warping in the piece you can end up with a tenon that won't fit in your jaws. Been there. I can then true up the tenon and base, flip it around, mount it in the chuck, put the cone in the tail stock end and start the second turn. Be sure and flatten the base and get a good clean mount ready for your chuck.
On this piece the neck was too large for my standard jaws so I used my jumbo jaws with rubber feet. It's important to make sure there is a shoulder for it to push against since the tail stock with cone and then the steady rest need to keep it tight against the head stock. If there's no shoulder then it will slip over the chuck from the pressure. I flatten up the base then clean up an area for the steady rest. Very light cuts taking only what's needed to get to a smooth surface. I ended up taking about 1/8 or less to true up the surface. Not bad re warping but I can see other areas further toward the top that will take a bit more. I believe I have plenty of material to work with since I turned this one a fat 3/4" thick. I left the base opening a bit thick to give me plenty of purchase area for the insert to glue to. Make sure the base and insert for the hole are perpendicular to get a good glue joint.
With this done I can turn an insert to glue into the base that will then give me a tenon to use for the second turning. Be sure and use dry, stable solid wood for the insert. I found a piece of walnut in my wood stash to use. It's a different tone but should work fine. A piece this size is not going to get picked up, turned over and examined up close like a small bowl or vase. Very likely no one will ever even look at the base of it.
