Bob its called a Donut chuck and you make them for the most part here is a pic and the instructions
http://www.turnedwood.com/tools&jigs.html
read the part about "Centering Donut Chuck for Reverse Turning"
if you dont have the chuck this guy has just use your chuck with the Donut you make.
Hope this helps
Eric
Eric...
Thanks for the link. Not only have I learned about different method of mounting semi finished stuff, but I also found, at that site, a reference to shop made graduated calipers that I had once seen but had lost the link. Thanks again.
...Bob