My favorite of all sanding mandrels. 
This 3pc set is a terrific idea developed by Kate Wolf for sanding and finishing the inside of rings. The larger diameter barrels work more quickly to do a better job. Sizes are 1/4 3/8 and 1/2 -6.35 8.9 12.65mm-. Shanks are 3mm on the two smaller ones and 4mm on the larger size. Made of stainless steel the mandrels are machined true and come with rubber bands to hold the emery paper in place. Shanks are 5/8 -16.50mm- long. Maximum 10 000 RPMs.