I do not have a good source for the indexed ring mandrel, but I can
offer a substitute which I have been using for many years and it
will cost you nothing.
Take a brass plate and cut out a disk of the diameter a bit larger
than the largest ring size you think you might need. With the
dividers inscribe progressively small circles graduating towards the
center. On mine each circle corresponds to ring size. Divide the
circles as required for application.
To achieve precision in dividing circles, use intersecting arcs
method. Start by drawing diameter ( any line through the center. Next
position one point of dividers on the intersection of diameter and
the circle, open dividers more than 1/2 of the diameter and draw an
arc which would cross the area where the center would be. ( no great
precision required ). Position dividers on another intersection of
the diameter with the circle and draw another arc in the same manner.
The arcs will intersect exactly opposite the center of the circle.
Using intersection of arcs and the center of the circle draw a line
until it intersects the circle on another side. You have divided
circle into 4 equal arcs.
Using the same method you subdivide each arc into required number of
subdivisions. Use sharp scriber and when done, mix some artist oil
paint with turpentine and rub into the marked lines. After it dries,
use emery stick to remove excess of paint and the scribed lines will
stand out sharply against the metal. Now you have a permanent
circular ruler which you can use to divide rings into any number of