wondering if any of you out there have a good simple metric method
for determing ring blank lengths. it would be much appreciated.
Take the millimeter diameter desired of the given ring size (the
diameter of the inside of the ring, or as measured at the appropriate
spot on a ring mandrel with a metric caliper.) Add to this the
thickness of the metal being used, also in millimeters. Multiply
that sum by pi (3.14159). That's your theoretically needed ring
shank blank length. Usually a good idea to add just a tad to that
measurement, to allow for some fitting of the ends in preparing the
seam, etc. Also, for very wide blanks, add a little bit more, again to
give you some adjustment options, as wide blanks don't stay
completely flat when you bend them around, sometimes needing a bit
more fooling with to get a well fitted seam.
In some European ring size systems, the ring size is just that
metric diameter, so if you're making a ring to such a ring size, then
you don't need to measure the mandrel. But be sure the sizing system
you're using is that way before just assuming such...
The same calculation, by the way, works fine for round bezels too.
And for ovals, add the width to the length, and divide by two. Use
that as a "diameter" in the above formula to get the length of bezel
stock needed for that oval. Works for properly elliptical ovals
only, not odd lumpy shapes.
Hope that helps.