Stretch the ring up an extra 1/2 to 3/4 of size and then shrink the
edges back down to the proper size.
Sometimes that works, but all too often the center of the inner
band, in which the spinning section rests, is very thin, and buckles
when you do any significant shrinking, so your end shrinking doesn’t
give a good surface for the spinner to ride in. That can be a
problem. Also, shrinking that thin a band means only the edges
shrink, causing the previously flat interior surface to become
domed/hollow, with the outer surface following that shape. That can
sometimes allow the spinning section to simply slide off, no longer
adequately trapped.
If the ring has no existing solder seams (check carefully), so that
it seems like it will stretch the required amount without cracking,
then here’s how I’d do it. First, very carefully measure the existing
ring size. Be exact. Now shrink the ring slightly, from one side
only. The ring in your video is flat enough that the shrinking die
should only contact the edge of the inner band, not the spinning
section. Choose the shrinking die you use to make sure of this.
Shrinking the inner band on one edge should allow you to slide the
spinner off without cutting. Now measure the ring size of the
spinner. Again, be exact, noting the exact difference in size between
the two. Stretch the spinner sectioh up, seperately, until it’s the
same amount larger than your desired end size, as was the original
difference in size. Annealing it first is a good idea. If find it
sometimes useful to go slightly more, and then lightly shrink back
down to the size, since the compression of the shrinking die seems to
lock in an exact round shape better than stretching does. Clean up,
polish as needed, etc, being sure there are no burs on the inner edge
of the spinner section. Now anneal the inner portion. Due to your
shrinking of the edge to seperate the two, it is likely more in need
of annealing. Now you can stretch the inner ring up to the desired
size, while holding the spinner in position over the center of the
inner ring. If you’ve done this right, the spinner should still be
loose and movable as you reach the desired end size. As with the
outer band, if there is any tendancy for the inner ring to not be
totally round, tweaking the edges slightly with shrinking dies, not
enough to really make anything smaller, just enough to true up the
shape, should help. Be careful not to overwork the metal with too
much up/down/up tweaking, or eventually you’ll tear somethng…
Making these smaller is somewhat similar. Again, measure the initial
size, shrink enough to seperate the rings. measure the difference,
and shrink the inner spinner to the needed size plus that difference.
Then size the inner ring smaller. Given the thin narrow center cross
section, you often will find it necessary to cut and solder this
part, not merely shrinking it, to avoid that doming effect. But if
it’s only going down a little, you may be able to shrink it. Again,
now shrink one edge enough to slip the spinner on, and stretch it
back up. This can be tricky, given the seam you’ve now got in the
ring, so be careful. At any sign of cracking, stop of fix before
stretching any more, since a slight hairling crack is a lot easier to
fix without freezing the two bands together than is a wider more
serious crack.
If you’re initial examination of the rings found solder joints, then
the whole thing becomes more complicated, as then you have to do it
mostly with conventional sizing, where getting both rings very round
so they not only lock together but still move, can be tricky. If
that’s the case, charge a lot more…
Peter