He says from the first generation metal ring to second metal ring,
there is 10% reduction in results. Is this % correct?
It's actually pretty easy to get even more shrink than that if you
really cook a traditional rubber mold while vulcanizing it, even with
low shrink rubber. 15 minutes too long and ten degrees too hot will
do the trick. The effect can be increased by cranking up the
temperature of the wax pot, squeezing the tar out of the mold while
it's being shot and using a heavy hand while finishing it. Using the
newer generation zero-shrink RTV (room temp. vulcanizing) rubber and
careful experimentation with wax temperature, air and mold pressure,
shrinkage of 5% or less is pretty easy to achieve, so it doesn't have
to be that high, even for someone with very little experience.
Cutting the mold and dealing with the results of that will usually
cause far more grief for the rookie than shrinkage.
Another consideration concerning shrinkage from master to 2nd
generation is that it is generally more of a volumetric kind of thing
as opposed to a dimensional thing. A 2nd G casting might be a half
ring size smaller and 5% lighter, but a bezel in a heavy part may
actually end up slightly larger in inside diameter as the wax / metal
shrink away from the hole, towards the center of the heavy part. In
other words, all of the solid parts get volumetrically smaller. When
using RTV for a single stone setting, I don't worry too much about
shrinkage. I just try to make sure the stones can be made to fit if
it doesn't shrink much and leave enough metal on the outside in case
Why? He wants to use a 6 mm stone down to a 5 mm in one rubber mold
You didn't say what kind of setting he might be thinking of, a prong
style or a bezel for instance, or how many stones are involved. A
setting for a single 5mm to 6mm shouldn't be that big of a deal for
either style, I don't think. If it's prongs, I'd make the master
using fairly heavy prongs, say 16 or even 14 gauge, taper the shape a
bit top to bottom, and construct it so that it will fit a 5mm stone
set low, with a shallow cut on the seat and a 6mm a little higher
with a heavier cut to the seat. For a bezel, just think of a tapered
tubular shape with an ID of something like 4.7mm and an OD of 6.8 or
so at the top on the master. If he doesn't want to cut and fit that
much, or if he wants something much more delicate and one-off
looking, then he might just be out of luck.
But we're talking about mass-production tradework, right? I mean,
times being what they are Gerry, if that's what the man wants, you
really ought to think about making it for him. If you don't want the
work or to be associated with "that kind" of work, I'll bet you can
find some takers here to pass it on to. Heck. PM me. I'm not that
proud, and I've got a payroll to meet. ;
In my humble opinion and certainly without the benefit of seeing the
project, a 5mm to 6mm tolerance shouldn't be completely out of the
realm of possibility unless there are extenuating circumstances like
multiple stones set girdle to girdle at the same height. Something
like that would make it a different story altogether.
Hope this helps, Gerry!