Steve,
How can I marry the hole to the donut perfectly?
A couple options. If by chance you happen to use a circle
punch/cutter to punch the center hole in the donut, and the same
cutter to produce the plug you’re trying to fit, you’ll find they’re
better than just a “decent” fit. That will help a lot. When
soldering, use a more active flux than things like Batterns self
pickling or the like. Those are fine for gold and even silver, but
the copper will need a more aggressive flux to ensure the joint stays
clean, a major reason why solder might not flow into an area (also be
sure the metal is clean and bright before you start to solder)
To get an even better, tighter fit, or if you’re not using such a
“matched” way to produce hole and plug, then start with silver for
the plug thats slightly thicker than the copper. Then fit it as well
as you can, and once fitted into place, planish (hammer) the silver
center. It will flatten to slightly thinner while doing this, and
will also expand. Done carefully, you can get a virtually perfect
fit. In fact, if you overdo it, you can get a fit so tight that
solder won’t easily flow into it, but in the case of copper and
silver, that’s not a problem. If well fluxed, with a reducing flame,
you can just heat the two pieces without solder until the contact
between the two fuse. That happens well before either the silver or
copper melt. Same principal, mostly as used in granulation, or in
making a mokume billet. To work, the metals need to be in close
contact, preferably almost a press/interference fit. As well, when
heating, heat the center slightly more than the copper donut. That
way, thermal expansion won’t cause the donut to expand out away from
a cooler silver center. All the above also applies if you add solder
to the joint, which you may wish to do in any case. Don’t use more
than you need for a clean result. And if you want to experiment, and
happen to be using thicker guage metals, shape the inner edge of the
hole in the copper with either a channel cut into the center of the
inner edge, or chamfer the top and bottom edges of the hole. Then
when you planish the (well annealed beforehand) silver, it can expand
either into that channel, or around the chamfered edges, forming a
mechanical joint. That can be very tight indeed., and may need no
solder at all. In effect, you’re forming a very wide, thin, rivet…
This is useful too as a means to do what you’re doing when one of the
metals is something you cannot solder, such as titanium…
Hope that helps
Peter Rowe