The first step is to use circles that are as round as possible. Use
a disk cutter or very good measuring with a circle template. First
dome the pieces to be soldered. Then set up the sandpaper on a small
sheet of acrylic sheet. This also insures a flat surface. I use blue
"painters" tape to secure the sandpaper. I wrap around two sides so
that there is a full surface with no seams on the top of the acrylic
sheet, and secure the tape on the back of the acrylic, holding the
sandpaper flat. Sand until sections match. Once the two sections of
the dome are even and fit together matching tightly, the soldering is
a breeze. I use a very soft paste solder if there is to be no
further soldering operations or another grade paste solder that
matches how many more soldering operations need to be done. The
application of the paste is done with a syringe and is applied to the
inside of the top dome, very close to the edge. The top is then
placed on the bottom and put on charcoal block for soldering.
Gravity will flow the paste down as long as you heat the project from
the bottom. If your block is not perfect, then you may need to
reshape block to hold the sphere to be or you can make a round ring
from thick binding wire to hold the bottom dome while soldering. The
top (with the solder in place) is popped onto the bottom and lined
up. With this method, there is no need for binding wire on the object
itself. Heat evenly, running flame around and around from the bottom.
Watch to see the flash from the solder and make sure it goes all
around the equator. You can solder closed forms this way BUT you can
only solder it one time if it is totally closed. Totally closed forms
will float. That always amazes my students, great way to keep them
guessing if metal can float. My company sells the proper temperature
and types of non drying paste solder for these operations. If you are
interested, please contact me directly. I will also be glad to answer
any questions. Please note that the sanded edges will be very flat
creating a nice even platform for the solder to connect the two
halves. If there is a gap in the edge, then you will not have a
totally closed form. You can also use this technique for soldering
domes to flat backs. Beth Katz