regulator, rubber hose, torch, and two or more
removable tips. This unit burns MAPP gas as a fuel in
the presence of oxygen. Figure 3-45 gives you an idea
of what the Presto-lite unit looks like.
When heating, apply heat to the fitting or thickest
part until it reaches the melting temperature of the
solder. Feed the solder at the edge of the fitting. When a
continuous ring of solder appears at the end of the
fitting, you have completed the joint.
After soldering is complete, clean the joints with a
wire brush, soap and water, or emery cloth. Exercise
caution to remove all flux from the joint after it is
soldered. Any flux left on a joint causes corrosion.
In plumbing, you may occasionally be called upon
to join copper pipe or tubing by silver brazing. You
may also use silver brazing in making repairs to air-
conditioning and refrigeration equipment, water
systems, galley equipment, and so on.
In SILVER BRAZING, also called SILVER
SOLDERING or HARD SOLDERING, joint
members are fused by heating with a gas flame and
silver alloy filler metal with a melting point above
800°F, but below the melting point of the base metal.
The filler metal is distributed in the joint by capillary
Since capillary attraction is important in the silver
brazing process, it may help you to understand what
this term means. Perhaps the best way to understand
Figure 3-45.Presto-lite heating unit.
capillary attraction is to consider some everyday
examples of the process. If you put one end of a strip of
cloth in a glass of water and allow the other end to hang
over the edge of the glass, the end of the cloth that is not
in the water becomes wet. Water rises in the cloth by
capillary attraction. The wick on an oil lamp can be lit
because the oil rises in the wick by capillary attraction.
In both examples, we have a LIQUID (water, oil) that
moves into an opening in a SOLID (cloth, wick) by a
process called capillary attraction. The basic rule of
capillary attraction is that the distance the liquid is
drawn into the opening in the joint depends on the size
of the opening in the joint-the smaller the opening,
the farther the liquid is drawn in.
In just the same way, capillary attraction causes
the melted filler metal used in silver brazing to be
drawn into the narrow clearance between the joining
members. Capillary attraction does not work unless the
filler metal is melted and unless the size of the opening
is quite small; therefore, the application of heat and the
use of a very small clearance between joining members
are essential to silver brazing. The heat is necessary to
melt the filler metal and to keep it molten; the small
clearance is necessary to allow capillary attraction to
draw the molten metal into the space between the joint
Silver-base alloys are commonly used as filler
metal for brazing. Although filler metals other than
silver-base alloys are often used, the technique for
making a brazed joint is basically the same. The main
difference is the amount of heat necessary to melt the
filler metal. In all instances, this temperature is below
the melting point of the base metals. Silver-brazed
joints have high strength on ferrous and nonferrous
metals. The strength of a joint that is made properly
exceeds that of the metals joined. On stainless steel, it
is possible to develop a joint tensile strength of
approximately 130,000 pounds per square inch (psi).
Since brazing with silver-based alloys is typical of
brazing in general, it is especially interesting at this
point to note the use of these materials as filler metal.
This information applies equally to brazing with other
filler metals that are distributed by capillary attraction.
Two methods are used to make joints between
tubes and fittings in piping systems with silver-base
brazing filler metal: the INSERT method and the
FEED-IN method. With either method, the parts must
be adequately supported during heating. The work
must be held firmly in position until the brazing filler
metal has completely solidified.