Buying Guide

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47
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1. The tube should be cut to desired length with a square cut,
preferably in a square-end sawing vise. The cutting wheel of the
type specifically designed for cutting copper tube will also do a
satisfactory job. The tube should be the exact length needed, so that
the tube will enter the cup of the fitting all the way to the shoulder
of the cup. Remove all slivers and burrs left from cutting the tube, by
reaming and filing, both inside and outside.
2. To make a proper brazing joint, the clearance between the solder cup
and the tube should be approximately 0.001" to 0.010" (0.0254mm to
0.254mm). Maintaining a good fit on parts to be brazed insures:
Ease of Application — Excessively wide tolerances tend to break
capillary force; and, as a result the alloy will either fail to flow
throughout the joint or may flush out of the joint.
Corrosion Resistance — There is also a direct relation between the
corrosion resistance of a joint and the clearance between members.
Economy — If brazing alloys are to be used economically, they,
of necessity, must be applied in the joint proper and in minimum
quantities, using merely enough alloy to fill the area between the
members.
3. The surfaces to be joined must be clean and free from oil, grease
and heavy oxides. The end of the tube need be cleaned only
for a distance slightly
more than it is to enter the
cup. Special wire brushes
designed to clean tube
ends may be used, but
they should be carefully
used so that an excessive
amount of metal will not
be removed from the tube.
Fine sand cloth or emery
cloth may also be used with the same precautions. The cleaning
should not be done with steel wool, because of the likelihood of
leaving small slivers of the steel or oil in the joint.
4. The cup of the fitting should be cleaned by methods similar to those
used for the tube end, and care should be observed in removing
residues of the cleaning medium.
Attempting to braze a contaminated
or an improperly cleaned surface
will result in an unsatisfactory joint.
Brazing alloys will not flow over or
bond to oxides; and oily or greasy
surfaces tend to repel fluxes, leaving
bare spots which will oxidize,
resulting in voids and inclusions.
5. Flux should be applied to the tube and solder cup sparingly and in a
fairly thin consistency. Avoid flux on areas not cleaned. Particularly
avoid getting excess flux into the inside of the tube itself. Flux has
three principal functions to perform:
A. It prevents the
oxidation of the metal
surfaces during the
heating operation by
excluding oxygen.
B. It absorbs and
dissolves residual
oxides that are on
the surface and
those oxides which
may form during the
heating operation.
C. It assists in the flow of the alloy by presenting a clean nascent
surface for the melted alloy to flow over. In addition, it is an
excellent temperature indicator, especially if an indicating flux is
used.
6. Immediately after fluxing, the parts to be brazed should be
assembled. If fluxed parts are allowed to stand, the water in
the flux will evaporate, and dried flux is liable to flake off, exposing
the metal surfaces to oxidation from the heat. Assemble the joint by
inserting the tube into the cup, hard against the stop. The assembly
should be firmly
supported so that
it will remain
in alignment
during the brazing
operation.
The Fine Art
of Brazing
Best results will be obtained by a skilled operator employing the step-by-step brazing technique that follows: