In addition to the
stop-offs mentioned last time, please note that milk of magnesia, readily and cheaply
available from many drug and food stores, is actually a decent stop-off that
may prove effective for many general-purpose needs. So, do some experimentation
and find out what kind of stop-off works best for you.
But – VERY
IMPORTANT – don't let the easy availability of stop-offs lull you into
carelessness in the brazing process, as discussed below!
CAUTIONS:
1. Overuse – Because of the effectiveness
of stop-offs, many brazing shops tend to lose concern about the amount of BFM
they apply to parts, merely saying: "Oh, don't worry about how much BFM is
used, just put on more stop-off." That is a VERY bad idea! Stop-off should
never be used as an excuse for allowing the use of too much BFM. Unfortunately,
I see too many shops that overuse brazing stop-offs rather than training their
people to back off from such overuse. Instead, they should only use the correct
amount of BFM that is needed to make a good brazement and only small amounts of
stop-off if it is absolutely necessary.
2. Correcting stop-off "mistakes"
– It is not uncommon for persons applying the stop-off to make a mistake and
accidentally apply it onto a surface that needs to be brazed. I've heard some
folks very softly say: "Uh, oh..." as they're putting stop-off onto
parts. When I look in their direction to see what they just did, I see them
taking a cloth and dipping it into a solution such as acetone or alcohol with
the intent of wiping the stop-off from the surfaces where it is not
supposed to be. THIS DOES NOT WORK!
Please understand
that the following saying is very true: "Once stopped-off, always
stopped-off." You cannot merely wipe the stop-off from the surface and
expect that surface will then become brazeable again. That will not occur.
Residues from the stop-off are still there, and the BFM will avoid wetting that
surface.
The only way to
effectively remove stop-off from surfaces is to either thoroughly
ultrasonically clean that surface or machine off the contaminated top surface
layer. Better yet, find
ways to eliminate the use of stop-off altogether. Too many shops plan on the
use of stop-offs on too many of their parts, thinking that it is always wise to
do so. No, it is not.
CONCLUSION:
Brazing stop-off can be a helpful tool for a brazing shop, but it should be
used sparingly and only when absolutely needed. Otherwise, overuse of stop-offs
can actually result in more-than-necessary scrap and rework of parts.
