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.
Using a "Stop-Off" in Brazing (part 2)
By Dan Kay

Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute in 1966 and his MBA from Michigan State University in 1982.
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