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Home » Blogs » Industrial Heating Experts Speak Blog » Using a "Stop-Off" in Brazing (part 2)
Dan-kay-200px

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.

Using a "Stop-Off" in Brazing (part 2)

August 18, 2011
Dan Kay
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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.                

Blog Topics

Dan Herring - Heat Treatment

David Pye - Metallurgy

Dan Kay - Brazing

Debbie Aliya - Failure Analysis

George Vander Voort - Metallography

Thomas Joseph - Intellectual Property

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Dan-kay-200px

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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