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Old 01-13-22, 06:53 AM
  #128  
Doug Fattic 
framebuilder
 
Join Date: Dec 2009
Location: Niles, Michigan
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Originally Posted by scarlson View Post
@Doug Fattic , I am sad to hear that. I probably won't make it, given what my next few years of career as a new PhD look like. But, equally likely to only build for myself and close friends, anyway. You were very helpful in your advice to get away from the Harris white flux.

Do you have any pointers for me based on the photos? Is no unsolicited advice from you a good sign or a bad sign? I will re-iterate that I appreciate all your advice. Here's a close-up of what I did. Unfortunately it's a little blurry, but you can at least see the flux and the coloring of the steel. The flux is Gasflux type H "Black Flux." You can see how much I applied, looking at the previous photo of the headtube with only the top lug brazed. Not enough?

Too much heat, again? By accident, I got some metal to dull red. Never bright cherry.
I did feel around inside the vent-holes with a soldering pick and could feel an internal shoreline. I used about 3/4 troy ounce of silver on the two lugs together.
By coincidence, the student in my class that started this week has already earned his PhD + more. It is always difficult to analyze the quality of a braze from just a picture. I can see you got it a bit too hot and didn't use enough flux. Why you got it so hot can't be known without seeing you braze in person. Were you using the same Mapp Turbo torch like you did a couple of years ago to put on cantilever bosses? That provides a kind of broad soaking heat pattern. You have to control heat with it like an on off switch. Flick on flick off. A very common student error is that their torch hand freezes when applying silver because they have not yet leaned to operate both hands independently. The problem is that it is easy enough to bring a joint up to brazing temperature but then when they start to add silver, it is very hard to keep that temperature window without going over some. Another way to control heat without flicking the flame on/off is to pull back with the flame so it is cooler farther away. The reason this is difficult for students is because in it normal that both of their hands want to go in as the silver holding hand moves closer as the silver melts. It isn't natural they can operate them independently enough at first so one hand is moving in while the other is moving back. That hakes practice.

3/4 of an once sounds like more than enough silver for 2 luged joints.

I'm not familiar with Gasflux's type H. I use Gasflux type G. It operates at 100║ higher temperature than their more common type U. Burning off the flux is a very common rookie error. I have students apply it way further up the tube then I need so they won't run out because it takes them much longer to braze a joint than me. Oops I have to go and get ready for another day of class. When I get a chance I can explain more about flux.
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