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Old 01-13-22, 11:47 AM
  #129  
scarlson 
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Originally Posted by Doug Fattic View Post
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.
Thank you for your comments!! Yeah, the turbo torch is what I'm using for this. Aside from the fork crown, these are my first two lugged joints. I do a lot of flicking the heat away as you say. I took much less time on the bottom lug (my second one) than I did on the top, and you can sorta see that in how far the heat got down the tubes, turning them blue. I'm somewhat better with the oxy-fuel, it's quicker. I was practicing your "back-the-flame-up-while-adding-filler" technique with oxy-fuel (and got some great results doing that with bronze!) but oxygen isn't allowed at the Harvard machine shop where I've been doing my work. Working in the shop is much easier - no passive-aggressive massachusetts neighbors calling the fire department on me!

3/4 of an once sounds like more than enough silver for 2 luged joints.
Good to know! Like I said, I could feel an internal shoreline when I ran a soldering pick inside the tubes. I could also feel a blob or two on the inside in at least one place. Better overdo the filler than leave a void, I figure!

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.
OK, thanks. I am interested! The black flux type H is advertised as being able to reheat multiple times. This is true in practice, and helps with sketchy torches like the one I'm using here. Also if I take a look at the joint and decide I don't like something, I can go back over an area. I also bought a tub of type G but I haven't used it yet.

I'll re-iterate, I really appreciate your advice. I'm doing this whole thing as a learning exercise. This is how I have fun: learning new things when the stakes are relatively low.
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