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Adding flux mid-braze

Old 05-17-12, 02:21 AM
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veryredbike
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Adding flux mid-braze

I'm watching some Paterek videos and he recommends not adding cold flux mid-braze, for fear of weakening the metal. I'm assuming that this is for the same reason that we don't quench hot pieces.

He, instead, recommends building up a pile nearby, so that it's naturally near brazing temperature, then scooping it over if you need it.

This all makes sense to me... buuut, I just brazed the little caps onto my seat stays that will join to the dropouts (I'm using these). I definitely brushed a bit of extra flux right out of the can on there.

My instinct is not to worry, since they're big old stays, and a brush of flux can't cool down metal that hot by that much... but I figured I'd get opinions.

Also: anyone know if frame forum got archived anywhere? I feel like I could probably find answers to some of these questions there without bugging folks.
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Old 05-17-12, 02:24 AM
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P.S. Someone needs to figure out a way to buy a forum a pizza and round of drinks so that I can properly thank you guys when this thing's ready to ride.
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Old 05-17-12, 03:33 AM
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I am no expert on brazing but I some people toss powered flux on a frame during brazing.
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Old 05-17-12, 04:20 AM
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In this case, we're talking goopy paste flux. The goopiest.
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Old 05-17-12, 04:44 AM
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As you initially heat the joint use a large flame on your flux so it'll get crusty and stay put without running off. Don't add flux, put some upstream on the tube so you can bring it down with the flame as you need it.
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Old 05-17-12, 09:00 AM
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I'm not sure if the "quench" would have an effect since you would be re-heating the area and would let it cool down normally afterwards?

You can try dipping the hot filler rod in the flux. It would work better with powdered flux (in which case it could be sprinkled on the work)
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Old 05-17-12, 09:19 AM
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Yes, it's probably fine for a seatstay cap on your own bike, but a bad practice in general...do what Craig said in the future.

BTW, the archives from the Phred/Google builder's list has a lot more info that Framforum ever did.
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Old 05-17-12, 09:22 AM
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I doubt that it quenches it to any degree, but it's really hard to get it to stick. Paterek is right, the best way is to put too much on beforehand and pull it into the braze later. It will tend to migrate towards the braze if it gets hot anyway.

I was just looking at the archive of the framebuilder's list yesterday. Fortunately there are a batch of people that copy email lists because I don't think the actual archives are very reliable. Note that the list was only recently moved to google, the good stuff was/is on phred

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Old 05-17-12, 10:29 AM
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I've seen two well known builders add paste flux when in the middle of brazing and they never spoke negatively about it cause any type of damage from the slight quench it might cause. More is probably rolling immediately off then what's actually staying put.
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Old 05-17-12, 01:08 PM
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Ok, so... I'm going cut it out in the future, but won't worry about the little joints I've already done. I'll also start hunting those archives.

Thanks!
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Old 05-24-12, 07:39 PM
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Instead of applying additional cold flux to the hot frame, heat the rod and dip it in the flux. The flux will stick to the rod and then come off with the molten brass/silver onto the frame at the proper temperature to avoid any thermal shock.
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