Framebuilders - A couple brazing questions.
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02-16-11, 06:27 PM
I am building my first frame. (4130 cromo, lugged) My original plan was mapp gas and 56% silver filler. Now I have an O/A rig and am wondering if I should stick with silver or go to brass. Is the O/A to hot for silver or would it be easier and more idiot proof b/c of the lower melting point? Also, if brass is the way is there anything wrong with the flux coated rods which I happen to have a wealth of or should I go with bare rods. I will of course be using paste flux as well either way.(I'm not that big an idiot;))
Any and all advice will be greatly appreciated.
02-16-11, 07:02 PM
silver would be fine. I've never used Mapp, but from what I can tell you are much more likely to cook the joint with MAPP than with O/A. Brazing is all about heat control. If your torch can barely support the heat you need, that's not control, that's pointing the torch at the frame. A practiced builder will generally be moving the torch quite a bit, and you have to know when to point it off into space.
People say to strip the flux off of the pre-fluxed rods and then clean the rods. I don't know if that is necessary, I haven't really tried pre-fluxed rods that much. Unless you are sure you are using compatible fluxes, it's best to only use one.
02-16-11, 07:23 PM
As long as you keep the joint clearances tight, silver will be plenty good and it is much easier to control than brass.
I've never liked flux-coated rods, but if you like them it's easy enough to make your own as you need them. Just heat the bare rod and stick it in some powdered flux. Be aware that the powder fluxes and those used for coated rods tend to be hard to remove afterward. By far the best brass flux I've used is the "Type B" paste flux from the Gasflux company. Works like a champ and comes right off with only hot water and a stiff brush.
For silver I use the "Type U" paste flux, also from Gasflux. Silver fluxes tend to be much more acrid than brass fluxes, so be sure you have some ventilation.
02-17-11, 09:22 AM
In terms of lug brazing, brass is a whole heck of a lot harder to nail than silver. I like to braze everything w/brass, even stainless, but in our class we teach w/silver because new folks have a much better time with it.
If you go with the brass, get 1/16" bare rod, low fuming. Scotch bright the rod to clean, and heat/dip it into powder flux as you go, as well as cover the joint in paste flux. Allstate 1113 paste flux is killer stuff, a bit more temp range than the Gasflux, and cleans easier.
My advise would be to practice with both. I find that bronze is a bit more challenging in the heat application: you need to blast the joint with big tips to sweat a BB or crown. I guess it teaches you to get in and out as fast as you can (which I'm not doing, eh), with the metal glowing red/orange and all. I like it personally. But otherwise the principles are the same, silver or bronze. Silver is also challenging as the flux can burn quickly if you're careless.
Never tried coated rods on lugs. I think I'd be annoyed by the thick and bubbly end (although most of the coating would be melted at brazing temp). I second using 1/16 bare rods.
Do you really braze stainless with brass? Isn't that a no no? I can imagine with a lug that if you stuff it in there it will make joint failure very difficult, but giving up adhesion would seem to be a problem, if that is what one is talking about.
02-17-11, 05:14 PM
I would like to get good at using brass for lugs. I tried it once and while it probably would have held up fine, I found it rough going. People have told me that brass is just as easy to learn as silver, but I'm not sure I believe that. I've seen some really horrible brazing jobs on bike-boom era European production frames. With silver, a good brazer can have a file-free lug at production speeds.
I have heard of people using brass on stainless, but I'd probably go with nickel silver. It costs about the same and you can use the same flux.
02-17-11, 05:16 PM
Sorry no, nickle bronze rather than ordinary LFB. Yes LFB would be risky.
But the Henry James sells seems wet out on most stainless where other bronzes will bead off... Which I have used on personal stainless lug bikes with no problems.
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