People often ask what sort of filler to use to braze together a lugged steel frame. When I was young, silver had a sort of mystique to it. The best tubesets available back then were susceptible to softening if they got too hot, so heaps of care was taken not to do so, including using silver filler, as it melts at a lower temperature than brass.
Then they started making frames with TIG, which actually melts the tube, so the tube makers came up with steels that could cope with the heat much better. So it's different now.
I use three fillers at the moment, depending on what I'm trying to achieve:
56% silver. I use this for lugs, where the fit-up of the join is very good. 56% silver is significantly weaker than other fillers, but has the advantage of being able to wick into incredibly tight places. If the gaps between tube and lug are well controlled and very small a join in 56% silver is as strong as one made in brass. 56% silver also has the advantage of wetting stainless well, so when I'm building with stainless I have to use this. A further advantage of 56% is that it melts at a relatively low 650 degrees C, where the tubing is barely starting to change colour - very dull brownish red. This is cool, because it makes for minimal distortion.
"System 48" filler. This is also a silver based filler. It's a little stronger than 56%, but doesn't flow as well. I use this predominantly on lugged bottom brackets, where it can be hard to get the lug sockets to fit the tubes really accurately. The system 48 gives me some insurance here, but at the expense of more cleanup to get a good shoreline. It flows at a similar temperature to 56%. You can sortof do fillets with system 48, so it's neat for fastback seat stays, though I've done these in 56% as well.
Low fuming bronze. I find myself using more LFB these days. This is a brass filler, so melts at a much higher 870 odd degrees. The tubes are glowing a healthy orange red by this point. It's significantly stronger than the silver alloys, so you can confidently fill gaps with it. It wets out really nicely and is a lot less fussy than silver in use. My main use for LFB at the moment is assemblies that are going to get brazed on to the frame - for example I used it to braze a piece into a chainstay bridge, that I then brazed into the frame with system 48. The second brazing operation was at a much lower temperature than the LFB melting point, so the LFB didn't reflow. LFB is also the filler of choice for fillet brazing and for forged dropouts. So far I've only done fillet brazed test pieces - I'm yet to do a fillet brazed frame. The disadvantage of LFB is the distortion that you can get from the higher temperatures. It's a lot harder to keep the frame in accurate alignment, and you're going to need a full set of tools - seat tube reamer, BB taps and facer, Head tube reamer etc to remove the distortion.
It's hard to make a recommendation. You can make a really good frame with silver, and you can make an equally good frame with brass. Similarly, you can make an awful mess with either. Silver is a weaker filler, so the fit must be tighter to compensate. Brass has distortion issues. Of course only silver works with stainless. There's no definitive agreement as to which is better even among the best framebuilders in the world. Richard Sachs and Llewellyn predominantly use silver. Nagasawa uses brass. I suspect what people use has more to do with what they learned with than anything else. I learned with silver, so changing to brass is a challenge.
So the rest of this tutorial will mainly talk about silver. I'll try to remember where those using brass should do things differently.
In summary, my favourite fillers are:
- 56% silver: Harris Stay-Silv 56 or Cycle Design brazage 56% or any other 56% silver filler - this stuff is made to standards and so is the same formulation from most all sources. Use 1/16th rod, and expect to pay through the nose.
- System 48: From Cycle Design. Also revoltingly expensive due to the high cost of silver.
- Low fuming bronze: From Cycle Design.
The flux used depends on both the filler and the application. For silver (56 or 48), I use either Harris stay-silv white, or Cycle design system 48. For work with stainless and 56% silver, I use Cycle design stainless light. When using low fuming bronze, I use Cycle design LFB flux.
Please don't take this as an exhaustive list of fillers and fluxes. These are simply the ones that I have experience with.