Skills · skill
Brazing
Joining · Intermediate
- Difficulty
- Intermediate
- Done When
- A full, smooth fillet all round with filler visible on the far side of the joint — proof it went through.
- Tools
- Oxy-acetylene, MAPP or propane torch; silver-brazing rod or brass rod; borax-based flux; wire brush; pickle (dilute acid) for cleanup.
- Category
- Joining
- Summary
- Joining metals with a filler that melts above 450 °C but below the parts — brass or silver alloy drawn into a tight gap by capillary action.
Safety
Eye protection against the flame and the flux glare. Ventilate — cadmium-bearing silver solder is toxic when hot; use cadmium-free. Hot metal looks cold within a minute.
Steps
Fit the parts to a 0.05–0.15 mm gap; too tight and the filler cannot enter, too loose and capillarity fails.
Clean to bright metal and flux both surfaces.
Heat the whole joint evenly to a dull red — heat the parts, not the rod.
Touch the rod to the joint; if the parts are hot enough it melts and runs in on its own.
Follow the joint round with the torch; the filler follows the heat.
Cool slowly, then pickle or scrub off the glassy flux.
The Science
The filler is pulled into a gap of a tenth of a millimetre by surface tension, and diffuses a little into both surfaces, giving a joint far stronger than solder — often stronger than the filler alloy itself, because the thin film is constrained. Flux dissolves oxide and keeps air off while the joint is red-hot.
How It Fails
Filler that balls up and will not run: parts too cold or dirty. Filler that runs everywhere but the joint: overheated. A joint that cracks: gap too wide, or brass filler on steel that was quenched.
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