Posted by David Sherman on Thursday, May 04, 2006 at 4:52PM :
In Reply to: I'm batting 1,000 since cranking up the oxy posted by chris case on Thursday, May 04, 2006 at 1:19PM :
Brazing is a bit more particular about the flame adjustment than welding mild steel is, but I think you've nailed the adjustment: oxidizing enough not to deposit carbon on the surface but reducing enough not to burn the zinc. That's another nice thing about the good old oxy-acetylene set: you can adjust the flame chemistry as needed. You can even use it to deliberately carbonize a surface if need be.
As for brazing cast, I've also been told you can never do a really good job on a broken gearbox because the oil has so thoroughly permeated the iron that the brazing will never really wet it thoroughly. Could be. I brazed mounting ears on a gearbox, but never tried brazing the oily part. I've brazed stuff that I've ground and it's taken and held up okay, but I can understand the theory about smearing the carbon around. Presumably a coarser, sharper wheel would be best for avoiding this, and a steel or carbide burr would be better yet.