On Sun, 11 Aug 2013 13:57:46 -0400, Bob Engelhardt
wrote:
I don't know why everybody says to vee the crack. The depth of the vee
will be so much less than the frame thickness that it can't be
significant. And the braze will wick into the crack without needing a
funnel.
The link Ed posted in the "Joining Stainless Wire" thread about silver
brazing (http://www.aws.org/wj/amwelder/9-00/fundamentals.html) had a
chart showing joint strength versus joint width. The maximum was for a
joint .0015 thick!! That thickness joint had a much higher tensile
strength than the silver itself (3X). Joint strength fell off
dramatically with increasing thickness. Thus a vee will be much weaker
than a close joint.
Bob
Bronze brazing (actually, it's brass) is a little different. It has a
higher bulk strength and works better in fillets. But it doesn't have
the high *joint* strength of silver in close joints.
I saw that graph and I don't doubt it, but it's a little sharper than
others I've seen in the past, and the usual recommendation for silver
brazing is to have joints of 0.002" or slightly less. That's in close
agreement with the graph from H&H, which AWS re-published.
There are silver braze materials (and special bronze brazing alloys)
formulated for use in fillets but their ultimate strength isn't quite
as high. The maximum joint strength for silver brazing that's usually
quoted is 120,000 psi, for close-fitting joints. As you say, that's
close to three times the bulk strength of the material.
When you silver-braze wires, and when they're in contact in a parallel
joint, enough of the joint is within the high-strength range that the
overall joint strength is quite high.
Keep in mind that there's a distinction between silver-brazing that's
done like soldering, wherein the braze wicks into a close-fitting
joint, and what is sometimes (inaccurately) called "braze-welding,"
which is mostly a matter of wetting the material on both sides of a
joint and building up a fillet. That's how Brit race-car builders made
their tubular space frames back in the '50s and '60s. In that case
there may be some wicking but you don't count on it.
Too bad Jim Rosen isn't around these days. He's pretty knowledgable
about silver-brazing.
--
Ed Huntress