John Rumm wrote:
Lobster wrote:
Prep and flux as usual, apply heat to the joint and wait a few secs,
then offer up the solder to the joint and touch it on to see if it is
hot enough to flow. Once it is, the solder will flow easily. Feed in
about half an inch or so (assuming you are using typical plumbing
thickness wire). You will actually see the joint suck the solder in.
Once you have added solder, withdraw the heat, wait for a few secs
and then wipe of any excess with a damp cloth for a neat finish.

Wipe off excess with a dry cloth. Having a damp cloth just makes
it more likely that you'll get a burn (steam), and shock cooling
is very bad for soldered joints.

Make sure your gas torch can heat the whole joint, one that's too
small is difficult to use.


Hmm. So far, I'm afraid all my prejudices have been confirmed...
Having armed myself with a length of 15mm tube and a few straight
end-feed couplings, I've been having a play today. One time, it did
seem to work OK, and I achieved the 'silver ring'; however, other
times the solder just ran off the tube like water off a duck's back
and landed on the floor, nothing being sucked up into the joint.

It sounds either like the pipe was either not chemically clean, or the
pipe was not hot enough, and you were just melting the solder in the
heat of the blow lamp.

The latter sounds most likely. The metal could be over- or under hot.


This is why it is important to heat the
pipe/joint, and allow the heat of the pipe to melt the solder and not
the flame.

For soldering to work, the joint must be physically clean (wirewool,
wire pipe cleaning brush etc - i.e. nice bright metal), it must also be
chemicaly clean (no oxide coatings, grease or other contaminants). That
is the job of the flux. It will do two things usually - clean the metal
in the first place (assemble joint with flux and twist it a bit to
transfer it to all the contact surfaces), and keep it from oxidizing as
you heat it. I would suggest something like a tub of Fry Metals
"Powerflow" flux - this is slightly aggressive and will clean the metal
well on contact.

Steel wool is good for cleaning tube. Wipe some flux around
only where you want the solder to go, a fingertip is good,
or a small brush. Unlike Mr. Rumm, I would absolutely avoid
Fry's "Powerflow" and go for a better flux such as Laco:

http://www.tooled-up.com/static/Lac1.asp


One other thing to avoid is overheating the metal and boiling off all
the flux. It should only take a few seconds of playing the flame over
the joint to get an end feed fitting hot enough. If it is glowing red or
giving a bright green flame you are overdoing it!


Heat the tube first/slightly more, it's easy to overheat a
little coupling or whatever, the tube takes a bit more. It's
surprising how little heat you need.