On Wed, 21 Jul 2004 12:33:50 +0000 (UTC), wrote:

On Wed, 14 Jul 2004 22:14:16 -0700, "william_b_noble"
wrote:

resistance solder units that I have output between 1 and 2.5 V

"Wild Bill" wrote in message
...
I dunno much about the specific outputs/capacity of the resistance
soldering
units, but I'll throw a few observations into the thread.
The units that I've seen in use in a starter/generator repair shop were
fairly compact units with a transformer housing of about a 6" cube. The
plier/tweezer-style handpiece appeared to have carbon jaws that made the
contact to the joint to be soldered. The heat was rapid, and they used
heavy
gauge solder, 1/8" maybe.

Some butt welding machines that I used to repair were used to weld ends of
heavy steel wire together (similar to a bandsaw blade welder). The sizes
ranged up to about 1/4" diameter.
The secondary of the transformer was only a couple of turns of flat
braided
cable securely clamped at the ends. The cable was the type that was used
as
engine ground strap in autos decades ago. This stuff would be good for
using
as a secondary winding in a modified transformer, easy to thread thru the
frame aand flat for fitting into a square shape.
For insulation, a good product would be fiberglas tape.. thin, high temp
resistant and an effective barrier/insulator.

WB
.................

"Eric R Snow" wrote in message
...
Greetings to all the microwave oven scroungers,
I have a job coming up that requires soldering the corners of brass
frames. These frames are made of 1 1/2 x 1 1/2 x 1/4 brass angle. In
the past I've used a torch to heat the brass. This leads to a little
distortion. This can be troublesome because the brass expands so much
that it moves the joint even though everything is clamped. The parts
must then be straightened when cool. Because of the application and
fit the frames must be straight within .010" in 36". I saw some
resistance soldering units in a catalog and the description of how
they operate says that only a small area is heated and gets to heat
fast. This would be ideal. However, the ones I saw were too small and
the price too large for me. But I've got several microwave
transformers and they seem like they might be perfect. A rough
calculation from the specs and pictures given in the catalog leads me
to believe that they output about 12 volts open circuit. Some have
variable outputs. So I have a few questions:
1) Does 12 volts sound reasonable? Would a different voltage be
better?
2) Is DC better than AC? Does it matter?
3)What would be good ways to limit the current? Would a lamp dimmer on
the input side of the xmfr work? Wouldn't that also lower the voltage?
Would that matter?
4) I have a timer that pulses a relay on and off. I can set the length
of the pulses. Sort of what a lamp dimmer does but much longer pulses
(1 second and up) and the voltage would be the same out as in. But the
brass would average the heating. Would this work almost as well as
lowering the current? Better?
Any other input is much appreciated.
Thank You,
Eric R Snow,
E T Precision Machine




Reading the original post it seems that it's soldering
that you need rather than flash or resistance butt welding
which would need a much higher peak power input.

It takes time for heat to travel and distribute itself
into a workpiece. The shorter the heating time the smaller
the volume of the workpiece affected and, for a defined
temperature rise, the smallest total heat input and smallest
heat affected volume. This means that the aim should be for
a high peak power input to permit a very short heating time.

Some resistance soldering units use either one or a
pair of carbon electrodes sharpened to a point. These can
apply intense local heat but it flows non uniformly into the
brass and can only melt the solder after heating a
relatively large volume of brass.

A better approach (and probably the one you're
already intending to try) is to resistance melt the solder
directly by clamping the transformer output leads to the
brass frame just either side of the joint and applying
pressure and time controlled current pulse for a second or
so.
If you can succeed in mostly filling with copper the
vacant space left by the high voltage secondary AND removing
any magnetic shunt pieces, a single microwave oven
transformer should be enough. You probably need about three
volts. These transformers are typically about 1 turn per
volt so two to four turns is the right range.

If there's a choice solder should be in the form of
flat preform lightly fluxed on both sides.

This is all partly informed guesswork so let us know
how you get on.
Jim
Thanks for the input Jim. You have hit exactly on the head what I'm
trying to do. Often it is hard, for me, to get an idea across
consisely. And I end up clarifying over and over. It must be hard to
be a teacher. Since I have already stripped out the secondaries and
shunts of two xmfrs I'll be wiring them in parallel. I will then
machine two copper electrodes to match the shape of the pieces to be
soldered and clamp the wires to them. I hope that getting the
electrodes within 1/4" of the joint will get me lots of heat fast. The
method of joining will be to use small .002" thick shim pieces in the
joint to provide capillary action. I have done this when using a torch
and it works very well.
Eric