Re: Using microwave xmfr for resistance soldering?

pentagrid_at_yahoo.com
Date: 07/21/04 

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

On Wed, 14 Jul 2004 22:14:16 -0700, "william_b_noble"
<nobody@nowhere.com> wrote:

>resistance solder units that I have output between 1 and 2.5 V
>
>"Wild Bill" <kwag98@usachoice.net> wrote in message
>news:40f5a7cf_5@127.0.0.1...
>> 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 *** 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" <etpm@whidbey.com> wrote in message
>> news:dov8f01q80ojajskvhj2o8kbh046sv2dd4@4ax.com...
>> > 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 *** 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

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