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Using microwave xmfr for resistance soldering?

E

Eric R Snow

Jan 1, 1970
0
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
 
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


How are you planning to connect the transformer to get 12 volts? Put
power to the high voltage secondary?

If you put power to the normal primary you get 10s of thousands of
volts at relatively low current. Only way to use a microwave
transformer for resistance soldering is to remove the HV secondary and
install a very husky secondary of several windings - to give you a
couple volts at very high current.

AC or DC is not important and a"motor duty" dimmer can be used to
control output. A variac works better.
 
R

Rich Grise

Jan 1, 1970
0
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".

All you need is to work on your torch technique. Try preheating.
There's a NG - where they discuss this
sort of thing. And don't use OA - use propane/air. (i.e. Bernz-O-
Matic o.e.)

I've seen spot welders (well, web pages with them) that used a
microwave transformer, and they replaced the secondary with
enough turns of wire to get 4V. That sounds like a good starting
ballpark. Use #2 or #4 weld cable, and big copper electrodes.

A simple timed on-off switch should work, like a 555 one-shot,
tranny, and relay (or SSR). You control the heat by on-time.
You might even be able to use the SSR out of the microwave
itself. :)
 
E

Eric R Snow

Jan 1, 1970
0
How are you planning to connect the transformer to get 12 volts? Put
power to the high voltage secondary?

If you put power to the normal primary you get 10s of thousands of
volts at relatively low current. Only way to use a microwave
transformer for resistance soldering is to remove the HV secondary and
install a very husky secondary of several windings - to give you a
couple volts at very high current.

AC or DC is not important and a"motor duty" dimmer can be used to
control output. A variac works better.
I will remove the secondary and wind my own. I have done this to make
a spot welder. It works well.
ERS
 
W

Wild Bill

Jan 1, 1970
0
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
..................
 
H

Howard Eisenhauer

Jan 1, 1970
0
Eric, I had a simillar idea a few months back when I needed a spot
welder. The main thing with using a microwave xformer based welder
is the power output, the "big" xformers are rated for around 1500 VAs.
That's O.K. for spot welding thin sheet metal & thats what the plans
available on the web seem to be aimed at, such as

http://www.5bears.com/welder.htm


1/4" brass is probably more than a single transformer home built will
handle. As a comparison, commercial units rated for spot welding
3/16" steel are rated around 2500 VAs w/ a 250 V primary. I ended up
buying a unit off of Ebay, they seem to go for about $70. Thats
probably a lot cheaper & certainly quicker than trying to roll my own.

If you're trying to butt weld the brass as I suspect then I really
have no idea how much juice you'll need but you're certainly talking
about a lot more than you'll get from a m/w trans. or the 70 bucks on
Ebay.

Howard.
 
D

Dan Caster

Jan 1, 1970
0
1. I suspect 12 volts is a bit high. 2. AC ought to be as good as DC.
3. Use fewer turns on the secondary. 4 Set the length of the pulse
so one pulse is the right amount of heat. 5. Use very heavy wire. 6
gauge or bigger. Might be easier to wind three or more #10 wires in
parallel.

Dan
 
W

william_b_noble

Jan 1, 1970
0
resistance solder units that I have output between 1 and 2.5 V
 
E

Eric R Snow

Jan 1, 1970
0
resistance solder units that I have output between 1 and 2.5 V
Thank You. That's just what I was looking for.
ERS
 
resistance solder units that I have output between 1 and 2.5 V

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
 
E

Eric R Snow

Jan 1, 1970
0
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
 
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