Resistance soldering

Discussion in 'Clock Repair' started by kinsler33, Jun 13, 2019.

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  1. kinsler33

    kinsler33 Registered User

    Aug 17, 2014
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    Opticians repair your broken metal glasses frames with a no-flame, no-iron device called a resistance soldering machine. Just a variable-voltage transformer, a pair of pointy electrodes (place one on each piece to be soldered together) and a foot pedal. The parts heat up a lot--nigh unto red or above--when you push down the pedal. You can, they say, do soft soldering, brazing, or silver soldering with this arrangement. American Beauty, among others, makes the machines, which are not cheap.

    Model railroaders like to use them to solder/braze/whatever their tiny tracks together. This fellow's web page has some interesting pictures: Resistance Soldering

    Has anyone tried this technique?

    Mark Kinsler
     
  2. R. Croswell

    R. Croswell Registered User

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    Not what I was expecting, but an interesting concept - using an electric current through the work to heat a carbon rod, sort of like a carbon soldering iron. I was expecting to see the parts heated directly by current through their own resistance. No, I've never used the described method. I have used resistance welding where a very high current is momentarily passed between parts under pressure to be welded. That equipment is very expensive as well. Not sure I see enough practical uses in clock repair to justify the cost. The last lot of pendulum suspension leaders & springs had the spring material spot welded to the leader. I would also like to hear from anyone who uses resistance soldering for clock work.

    RC
     
  3. Willie X

    Willie X Registered User

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    The part to be soldered heats up. The carbon rod/s are just for electrical contact. This method was used in the jewelry business for quite a while. They usually used a car battery and foot operated reostat. It's trickey and takes a lot of practice. I would definately not recommend it when working with something that can't be replaced.

    Not to be confused with fusion welding, where a quickly applied current actually melts the two parts together at the contact point. This has become very common now in the jewelry trade, not so much In repair as production work. In sheet metal work and welding up car bodies it's always been called 'spot' welding.

    Not to be confused with induction welding. This requires huge amounts of current through a special coil to instantly raise metal to the welding point. This energy can be carefully controlled and concentrated in a very small area. Used mostly in industrial production.

    Willie
     
  4. shutterbug

    shutterbug Moderator
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    I made my living in an optical laboratory, and in the days of gold filled frames and parts we soldered with exactly that type of solder machine and gold solder. When gold stopped being used, we converted to flame soldering.
     
  5. sharukh

    sharukh Registered User
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    This is used to solder dial feet in watches. I had one made for me based on a diagram from the net.

    I've used it a few times and works very well. I've also used it to solder the tabs in the bezels of wall clocks to hold the glass. The work is neat and does not discolor the thin brass bezels. Of course you need to practice on some scrap material first.

    Search Google for the schematics. There's a Youtube video showing how the dial feet soldering machine is to be used.

    Sharukh
     
  6. kinsler33

    kinsler33 Registered User

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    Being stranded in a Pittsburgh hotel room I took the opportunity to do some research on resistance soldering. It looks like there are some advantages here, and that there are any number of ways to set up a soldering job. His setup uses neither carbon nor stainless steel nor anything much but Tix solder and flux. Opinions are solicited. I don't know the fellow who made the video, but he did refer to 'the group' having had a controversy over resistance soldering.


    Mark Kinsler
     
  7. sharukh

    sharukh Registered User
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    Technically I don't think that is "resistance" soldering.

    He's directly heating the piece to be soldered. That is a Weller type soldering gun. He cut off the tip but by touching the two ends to the piece to be soldered, he recreated the original structure and function of the tip. It would have worked just as well without cutting the tip off in the first place.

    Sharukh.
     
  8. R. Croswell

    R. Croswell Registered User

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    Current flowing through the part to be soldered causes it to heat so I would say it qualifies as resistance soldering. Unfortunately most of the current is through just one part and that part has to heat the other. Tix flux shown is corrosive so unless solder completely fills the joint it will be difficult to completely remove the flux. I don't see much advantage but something to keep in mind if the solder gun tip burns out and a replacement isn't immediately available.

    RC
     
  9. Willie X

    Willie X Registered User

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    Yep, that's resistance soldering but not practical for two reasons. No control except the low and high on the gun switch and not enough current except for very small work.

    Now, if you used a much larger 6.3v transformer with a large foot operated reostat, you would have something usable.

    But ... using the Weller gun as intended would probably be a much better approach. Ha

    Willie X
     
  10. kinsler33

    kinsler33 Registered User

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    I can only submit that the job shown in the video was exceptionally neat and not what you'd get if the soldering gun tip was used. Note that the heat was confined to the brass tab.
     
  11. R. Croswell

    R. Croswell Registered User

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    If that were true there would be no solder bond. Both pieces have to reach the solder melting point regardless of how the heat is applied. In my opinion the operator in the video did not adequately clean the parts to be joined. He also pre-tinned only the brass tab which would have required a different tool. If both parts were totally cleaned bright an solder such as radio solder with a noncorrosive flux coul dhave been used. The one big advantage I see in using the resistance heating method demonstrated has not been mentioned and that is that pressure can be maintained while the solder hardens with less chance of the solder gun tip becoming soldered to the tab.

    RC
     
  12. shutterbug

    shutterbug Moderator
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    The one I used had a large metal piece that supplied half of the current, like an anvil of sorts. Then the part to be applied was held with a small clamp. With the corners of the "anvil" one could concentrate a lot of heat in a very small area.
     
  13. Dick Feldman

    Dick Feldman Registered User

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    I have one of those soldering irons similar to the one in the video with two terminals for connection of the tip.
    It has ratings of 100/140 Watts, 120 V, 60 CY, \ 1.2A
    I am assuming those are primary ratings (utility mains).
    Using the formula (V) (A) =W/PF where V=Volts, A=Amps and PF= Power Factor. Restated, that formula is A= (W/PF)/ V
    From what I could find, the PF of normal appliances is .6 to .9. It seems the higher PF appliances are resistance type and the lowest are motors running at less than rated loads.
    It was stated in earlier posts that resistance soldering machines use low voltage and high amperage to achieve the goals.
    When I measure voltage (AC) across the two output terminals of my iron at the highest power setting, I get a reading of .1V (The reading is the same with the tip installed and not installed).
    Using: .6 PF, 140 W and .1 V the formula above should give calculated amperage of 2000 amps at the tip. [2000A = (120/.6)/.1]
    Using: .9 PF, 140 W and .1 V the formula above should give calculated amperage of 1333 amps at the tip. [1333 A = (120/.9)/.1]
    Is this a reasonable assumption/estimation of amperage?
    Or, have I multiplied apples times oranges?
    Best,

    Dick
     
  14. Willie X

    Willie X Registered User

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    You have something is wrong there somewhere. You should have several volts across the open tip terminals. This should drop to near zero when the tip is installed. These guns are not designed for continuous use. The heat output on your gun is 140 watts max. I would expect the duty cycle to be about 50% or thereabouts.

    Willie X
     
  15. Dick Feldman

    Dick Feldman Registered User

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    The soldering iron may have some hidden device that controls duty cycle.
    Duty cycle - Wikipedia
    The amperage draw should be the same, only possibly interrupted by some unknown internal device.
    The voltage between the contacts does not change with the tip installed.
    It was measured at .1 V both ways.
    If there is something wrong, please tell me what it is.
    D
     
  16. kinsler33

    kinsler33 Registered User

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    Your formulas and reasoning are correct, but they assume an ideal transformer, which is not the case here. I strongly suspect--though I haven't measured it or anything--that the power factor of your standard old Weller is far lower than 0.6. That's because vast amounts of magnetic flux leak out of the primary coil (that's the heavy roll of wire enclosed in the body of the soldering gun) instead of being transferred efficiently to the secondary coil. (In this case, the secondary coil is just one turn of copper wire that includes the wire tip.)

    The power factor of a pure resistance is 1.0, as is the power factor of an ideal transformer connected to that pure resistance. But if the transformer allows much of its magnetism to leak out into space, then it becomes much more like an inductor and its power factor is reduced, sometimes drastically.

    An inductor--just a coil of wire--has a power factor of zero--that is, if you were to connect a very low-resistance coil of wire to an AC electrical outlet, it would not get warm even though the current through it is very high and the voltage across it is very high as well. That is because the phase of the AC current is 90 degrees out of phase with the AC voltage waveform. Effectively this last statement means that whenever the current is at its maximum, the voltage is zero, and vice versa. (Technically the power factor equals the cosine of the angle between voltage and current waveforms, assuming a sinusoidal source.)

    The same is true for a pure capacitance: it dissipates no power from an AC source.

    If we wanted to determine the actual current through the soldering gun tip we'd have to do some extra work. One way would be to find the actual power dissipated by the tip by finding its heating ability and measuring its resistance, because power = resistance x current. (This is actually how we measure the power of weird things like radar transmitters.) Another way of determining the current through any conductor is to divide the voltage across each end of the conductor by the resistance of that stretch of conductor. Note, however, that the resistance of your soldering iron tip is in the micro-ohm range and thus rather difficult to measure. Finally--and I don't quite know how this is done--the electric current through a conductor can be measured directly by determining the magnetic field it produces around the conductor.

    We do a great deal of this sort of thing in electric power transmission work. We know the lengths of the conductors and their electric resistance per foot, plus their inductance per foot. With this sort of information our instruments at the substation can determine the location of a short circuit and trip the appropriate circuit breaker to protect the system.

    M Kinsler
     

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