For many of us the methods and tools that we use in this wonderful hobby are very personal, possibly even idiosyncratic.  Therefore, I believe that if your methods work for you then use them.  However, one does need to be aware of technical innovations and improvements to accepted practice.  Simply because we have done something in a particular way for 25 years is no reason to continue with it if a better method emerges.

There are a number of myths about resistance soldering; possibly even more so than there are about soldering with an iron, all of them nonsense.  I learned the ‘black art’ of soldering by reading, trying it out and comparing notes with other modellers.  One aspect of soldering that I really dislike is cleaning off excess solder.  It is a waste of time and solder and potentially dangerous.  More than once has the paring tool embedded itself in my hand.

The following views are based entirely upon my own knowledge and experience and, naturally, reflect my own prejudices.  However, if they help others to speed up and gain greater enjoyment from their modelling, my efforts will be well rewarded.

A definition of soldering could be: "persuading two or more pieces of (frequently dissimilar) metal to stick together permanently at room temperature".  Over time a number of ways have been devised to achieve this end; for instance, gas torches, blow lamps, soldering irons of many and various types and RSUs (resistance soldering units).  Even the gas hob has been pressed into service, though I do not recommend this latter as is it likely to upset the domestic authorities!

Aside from the gas hob these methods require the effective use of a tool.  Granted, neither an iron nor an RSU is necessarily a simple tool, but it is a tool nevertheless.  To use a tool successfully, one needs to know how to use it; what it can, and cannot, do.  A tool of itself is never ‘bad’, only its usage.

The only use I have for a soldering iron is electrical work.  The tools I use exclusively for all other soldering work are an RSU, plus a gas torch for rare use with heavy metal sections.  More than a few people have told me that an RSU cannot be used to perform this or that soldering task.  I disagree totally; it is simply a technological improvement on the soldering iron, provided that one takes the trouble to learn how to use it, to discover of what it is capable.     Back to top

The RSU is an improvement over the soldering iron for a number of reasons: No need of various shaped bits, no need for special cleaning of coated tips, no tinning of parts for sweated together, little or no excess solder to remove, uses less solder and electrical power is required only when using the probe, no iron sitting on the bench all day (or all night because one forgot to turn it off!) burning the pennies and no more burned fingers!

The ideas for the base plate and magnets discussed later are based upon methods expounded by the late, lamented, Bernard Weller.  I use the London Road Models unit, 179% cream and wire lead solder and, at times, standard 12% phosphoric acid flux.  Rare earth magnets are also effectively an extra pair of hands.

How does an RSU work?  The box contains a transformer with several output windings that produce - in this case - outputs of 1, 1.5, 2, 3 & 4.5 volts.  However, it does so at very high amperages, in the order of 40 amps for the higher settings.  The power is controlled by an on/off foot switch.

The negative terminal is attached to a steel base plate on which the items to be soldered sit and the positive to a carbon probe.  Aluminium is completely useless as a negative terminal as it oxidizes rapidly, preventing good conduction, and will not of course allow the use of magnets to hold parts in place.  When the probe is placed upon the work piece and the foot pedal depressed, it completes an electrical circuit and this produces a high temperature almost instantaneously but much localized around the point of the probe due to the inherent resistance at the point of contact.  This is the reason why there must be no loose connections in the circuit, which would create local resistance spots and reduce the output where it is needed.

I never use clips of any kind to transmit electricity to a model because I have in the past burned some neat sets of teeth marks into models.  When using the chassis jig I have an extra negative lead bolted to the jig and am seriously considering making another to bolt to the bench vice for soldering laminated rods together.

At its simplest one holds items to be joined on the plate with the probe and apply heat using the foot switch.  The solder cream between the parts melts very rapidly and joins the parts.  As soon as the solder has flashed silver and or has stopped smoking, switch off the power, still holding the parts together.  With good quality solder cream the joint will 'go off' in seconds and the joint will be strong (I strongly advise against coarse grained solder pastes; having used them and found that often the joint fails after a very few years).
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It is normal to produce joints that have little or no solder visible, making cleaning up almost redundant.  Solder cream was designed for use in the electronics industry, it has no acid based flux but contains 2% silver to aid the flow.

Cleanliness is important and it is critical that no power is lost in the chain from the unit to the work piece, so electrical joints must be tight.  Here you can see that the plugs that go into the unit are cleaned to a bright shine.  It is also important to keep the springs opened out to ensure that they fit in the sockets tightly.  The negative terminal for my RSU is a piece of 16th inch steel plate screwed to a piece of MDF.  A bolt is passed through one corner so that the heavy brass tag with the negative lead firmly soldered into a pre-drilled hole (ironically, of course, with a heavy soldering iron) can be bolted tightly to the steel plate.  Notice that all metal parts are bright before clamping together.  The tag is then bolted down firmly.
For the probe, which is a piece of brazing rod about 4 inches long, most of the copper is stripped back and the carbon sharpened in a mechanical pencil sharpener.  It can be kept clean with a coarse file until it needs re-sharpening again The flux from the solder cream does cause a build up of dirt that tends to reduce conductivity.  Use a fibre glass brush to buff up the remaining copper to ensure good conductivity.  The rod is pushed well down the holder and the brass ferrule forced down hard so that there is no wobble in the probe.

Let us begin with the basics, soldering two pieces of metal together.  Here just pieces of scrap. 







Clean the surfaces to be bonded.  One part is anchored with magnets and a few tiny spots of solder cream added.  The other piece is laid on top and anchored with another magnet.     Back to top






All that is required is heat, applied until the solder flashes silver and stops smoking.  Once the power is switched off the solder goes ‘off’ in seconds, then the probe can be removed.










Finally, we have ‘two pieces of metal persuaded to stick together at room temperature’.
















Now we will try laminating.  Here the steel plate has two holes drilled in it to take lengths of 0.5mm wire at the spacing required for the spring.  The laminates are threaded onto the wires, with the etched leaves face down.     Back to top








A few tiny spots of solder cream are placed between the layers as they are threaded into place.







Heat is then applied with the probe.  I would normally hold this down with a scalpel or similar but the other hand was operating the camera.  The discoloration is normal and will disappear with normal cleaning ready for painting.







The finished article, as you can see, needs no excess solder removing.     Back to top









Now we shall try our hand at edge soldering two pieces of metal at right angles using a seam joint.  For this the magnets hold the work, together with a small engineer's square; we will also need wire solder and flux.








The parts to be joined should be clean, including the back of the piece that will lie on the plate.











It is important to ensure that there is as little resistance as possible in the path so that the probe will heat up the metal thoroughly and rapidly.  The magnets hold the base down and the support upright, which is also kept vertical by the engineer's square.

























To this we add a few small blobs of solder cream.  The solder cream is relatively expensive but one uses very little (about £20 a tube however, one tube will suffice for at least half a dozen engines or a myriad wagons).  I have been quite generous with this joint because I want the solder later to run through the joint.








Here you can see that the probe is in action; it is set at 3 volts.  The tip must touch both pieces of the work and the heat is kept on only long enough for the solder to join both pieces.  Once this is done - it takes seconds - let it cool, remove the magnets and check for square along its length.










Now, set it up again on the plate to finish the seam joint.  Run a generous bead of flux along the whole of the joint area.     Back to top













Now the solder blobs are re-melted and they will run into the length of the joint.  Simply run the probe gently along, not too fast.  One can go over it several times if necessary.  Since the part is now securely held together, the problems of expansion are much reduced.  The solder has run along the joint and the back of the joint is clean but could be neater.





For some joints this will be enough but often it will need to have a strengthening fillet added.  The cream has little gap filling capacity.  To add a fillet use wire solder and run the probe along it and into the joint.  There are two ways this can be done; with the wire being laid in the corner as the probe is advanced or, cut short lengths and lay them in the corner using the probe to melt them and let the solder run along as you go.















It may be necessary to add some flux too but the silver in the solder ensures that it runs well and, as with the solder cream, it 'goes-off' within seconds of the power being switched off.  The finished joint, neat and accurate. Back to top












I said earlier that I use this method for all my soldering and that includes white metal castings fitted to brass or nickel silver.  It is not nearly as scary as it sounds but I would suggest some practice with scrap castings, testing them to destruction, to get the ‘feel’ for how the process works.


Begin by cleaning the brass where the casting will go and the back of the casting with a fibre glass brush.  Apply a few tiny dabs of solder cream to the brass and a good helping of flux to the back of the castings.  Place the casting on the brass and manoeuvre into place; ensure that you can see some small fraction of the solder cream.     Back to top
Now, holding the casting in place apply the probe to the back of the brass and apply power.  If it is not possible to apply the probe to the back, then apply it to the side at the front, not to the casting itself.  As soon as the solder flashes silver switch off the power.  Check that it is fixed securely and move on to the next one.  If necessary, use clamps made from aluminium hair grips to hold the piece in place because, unlike using low melt solder, one cannot remove this by dunking it in hot water.  However, on occasion even I chicken out if it is a particularly fragile casting and use Loctite 408 instead.  Take your choice.

CONCLUSION: Keep the work clean.  The probe degrades with use and the flux in the solder builds up a film that can stop current flow.  Keep it clean and sharp.  However, it is less onerous than keeping a soldering iron tip clean.

If it is necessary to use high heat for thick material, round off the probe to spread the heat to both parts of the metal to be joined.  Always apply power after the probe is in place and before removing it to avoid arcing.

Do practice on scrap material of varying thickness and determine the best temperatures to use without burning holes in the metal.

The steel plate will get dirty so clean it with isopropyl to remove flux residues and a large fibre glass brush to remove rust and stains.  Generally, the plate needs a good scrub with 240 grit wet & dry about every three months.

A combination of drilled holes in the plate together with the magnets allows the construction of a variety of jigs to hold work, thus releasing the hands for other duties.

Do ensure adequate ventilation; inhaling the fumes from solder and flux is not recommended.

I hope that that dispels some of the myths and disinformation circulating about the use of the RSU, lowered your resistance to using that heavy box you have lurking in a cupboard somewhere and also helps you better enjoy your modelling.

The unit I use is available from: Hobby Holidays, http://www.hobbyholidays.co.uk.
Cream & wire leaded solder with 2% silver, plus 12% phosphoric acid flux, are available from: C&L, http://www.finescale.org.uk/.
Rare earth magnets are available from: Magnet Sales, http://www.magnetsales.co.uk/deep.htm the reference is: deep pot SMDP00103 13mm diameter x 20 mm deep.

I have no connection with any of these traders other than that of a satisfied customer.
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