Thanks to John H. for this informative article about soldering. It's a helpful guide about what and how to do this, in simple terms. As model makers, many of us do a lot of work with metal, and there's always something to learn. As with all shop work, work safely and use protective gear. Enjoy your joints!!
When you want to bond pieces of copper, brass, tin, or even steel together, other than by spot welding, think "soldering". That is heating until hot enough for a third metal to melt and bond to both surfaces.
Low Temperature Soldering
Solder is typically an alloy of lead and tin (although lead is now often replaced with some other metal). It must melt at a lower temperature than the two metals to be joined. The location of the joint must be clean of all grease and surface oxides. This can be achieved with fine-grain sandpaper, and methyl hydrate as a solvent (acetone also works). Copper, brass, tin and steel can all be soldered together, even though the two metals may be different.
A flux is used to prevent the joint from forming oxides as it is heated. Sometimes the flux is contained in the solder itself. Electrical solder has a core of resinous material and, usually, when the solder joint is made the joint does not require further cleaning. Solder for plumbing and mechanical joints requires an acid core or separate flux paste or acid to be applied to the joint. This must be scrubbed off and cleaned with solvent when the joint is completed, to prevent later corrosion. A mixture of hydrochloric acid and zinc sulphate may also be used as a flux (don’t get it on bare skin!).
Examples of low-temperature solder and flux
For best results, “tin” both surfaces before attempting to bond them together (Apply a small quantity of solder to the surface of each part and then quickly wipe off any excess with a rag, to leave a shiny looking surface). When these two surfaces are brought together and additional solder applied, it is much easier to get the solder to flow into the joint and create a bond. When tinning is used on steel, it is referred to as “sweat soldering”.
To heat, use a butane or propane torch, or an electric soldering iron. Low-wattage electric irons are used for electronic circuits with heat-sensitive components, whereas the pistol-type on/off soldering “guns” are for tinning electrical wires and attaching terminal lugs. To prevent damage to heat-sensitive components or adjacent solder joints, use a heat-sink (such as an alligator clip or a temporary cover of silicone sealant) to prevent heat migration to the sensitive area.
Examples of heat sources.
It is also important to “tin” the tip of a soldering iron, and wipe off the excess. Over time, heat will erode the tip of the soldering iron and it may have to be filed to expose fresh metal, or replaced. As joints are made, the tip may acquire excess solder that needs to be wiped off (don't shake the iron to get rid of the excess, molten solder can be splashed about). Parts should be held imobile while the joint is made. “Third hand” alligator clip fixtures are very handy for this. Joints should be heated before solder is brought into contact. If the joint is hot enough, the solder will melt and flow to create a bond. Heat and solder should be applied on opposite sides of a joint, so as to “draw” the solder into the joint. Only a very small amount of solder is required for a strong joint. Excessive solder is unsightly and often has to be filed off.
If the solder wire you are using is physically too large, you can flatten it with a hammer and cut small pieces (with tin snips or heavy scissors) to lay in the joint.
A gas torch is best used to heat large surfaces or thick materials, or metals/alloys needing high temperatures, but the need for heat shielding of surrounding material may make such use impossible. Soldering irons are often used for small joints but, again, the problem of heat damage to surrounding material may negate their use.
By passing a strong electrical current to a carbon electrode touching the joint, for a very short time, intense heat can be applied to a very small area. Solder at this point melts into a tiny droplet,making a very clean joint. Heat dissipation is minimal. A low voltage and high amperage power source is required to do this.
I have used both a simple 12V auto battery charger and a 12V 7 Amp-hour gel cell to resistance solder. The negative electrode wire was attached to one side of the joint and power passed through a foot-operated pushbutton switch to the positive carbon electrode. The push-button switch should only be activated for ½-1 second to avoid burning out the switch (you can buy an automotive pushbutton for about $15 rated for 25 amps). With the charger, I have measured a current of 20-30 amps, which is hard on the charger and the switch, but it is of a very short duration. My charger has an overload protector to avoid overheating. With the battery, I get much higher current and the connecting wires actually get red hot (overkill). To avoid arcing, do not to remove the positive electrode until the circuit is broken by the switch.
Model railings and ladders require vertical posts and horizontal rails, and while it may be possible to make butt joints that are most realistic, they can be very difficult to achieve. The vertical posts should be of continuous length (for strength), with the rails being short pieces in-between. Except for the end posts, you have to solder two horizontal pieces to the post at the same time, one on either side. Perhaps a fixture could be made to hold things in place, but usually you are working with small dimensions and a confined space. I find it easier to use lap joints which are much easier.
As an example, I use 20 or 22 gauge copper or brass wire (be sure to check that the wire is actually an electrical conductor, I bought some 24 and 28 gauge "copper "utility wire that was non-conductive and I have no idea what it was made of). I cut appropriate long lengths of wire and pinched one end in a vise. I held the other end with vise grips, and clean the wire with fine emery paper. Then I pulled and stretch the wire. This stiffens the wire and straightens it.
I made a paper pattern of the railing and put it on a board. Then I stretched the horizontal rails over the pattern with one end attached to small springs and clamped at the other end. That keeps tension on the wire – it will expand slightly when heated. The I slipped the posts under the rails at the required spacing, above the paper pattern.
I clipped a ground (negative) wire to the end of the rail I wished to solder, and hand-held the positive carbon electrode against the joint. With the other hand, I touched a fine solder wire to the joint and depressed the power switch with my foot. The end of the solder instantly melted as a droplet (be careful not to push more solder into the joint). Note that feed wires to the electrodes got hot!
(See also: http://www.raymondwalley.com or http://technitoys.com/diy-resistance-soldering-outfit)
Typical fixture for soldering a railing.
I tried using a brass rod as an electrode, instead of the carbon rod; it doesn’t work! All that happened was that the insulation melted off of one my feed wires. Because it was a dead short, it overheated the smallest wire in the circuit. The small resistance in the carbon is just enough to heat the joint.
High Temperature (Silver) Soldering
For a much stronger joint, silver-bearing solder is used, but it requires high temperature before it will melt. This solder is very much more expensive. Commercially available flux or a borax/water paste is used as a flux, and cleanliness is very important. The flux residue after making the joint is also much more difficult to remove. I use fine grain sandpaper to clean the surfaces, and lemon juice to remove any fingerprint oils.
Usually the metals to be joined must be heat-red before this solder will melt. Propane or Mapp gas is adequate for very small joints, but for big joints a combination of propane/oxygen, mapp/oxygen or acetylene/oxygen may be necessary to provide enough heat. Sometimes a heat trap or reflector/shelter made from firebrick can be employed to improve the heating efficiency.
Silver Soldering equipment and materials.