Nick Baines

Most of my soldering is done using a temperature controlled iron rated at 80W. It has interchangeable bits but I only ever use two. Both as the usual "screwdriver" shape, one is about 3mm wide for the big stuff including frames and spacers, and one is about 1.5mm wide for fine stuff. Just occasionally I use a gas torch when some serious heating is required, or when access with the soldering iron is difficult. I have never used a resistance soldering unit (RSU) so I cannot say anything about them, other than to comment that since the safety regulations governing the sale and disposal of all mains electrical equipment were tightened up, it has been difficult to obtain them.

For most soldering, I use resin-cored solder, mainly because I possess a large quantity of it. Occasionally I use low temperature solder (145°C) when it is necessary to ensure existing joints in the vicinity are not disturbed, but by proper planning of the work, that can usually be avoided. With both solders, Carr's red label flux is helpful, but I'm careful to use it in very small quantities. That way I don't have lots of evaporated flux that rusts any piece of steel within reach, or have to clean up lots of flux residues.

There is not a great deal of advice I can pass on about soldering, because it really is just a matter of practice. The two essentials are cleanliness and heat. Using a liquid flux does mean that the surfaces do not have to be quite as clean as they otherwise should be, because its main purpose is to render the surfaces chemically clean so that the solder bonds firmly, but I do try to give them a good polish with wet-and-dry paper or a glass fibre brush, and then to avoid handling them, before soldering. Flux is applied with a small paintbrush. The aim should be to use just enough flux, and that means there should be a small liquid meniscus visible where the join is to be made, but no more than that. A big puddle of flux does not make soldering easier or better, and runs the risk of spreading fumes or droplets of flux about the workbench where it can caust rusting. Excess flux can be removed before soldering by first wiping the brush on a rag to remove any flux, and applying it dry to the work.

 

It is then a matter of applying sufficient heat to ensure that the solder melts and flows right through the join. It is hot enough when the solder is mobile and has a silvery surface. Capiliary action draws the molten solder between the surfaces to be joined. It is the solder in the joint itself, not the fillets of solder on the outside surfaces, that give the joint its strength. The aim should be to use enough solder to fill the join, and that will be sufficient.

 

Melting some solder on the bit before bringing it to the work is important, because that makes for a better contact between the bit and the work, and enables the work to heat up more quickly. For small parts, the molten solder on the bit can then be allowed to flow into the join, and that should be enough solder to make the join. For larger parts, it is necessary to add solder to the work while it is hot enough to melt the solder. That, of course, means you need two hands, one to hold the iron and one to hold the solder, so there is not a hand free to hold the work. In such circumstances, some form of clamping is essential.

The clamping plate

This has proved to be an invaluable tool for holding parts together for soldering, and occasionally for other assembly purposes. It is made from an off-cut of hardwood, planed and sanded to a smooth and very level surface. Wood is a good thermal insulator, which means less heat is required to solder the join, and the plate never gets too hot to handle. Into this plate are set a number of bushes of brass or aluminium, which are drilled and tapped for set screws. The clamps themselves are located where they are required on the plate, and held in place by the set screws. The clamps have balancing screws with large, flat heads, so as to be able to clamp materials of different thicknesses. The clamps are made from stainless steel, which is ideal for the job because it does not rust even when exposed to flux, and does not conduct heat very well. I wish I could say that I thought about that in advance, but in reality I happened to have some strips of stainless steel of just the right size in my scrap box. I also have a couple of additional strips of hardwood, planed flat and square, that can be used to hold parts at right angles to each other for soldering. Where necessary, I can also use small toolmakers clamps for additional holding.

 

The best way to clean up after soldering is to avoid having to do most of it! Do not use more solder than is necessary to make the join. Plan the assembly so as to solder from the inside or the underside of the finished model, where it will not be seen. The solder should be between the joint faces, and having large fillets of solder outside the join adds little strength and looks ugly. Ideally the solder should be all but invisible. Inevitably, however, there will be some solder to clean up. I do not like using a file for the purpose, because the solder clogs it and it is necessary to stop and clean the file all too often. A fine grade of wet-and-dry paper is good, and for cleaning up joints made on flat surfaces (as opposed to corners), I have a sheet of such paper stuck to a flat board on which I can rub the joined parts. (The same sheet is useful for cleaning up parts before soldering, and for giving machined or filed surfaces and edges a final polish. I stick it down with double-sided tape and replace it when it gets worn.) A glass fibre brush is also useful for removing small quantities of solder in difficult locations.

The best tool, however, is a chisel, not the woodworking type, but one specially made for the purpose. This is just a piece of square or rectangular hard brass or nickel silver with one end filed off obliquely, keeping the edges and corners sharp. This can then be used to remove excess solder from surfaces and corners. Some form of handle is required to be able to manipulate the tool. It should not be necessary to use a lot of force, if you find you are, you are most likely either trying to remove too much solder at a time, or the cutting edge is blunt. It does get blunt fairly quickly, but can easily be resharpened with a file, followed by a polish with the wet-and-dry.

 

 

Just recently I had to remove some solder from an acute corner between a splasher and the boiler, and the chisel bit would not reach in there. I therefore made a triangular bit, filed up from square stock brass. It only takes a few minutes to make bits of any shape you need.

The reason for choosing brass or nickel silver is that it will not mark the underlying material. I also have similar scrapers made from old needle files, but being much a harder material, they tend to scratch the parts that have been soldered as well as removing the solder. Such scratches can be difficult to polish out subsequently.