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workshop by Handyman
Benchwise: Silver solder and brazing
ALTHOUGH the practice today seems to be wherever possible to weld with steel rod or hard brass in order to obtain the most secure join, in some cases this can be a disadvantage as with steel there is no possibility of a reheat to disconnect the joined items without damage. With tubular operating rods, stays, pipes, etc, whereresetting may be desirable from time to time, a ncw tube or frame stay could be replaced with no more heat than that obtainable from a good blow-lamp, there is something to be said for the use of silver solder, or at least low temperature brazed joints. The necessary skill is well worth acquiring and is not at all difficult.
Silver solder, often termed spelter, is usually an alloy of brass and silver and will melt at a lower temperature than pure brass. A flux is needed, and the basis of this is borax, either in powder form or as a paste. Although reliable proprietary brands are available, one important point must be remembered. It is that the flux sets glass hard if left to cool and can leave you with the problem of removing it; using a chisel or hammer can damage the work.
Brush the flux
The answer is to brush away the flux just as soon as the work has cooled to below red heat and the solder has begun to firm or set; if you allow the work to go cold you will be Left with that chipping job, or a rather tough filing job to trim the join.
Although the heavy-duty blow-lamp was the mainstay for this type of soldering for so long, today it is more than likely that the wry-acetylene torch will supply the heat. If so, a much larger jet should be used than for welding, and the trick to learn with the torch is to keep the envelope of the flame and not the higher temperature cone in contact with the work; also keep the flame constantly on the move over the largest possible area of the join in order to achieve a good single run, clear of breaks.
At times it may be a difficult matter to coat completely the work area with flux, but it is no use going to the next stage until the base area is coated. To do this properly, it is necessary to apply a little heat as the flux is spread or painted on, and the liquid part of the flux will dry off leaving the actual flux compound exactly where required.
Try to arrange things so that the runs are slightly downhill as gravity can assist the capillary action in filling the join.
Knowing the right moment to apply the solder is difficult for the beginner. A very good guide is to watch the flux itself as it changes appearance under increasing heat. As the heating up proceeds the flux begins to bubble, and then gradually settles into a thin clear liquid—a second or two later the correct temperature will be reached and that is the time to apply the solder.
The feeding stick or strip should be applied firmly to the join and it should begin to melt at once and can be fed along the join. This should be done steadily and the aim should be to over-fill slightly the join, as this will counteract the effects of shrinkage as the work cools down.
The main point to remember at all times is that the melted spelter should never be dropped on the join; unfortunately, trainees tend to do just this which then undoes the good work of the flux and allows air to get at the spelter. Even a fraction of a second is critical; once the flux shows clear and thin, let the spelter strip down in contact with the work so that the transfer of material is achieved cleanly under the envelope of flux and flame.
However, another little trick of the trade—it may seem something of a contradiction to what I have just said about the flame envelope—is to keep the main flame a little ahead of the spelter, but not to leave the spelter out of the envelope. The idea of this is that the slightly hotter area just ahead of the spelter will, in fact, have a strong attraction for it, and it can be seen that the spelter will follow the flame closely.
This is something that needs a little practice, as you are watching several things happening at the same time, but once this little trick has been perfected, whereby the flux is clear, the spelter is down on the work, and the flame envelope is ahead and just covering the spelter, you will be surprised at the pace at which you can go. What's more, you will be left with a perfectly soldered join, quite clear of air bubbles, cracks, etc, and solid right through.
This trick of leading the spelter with the flame has many applications, and will be a big help in circular tasks, and for sealing joints and pipe seatings on tanks, etc. Again I must mention that many a job will be spoiled if clamped in a steel vice, etc, because the heat then dissipates too rapidly for the work to retain its correct heat. Quite often the trainee will argue that if he is doing a fine job on some branch pipe, etc, he must use the bench vice to hold the work steady or at the angle called for. However, I still believe in the fire brick covered work area used in conjunction with a mole or vice grip to hold the article as the best answer, and I have seen good use made of suspended alligator clips to hold the work above but in contact with the fire bricks.