Metalworking
Practical Action
until the action ceases. A little extra acid added after bottling improves the cleaning power of
the flux.
Brazing is a process similar to soldering, in principle, (and is often called hard soldering).
Instead of using soft solder (melting point 200°C), brass is used (melting point 900°C).
Flame heating is almost always used, although the use of a forge is possible. A gas-air torch
or a powerful paraffin blowlamp is convenient sources of heat, as is oxyacetylene equipment if
it is available. As flux, proprietary preparations and borax are equally effective.
A brazed joint is often as strong as the surrounding metal, and in copper work especially it is
an excellent and neat joint.
Welding is the most generally used jointing process. It consists of melting together the joint
edges and allowing the result to solidify, with or without additional (filler) metal.
Oxyacetylene equipment is the most versatile, as it can be used for welding, brazing, cutting
and heating. Steel down to about 0.5 mm can be welded and gaps filled easily, but distortion
tends to be a difficulty because of the somewhat diffuse heat. The main expenses are in the
rental of the cylinder and the supply of the gases. The need for a nearby supply network
limits the usefulness of the oxyacetylene process to industrialised areas.
Metallic arc welding (”stick welding”) is economical and versatile. A transformer type welder
of 180 amps, will weld steel of thickness between 1.5 and 10 mm relatively easily, and with
care it is possible to weld steel outside this range of thickness. With special electrodes other
ferrous metals can be welded with this equipment. A welding generator with D.C. output can
weld all metals in common use, but the equipment cost is much higher. Consumable
electrodes are the main material expense.
With a carbon arc torch, an arc welder can be used on jobs normally tackled with
oxyacetylene equipment, but the stability of the arc flame tends to be poor, making it difficult
to weld thin material. However, for brazing, it is quite adequate.
Contact Practical Action Southern Africa for a design of a low-cost welding machine.
Finishing
Any product will look much more attractive if a suitable finish has been added and it will be
easier to sell. There are a number of finishes that can be applied to metal products. In most
cases, the finishes will help protect the product from corrosion. The simplest is to paint them.
Other finishes are:
Galvanising
Galvanising of steel is done by emersion into a bath of molten zinc which will chemically
bond to the surface. This protects the steel from corrosion.
Electroplating
Electroplating produces a deposited layer of metal onto a product by the application of an
electric current so a suitable solution.
Enamelling
Products can be given a hardwearing but attractive finish by enamelling them. Once the
product has been coasted then it needs to be heated in a kiln.
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