When it comes to joining two pieces of metal, tradespeople have a wide selection: mechanical fasteners, adhesives, soldering, welding and brazing. The first three options produce weaker joints, which are preferable in certain circumstances. Consider an assembly in which a pump must be connected to pipes. Because the pump has a finite lifetime and will eventually need replacement, it makes little sense to use a permanent joining technique. Instead, metalworkers would opt for a mechanical fastener, which could be easily disassembled when the pump failed.
If the goal, however, is to create a strong, permanent joint -- one that has superior resistance to shocks, vibrations and leaking -- the best candidates are welding and brazing. Knowing which technique to choose depends on the requirements of the project. One important consideration is the overall size of the finished piece. Metalworkers often choose welding if they're producing large assemblies, brazing if they're producing smaller assemblies. Why? Because brazing can only be achieved by heating all or most of the base assembly to the temperature at which the filler metal flows. If the assembly gets too large, heat dissipates more quickly than it builds up. Welding, on the other hand, doesn't rely on large-scale heating. In fact, a strong welded joint can be accomplished with intense, localized heating only.
Next, metalworkers must be concerned with the types of metals being joined. Thin sections, for example, are more likely to warp than thick sections. And the composition of the metals is just as important. Welding works better if someone is trying to join similar types of metal. That's because the welding process melts both the base metals and the filler. If a project calls for joining two wildly different metals -- say copper and stainless steel -- welding would melt one metal long before the other. Brazing, however, can readily join dissimilar metals because it's possible to find a filler that's compatible with both base metals and has a melting point lower than the two.
Finally, a metalworker must consider production requirements. Does he need 10 finished pieces or 10,000? Both welding and brazing can be done manually, but brazing is far more suitable for automation. Automated welding can be accomplished, but it takes sophisticated equipment and a highly repeatable fabrication process. Brazing offers much more flexibility and, as a result, can be set up quickly and cost-effectively.
All of which brings us back to those $200 aviator sunglasses -- and why companies such as Ray-Ban choose brazing in their manufacturing plants. Sunglasses possess all of the hallmarks of a braze-friendly assembly. Strong joints? Check. Thin, delicate pieces of metal? Check. Millions of units produced in an automated environment? Check. Now you can impress your friends with your cool shades and wow them with your knowledge of ancient metallurgical techniques.
Author's Note: How Brazing Works
When I was a kid, my next-door neighbor's father worked as a welder on the Washington Metro rapid transit system, so I learned a bit about the process from him. And I saw my father use a soldering iron in the garage. But until I began working on this article, I hadn't heard of brazing or thought much about the complex manufacturing required to produce something as outwardly simple as a pair of sunglasses.
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