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Joining metal parts is critical for making and repairing your instrument, but most players don’t know the terminology or the reasons for each type of joint.
The soldered joint
Soldering is joining metal parts by applying heat and flowing solder into that joint. Solder is an alloy of metals that melt at a lower temperature than the work pieces. When the alloy cools and crystallizes it forms a bond between surfaces.]
Soft solder is just that, a fairly soft alloy of metals which often contains lead and tin. A soft solder joint is appropriate for joining fairly large surfaces that don’t take heavy forces or shocks. A shock can break the soft solder.
Hard solder, or bearing solder, contains metals that have higher tensile strength, and is often used for small surface area joints that support shearing forces, such as water key saddles. Bearing solder is used more and more in instrument construction because it does not contain lead.
The brazed joint
Brazing (not braising) is a high temperature process that is accomplished very much like soldering, but with much higher temperatures and with elemental or alloyed metal that has extremely high tensile strength, such as thin silver wire. Brazing is done where the joint has to be extremely strong, sometimes stronger than the component parts. Sheet metal will often be brazed at a joint, as will nickel parts such as key levers.
The welded joint
Welding is an extremely high temperature process, usually done between parts of the same elemental metal, that actually melts the two surfaces and fills the joint with the molten material and filler. Welding is rarely used in instrument repair. It is most often done during fabrication, such as in joining sheet metal to form tubes. A welded joint is permanent and is as strong as the component parts.
Most often your repair technician will solder parts together. But, no matter which process is used surface preparation is absolutely critical to a good bond. A broken joint that is simply reheated to allow the solder to flow again is not likely to be a permanent joint.
Oxidized solder, dirt, and oil can all inhibit a good, strong bond. If you have a solder joint that breaks repeatedly your tech has to ascertain the forces pulling the joint apart, and to re-assemble the part appropriately to prevent further failures.
Your technician should always know both how and why to form the proper joint between parts.
