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JOINT DESIGN

In brazing, the filler metal is distributed by capillary action. This requires the joints to have close tolerances and a good fit to produce a strong bond. Brazing has three basic joint designs (fig. 6-13): lap, butt, and scarf. These joints can be found in flat, round, tubular, or irregular shapes.

Lap Joints

The lap joint is one of the strongest and most fre­quently used joint in brazing, especially in pipe work The primary disadvantage of the lap joint is the increase

Figure 6-13.-Three types of common joint designs for brazing.

in thickness of the final product. For maximum strength, the overlap should be at least three times the thickness of the metal. A 0.001-inch to 0.003-inch clearance be­tween the joint members provides the greatest strength with silver-based brazing filler metals. You should take precautions to prevent heat expansion from closing joints that have initial close tolerances.

Butt Joints

Butt joints are limited in size to that of the thinnest section so maximum joint strength is impossible. Butt joint strength can be maximized by maintaining a joint clearance of 0.001 to 0.003 of an inch in the finished braze. The edges of the joint must be perfectly square to maintain a uniform clearance between all parts of the joint. Butt joints are usually used where the double thickness of a lap joint is undesirable. When double­metal thickness is objectionable and you need more strength, the scarf joint is a good choice.

Scarf Joints

A scarf joint provides an increased area of bond without increasing the thickness of the joint. The area of bond depends on the scarf angle cut for the joint. Usu­ally, an area of bond two to three times that of a butt joint is desirable. A scarf angle of 30 degrees gives a bond area twice that of a 90-degree butt joint, and an angle of 19 1/2 degrees increases the bond area three times.

Figure 6-14.-Joints designed to produce good brazing results.

Figure 6-15.-Some well-designed joints that have been prepared for brazing, and some poorly designed joints shown for comparison

Figure 6-14 shows some variations of butt and lap joints designed to produce good brazing results. A com­parison of good and bad designed joints is shown in figure 6-15.

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