IGLESIAS Y EL FIN DEL TIEMPO

This concept can be represented by the simple formula:

H a I² Rt

Attempts to calculate the amount of heat developed by conventional math-ematical techniques are doomed to failure.

A brief glance at Figure 6.21 will show that the total resistance in the circuit will always be unknown. In this case the fact that one can determine the electrical resistance of the components and the brazing filler material with a high degree of accuracy is of only passing interest. The source of the difficulty is that the individual contact resistance that exists between adjacent members of the joint will vary from case to case. Moreover, these values change while the brazing process is in progress

In situations where dissimilar materials are used as electrodes the formula for total resistance given in Figure 6.21 would need to be modified to take account of that fact. Since the mathematical determination of the total resis-tance in the circuit is impossible to calculate, such considerations are really only of academic interest..

6.2.1 Types of Resistance Heating in Common Use

For practical purposes there are two different methods of resistance heating.

In the first, the electrodes are made of carbon, and this has a relatively high electrical resistivity. This method is known as carbon resistance heating.

With the second method electrodes of low electrical resistance are employed. Examples include copper; copper-chromium; molybdenum; or one of the sintered-metal products such as copper-tungsten, silver-tung-sten, or silver-tin oxide. With these types of electrodes the bulk of the heat is generated by the passage of current through the work and as a result of the contact resistance between the face of the electrode and the work.

This technique is known as either direct resistance or direct interface resistance heating.

Figure 6.22 shows the electrode arrangements for carbon resistance and direct resistance heating. A combination is shown in which one carbon and one copper-alloy electrode are used to achieve uniform heating of two com-ponents that have dissimilar resistances or different heat capacities. As can be seen, the carbon electrode is placed in contact with the material having the lower electrical resistance or the higher heat capacity.

6.2.1.1 Carbon Resistance Heating

The handheld tongs illustrated in Figure 6.23(a) exemplify the simplest type of system for the application of carbon resistance heating. These find rela-tively wide use in the plumbing industry for the brazing or soldering of copper tubes.

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FIGURE 6.21

Sketch showing the various factors that influence the total electrical resistance of an assembly that is to be brazed by resistance heating.

FIGURE 6.22

Some suggested electrode arrangements.

Contact resistance = R₁ Parent material 1 = R₂ Contact resistance = R₃ Filler Material resistance = R₄ Contact resistance = R₅

Parent material 2 = R₆ Contact resistance = R₇

Bottom electrode

Total Resistance = 2R1 +(R₁+ R₂ + R₃ + R₄ + R₅+ R_{6 +} R₇).

Resistance of electrode material

R1

Copper

Carbon Tungsten Copper

Molybdenum Alloy

Sintered copper-tungsten Copper

Pure copper Copper alloy

Figure 6.23(b) shows a slightly more complex carbon resistance heating machine. This bench-mounted machine is simple in design and suited for the general application of the technique.

Figure 6.23(c) shows a machine that is based on the concept of a conventional resistance welding machine. The carbon electrodes, which are usually shaped to the size and form of the component being brazed, are held in water-cooled, copper-alloy clamps. The provision of water cooling prevents the heat from the electrodes being conducted to the moving parts of the machine.

As illustrated in Figure 6.23(b) and Figure 6.23(c), machines can be con-structed such that the electrodes move in either the horizontal or the vertical plane. In the latter case it is sometimes arranged that only the upper electrode be made from carbon, and the lower be made from a shaped block of metal.

In such circumstances the block serves not only as an electrode, but also as the fixture for the assembly being produced. Movement of the electrodes is normally achieved by mechanical means, leaving the operator free to apply the flux and brazing material when these have not been preplaced.

As a general rule, flux is not preplaced in the joint. This is because until it melts flux is a truly excellent electrical insulator. If it were to be preplaced in the joint there would be a very high probability that its presence would

(a)

(b) FIGURE 6.23

(a) A series of handheld tools specifically designed for carbon resistance heating. (b) A fine example of a bench-mounted carbon resistance brazing machine. (Photos courtesy of Solbraze Limited, Erith, Kent, U.K.)

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prevent current flow in the circuit. This would prevent heating of the carbon electrodes and brazing would not occur. When the design of a machine intended for resistance heating calls for the current to flow from one elec-trode, through the joint area, and into the other elecelec-trode, the use of flux must be avoided. The use of equipment of this type in these conditions should be restricted to those cases where copper-to-copper joints are required to be made and where one of the self-fluxing, phosphorus-bearing alloys can be used (e.g., EN1044 Type CP102).

(c) FIGURE 6.23C

A heavy-duty carbon resistance brazing machine where, unlike the machine shown in Figure 6.23(b), the electrodes move in the vertical plane. (Photo courtesy of Solbraze Limited, Erith, Kent, U.K.)