Mechanized Induction Brazing

Throughout this chapter it has been stressed that for maximum heating efficiency the inductor should surround the work. Where relatively complex parts have to be brazed and fixtures have to be employed (to maintain the alignment of one component with respect to another) it is sometimes very difficult (and even impossible) to arrange for the inductor to surround the joint. Under these circumstances, and assuming that the joint area is circular, two C-shaped coils that can be moved toward each other so as to more or less surround the joint area might provide a solution to this problem. As pointed out in Chapter 5, in most mass production brazing procedures and to attain the necessary output rate, it is very often necessary to heat the parts over a series of stations.

In cases of this type, and particularly if the inductors have to be of complex shape, they will need to be mounted on hydraulically operated reciprocating slides that move them into and away from their heating position. Such a machine would possess a very high level of complexity, and the effect of this would be a high capital cost. It would not be unusual to find that a machine of the type described could be nearly twice the cost of a machine that could braze the parts at the same rate, but where flame, rather than induction, had been chosen as the heating source. As a result of these considerations, rotary

FIGURE 6.15

Two alternative forms of inductors for series heating. (From Roberts, P.M., Industrial Brazing, Newnes-Butterworth, 1975. With permission.)

indexing brazing machines that employ multistation induction as the heat source are a comparative rarity.

The foregoing does not mean that mechanized brazing with induction heating is always economically impossible to justify. There are some excellent examples to be found, and the four-station rotary indexing machine illus-trated schematically in Figure 6.16 is a prime example.

This machine was specially developed to braze carbide-tipped rotary burs to their steel supports. A diagrammatic representation of the type of com-ponent being produced is shown in Figure 6.17.

This machine and its mode of operation is another excellent example of the application of the fundamental principles of a technical process analysis to the solution of a relatively complex brazing problem. It will be interesting to list the special factors that had to be taken into account before a cost-effective and technically sound solution was developed and a machine was built (see Figure 6.18).

It is important to keep the following in mind:

1. There is differential expansion between the tungsten carbide bur and the steel shank. This leads to the generation of high levels of stress in the brazed joint during the cooling stage of the process. The larger

FIGURE 6.16

Schematic diagram of a four-station tungsten carbide-tipped bur brazing machine.

FIGURE 6.17

A diagrammatic representation of the parts being brazed on the machine pictured in Figure 6.18.

Tip centralization

Heating Automatic brazed-part removal

with optical pyrometer temperature control

Loading

Carbide Bur

Brazing filler material

Steel shank

2112_book.fm Page 162 Tuesday, November 4, 2003 1:07 PM

FIGURE 6.18

A four-station rotary indexing machine fitted with induction heating for the brazing of tungsten carbide-tipped burs. (Courtesy of VerMoTec GmbH, St. Ingbert, Germany.)

the diameter of the bur, the greater the facet of the problem (see Chapter 10, Section 10.6.22).

2. The appropriate filler material and flux must be selected (see Chap-ter 10, Section 10.6.2.4).

3. Because the coupling factor in uncontrolled induction heating would lead to severe overheating of the outer edges of the steel pin, close temperature regulation of the assembly is a mandatory requirement (see Section 6.1.3.1).

4. The requirement for the finished assembly demands that its tip be automatically centralized on its support with an axial symmetry of 0.08 mm. To achieve this requires that the tip be moved on its shank under tightly controlled conditions while the filler alloy is molten.

5. Because the parts are hot when they leave the tip-centralization station, automatic unloading of the brazed parts from the machine by a pick-and-place mechanism is mandatory.

6. Because of the risk of flux entrapment within the joint it is necessary to move the tip with respect to the shank while the alloy and flux are molten. To meet this need the components have to be assembled so that their respective axes are eccentric with respect to each other.

This provides for a sufficient amount of tip movement during auto-matic tip centralization.

7. The carbide bur is balanced on the pin with a minimum of ancillary fixturing. Very smooth motion of the rotary table is mandatory (see Chapter 5).

Figure 6.19 shows a tungsten carbide tipped bur at brazing temperature in its fixture on the machine illustrated in Figure 6.18.

It is interesting that four different versions of the machine described above are available; the operational parameters are described in Table 6.1.

As can be seen from this table, machines with two, three, four, or five stations are available. As might be expected, and as already implied in Table 5.1, the complexity level of these four machines increases as the number of stations increases.

The cycle time of each machine is not only related to the time taken to heat the parts to brazing temperature, but is also dependent to some extent on the efficiency of the operator and the weight of the parts that are to be brazed. The following comments related to each of the options underline this point:

1. Two-station machine: Depending upon the size of bur to be pro-cessed, this machine operates with a cycle time of between a mini-mum of 20 sec and a maximini-mum of 30 sec. To some extent, the cycle time is a machine parameter rather than an operator parameter.

2. Three-station machine: The cycle time is a minimum of 15 sec.

2112_book.fm Page 164 Tuesday, November 4, 2003 1:07 PM

3. Four-station machine: Because the finished burs are removed from the machine automatically a minimum cycle time of 12 sec is readily achievable.

4. Five-station machine: Because of the relatively high level of automa-tion present on this type of machine it is physically the largest in the range. Since indexing time and the tip centering are machine variables, it is normally the case that the machine runs with a cycle time between 12 and 20 sec.