1. Field of the Invention
The present invention relates in general to an improvement of a method for preparation of an electron gun of a color cathode-ray tube, and more particularly to an improved laser welding method for efficiently assembling all of the parts of the electron gun in which a coining part is formed on a weld zone prior to the laser welding.
2. Description of the Prior Art
As is well known to those skilled in the art, a color cathode-ray tube (or a color picture tube) has an electron gun which emits electrons, accelerates these electrons in order to make these electrons become an electron beam, and thereafter, focuses the electron beam on a phosphor screen.
With reference to FIG. 1 showing a construction of a conventional electron gun of a color cathode-ray tube, the electron gun includes a cathode 1 for emitting electrons which are to become an electron beam. This cathode 1 includes a heater therein and is supported by a cathode supporter 2 which is arranged at the front of the cathode 1. The electron gun 1 further includes four anodes, first to fourth anodes 3a to 3d, for condensing and accelerating these electrons emitted from the cathode 1 and making these electrons become the electron beam, and thereafter, focusing the electron beam on a phosphor screen (not shown) of the color cathode-ray tube. In order to mount this electron gun on a neck part of the cathode-ray tube, this electron gun also includes a shield cup 4 at the front of the fourth anode 3d. This shield cup 4 has a valve space contact and a getter spring which are welded thereto.
Here, the aforementioned parts 1, 3a, 3b, 3c, 3d and 4 of a conventional electron gun are arranged in a line and connected, using electric insulation glass beads, to each other with predetermined intervals therebetween. These parts of the electron gun are, for the most part, made of nonmagnetic stainless steel thin plates and assembled into the electron gun by welding at about 100 weld zones.
In preparation of the electron gun of the color cathode-ray tube, the parts of the electron gun have been generally welded using a conventional electric resistance welding.
FIG. 2 shows a representative embodiment of conventional electric resistance welding. As depicted in this drawing, an upper metal part 6a and a lower metal part 6b to be welded to each other are arranged between the upper and lower electrodes 5a and 5b in such a manner that the one metal 6a lies upon the other metal 6b. At this state, turning power on simultaneously with compressing the metals 6a and 6b generates electric resistance heat between the metals 6a and 6b and this causes a weld nugget 7 to be formed between the upper and lower parts 6a and 6b. As a result, the metals 6a and 6b are welded to each other.
However in such electric resistance welding, the upper metal 6a is overheated when power for the electrodes 5a and 5b is turned on and this causes the liquid metal of the upper part 6a to disperse in all directions in order to form a plurality of metal splashes. In this respect, the use of conventional electric resistance welding in preparation of the electron gun presents a problem to the result electron gun. Otherwise stated, each of the metal splashes resulted from the electric resistance welding has electric charge which causes a discharge inferiority of the electron gun. Furthermore, since the metal parts of the electron gun should be compressed during the electric resistance welding, the metal parts are apt to be deformed and, as a result, desired accuracy of the metal parts may be not accomplished. Hence, another problem of this electric resistance welding is resides in that it causes the operational performance of the result color cathode-ray tube combined with the electron gun to be necessarily deteriorated.
In order to overcome the problems introduced in using the conventional electric resistance welding for preparation of the electron gun, there has been proposed the use of laser welding.
In conventional laser welding, a laser medium arranged between two reflectors of a laser generator is energized in order to emit a light which in turn becomes an intense laser beam of high energy by reciprocating between the reflectors. This intense laser beam is then focused, using a focusing lens, on a desired weld zone of the metal parts to be welded. As a result, a weld nugget is formed at the weld zone and the metal parts are welded to each other by the weld nugget.
With reference to FIGS. 3A and 3B showing conventional laser welding for welding an upper part 8a to a lower part 8b in order to provide a cap-shaped product, respectively, a laser beam B is focused on a weld zone between the parts 8a and 8b using a focusing lens 10.
Here, FIG. 3A shows an embodiment of the conventional laser welding wherein the laser beam B is laterally focused on the weld zone, while FIG. 3B shows another embodiment of the conventional laser welding wherein the laser beam B is downwardly inclinedly focused on the weld zone.
In these conventional laser weldings, when the thickness of the upper part 8a is at least similar to that of the lower part 8b and the weld zone is provided, similarly to a conventional butt welding, at the corner of the juncture of the parts 8a and 8b as depicted in FIGS. 3A and 3B, the weld penetration is formed between the upper and lower parts 8a and 8b with at least similar penetration depth and, in this respect, there is no problem in providing a desired weld strength for the result cap-shaped product.
However, when the thickness of the upper part 8a is different from that of the lower part 8b as shown in FIGS. 6A and 6B, it is very important to determine the diameter d (d.sub.1 and d.sub.2) of the weld nugget of the juncture of the parts 8a and 8b in order to provide the desired weld strength for the result product.
If described in detail, it is noted that the weld nugget diameter d (d.sub.1 and d.sub.2) is a very important factor in determining the weld strength P of a spot welding using the laser beam as represented in the following expressions. That is, when you let the area of the weld zone be A and the shearing strength of the material of the parts 8a and 8b be .sigma.s, the weld strength P is represented by the expressions: EQU A=.pi..multidot.(d/2).sup.2 EQU P=A.multidot..sigma.s=.pi..multidot.(d/2).sup.2 .multidot..sigma.s
wherein d is the weld nugget diameter at the juncture of the parts 8a and 8b.
Conventionally, the parts of the electron gun mostly have thicknesses ranged from 0.1 mm to 1.0 mm, respectively. In addition, as depicted in FIGS. 4, 6a and 6b, the known spot welding using the laser beam generally causes an evaporation section 9b having a diameter D to be formed on a surface of a part, for example, the upper part 8a, on which the laser beam is to be focused by the focusing lens 10. The weld nugget diameter d (d.sub.1 and d.sub.2) is rapidly reduced in proportion to the depth H of the weld penetration 9a.
Particularly, when the upper part 8a has a thickness larger than that of the lower part 8b as depicted in FIG. 6A, the weld nugget has a very small diameter d.sub.1 at the juncture between the upper and lower parts 8a and 8b. In this respect, it is difficult to determine the welding condition when the thickness of the upper part 8a is larger than that of the lower part 8b as described above.
For example, when the thickness of the upper part 8a is 1.0 mm and the thickness of the lower part 8b is 0.2 mm, the diameter d.sub.1 of the weld nugget at the juncture between the upper and lower parts 8a and 8b is only about 1/3 of the diameter d.sub.2 of the weld nugget at the juncture when the thickness of the upper part 8a is 0.2 mm and the thickness of the lower part 8b is 1.0 mm as shown in FIG. 6B. Thus, it is noted that the weld strength is deteriorated when the thickness of the upper part 8a is larger than that of the lower part 8b.
Furthermore, in order to determine the optimum weld strength of the parts 8a and 8b in the known laser welding for preparation of the electron gun, there is a requirement to control the capacity of the laser generator one by one in accordance with the thicknesses of the parts 8a and 8b. Otherwise, the existing focusing lens 10 should be substituted with another focusing lens having a different radius of curvature .rho. and satisfying the optimum welding condition. In this regard, another problem of the known laser welding is resides in that it is very difficult to determine the welding condition.
In accordance with the above, it is noted that the known laser welding can be used only when the thickness of the upper part 8a is at least similar to that of the lower part 8b and the weld zone is provided, similarly to the conventional butt welding, at the corner of the juncture of the parts 8a and 8b as depicted in FIGS. 3A and 3B or when the thickness of the upper part 8a is smaller than that of the lower part 8b. Another disadvantage of this known laser welding is resides in that it cannot be used when the thickness of the upper part 8a is larger than that of the lower part 8b.