JP2001345045A - Forming method and device of beam hole in electrode of electron gun - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a beam hole which has an expanding diameter part of a predetermined shape with a good precision without giving an adverse effect to a surface shape of electrode material. SOLUTION: At the time of forming a beam hole in the electrode material, after a concave part having an expanding diameter part 22 is formed in an electrode material surface using the first punch 20 beforehand and a shape forming process is performed in which the material of a share of concave is made to escape to a relief hole 12 of the first die 14, the concave part is punched using the second punch to form the beam hole. Further, this is a beam hole forming device, and equipped with the first die 14, the first punch 20, and the second punch.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for forming a beam hole for passing an electron beam through electrodes constituting an electron gun.

[0002]

2. Description of the Related Art Generally, a plurality of electrodes are arranged on an electron gun (for example, an electron gun for a projection tube, a color electron gun, a mini electron gun, etc.), and a beam hole for passing an electron beam is provided at the center of each electrode. Are formed. Further, in the beam hole of a specific electrode (generally, the second grid), the diameter of the hole formed on the periphery of the hole close to the adjacent high-voltage side electrode (generally, the third grid) is formed by a tapered surface or a curved surface having an arc-shaped cross section. It is common to provide a part. Due to the formation of the enlarged diameter portion, the concentration of the magnetic field is reduced, and the distance between the high voltage side electrode and the adjacent high voltage side electrode can be increased. As a result, the focus characteristics and the withstand voltage characteristics of the cathode ray tube can be improved. it can.

[0003] Conventionally, as a method of forming a beam hole with an enlarged diameter portion as described above, for example, Japanese Patent Application Laid-Open No. 7-5 / 1995.
No. 7657 discloses that the electrode material 92 is placed on a die 90 having an escape hole 93 as shown in FIG. 9 and a processing punch 94 having a tapered surface forming portion 94a is lowered. A method is disclosed in which a through hole, that is, a beam hole is formed in the electrode material 92, and at the same time, the tapered surface forming portion 94a is pressed against a peripheral portion thereof to form a tapered surface which is an enlarged diameter portion.

[0004]

In the method disclosed in the above publication, focusing on the moment when the tip of the working punch 94 is pressed against the surface 92a of the electrode material 92, the pressing causes the surface portion of the electrode material 92 to be pressed. However, before the material for the recesses escapes to the escape holes 93 below, the working punch 94 punches the electrode material 92 as it is, so that the “material for the recess” is applied to the peripheral portion of the working punch 94. It will be pushed out. For this reason, after the perforation work, the peripheral edge of the through-hole rises toward the surface 92a of the electrode material 92, and there is a possibility that the shape accuracy (flatness) of the electrode material surface 92a is reduced.

The present invention has been made in view of the above circumstances, and provides a method and apparatus capable of accurately forming a beam hole having an enlarged diameter portion having a predetermined shape without adversely affecting the surface shape of an electrode material. The purpose is to do.

[0006]

According to one aspect of the present invention, there is provided a method of forming a beam hole for passing an electron beam through an electrode of an electron gun. An electrode material is mounted on the mounting surface of a die having a first relief hole opened on the mounting surface, and a first diameter substantially equal to the diameter of the first relief hole is formed on the surface of the electrode material. By pressing a first punch having a circular tip surface and a diameter-enlarging portion whose diameter increases rearward from the periphery of the tip surface, the shape of the tip surface and the diameter-enlarging portion of the punch on the surface of the electrode material Forming a concave portion having a shape corresponding to the shape and releasing the material corresponding to the concave portion into the release hole; and a mounting surface and a second opening having a diameter larger than the first diameter and opened to the mounting surface. 2 on the mounting surface of a die having two escape holes. Electrode material is placed, is intended to include a punching step of punching the second punch having a second diameter smaller than the maximum diameter of the recess in the first diameter or more and the recess of the electrode material.

According to this method, before the actual punching (punching step) is performed, a molding step of forming a concave portion using the first punch is performed in advance, and at this stage, the first punch is expanded. A concave portion having an enlarged diameter portion corresponding to the diameter portion is formed, and the material of the concave portion is released into the relief hole of the die. Therefore, there is no possibility that the meat is raised on the surface side of the electrode material, and the surface shape can be kept good. After this molding step, a punching step using a second punch having a second diameter larger than the diameter (first diameter) of the front end face of the first punch and smaller than the maximum diameter of the recess is performed. As a result, a beam hole having a second diameter is formed in the electrode material while leaving the enlarged portion formed on the electrode surface by the enlarged portion of the first punch on the electrode surface side.

In this method, if the first punch having a diameter-expanding portion that expands in a circular arc shape in a direction protruding inward from the front end surface is used, the diameter of the beam hole is reduced gradually. An object having a curved surface can be processed with high accuracy, and the focus characteristic and the withstand voltage characteristic can be made higher.

The present invention also relates to a method for forming a beam hole for passing an electron beam through an electrode of an electron gun, comprising a mounting surface and a die having a first escape hole opened in the mounting surface. An electrode material is mounted on the mounting surface, and a circular tip surface having a first diameter substantially equal to the diameter of the escape hole is formed on the surface of the electrode material, and the diameter of the electrode material is increased rearward from the periphery of the tip surface. By pressing a first punch having a first enlarged portion to be formed, a concave portion having a shape corresponding to the shape of the distal end surface of the punch and the first enlarged portion is formed on the surface of the electrode material, and the concave portion is formed. A molding step of releasing a portion of the material into the release hole; and a mounting surface of the die having a mounting surface and a second release hole having a diameter larger than the first diameter and opened to the mounting surface. The electrode material is placed on top of the electrode material, and the concave portion of the electrode material is Perforation of a second punch having a perforated portion having a second diameter greater than or equal to a first diameter and smaller than the maximum diameter of the concave portion, and a second enlarged diameter portion that increases in diameter rearward from the perforated portion. A second punch punched out of the concave portion proceeds in the punching direction as it is, and presses the second enlarged diameter portion against the peripheral portion of the through-hole to form the second peripheral portion into the second peripheral portion. And a shaping step of shaping into a shape corresponding to the enlarged diameter portion.

According to this method, the second punch used in the punching step is advanced in the punching direction as it is, following the above-described molding step and punching step.
The peripheral portion of the through hole formed in the punching step can be shaped into a shape corresponding to the second enlarged diameter portion without increasing the number of steps. With this shaping, the centering accuracy between the center of the beam hole and the center of the enlarged diameter portion of the beam hole can be improved.

In this method, as the first punch, a first enlarged portion having an arc-shaped cross-section protruding inward from the tip end surface and expanding into a cross-section having a first radius of curvature. The second punch has an arcuate cross-section in a direction protruding inward from the perforated portion and has a first shape as the second punch.
By using a member having a second enlarged portion having an arc-shaped cross section having a second radius of curvature equal to or less than the radius of curvature of
Eventually, a beam portion having a gently curved surface can be machined with high precision as the enlarged diameter portion of the beam hole, and the focus characteristics and the withstand voltage characteristics can be further improved.

According to the present invention, there is provided an apparatus for forming a beam hole for passing an electron beam through an electrode of an electron gun, comprising: a mounting surface on which an electrode material is mounted; and a first opening formed on the mounting surface. A first die having a relief hole, a circular tip surface having a first diameter substantially equal to the diameter of the first relief hole, and a diameter-enlarging portion increasing in diameter from the periphery of the tip surface toward the rear. A first punch having a concave portion having a shape corresponding to the shape of the distal end surface and the enlarged diameter portion formed on the surface by being pressed against the surface of the electrode material; and a mounting surface on which the electrode material is mounted. A second die having an opening in the mounting surface and having a second relief hole having a diameter larger than the first diameter; and a second die having a diameter larger than the first diameter and smaller than a maximum diameter of the recess. A second punch having a diameter of It is those with a door.

In this device, the first punch is:
It is preferable to use a member having a diameter-enlarging portion that expands in a circular arc shape in a section protruding inward from the tip end surface. The use of the first punch makes it possible to accurately process a beam having a gently curved surface as the enlarged diameter portion of the beam hole, thereby improving focus characteristics and withstand voltage characteristics.

Further, the second punch has a perforated portion having the second diameter, and a second enlarged portion which increases in diameter from the perforated portion to the rear, and an electrode formed by the perforated portion. The second portion is formed on the periphery of the through hole formed by punching the material.
It is preferable that the enlarged diameter portion is pressed to shape the peripheral portion into a shape corresponding to the second enlarged diameter portion. If you use this second punch,
The punching step and the shaping step can be performed at once.

In the second punch, the first enlarged-diameter portion has an arc-shaped cross-section in a direction protruding inward from a tip end surface of the first punch and has a first arc-shaped radius of curvature. The second enlarged-diameter portion has an arc-shaped cross-section that is convex inward from the tip end surface of the second punch, and
It is more preferable that the diameter be expanded in a circular arc shape having a second radius of curvature equal to or less than the radius of curvature of. By using the second punch, finally, a beam hole having a gentle curved surface can be accurately processed as the enlarged diameter portion of the beam hole, and the focus characteristic and the withstand voltage characteristic can be further improved. .

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the drawings. Although this embodiment shows an example in which the present invention is applied to the second electrode G as shown in FIGS. 8A and 8B, the present invention can also be applied to the formation of a beam hole in another electrode. Things.

The second electrode G shown in FIG.
As shown in FIG. 1, a disk-shaped top wall 10 and a peripheral wall 11 extending downward from the outer peripheral edge of the top wall 10 and continuing over the entire circumference are integrally provided. A beam hole 40 that penetrates the top wall 10 is provided in the center of the top wall 10, and the electron beam passes through the beam hole 40. The side of the inner peripheral surface of the beam hole 40 facing the third electrode (FIG. 8)
(Right side in FIG. 3), an electrode-side enlarged diameter portion 42 is formed as shown in FIG.

Side holes 50, which also penetrate the top wall 10 and have a larger diameter than the beam holes 40, are provided on both left and right sides of the beam holes 40, and these side holes 50 are used for assembling an electron gun (not shown). The positioning pin of the jig is inserted.

Next, a method of forming the beam hole 40 in the top wall 10 of the second electrode G will be described. This method includes the following molding, punching, and shaping steps.

1) Molding Step (FIGS. 1A and 1B) In this molding step, an apparatus having a first die 14 and a first punch 20 is used.

The first die 14 has a flat mounting surface 13,
A first relief hole 1 having a diameter d1 and opening in the mounting surface 13
And 2.

The first punch 20 is made of a material harder than the electrode material (generally, a super hard material is preferable). The first punch 20 has a circular tip surface 17 having a first diameter D1 and a tip surface having the first diameter D1. 17 has an enlarged diameter portion 18 that increases in diameter from the outer periphery toward the rear (upward in the figure).

The first diameter D1 is substantially equal to the diameter d1 of the first relief hole 12 (more preferably, equal to or slightly larger than the diameter d1).
And the diameters D1 and d1 are set to be smaller than the target diameter of the beam hole 40.

In this embodiment, the enlarged diameter portion 18 has a shape extending from the outer periphery of the front end face 17 to an inwardly projecting circular cross section to reach the diameter of the punch body, and has a first radius of curvature. R1.

In this apparatus, the top wall (electrode material) 10 of the second electrode G is mounted on the mounting surface 13 of the die 14, and the central portion of the top wall 10 has the first escape hole 12. (FIG. 1A). Then, the first punch 20 is lowered from above, and the tip face 17 thereof is lowered.
Then, the enlarged diameter portion 18 is pressed against the surface of the ceiling wall 10 (FIG. 2B).

As a result, a concave portion having a bottom surface 21 and an electrode-side enlarged-diameter portion 22 having a shape corresponding to the tip end surface 17 and the enlarged-diameter portion 18 is formed on the front side (upper side in the figure) of the top wall 10. In other words, the surface-side portion of the top wall 10 is dented, but the material of the dented portion is released into the first relief hole 12 immediately below the concave portion. That is, on the back surface of the top wall 10, the protruding portion 24 protruding into the first escape hole 12 by an amount corresponding to the concave portion is formed. In this way, the material of the dent is released to the back side, so that the meat is prevented from rising on the front side (the upper side in the figure) of the top wall 10, and a good surface shape is maintained.

2) Punching Step (FIGS. 2A and 2B) In this punching step and a shaping step described later, the second die 2 is used.
An apparatus provided with the sixth and second punches 30 is used.

The second die 26 has a flat mounting surface 27,
A second relief hole 2 having a diameter d2 opened in the mounting surface 27
8 is provided.

The second punch 30 is provided with the first punch 2
As in the case of 0, it is made of a material harder than the electrode material (generally, a super-hard material is preferable), and has a cylindrical perforated portion 32 having a second diameter D2, and a rear portion (upward in the figure) from the perforated portion 32.
The diameter increasing portion 34 increases in diameter toward
4 and a main body 33 following the main body 4.

The second diameter D2 is a diameter corresponding to the target diameter of the beam hole 40. Therefore, the second diameter D2 is larger than the first diameter D1 and smaller than the maximum diameter of the concave portion (the outer diameter of the electrode-side enlarged portion 22). The diameter d2 of the escape hole 28 is the second
Is set slightly larger than the diameter D2.

In this embodiment, the diameter-enlarging portion 34 has such a shape that the diameter thereof expands in an arc-shaped cross section in a direction protruding inward from the rear end of the perforated portion 32 to reach the diameter D 3 of the main body portion 33. The radius of curvature is a second radius of curvature R2 slightly smaller than the first radius of curvature R1.

In this apparatus, the top wall (electrode material) 10 of the second electrode G is connected to the projection 24 formed on the back surface thereof.
Is mounted on the mounting surface 13 of the die 14 at a position where it fits into the second escape hole 28 (FIG. 2A). Then, the second punch 30 is lowered from above, and the center of the concave portion is punched by the perforated portion 32 in the thickness direction of the ceiling wall. As a result, a beam hole 40 having a diameter corresponding to the second diameter D2 is formed in the center of the top wall 10, and the electrode-side enlarged portion 22 is left in the peripheral portion on the surface side (FIG. )).

3) Shaping Step (FIG. 3) This shaping step is performed at a stretch following the punching step. That is, in this shaping step, the second punch 30 punched from the top wall 10 is advanced in the punching direction (downward in the drawing) as it is, and the enlarged diameter portion 34 of the punch 30 is already formed on the top wall 10. This is a step of pressing the electrode-side enlarged diameter portion 22 from above. By this pressing, the electrode-side enlarged-diameter portion 22 is shaped, and the electrode-side enlarged-diameter portion 42 having a radius of curvature corresponding to the second radius of curvature R2 is formed (see FIG. 4).

If this shaping step is performed, even if the axis of the concave portion formed in the molding step before the step slightly deviates from the axis of the second punch 30 used in the punching step, the final step is performed. The centering of the electrode-side large-diameter portion 42 to be formed can be accurately performed, and the shape accuracy of the beam hole 40 can be further improved. However, this shaping step can be omitted depending on the shape accuracy required for the beam hole 40.

In this embodiment, the electrode-side enlarged portion 42 having an arc-shaped cross section is formed at the periphery of the beam hole. However, the shape of the enlarged portion can be set as appropriate, for example, a tapered shape. It may be. In this case, as shown in FIG. 5A, a bottom surface 21 and a tapered electrode-side enlarged diameter portion are formed by using a first punch 20 'having a tapered first electrode-side enlarged diameter portion 22'. After the formation of the recess with 22 ', FIG.
As shown in (b), the central part is the second punch 30 '.
In this case, the punching may be performed at the perforated portion 32. Further, the first electrode-side enlarged-diameter portion 22 'is attached to the second punch 30'.
If a second enlarged diameter portion 34 'having a taper angle substantially equal to that of the second enlarged diameter portion 34' is formed and the second enlarged diameter portion 34 'is pressed against the electrode-side enlarged diameter portion 22', the electrode can be formed. Side enlarged diameter part 2
The electrode-side enlarged portion 42 'can be formed by shaping 2' (see FIG. 6).

In addition, the present invention can take the following embodiments, for example.

In the above embodiment, the first enlarged diameter portion 18
By pressing the second enlarged diameter portion 34 to the end (that is, to a depth equivalent to the respective radii of curvature) to the end, the electrode side finally having a depth equivalent to the second radius of curvature R2 is pressed. Although the method of forming the enlarged diameter portion 42 has been described, in the present invention, the enlarged diameter portions 18 and 34 do not necessarily have to be completely pressed against the electrode material, and only a part thereof is pressed (that is, the enlarged diameter portions 18 and 34). 34 may be pressed shallowly). In this case, as shown in FIG. 7, the electrode-side enlarged portion 42 having the depth dimension δ smaller than the second radius of curvature R2 is finally formed.

In the present invention, regardless of the number of beam holes formed in one electrode, for example, a plurality of beam holes can be formed simultaneously using a plurality of punches.

The “first die” and “second die” according to the present invention are not necessarily separate members, and a single die is referred to as “first die” and “second die”. May be shared. That is, the first release hole and the second release hole are provided in a single die, and the electrode material is positioned in the first release hole in the molding step, and the electrode material is positioned in the second release hole in the punching step. It may be performed.

[0040]

EXAMPLE For a 0.4 mm thick electrode material, a first diameter D
1 is 0.5 mm and the first radius of curvature R1 is 0.12 mm.
By performing a punching step and a shaping step with a second punch having a diameter of 0.592 mm and a second radius of curvature R2 of 0.10 mm,
It was confirmed that a good beam hole was formed without disturbing the shape on the surface side of the electrode material.

[0041]

As described above, according to the present invention, there is provided a molding step in which a concave portion having an enlarged diameter portion is previously formed on the surface of an electrode material with a first punch, and the material of the concave portion is released into a relief hole of a first die. Is performed, and then the recess is punched out with a second punch, so that unlike the conventional method of punching out the electrode material suddenly, the electrode material that has lost the escape space by pressing the punch is on the electrode surface side. Inconveniences such as swelling can be avoided beforehand, and there is an effect that a desired beam hole can be accurately formed while maintaining the electrode surface in a good shape.

[Brief description of the drawings]

FIGS. 1A and 1B are cross-sectional views showing a molding step according to an embodiment of the present invention.

FIGS. 2A and 2B are cross-sectional views showing a punching step according to the embodiment of the present invention.

FIG. 3 is a cross-sectional view showing a shaping step according to the embodiment of the present invention.

FIG. 4 is a sectional view showing the shape of an electrode-side enlarged-diameter portion formed by the molding step and the shaping step, respectively.

FIG. 5A is a cross-sectional view illustrating a molding step according to another embodiment of the present invention, and FIG. 5B is a cross-sectional view illustrating a subsequent shaping step.

6 is a cross-sectional view showing a shape of an electrode-side enlarged-diameter portion formed by a molding step and a shaping step shown in FIG.

FIG. 7 is a cross-sectional view showing the shape of an electrode-side enlarged portion formed by a molding step and a shaping step according to another embodiment.

8A is a cross-sectional view showing an example of an electrode to which the present invention is applied, and FIG. 8B is an enlarged view of a portion A in FIG.

FIG. 9 is a cross-sectional view showing a conventional beam hole forming method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Top wall (electrode material) 12 1st relief hole 13 1st die mounting surface 14 1st die 17 Tip surface 18 1st enlarged diameter part 20 1st punch 22 Electrode side enlarged diameter part 26th 2 die 27 27 mounting surface of 2nd die 28 2nd relief hole 30 2nd punch 32 perforated section 34 2nd enlarged diameter section 40 beam hole D1 1st diameter D2 2nd diameter G electrode R1 1st 1st radius of curvature R2 2nd radius of curvature

Claims (8)

[Claims] 1. A method of forming a beam hole for passing an electron beam through an electrode of an electron gun, comprising: a mounting surface; and a mounting surface of a die having a first relief hole opened in the mounting surface. An electrode material is placed thereon, and a circular tip surface having a first diameter substantially equal to the diameter of the first relief hole is formed on the surface of the electrode material, and the diameter increases toward the rear from the periphery of the tip surface. By pressing a first punch having an enlarged diameter portion to form a concave portion having a shape corresponding to the shape of the distal end surface of the punch and the enlarged diameter portion on the surface of the electrode material, the material corresponding to the concave portion is formed by the above-mentioned method. A molding step of releasing the electrode material onto the mounting surface and a die having a second relief hole having a diameter larger than the first diameter and having an opening in the mounting surface and the mounting surface; Placed, and the concave portion of the electrode material is not less than the first diameter and A second smaller than the maximum diameter of the recess;
And a punching step of punching with a second punch having a diameter of: 2. A method for forming a beam hole in an electrode of an electron gun according to claim 1, wherein the first punch comprises:
A method of forming a beam hole in an electrode of an electron gun, characterized in that the beam hole has a diameter-enlarging portion which expands in a circular arc shape in a direction protruding inward from the tip surface. 3. A method for forming a beam hole for passing an electron beam through an electrode of an electron gun, comprising: a mounting surface; and a mounting surface of a die having a first relief hole opened in the mounting surface. An electrode material is placed thereon, and a circular tip surface having a first diameter substantially equal to the diameter of the relief hole is provided on the surface of the electrode material, and a first diameter is increased from the periphery of the tip surface toward the rear. By pressing a first punch having an enlarged diameter portion, a concave portion having a shape corresponding to the shape of the tip end surface of the punch and the first enlarged diameter portion is formed on the surface of the electrode material, and the concave portion is formed. A molding step of releasing material into the relief hole; and a mounting surface and a die having a second relief hole having a diameter larger than the first diameter and having an opening in the mounting surface. The electrode material is placed, and the concave portion of the electrode material is set to the first diameter or less. Punching the second punch having a perforated portion having a second diameter smaller than the maximum diameter of the concave portion above and a second enlarged diameter portion increasing in diameter from the perforated portion to the rear side with the perforated portion. Process, the second punch punched out of the concave portion proceeds in the punching direction as it is, and the second enlarged portion is pressed against the peripheral portion of the through hole, so that the peripheral portion becomes the second enlarged portion. Forming a beam hole in an electrode of an electron gun. 4. A method for forming a beam hole in an electrode of an electron gun according to claim 3, wherein said first punch comprises:
The first punch having a first enlarged diameter portion having an arc-shaped cross-section protruding inward from the front end face and having a first radius of curvature and expanding in a cross-section arc is used as the second punch. Characterized in that it has a second enlarged-diameter portion having an arc-shaped cross-section protruding inward from the portion and having a second arc-shaped cross-section having a second radius of curvature equal to or less than the first radius of curvature. A method for forming a beam hole in an electrode of an electron gun. 5. An apparatus for forming a beam hole for letting an electron beam pass through an electrode of an electron gun, comprising: a mounting surface on which an electrode material is mounted; and a first escape hole opened in the mounting surface. A first die having a circular tip surface having a first diameter substantially equal to the diameter of the first relief hole, and a diameter-enlarging portion that expands rearward from the periphery of the tip surface. A first punch that is pressed against the surface of the electrode material to form a recess having a shape corresponding to the shape of the tip end surface and the enlarged diameter portion on the surface, a mounting surface on which the electrode material is mounted, and a mounting surface on which the electrode material is mounted; A second die having an opening in the mounting surface and having a second escape hole having a diameter larger than the first diameter; and a second die having a diameter larger than the first diameter and smaller than a maximum diameter of the recess. And a second punch for punching a recess formed in the electrode material. An apparatus for forming a beam hole in an electrode of an electron gun. 6. An apparatus for forming a beam hole in an electrode of an electron gun according to claim 5, wherein said first punch has a diameter-enlarging portion which expands in a circular arc shape in a direction inwardly projecting from a tip end surface thereof. An apparatus for forming a beam hole in an electrode of an electron gun, comprising: 7. An apparatus for forming a beam hole in an electrode of an electron gun according to claim 5, wherein said second punch has a pierced portion having said second diameter and a diameter increasing from said pierced portion toward the rear. A second enlarged-diameter portion that presses the peripheral portion of the through-hole formed by punching out the electrode material by the perforated portion, so that the peripheral edge is formed by the second enlarged-diameter portion. An apparatus for forming a beam hole in an electrode of an electron gun, wherein the apparatus is configured to be shaped into a shape corresponding to an enlarged diameter portion. 8. An apparatus for forming a beam hole in an electrode of an electron gun according to claim 7, wherein said first enlarged diameter portion has an arc-shaped cross section projecting inward from a tip end surface of said first punch. The second enlarged diameter portion has an arc-shaped cross-section that is convex inward from the tip end surface of the second punch, and the first enlarged diameter portion has an arc-shaped cross section having a first radius of curvature. A beam hole forming device for an electrode of an electron gun, wherein the diameter of the beam hole is increased in a circular arc shape having a second radius of curvature equal to or smaller than the radius of curvature of the electron gun.