Protective device for centerless grinding machine

Abstract

The utility model discloses a protective device for a centerless grinding machine, which comprises a machine body, a guide wheel, a grinding wheel and a guide plate, the guide wheel and the grinding wheel are arranged on the machine body, the guide plate is positioned between the guide wheel and the grinding wheel, a protective cover is arranged on the machine body and covers the guide wheel and the grinding wheel, at least two supports are arranged on the protective cover, and scale positioning rods are arranged on the supports. The lower end of the scale positioning rod extends into the protective cover and is connected with a limiting plate, and the limiting plate is horizontally arranged above the guide plate and extends in the length direction of the guide plate. According to the protective device for the centerless grinding machine in the technical scheme, a product is arranged on the guide plate and is fed in the length direction of the guide plate, the product is limited through the limiting plate arranged above the product, and potential safety hazards such as grinding wheel jamming caused by the fact that the product jumps excessively or is clamped into a grinding wheel are prevented.

Description

Technical Field

[0001] This utility model relates to the field of centerless grinders, and in particular to a protective device for centerless grinders. Background Technology

[0002] A centerless grinder is a type of grinder that does not require the workpiece to be positioned by an axis for grinding. It mainly consists of three mechanisms: a grinding wheel, a guide wheel, and a guide plate. The grinding wheel is responsible for the actual grinding work, the guide wheel controls the rotation of the workpiece and the feed rate of the workpiece, and the guide plate supports the workpiece during grinding. These three components can be combined in several ways, except when grinding is stopped, but the principle is the same.

[0003] The characteristic of centerless grinders is that the workpiece does not need to be clamped; it can be directly pushed between the guide wheel and the grinding wheel for grinding. However, since the product itself cannot be clamped and fixed, some blanks that are deformed or products with a small height-to-diameter ratio may jump during the entire movement process as the guide wheel pushes the workpiece close to the grinding wheel for grinding and cutting. In severe cases, the product may even fly out or get stuck in the grinding wheel, posing a safety hazard.

[0004] For example, Chinese Patent Publication No. CN106736911B, published on July 3, 2020, entitled "Centerless Grinding Machine", includes a dressing device for performing a dressing process of a grinding wheel and an adjusting wheel. The dressing device includes a single dressing unit arranged at a lower position between the grinding wheel and the adjusting wheel. The dressing unit includes a dresser that is vertically movable between a standby position and a dressing position. The dresser is also movable relative to the grinding wheel and the adjusting wheel in a longitudinal axial direction to dress the grinding wheel and the adjusting wheel at the dressing position.

[0005] The drawback of existing patents is that, during the grinding and cutting process of existing centerless grinders, some blanks that are deformed or products with a small height-to-diameter ratio will jump during the entire movement. In severe cases, the product may fly out or get stuck in the grinding wheel, posing a safety hazard. Utility Model Content

[0006] The purpose of this invention is to solve the problem that in existing centerless grinders, the blank may deform or the product with a small height-to-diameter ratio may jump or get stuck in the grinding wheel during the processing, causing safety hazards. This invention provides a protective device for centerless grinders to prevent the product from jumping or getting stuck in the grinding wheel during the processing of centerless grinders.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A protective device for a centerless grinder includes a machine bed, a guide wheel and a grinding wheel on the machine bed, and a guide plate located between the guide wheel and the grinding wheel. A protective cover is provided on the machine bed, covering the guide wheel and grinding wheel. At least two supports are provided on the protective cover, and a scale positioning rod is provided on each support. The lower end of the scale positioning rod extends into the protective cover and is connected to a limit plate. The limit plate is horizontally positioned above the guide plate and extends along the length of the guide plate. In this protective device for a centerless grinder, the product is placed on the guide plate and fed along the length of the guide plate. The limit plate above the product limits the product, preventing excessive product movement or jamming in the grinding wheel, which could cause safety hazards such as grinding wheel jamming.

[0009] Before centerless grinding, adjust the scale positioning rods mounted on the support to ensure that the scale of the scale positioning rods connected to the limiting plate is uniform, thus ensuring that the limiting plate is horizontally positioned above the guide plate. The scale positioning rods, positioned above the guide plate, facilitate easy retrofitting of existing centerless grinders. Their simple structure and intuitive operation ensure the limiting plate remains horizontal, preventing significant product movement on the guide plate and reducing safety hazards. During setup, the scale positioning rods limit the limiting plate to a certain distance above the product.

[0010] The protective cover is installed over the grinding wheel and guide wheel. The ruler positioning rod is mounted above the protective cover via a bracket, which is also installed over the grinding wheel and guide wheel along with the protective cover. The protective cover serves two purposes: firstly, it prevents debris from flying and reduces safety hazards; secondly, it facilitates the installation of the ruler positioning rod.

[0011] Preferably, the lower end face of the limiting plate is an arc surface with an opening facing downwards, and the arc surface extends along the length direction of the guide plate. The arc surface is compatible with the product.

[0012] Preferably, the ruler positioning rod is provided with a sliding groove extending along its length. The ruler positioning rod is mounted on the bracket via a locking member, the locking end of which passes through the sliding groove and connects to the bracket. The locking member can be a bolt, a Torx screw, etc. The locking end of the locking member passes through the sliding groove and connects to the bracket, while the limiting end of the locking member presses the ruler positioning rod firmly onto the bracket, facilitating easy and intuitive adjustment of the ruler positioning rod's height.

[0013] Preferably, the scale positioning rod is provided with graduations distributed along the flight direction of the scale positioning rod, and the graduations cooperate with the bracket.

[0014] Preferably, the width of the limiting plate is smaller than the minimum distance between the outer peripheral walls of the guide wheel and the outer peripheral walls of the grinding wheel. This prevents the grinding wheel and guide wheel from damaging the limiting part during processing.

[0015] Preferably, the length of the limiting plate is greater than the length of the guide plate.

[0016] Preferably, there are two brackets, each positioned above one end of the limiting plate, so that the limiting plate is arranged in parallel.

[0017] Preferably, the bracket has three sections, with two sections positioned above both ends of the limiting plate and the third section positioned above the center of the limiting plate. This prevents damage to the limiting plate caused by the product impacting its center if the limiting plate is too long.

[0018] Preferably, the bracket includes two supports and a connecting rod connecting the two supports, and the scale positioning rod is disposed on the connecting rod.

[0019] Preferably, the limiting plate includes a rotating shaft supported above the guide plate by a ruler positioning rod and two limiting plates rotatably mounted on the rotating shaft. The limiting plates extend along the length of the guide plate, and a torsion spring hinge is provided at the connection point of the lower end faces of the two limiting plates to make the two limiting plates form an inverted V shape. The rotating shaft is fixedly mounted at the lower end of the ruler positioning rod, and the limiting plates are rotatably mounted on the rotating shaft. The connection between the limiting plates and the rotating shaft can be referenced to the hinge. A torsion spring hinge is provided at the connection point of the lower end faces of the two limiting plates, which keeps the two limiting plates in an inverted V shape at a certain angle, so that the two limiting plates have a certain elasticity and that the limiting plate and the product itself will not be affected by the product impact.

[0020] Therefore, this utility model has the following beneficial effects: the product is set on the guide plate and fed along the length of the guide plate. The product is limited by the limiting plate set above the product to prevent the product from jumping too much or getting stuck in the grinding wheel, causing safety hazards such as grinding wheel jamming; the protective cover serves to prevent debris from flying and reduce safety hazards; on the other hand, it facilitates the setting of the ruler positioning rod, with a simple structure and convenient operation. Attached Figure Description

[0021] Figure 1 This is a structural schematic diagram of a second embodiment of the present invention.

[0022] Figure 2 This is a side view of a second embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of a bracket, a ruler positioning rod, and a limiting plate in Embodiment 2 of this utility model.

[0024] Figure 4 This is a schematic diagram of a scale positioning rod and a limiting plate in Embodiment 2 of this utility model.

[0025] Figure 5 This is a structural schematic diagram of Embodiment 4 of the present invention.

[0026] Figure 6 This is a schematic diagram of a scale positioning rod and a limiting plate in Embodiment 5 of this utility model.

[0027] As shown in the picture:

[0028] 1. Bed frame; 2. Guide rollers; 3. Grinding wheel; 4. Guide plate; 5. Protective cover.

[0029] 6. Bracket 6.1, Support 6.2, Connecting rod 6.

[0030] Scale positioning rod 7, slide groove 7.1,

[0031] Limit plate 8, pivot 8.1, limit plate 8.2, twist hinge 8.3,

[0032] Locking component 9. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the utility model will be further described below in conjunction with the accompanying drawings and specific implementation methods.

[0034] Example 1, as Figure 1 , Figure 2 , Figure 3 , Figure 4 The protective device for a centerless grinder shown includes a bed 1, a guide wheel 2 and a grinding wheel 3 on the bed 1, and a guide plate 4 located between the guide wheel 2 and the grinding wheel 3. A protective cover 5 is provided on the bed 1, covering the guide wheel 2 and the grinding wheel 3. At least two brackets 6 are provided on the protective cover 5, and a scale positioning rod 7 is provided on the bracket 6. The lower end of the scale positioning rod 7 extends into the protective cover 5 and is connected to a limit plate 8. The limit plate 8 is horizontally arranged above the guide plate 4 and extends along the length direction of the guide plate 4.

[0035] A centerless grinder is a type of grinder that does not require workpiece axis positioning for grinding. It mainly consists of three mechanisms: a grinding wheel (3), a guide wheel (2), and a guide plate (4). The grinding wheel (3) performs the actual grinding work, the guide wheel (2) controls the workpiece rotation and feed rate, and the guide plate (4) supports the workpiece during grinding. These three components can be combined in several ways, except when grinding is stopped; the principle is the same. The characteristic of a centerless grinder is that it does not require workpiece clamping; the workpiece is directly pushed between the guide wheel (2) and the grinding wheel (3) for grinding. However, because the workpiece itself cannot be clamped and fixed, some blanks that are deformed or have a small height-to-diameter ratio may jump during the grinding process as the guide wheel (2) pushes the workpiece closer to the grinding wheel (3). In severe cases, the workpiece may fly out or become stuck in the grinding wheel (3), posing a safety hazard.

[0036] In order to solve the problem that the blank deformation or products with small height-to-diameter ratio may jump or get stuck in the grinding wheel 3 during the processing of existing centerless grinders, which may cause safety hazards, a protective device for centerless grinders is provided to prevent products from jumping or getting stuck in the grinding wheel 3 during the processing of centerless grinders.

[0037] In this technical solution, a protective device for a centerless grinder is provided. The product is placed on a guide plate 4 and fed along the length of the guide plate 4. The product is limited by a limiting plate 8 located above the product to prevent excessive product movement or jamming in the grinding wheel 3, which could cause the grinding wheel 3 to jam and other safety hazards.

[0038] Before centerless grinding, adjust the scale positioning rod 7 mounted on the support 6 to ensure that the scale of the scale positioning rod 7 connected to the limiting plate 8 is uniform, so that the limiting plate 8 is horizontally positioned above the guide plate 4. Positioning the scale positioning rod 7 above the guide plate 4 facilitates retrofitting of existing centerless grinders. The scale positioning rod 7 has a simple structure and is intuitive and easy to operate, ensuring that the limiting plate 8 remains horizontal, preventing significant product movement on the guide plate 4, and reducing safety hazards. During setup, the scale positioning rod 7 limits the limiting plate 8 to a certain distance above the product.

[0039] The protective cover 5 is installed over the grinding wheel 3 and the guide wheel 2. The scale positioning rod 7 is mounted above the protective cover 5 via a bracket 6, which is also installed over the grinding wheel 3 and the guide wheel 2 along with the protective cover 5. The protective cover 5 serves two purposes: firstly, to prevent debris from flying and reduce safety hazards; and secondly, to facilitate the installation of the scale positioning rod 7.

[0040] Specifically, the bracket 6, the ruler positioning rod 7, and the limit rod can all be made of alloy tool steel as the main material to improve wear resistance.

[0041] Example 2, as Figure 1 , Figure 2 , Figure 3 , Figure 4 The protective device for a centerless grinder shown includes a bed 1, a guide wheel 2 and a grinding wheel 3 mounted on the bed 1, and a guide plate 4 located between the guide wheel 2 and the grinding wheel 3. A protective cover 5 is mounted on the bed 1, covering the guide wheel 2 and the grinding wheel 3. At least two supports 6 are mounted on the protective cover 5, and a scale positioning rod 7 is mounted on each support 6. The lower end of the scale positioning rod 7 extends into the protective cover 5 and is connected to a limiting plate 8. The limiting plate 8 is horizontally positioned above the guide plate 4 and extends along the length of the guide plate 4. The lower end face of the limiting plate 8 is a downward-facing arc surface, extending along the length of the guide plate 4. The arc surface adapts to the product and guides the product, preventing it from moving laterally.

[0042] The ruler positioning rod 7 is further optimized. It features a groove 7.1 extending along its length. The ruler positioning rod 7 is mounted on the bracket 6 via a locking element 9. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6. The locking element 9 can be a bolt, a Torx screw, etc. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6, while the limiting end of the locking element 9 presses the ruler positioning rod 7 firmly onto the bracket 6, facilitating easy and intuitive adjustment of the ruler positioning rod 7's height.

[0043] The scale positioning rod 7 is further optimized. The scale positioning rod 7 is provided with scales distributed along the flight direction of the scale positioning rod 7, and the scales cooperate with the bracket 6.

[0044] The limiting plate 8 is further optimized so that its width is less than the minimum distance between the outer peripheral walls of the guide wheel 2 and the outer peripheral walls of the grinding wheel 3. This prevents the grinding wheel 3 and the guide wheel 2 from damaging the limiting part during processing.

[0045] The limiting plate 8 is further optimized, and its length is greater than that of the guide plate 4.

[0046] The bracket 6 is further optimized. The bracket 6 includes two supports 6.1 and a connecting rod 6.2 connecting the two supports 6.1. The scale positioning rod 7 is set on the connecting rod 6.2.

[0047] Example 3, as Figure 1 , Figure 2 , Figure 3 , Figure 4 The protective device for a centerless grinder shown includes a bed 1, a guide wheel 2 and a grinding wheel 3 on the bed 1, and a guide plate 4 located between the guide wheel 2 and the grinding wheel 3. A protective cover 5 is provided on the bed 1, covering the guide wheel 2 and the grinding wheel 3. At least two brackets 6 are provided on the protective cover 5, and a scale positioning rod 7 is provided on the bracket 6. The lower end of the scale positioning rod 7 extends into the protective cover 5 and is connected to a limit plate 8. The limit plate 8 is horizontally arranged above the guide plate 4 and extends along the length direction of the guide plate 4.

[0048] In this embodiment, there are two brackets 6, which are respectively positioned above both ends of the limiting plate 8 to ensure that the limiting plate 8 is arranged in parallel.

[0049] The limiting plate 8 has been further optimized. Its lower end face is an arc surface with an opening facing downwards, extending along the length of the guide plate 4. The arc surface adapts to the product and guides it, preventing it from moving laterally.

[0050] The ruler positioning rod 7 is further optimized. It features a groove 7.1 extending along its length. The ruler positioning rod 7 is mounted on the bracket 6 via a locking element 9. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6. The locking element 9 can be a bolt, a Torx screw, etc. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6, while the limiting end of the locking element 9 presses the ruler positioning rod 7 firmly onto the bracket 6, facilitating easy and intuitive adjustment of the ruler positioning rod 7's height.

[0051] The scale positioning rod 7 is further optimized. The scale positioning rod 7 is provided with scales distributed along the flight direction of the scale positioning rod 7, and the scales cooperate with the bracket 6.

[0052] The limiting plate 8 is further optimized so that its width is less than the minimum distance between the outer peripheral walls of the guide wheel 2 and the outer peripheral walls of the grinding wheel 3. This prevents the grinding wheel 3 and the guide wheel 2 from damaging the limiting part during processing.

[0053] The limiting plate 8 is further optimized, and its length is greater than that of the guide plate 4.

[0054] The bracket 6 is further optimized. The bracket 6 includes two supports 6.1 and a connecting rod 6.2 connecting the two supports 6.1. The scale positioning rod 7 is set on the connecting rod 6.2.

[0055] Example 4, as Figure 5 The protective device for a centerless grinder shown includes a bed 1, a guide wheel 2 and a grinding wheel 3 on the bed 1, and a guide plate 4 located between the guide wheel 2 and the grinding wheel 3. A protective cover 5 is provided on the bed 1, covering the guide wheel 2 and the grinding wheel 3. At least two brackets 6 are provided on the protective cover 5, and a scale positioning rod 7 is provided on the bracket 6. The lower end of the scale positioning rod 7 extends into the protective cover 5 and is connected to a limit plate 8. The limit plate 8 is horizontally arranged above the guide plate 4 and extends along the length direction of the guide plate 4.

[0056] In this embodiment, the support 6 has three sections, with two sections 6 positioned above the two ends of the limiting plate 8, and the other section 6 positioned above the middle of the limiting plate 8. This is to prevent damage to the limiting plate 8 if the product impacts its middle section due to the limiting plate 8 being too long.

[0057] The limiting plate 8 has been further optimized. Its lower end face is an arc surface with an opening facing downwards, extending along the length of the guide plate 4. The arc surface adapts to the product and guides it, preventing it from moving laterally.

[0058] The ruler positioning rod 7 is further optimized. It features a groove 7.1 extending along its length. The ruler positioning rod 7 is mounted on the bracket 6 via a locking element 9. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6. The locking element 9 can be a bolt, a Torx screw, etc. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6, while the limiting end of the locking element 9 presses the ruler positioning rod 7 firmly onto the bracket 6, facilitating easy and intuitive adjustment of the ruler positioning rod 7's height.

[0059] The scale positioning rod 7 is further optimized. The scale positioning rod 7 is provided with scales distributed along the flight direction of the scale positioning rod 7, and the scales cooperate with the bracket 6.

[0060] The limiting plate 8 is further optimized so that its width is less than the minimum distance between the outer peripheral walls of the guide wheel 2 and the outer peripheral walls of the grinding wheel 3. This prevents the grinding wheel 3 and the guide wheel 2 from damaging the limiting part during processing.

[0061] The limiting plate 8 is further optimized, and its length is greater than that of the guide plate 4.

[0062] The bracket 6 is further optimized. The bracket 6 includes two supports 6.1 and a connecting rod 6.2 connecting the two supports 6.1. The scale positioning rod 7 is set on the connecting rod 6.2.

[0063] Based on Embodiment 1, Embodiment 3, or Embodiment 4, Embodiment 5 further optimizes the limiting plate 8, such as... Figure 6 The protective device for a centerless grinder shown includes a bed 1, a guide wheel 2 and a grinding wheel 3 on the bed 1, and a guide plate 4 located between the guide wheel 2 and the grinding wheel 3. A protective cover 5 is provided on the bed 1, covering the guide wheel 2 and the grinding wheel 3. At least two brackets 6 are provided on the protective cover 5, and a scale positioning rod 7 is provided on the bracket 6. The lower end of the scale positioning rod 7 extends into the protective cover 5 and is connected to a limit plate 8. The limit plate 8 is horizontally arranged above the guide plate 4 and extends along the length direction of the guide plate 4.

[0064] The limiting plate 8 includes a rotating shaft 8.1 supported above the guide plate 4 by a scale positioning rod 7, and two limiting plates 8.2 rotatably mounted on the rotating shaft 8.1. The limiting plates 8.2 extend along the length of the guide plate 4. A torsion spring hinge 8.3 is provided at the connection of the lower end faces of the two limiting plates 8.2 to make the two limiting plates 8.2 form an inverted V shape. The rotating shaft 8.1 is fixedly mounted at the lower end of the scale positioning rod 7, and the limiting plates 8.2 are rotatably mounted on the rotating shaft 8.1. The connection between the limiting plates 8.2 and the rotating shaft 8.1 can be referenced to a hinge. A torsion spring hinge 8.3 is provided at the connection of the lower end faces of the two limiting plates 8.2. The torsion spring hinge 8.3 keeps the two limiting plates 8.2 inverted V shape at a certain angle, so that the two limiting plates 8.2 have a certain elasticity, so that the limiting plate 8 and the product itself will not be affected when the product is impacted.

[0065] The limiting plate 8 has been further optimized. Its lower end face is an arc surface with an opening facing downwards, extending along the length of the guide plate 4. The arc surface adapts to the product and guides it, preventing it from moving laterally.

[0066] The ruler positioning rod 7 is further optimized. It features a groove 7.1 extending along its length. The ruler positioning rod 7 is mounted on the bracket 6 via a locking element 9. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6. The locking element 9 can be a bolt, a Torx screw, etc. The locking end of the locking element 9 passes through the groove 7.1 and connects to the bracket 6, while the limiting end of the locking element 9 presses the ruler positioning rod 7 firmly onto the bracket 6, facilitating easy and intuitive adjustment of the ruler positioning rod 7's height.

[0067] The scale positioning rod 7 is further optimized. The scale positioning rod 7 is provided with scales distributed along the flight direction of the scale positioning rod 7, and the scales cooperate with the bracket 6.

[0068] The limiting plate 8 is further optimized so that its width is less than the minimum distance between the outer peripheral walls of the guide wheel 2 and the outer peripheral walls of the grinding wheel 3. This prevents the grinding wheel 3 and the guide wheel 2 from damaging the limiting part during processing.

[0069] The limiting plate 8 is further optimized, and its length is greater than that of the guide plate 4.

[0070] The bracket 6 is further optimized. The bracket 6 includes two supports 6.1 and a connecting rod 6.2 connecting the two supports 6.1. The scale positioning rod 7 is set on the connecting rod 6.2.

[0071] The specific embodiments described above are merely preferred embodiments of this utility model and are not intended to limit the specific scope of implementation of this utility model. All equivalent changes made to the shape and structure of this utility model should be included within the protection scope of this utility model.

Claims

1. A protective device for a centerless grinder, comprising a bed, a guide wheel and a grinding wheel on the bed, and a guide plate located between the guide wheel and the grinding wheel, characterized in that, The bed is equipped with a protective cover that covers the guide wheel and the grinding wheel. The protective cover is equipped with at least two brackets, and a ruler positioning rod is provided on the bracket. The lower end of the ruler positioning rod extends into the protective cover and is connected to a limit plate. The limit plate is horizontally positioned above the guide plate and extends along the length of the guide plate.

2. The protective device for a centerless grinder according to claim 1, characterized in that, The lower end face of the limiting plate is an arc surface with the opening facing downwards, and the arc surface extends along the length direction of the guide plate.

3. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The ruler positioning rod is provided with a sliding groove extending along the length of the ruler positioning rod. The ruler positioning rod is mounted on the bracket by a locking member, and the locking end of the locking member passes through the sliding groove and is connected to the bracket.

4. A protective device for a centerless grinder according to claim 3, characterized in that, The ruler positioning rod is provided with graduations distributed along the direction of the ruler positioning rod, and the graduations cooperate with the bracket.

5. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The width of the limiting plate is less than the minimum distance between the outer peripheral wall of the guide wheel and the outer peripheral wall of the grinding wheel.

6. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The length of the limiting plate is greater than the length of the guide plate.

7. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, There are two brackets, which are respectively positioned above both ends of the limiting plate.

8. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The bracket has three lengths, with two brackets respectively located above both ends of the limiting plate, and the other bracket located above the middle of the limiting plate.

9. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The bracket includes two supports and a connecting rod connecting the two supports, and the scale positioning rod is mounted on the connecting rod.

10. A protective device for a centerless grinder according to claim 1 or 2, characterized in that, The limiting plate includes a rotating shaft supported above the guide plate by a ruler positioning rod and two limiting plates rotatably mounted on the rotating shaft. The limiting plates extend along the length of the guide plate, and a torsion spring hinge is provided at the connection of the lower end faces of the two limiting plates so that the two limiting plates are inverted V shape.

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