Tool for measuring brittle fracture of diamond wire coating

Abstract

The utility model relates to a tool for measuring diamond wire coating brittle failure, which comprises a base, a first support plate, a second support plate, a suspension fixing structure and a weighing structure, and is characterized in that the first support plate and the second support plate are vertically connected to two opposite ends of the base respectively; the suspension fixing structure is horizontally arranged, two ends of the suspension fixing structure are respectively connected with the first supporting plate and the second supporting plate, and the suspension fixing structure is used for fixing a sample line to be tested; and the weighing structure is used for applying a vertically downward pulling force to the sample line to be measured. According to the utility model, the to-be-measured sample wire is fixed through the suspension fixing structure, and a certain vertical downward pulling force is applied to the to-be-measured sample wire through the weighing structure, so that the brittle fracture of the diamond wire coating is realized, the ductile fracture of the bus is avoided, and the influence of the ductile fracture of the bus on the brittleness measurement of the diamond wire coating is eliminated; the brittle fracture measurement result obtained by the tool provided by the utility model reflects the brittleness of the diamond wire coating, and can represent the brittle fracture of the diamond wire coating.

Description

Technical Field

[0001] The utility model belongs to the technical field of diamond wires, and in particular relates to a tool for measuring the brittle fracture of a diamond wire coating. Background Art

[0002] Diamond wire is made by uniformly consolidating diamond powder particles on a mother wire (high carbon steel wire or tungsten wire) with a certain distribution density. The cutting purpose is achieved by high-speed grinding between the diamond wire and the object to be cut. Diamond wire was first used in sapphire cutting, and began to be used in photovoltaics around 2010. It is mainly used for silicon wafer cutting (including truncation and slicing, mainly slicing). Its cutting performance directly affects the quality of silicon wafers and battery efficiency. Brittle fracture is one of the indicators to measure the performance of diamond wire coating. Brittle fracture refers to the fracture of metal parts without obvious deformation. When breaking, the metal material breaks directly without any plastic deformation. The brittle fracture of diamond wire mainly refers to the excessive brittleness of its coating, which leads to cracking and even peeling of the coating.

[0003] Conventional methods for testing the brittleness of coatings include: quantitative determination of coating brittleness by cupping tester, Hastelloy sheet bending method, etc., but none of them are suitable for testing the brittleness of diamond wire coatings. At present, the main method for measuring the brittleness of diamond wire coatings is the mold method, which is to fold the diamond wire in half and pass it through a mold with a certain aperture size, and use the mold hole to bend or break the folded part of the diamond wire to determine whether the diamond wire coating is brittle.

[0004] However, since the diamond wire is divided into a main wire and a coating wrapped outside the main wire, the main wire is usually a high-carbon steel wire that has been drawn multiple times. The fracture morphology after bending and breaking using the die method is usually a tough dimple, that is, the main wire has a ductile fracture, and only the coating has a brittle fracture. Therefore, the test results of the die method include the influence of the ductile fracture of the main wire, and cannot fully characterize the brittle fracture of the diamond wire coating. Utility Model Content

[0005] In order to solve the above problems existing in the prior art, the utility model provides a tool for measuring the brittle fracture of diamond wire coating. The technical problem to be solved by the utility model is achieved through the following technical solutions:

[0006] A tool for measuring the brittle fracture of diamond wire coating, comprising: a base, a first support plate, a second support plate, a hanging fixing structure and a weighing structure, wherein:

[0007] The first support plate and the second support plate are vertically connected to opposite ends of the base respectively;

[0008] The hanging fixing structure is arranged horizontally, and its two ends are respectively connected to the first supporting plate and the second supporting plate, and is used to fix the sample line to be tested;

[0009] The weighing structure is used to apply a vertical downward pulling force to the sample line to be tested.

[0010] In a specific embodiment, it also includes a positioning plate;

[0011] The positioning plate is horizontally arranged and movably connected to the base.

[0012] In a specific embodiment, the base includes a bottom plate and a first side plate and a second side plate arranged at two ends of the bottom plate;

[0013] The first supporting plate is arranged at an end of the first side plate away from the bottom plate;

[0014] The second supporting plate is arranged at an end of the second side plate away from the bottom plate;

[0015] The opposing surfaces of the first side plate and the second side plate are provided with a plurality of limiting grooves corresponding to each other in the vertical direction;

[0016] Two ends of the positioning plate are respectively clamped in the limiting grooves of the first side plate and the second side plate.

[0017] In a specific embodiment, a connecting portion is formed on the top of the first support plate and the top of the second support plate;

[0018] Two ends of the suspension fixing structure are respectively arranged in the connection part of the first supporting plate and the second supporting plate.

[0019] In a specific embodiment, the connecting portion is a U-shaped groove.

[0020] In a specific embodiment, the suspension fixing structure includes a suspension rod, a butterfly nut and a fixing plate;

[0021] The hanging rod passes through the central through hole of the fixing plate;

[0022] The butterfly nut is threadedly connected to the suspension rod, and the butterfly nut can move along the axis of the suspension fixing structure to move away from or close to the fixing plate.

[0023] In a specific embodiment, the weighing structure includes a bottom tray, a connecting frame and a suspension ring;

[0024] A plurality of limiting holes for placing weights are provided at the geometric center of the bottom tray;

[0025] The two ends of the connecting frame are respectively fixedly connected to the opposite two sides of the bottom tray;

[0026] The suspension ring is fixedly arranged at the middle position of the top of the connection frame.

[0027] In a specific embodiment, the suspension ring is a ring-shaped iron wire;

[0028] The annular iron wire is connected to the connecting frame via a threaded fastener.

[0029] In a specific embodiment, the weighing structure includes a cavity and a hanging wire;

[0030] A plurality of limiting holes for placing weights are provided at the geometric center position inside the cavity;

[0031] The opposite ends of the top of the cavity are both provided with ear plates, and the two ends of the hanging wire are respectively connected to the oppositely arranged ear plates.

[0032] In a specific embodiment, the centers of the plurality of limiting holes overlap and their diameters decrease sequentially from the outside to the inside.

[0033] Compared with the prior art, the utility model has the following beneficial effects:

[0034] The utility model fixes the sample wire to be tested by a hanging fixing structure, and applies a certain magnitude of vertical downward pulling force to the sample wire to be tested by a weighing structure, so as to achieve brittle fracture of the diamond wire coating, while the busbar does not suffer from ductile fracture, thereby eliminating the influence of the ductile fracture of the busbar on the brittleness measurement of the diamond wire coating. The brittle fracture measurement result obtained by the tooling provided by the utility model reflects the brittleness of the diamond wire coating, and can characterize the brittle fracture of the diamond wire coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a structural schematic diagram of a tool for measuring the brittle fracture of diamond wire coating provided by an embodiment of the utility model;

[0036] Figure 2 It is a structural schematic diagram of another tool for measuring the brittle fracture of diamond wire coating provided by an embodiment of the utility model;

[0037] Figure 3 It is a structural schematic diagram of a weighing structure provided by an embodiment of the utility model;

[0038] Figure 4 It is a structural schematic diagram of another weighing structure provided by an embodiment of the utility model.

[0039] Reference numerals:

[0040] 1: base; 2: first support plate; 3: second support plate; 4: hanging fixing structure; 5: weighing structure; 6: positioning plate; 201: connecting part; 511: bottom tray; 512: connecting frame; 513: hanging ring; 521: cavity; 522: hanging wire. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with specific embodiments, but the implementation methods of the present invention are not limited thereto.

[0042] Embodiment 1

[0043] See also Figure 1 , Figure 1 It is a structural schematic diagram of a tool for measuring the brittle fracture of diamond wire coating provided by an embodiment of the utility model.

[0044] The present embodiment provides a tool for measuring the brittle fracture of diamond wire coating, comprising: a base 1, a first support plate 2, a second support plate 3, a suspension fixing structure 4 and a weighing structure 5, wherein the first support plate 2 and the second support plate 3 are respectively vertically connected to the opposite ends of the base 1, and the suspension fixing structure 4 is horizontally arranged, and the two ends are respectively connected to the first support plate 2 and the second support plate 3, for fixing the sample wire to be tested. The weighing structure 5 is used to apply a vertical downward pulling force to the sample wire to be tested.

[0045] Specifically, when measuring, the sample line to be tested is passed through the weighing structure 5 and the two ends of the sample line to be tested are fixed on the suspension fixing structure 4. Since the suspension fixing structure 4 is arranged horizontally, the weighing structure 5 can apply a vertical downward pulling force to the sample line to be tested. By changing the weight of the weighing structure 5, the magnitude of the pulling force on the sample line to be tested can be changed to ensure that the pulling force on the sample line to be tested can only achieve the brittle fracture of the diamond wire coating, and the mother wire does not have a ductile fracture. After the weighing structure 5 applies a vertical downward pulling force to the sample line to be tested for at least 10 seconds, the sample line to be tested is removed and the sample line coating is observed under an electron microscope to see whether a brittle fracture occurs. If a brittle fracture occurs, the morphology of the brittle fracture is further observed.

[0046] Specifically, a connection portion 201 is provided at the top of the first support plate 2 and the top of the second support plate 3, and both ends of the suspension fixing structure 4 are respectively disposed in the connection portion 201 of the first support plate 2 and the second support plate 3. More specifically, the connection portion 201 is a U-shaped groove.

[0047] Further, the suspension fixing structure 4 comprises a suspension rod, a butterfly nut and a fixing plate. The suspension rod passes through the central through hole of the fixing plate. The butterfly nut is threadedly connected to the suspension rod, and the butterfly nut can move along the axis of the suspension rod to move away from or close to the fixing plate.

[0048] Specifically, one end of the hanging rod is fixed in a U-shaped groove opened at the top of the first support plate 2, and the other end of the hanging rod is fixed in a U-shaped groove opened at the top of the second support plate 3. The hanging rod passes through the central through hole of the fixing plate, and the fixing plate is fixed on the hanging rod. The two ends of the sample line to be tested are placed between the fixing plate and the butterfly nut. The butterfly nut moves along the axis of the hanging rod and cooperates with the fixing plate to fix the two ends of the sample line to be tested. When the sample line to be tested needs to be replaced, the butterfly nut is rotated so that the butterfly nut moves in a direction away from the fixing plate to release the fixing of the fixing plate and the butterfly nut to the sample line to be tested, and then the sample line to be tested is replaced. Preferably, threads are provided at both ends of the hanging rod. After the two ends of the hanging rod are respectively placed in the U-shaped grooves opened at the tops of the first support plate 2 and the second support plate 3, the hanging rod is fixed to the first support plate 2 and the second support plate 3 by nuts.

[0049] The present embodiment provides a tool for measuring the brittle fracture of diamond wire coating, which fixes the sample wire to be measured by a hanging fixing structure 4, and applies a certain magnitude of vertical downward pulling force to the sample wire to be measured by a weighing structure 5, so as to achieve the brittle fracture of the diamond wire coating without causing ductile fracture of the busbar, thereby eliminating the influence of the ductile fracture of the busbar on the brittle fracture measurement of the diamond wire coating. The brittle fracture measurement result obtained by the tool provided by the present embodiment reflects the brittleness of the diamond wire coating and can characterize the brittle fracture of the diamond wire coating.

[0050] Embodiment 2

[0051] Based on Example 1, see Figure 2 , Figure 2 It is a structural schematic diagram of another tool for measuring the brittle fracture of diamond wire coating provided by an embodiment of the utility model.

[0052] Another tooling for measuring the brittle fracture of diamond wire coating provided in this embodiment includes a base 1, a first support plate 2, a second support plate 3, a hanging fixing structure 4, a weighing structure 5 and a positioning plate 6. The relative position relationship of the base 1, the first support plate 2, the second support plate 3, the hanging fixing structure 4 and the weighing structure 5 can be found in the first embodiment, which will not be described in detail in this embodiment. The positioning plate 6 is horizontally arranged and movably connected to the base 1.

[0053] Specifically, the base 1 includes a bottom plate and a first side plate and a second side plate arranged at two ends of the bottom plate. The first support plate 2 is arranged at one end of the first side plate away from the bottom plate, and the second support plate 3 is arranged at one end of the second side plate away from the bottom plate. The surfaces opposite to each other of the first side plate and the second side plate are provided with a plurality of limit grooves corresponding to each other in the vertical direction, and the two ends of the positioning plate 6 are respectively clamped in the limit grooves of the first side plate and the second side plate.

[0054] Specifically, the bottom plate is arranged horizontally, the first side plate and the second side plate are vertically fixed at opposite ends of the bottom plate, the first support plate 2 is arranged at the top of the first side plate, and the second support plate 3 is arranged at the top of the second side plate. The relative surfaces of the first side plate and the second side plate are provided with a plurality of limit grooves at different heights, and the limit grooves on the surface of the first side plate correspond to the limit grooves on the surface of the second side plate one by one, and the two ends of the positioning plate 6 are respectively clamped in the limit grooves corresponding to the heights of the first side plate and the second side plate. When measuring, the positioning plate 6 is clamped in the limit grooves of appropriate height according to the length of the sample line to be measured, and the weighing structure 5 is placed on the positioning plate 6, and then the sample line to be measured is passed through the weighing structure 5, and the two ends of the sample line to be measured are fixed on the hanging fixing structure 4. After completing all the preparations before measurement, the positioning plate 6 is taken out from the limit groove, and the weighing structure 5 applies a vertical downward pulling force to the sample line to be measured under the action of gravity.

[0055] In this embodiment, a positioning plate 6 is provided, and the weighing structure 5 is placed on the positioning plate 6 before measurement, so that the sample line is straightened and free from stress between the weighing structure 5 and the hanging fixing structure 4, thereby ensuring the accuracy of the measurement result.

[0056] Preferably, corresponding threaded holes are provided at both ends of the positioning plate 6 and on the limiting grooves of the first side plate and the second side plate. When the positioning plate 6 is clamped into the limiting groove, the positioning plate 6 is fixed to the first side plate and the second side plate by bolts.

[0057] The present embodiment provides a tool for measuring the brittle fracture of diamond wire coating, which fixes the sample wire to be tested by a hanging fixed structure 4, and applies a certain amount of vertical downward pulling force to the sample wire to be tested by a weighing structure 5, so as to achieve the brittle fracture of the diamond wire coating, while the mother wire does not have a ductile fracture, thereby eliminating the influence of the ductile fracture of the mother wire on the brittle measurement of the diamond wire coating, and by placing the weighing structure 5 on the positioning plate 6, the sample wire is straightened and not subjected to force between the weighing structure 5 and the hanging fixed structure 4, thereby ensuring the accuracy of the measurement result. Therefore, the brittle fracture measurement result obtained by the tool provided by the present embodiment reflects the brittleness of the diamond wire coating and can characterize the brittle fracture of the diamond wire coating.

[0058] Embodiment 3

[0059] Based on Example 2, see Figure 3 , Figure 3 It is a structural schematic diagram of a weighing structure provided in an embodiment of the utility model.

[0060] The weighing structure 5 of this embodiment includes a bottom tray 511, a connecting frame 512 and a suspension ring 513. A plurality of limiting holes for placing weights are provided at the geometric center of the bottom tray 511, two ends of the connecting frame 512 are respectively fixedly connected to the opposite sides of the bottom tray 511, and the suspension ring 513 is fixedly provided at the middle position of the top of the connecting frame 512. The suspension ring 513 is a ring-shaped iron wire, and the ring-shaped iron wire is connected to the connecting frame 512 by a threaded fastener. The centers of the plurality of limiting holes overlap and the diameters decrease from the outside to the inside.

[0061] Specifically, the diameters of the plurality of limiting holes at the geometric center of the bottom tray 511 correspond to the diameters of weights of different weights, the centers of the plurality of limiting holes are all located at the geometric center of the bottom tray 511, and the limiting hole located at the outermost surface of the bottom tray 511 has the largest diameter, and the limiting hole located at the innermost portion of the bottom tray 511 has the smallest diameter, thereby ensuring that the weights can be placed at the geometric center of the bottom tray 511, thereby applying a vertical downward pulling force to the sample line to be tested. Preferably, the two ends of the connecting frame 512 are respectively fixed at the center of the opposite sides of the bottom tray 511, and a fixing hole is provided at the center of the top of the connecting frame 512 to allow the ring wire to pass through, a threaded hole is provided on one side of the fixing hole, and the two ends of the ring wire are located in the fixing hole, and the butterfly bolt passes through the threaded hole to fix the ring wire.

[0062] The steps of measuring the brittleness of the coating of the diamond wire to be tested using the tooling for measuring the brittle fracture of the diamond wire coating provided in this embodiment are as follows:

[0063] Step 1: Take 10 sections of diamond wire samples from the diamond wire produced by the same production line equipment as the sample group.

[0064] Step 2: Select a sample line from the sample group as the sample line to be tested, clamp the positioning plate 6 in the limit groove of appropriate height according to the length of the sample line to be tested, and fix the two ends of the positioning plate 6 to the first side plate and the second side plate respectively by bolts.

[0065] Step three: Place the weights in the limiting holes with matching diameters, and place the weighing structure 5 with the weights on the positioning plate 6 .

[0066] Step 4: Pass the sample line to be tested through the ring wire.

[0067] Step 5: Place the two ends of the sample line to be tested between the fixing plate and the butterfly nut, and rotate the butterfly nut to move the butterfly nut toward the fixing plate until the butterfly nut and the fixing plate fix the sample line to be tested.

[0068] Step 6: Release the bolt connection between the two ends of the positioning plate 6 and the first side plate and the second side plate, and take the two ends of the positioning plate 6 out of the limit grooves. After the positioning plate 6 is taken out, the weighing structure 5 applies a vertical downward pulling force to the sample line to be measured.

[0069] Step 7: After holding for at least 10 seconds, rotate the butterfly nut to move it away from the fixing plate to release the fixing plate and the butterfly nut from fixing the sample line to be tested, then replace the sample line to be tested, and repeat steps 2 to 7 until all diamond wire sample lines in the sample group have completed the measurement.

[0070] Step 8: Observe under a 3D optical microscope whether the coating of all the diamond wire samples that have been measured has brittle fracture. If brittle fracture has occurred, further observe the morphology of the brittle fracture.

[0071] It should be noted that this tool is used to measure the brittle fracture rate of the diamond wire coating. For example, for 10 sample wires, if 2 of them are brittle fractured under a specified tension, then under this tension, the brittle fracture rate of the diamond wire coating is 20%.

[0072] Furthermore, for the diamond wire samples selected from the diamond wire produced by the same production line equipment, weights of different weights can be used for brittle fracture test, and the brittle fracture rate of the same batch of products under different tensile forces (tensile force value is related to the sum of the gravity of the weight and the weighing structure) can be obtained respectively.

[0073] The present embodiment provides a tool for measuring the brittle fracture of diamond wire coating, which fixes the sample wire to be tested by a hanging fixing structure 4, and sets a plurality of limit holes at the geometric center of the bottom tray 511, thereby ensuring that the weight can be placed at the geometric center of the bottom tray 511, and then applying a vertical downward pulling force to the sample wire to be tested, thereby achieving the brittle fracture of the diamond wire coating, while the busbar does not undergo ductile fracture, thereby eliminating the influence of the ductile fracture of the busbar on the brittle measurement of the diamond wire coating; therefore, the brittle fracture measurement result obtained by the tool provided by the present embodiment reflects the brittleness of the diamond wire coating, and can characterize the brittle fracture of the diamond wire coating.

[0074] Embodiment 4

[0075] Based on Example 2, see Figure 4 , Figure 4 It is a structural schematic diagram of another weighing structure provided by an embodiment of the utility model.

[0076] The weighing structure 5 of this embodiment includes a cavity 521 and a hanging wire 522. A plurality of limiting holes for placing weights are provided at the geometric center of the cavity 521. Ear plates are provided at opposite ends of the top of the cavity 521, and the two ends of the hanging wire 522 are respectively connected to the oppositely arranged ear plates. The centers of the plurality of limiting holes overlap and the diameters decrease from the outside to the inside.

[0077] Specifically, the diameters of the plurality of limiting holes at the geometric center of the bottom surface of the cavity 521 correspond to the diameters of weights of different weights, and the centers of the plurality of limiting holes are all located at the geometric center of the bottom surface of the cavity 521, thereby ensuring that the weights can be placed at the geometric center of the cavity 521, thereby applying a vertical downward pulling force to the sample line to be tested. Preferably, the two ear plates are correspondingly arranged at the middle positions of the opposite ends of the top of the cavity 521, and the two ends of the hanging wire 522 are respectively fixed to the two ear plates by hexagon socket bolts. It should be noted that the hanging wire 522 is not completely straightened, and a certain margin is left.

[0078] The present embodiment provides a tool for measuring the brittle fracture of diamond wire coating, which fixes the sample wire to be tested by a hanging fixing structure 4, and sets a plurality of limit holes at the geometric center inside the cavity 521, thereby ensuring that the weight can be placed at the geometric center of the bottom tray 511, and then applying a vertical downward pulling force to the sample wire to be tested, so as to achieve the brittle fracture of the diamond wire coating, while the busbar does not have a ductile fracture, thereby eliminating the influence of the ductile fracture of the busbar on the brittleness measurement of the diamond wire coating; therefore, the brittle fracture measurement result obtained by the tool provided by the present embodiment reflects the brittleness of the diamond wire coating and can characterize the brittle fracture of the diamond wire coating.

[0079] The above contents are further detailed descriptions of the present invention in combination with specific preferred implementations, and the specific implementation of the present invention cannot be considered to be limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A tool for measuring the brittle fracture of diamond wire coating, characterized in that: include: A base (1), a first support plate (2), a second support plate (3), a hanging fixing structure (4) and a weighing structure (5), wherein: The first support plate (2) and the second support plate (3) are respectively vertically connected to two opposite ends of the base (1); The hanging fixing structure (4) is arranged horizontally, and its two ends are respectively connected to the first supporting plate (2) and the second supporting plate (3), and is used to fix the sample line to be tested; The weighing structure (5) is used to apply a vertical downward pulling force to the sample line to be tested; The weighing structure (5) comprises a bottom tray (511), a connecting frame (512) and a hanging ring (513); A plurality of limiting holes for placing weights are arranged at the geometric center of the bottom tray (511); Two ends of the connecting frame (512) are respectively fixedly connected to opposite sides of the bottom tray (511); The suspension ring (513) is fixedly arranged at the middle position of the top of the connection frame (512); Alternatively, the weighing structure (5) comprises a cavity (521) and a hanging wire (522); A plurality of limiting holes for placing weights are arranged at the geometric center position inside the cavity (521); Ear plates are provided at opposite ends of the top of the cavity (521), and the two ends of the hanging wire (522) are respectively connected to the opposite ear plates.

2. A tool for measuring brittle fracture of diamond wire coating according to claim 1, characterized in that: Also includes a positioning plate (6); The positioning plate (6) is arranged horizontally and is movably connected to the base (1).

3. A tool for measuring brittle fracture of diamond wire coating according to claim 2, characterized in that: The base (1) comprises a bottom plate and a first side plate and a second side plate arranged at two ends of the bottom plate in opposite directions; The first supporting plate (2) is arranged at an end of the first side plate away from the bottom plate; The second supporting plate (3) is arranged at an end of the second side plate away from the bottom plate; The opposing surfaces of the first side plate and the second side plate are provided with a plurality of limiting grooves corresponding to each other in the vertical direction; Two ends of the positioning plate (6) are respectively clamped in the limiting grooves of the first side plate and the second side plate.

4. A tool for measuring brittle fracture of diamond wire coating according to claim 3, characterized in that: A connecting portion (201) is provided on the top of the first supporting plate (2) and the top of the second supporting plate (3); Two ends of the suspension fixing structure (4) are respectively arranged in the connection part (201) of the first supporting plate and the second supporting plate.

5. A tool for measuring brittle fracture of diamond wire coating according to claim 4, characterized in that: The connecting portion (201) is a U-shaped groove.

6. A tool for measuring brittle fracture of diamond wire coating according to claim 1, characterized in that: The suspension fixing structure (4) comprises a suspension rod, a butterfly nut and a fixing plate; The hanging rod passes through the central through hole of the fixing plate; The butterfly nut is threadedly connected to the suspension rod, and the butterfly nut can move along the axis of the suspension fixing structure (4) to move away from or closer to the fixing plate.

7. A tool for measuring brittle fracture of diamond wire coating according to claim 1, characterized in that: The suspension ring (513) is a ring-shaped iron wire; The annular iron wire is connected to the connecting frame via a threaded fastener.

8. The tool for measuring brittle fracture of diamond wire coating according to claim 1, characterized in that: The centers of the plurality of limiting holes overlap and their diameters decrease sequentially from the outside to the inside.

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