Indexable turning insert for heavy roughing

Abstract

The present application belongs to the technical field of cutting tools, and relates to a rotatable turning blade for heavy rough machining, which comprises an upper surface, a lower surface, and a through hole penetrating through the upper surface and the lower surface; the upper surface and a side surface form a cutting edge at an intersection; the upper surface comprises the cutting edge, a recessed chip breaker groove, and a reinforcing ring; the reinforcing ring is continuously arranged along the periphery of the through hole in the middle of the blade, has a convex shape in cross section, and has a top that is a flat surface or an arbitrary curved surface; the width of the reinforcing ring is 1-4 mm, and the height of the reinforcing ring is greater than or equal to 0.4 mm; the reinforcing ring and the chip breaker groove are connected and transitioned by a slope, and the angle between the slope and the lower surface is 20-70 degrees; the reinforcing ring is arranged at the periphery of the through hole in the middle of the blade, the cross-sectional area is increased, and the risk of a half fracture of the blade is reduced; the blade of the present application hardly occupies the chip space of the chip breaker groove, does not affect the chip breaking effect of the chip breaker groove, increases the cross-sectional area of the middle section of the blade, increases the strength of the blade, reduces the risk of a half fracture of the blade, and prolongs the service life of the blade.

Description

Technical Field

[0001] This invention belongs to the field of cutting tool technology, specifically a type of indexable turning insert for heavy roughing. Background Technology

[0002] The primary purpose of heavy roughing is to achieve a high metal removal rate. To achieve this, the design requirements for the chip breaker groove differ significantly from those for finishing and intermediate machining. A large chip space in the chip breaker groove is a crucial characteristic. However, a large chip space reduces the strength of the cutting insert. Furthermore, the through-hole in the center of the insert also reduces its strength. When the insert's strength is reduced, many inserts fail by breaking in half under conditions of high cutting forces and high heat generation. To reduce this failure mode, various methods have been employed in the design of existing cutting inserts, such as increasing the strength of the insert material, changing the insert's geometry, optimizing the chip breaker groove, and utilizing coolant to reduce heat generation. These methods have shown some effectiveness, but they primarily rely on ensuring a large chip space in the chip breaker groove of heavy roughing inserts. Therefore, their effectiveness in addressing the failure mode of half-fracture in heavy roughing inserts is insufficient to fully meet the demands of heavy roughing operations with high cutting forces and high heat generation. Summary of the Invention

[0003] To address the problems existing in the prior art, the main objective of this invention is to propose an indexable turning insert for heavy roughing.

[0004] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0005] A heavy roughing indexable turning insert includes an upper surface, a lower surface, multiple side surfaces connecting the upper and lower surfaces, and a through hole passing through the upper and lower surfaces. A cutting edge is formed at the intersection of the upper surface and the side surfaces. The upper surface includes the cutting edge, a recessed chip breaker groove, and a reinforcing ring. The reinforcing ring is continuously arranged around the periphery of the through hole in the middle of the insert, and its cross-sectional shape is a boss. The top of the reinforcing ring is a plane or a curved surface of any shape. The width of the reinforcing ring is 1-4 mm, and the height of the reinforcing ring is ≥0.4 mm. The reinforcing ring and the chip breaker groove are connected and transitioned by an inclined plane, and the angle between the inclined plane and the lower surface is 20-70°.

[0006] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, the cutting edge includes multiple main cutting edges and multiple tip cutting edges, with the cylindrical surface intersecting the upper surface to form the tip cutting edge.

[0007] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, the arcs of multiple cutting edges are on the same plane, and the plane is parallel to the lower surface.

[0008] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, each main cutting edge is composed of a concave arc extending downwards and a straight line connecting each end of the arc, or is composed of multiple straight lines, or is composed of only one arc. The distance from any point on the main cutting edge to the lower surface is not greater than the distance from the cutting edge tip to the lower surface.

[0009] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, the top of the reinforcing ring is L lower than the cutting edge of the tool tip by a certain distance, and 0.1mm≤L≤1mm.

[0010] As a preferred embodiment of the indexable turning insert for heavy roughing according to the present invention, the lower surface is a ring plane; the width of the ring on the lower surface is ≥3mm.

[0011] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, two adjacent sides are connected and transitioned by a cylindrical surface.

[0012] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, the distance between the upper surface and the lower surface forms the thickness H of the insert, and 5.8mm≤H≤9.8mm.

[0013] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, the through hole passes through the center of the upper and lower surfaces, and the diameter of the through hole is d, and 7.7mm≤d≤10mm.

[0014] As a preferred embodiment of the indexable turning insert for heavy roughing described in this invention, two adjacent sides are connected and transitioned by a cylindrical surface, the radius of which is R, and 1.5mm≤R≤3.3mm.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention proposes an indexable turning insert for heavy roughing, comprising an upper surface, a lower surface, and a through hole passing through the upper and lower surfaces; a cutting edge is formed at the intersection of the upper surface and the side surface, the upper surface including the cutting edge, a recessed chip breaker groove, and a reinforcing ring, the reinforcing ring being continuously arranged around the periphery of the through hole in the middle of the insert, its cross-sectional shape being a boss shape, the top of the reinforcing ring being a plane or a curved surface of arbitrary shape, the width of the reinforcing ring being 1-4mm, and the height of the reinforcing ring being ≥0.4mm; the reinforcing ring and the chip breaker groove are connected and transitioned by an inclined plane, and the angle between the inclined plane and the lower surface is 20-70°. The reinforcing ring is set around the periphery of the through hole in the middle of the insert to increase the cross-sectional area and resist hemi-fracture. The insert of this invention almost does not encroach on the chip space of the chip breaker groove, does not affect the chip breaking effect of the chip breaker groove, increases the cross-sectional area of ​​the middle section of the insert, increases the strength of the insert, reduces the probability of hemi-fracture failure mode of the insert, and improves the service life of the insert. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0018] Figure 1 This is a three-dimensional structural schematic diagram of an indexable turning insert according to an embodiment of the present invention;

[0019] Figure 2 This is a top view of an indexable turning insert according to an embodiment of the present invention;

[0020] Figure 3 It is along Figure 2 An enlarged schematic diagram of the cross-section of the MM.

[0021] Figure 4 This is a three-dimensional structural schematic diagram of an indexable turning insert according to another embodiment of the present invention.

[0022] Among them, 1-upper surface, 2-lower surface, 3-side surface, 4-through hole, 5-cylindrical surface, 6-cutting edge, 7-chip breaker groove, 8-reinforcing ring.

[0023] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0024] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention proposes an indexable turning insert for heavy roughing. Addressing the common technical need in existing technologies to ensure a large chip breaker space in the chip breaker groove of turning inserts for heavy roughing, methods such as increasing the strength of the insert material, changing the insert geometry, optimizing the chip breaker groove, and fully utilizing coolant to reduce heat generation have all shown some effectiveness. However, their effectiveness in addressing the failure mode of partial fracture is insufficient to meet the demands of heavy roughing operations with high cutting forces and high heat generation. The applicant's research has revealed that during the cutting process, the cutting chips from the heavy roughing turning insert do not reach the edge of the through hole through the chip breaker groove. Therefore, the cross-sectional area of ​​the cutting tool can be increased by utilizing the position of the through hole edge. This overcomes the technical bias in the prior art that requires ensuring a large chip breaker groove without utilizing the position of the through hole edge in the center of the cutting tool to reinforce the cutting tool. After accumulating material around the through hole in the center of the cutting tool to form a reinforcing ring, the cross-sectional area of ​​the cutting tool's middle section is increased, thereby increasing the strength of the cutting tool, reducing the probability of the cutting tool's failure mode of partial fracture, and improving the cutting tool's service life. Moreover, the reinforcement ring formed by accumulating material around the through hole in the center of the cutting tool does not affect the chip breaker's ...

[0026] As is well known, fractures tend to occur at locations with smaller cross-sectional areas, and the minimum cross-sectional area for heavy roughing inserts is located at the halfway point of the insert. To reduce this failure mode of half-fracture, this invention incorporates a reinforcing ring around the central through-hole of the insert. This ring increases the cross-sectional area, resisting half-fracture. The insert of this invention barely encroaches on the chip breaker's space and does not affect the chip breaker's chip-breaking effect. It increases the cross-sectional area of ​​the insert's central section, thereby increasing the insert's strength.

[0027] like Figure 1-3 As shown, an indexable turning insert for heavy roughing according to an embodiment of the present invention includes an upper surface 1, a lower surface 2, four side surfaces 3 connecting the upper surface 1 and the lower surface 2, and a through hole 4 passing through the upper surface 1 and the lower surface 2; a cutting edge 6 is formed at the intersection of the upper surface 1 and the side surfaces 3, and the upper surface 1 includes the cutting edge 6, a recessed chip breaker groove 7, and a reinforcing ring 8; the lower surface 2 is a large annular plane; adjacent two side surfaces 3 are connected and transitioned by a cylindrical surface 5.

[0028] In this embodiment, the reinforcing ring 8 has a basically boss-shaped cross-section and is continuously arranged around the through hole 4 in the middle of the cutting tool. Its function is to increase the cross-sectional area of ​​the middle section of the cutting tool, thereby increasing the strength of the cutting tool and reducing the failure mode of the cutting tool breaking in half. The reinforcing ring 8 and the chip breaker groove 7 are connected and transitioned by an inclined plane, and the angle between the inclined plane and the lower surface 2 is 20-70°, preferably 30-60°. The top of the reinforcing ring 8 can be a flat plane or a curved surface of any shape. Any point on its top is L lower than the cutting edge of the cutting tool tip, and 0.1mm≤L≤1mm. The width of the reinforcing ring 8 is generally 1-4mm, which ensures the strength of the cutting tool while also ensuring the chip space of the turning tool for heavy roughing. The height of the reinforcing ring 8 is generally ≥0.4mm. Specifically, the reinforcing ring 8 and the chip breaker groove 7 are connected and transitioned by an inclined plane. The angle between the inclined plane and the lower surface 2 can be, for example, but not limited to, any one or any two of 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, and 70°. Any point on the top is L lower than the cutting edge of the tool tip. L can be, for example, but not limited to, any one or any two of 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, and 1mm. The width of the reinforcing ring 8 can be, for example, but not limited to, any one or any two of 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, and 4mm. The height of the reinforcing ring 8 can be, for example, but not limited to, any one or any two of 0.4mm, 0.5mm, 0.6mm, and 0.7mm.

[0029] The purpose of the cutting edge 6 is to remove excess material from the workpiece. It includes multiple main cutting edges and multiple tip cutting edges. The cylindrical surface 5 intersects with the upper surface 1 to form the tip cutting edge. The arcs of the multiple tip cutting edges are on the same plane, and this plane is parallel to the lower surface 2. Each main cutting edge is generally composed of an arc that is concave towards the lower surface 2 and a straight line connecting each end of the arc, or it is composed of only multiple straight lines, or it is composed of only one arc. The distance from any point on the main cutting edge to the lower surface 2 is no greater than the distance from the tip cutting edge to the lower surface 2.

[0030] The chip breaker groove 7 is arranged in a ring along the cutting edge 6 to form short chips. Long chips will entangle the workpiece and the tool, preventing the machining process from continuing and potentially injuring the operator. The cross-section of the chip breaker groove 7 is basically a groove or a U-shape. The chips are first cut off by the cutting edge, flow through the rake face of the chip breaker groove 7, then reach the bottom of the groove, and are finally broken by the inclined surface between the chip breaker groove 7 and the reinforcing ring 8.

[0031] The lower surface 2 is a large annular plane, with a width typically ≥3mm. Due to the high cutting force of the cutting tool in heavy roughing operations, the lower surface 2 requires a large plane for support to achieve a relatively small pressure per unit area. The distance between the upper surface 1 and the lower surface 2 forms the cutting tool thickness H, where 5.8mm ≤ H ≤ 9.8mm. Specifically, the width of the ring can be, for example, but not limited to, any one or any two of 3mm, 3.2mm, 3.5mm, 3.7mm, 4.0mm; the distance between the upper surface 1 and the lower surface 2 forms the cutting tool thickness H, which can be, for example, but not limited to, any one or any two of 5.8mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm, 8.5mm, 9.0mm, 9.5mm, 9.8mm.

[0032] The upper surface 1 and the lower surface 2 are connected to form a side surface 3, which forms the flank face of the cutting tool. Side surface 3 is a large flat surface, and its function is to provide positioning during installation. Sometimes, the tip of a cutting chip hits the flank face, allowing the chip to be broken. The number of side surfaces 3 can be set as needed. In this embodiment, there are four side surfaces 3, forming a rhombus. The inscribed circle of the rhombus is D, and 18.5mm ≤ D ≤ 38.5mm. The included angle between any two adjacent side surfaces is α, and 78° ≤ α ≤ 102°. Specifically, the inscribed circle of the rhombus is D, which can be any one or a range between any two of the following: 18.5mm, 20mm, 22.5mm, 25mm, 27.5mm, 30mm, 32.5mm, 35mm, 38.5mm; the included angle between any two adjacent lateral faces is α, which can be any one or a range between any two of the following: 78°, 80°, 85°, 90°, 95°, 100°, 102°.

[0033] The through hole 4 passes through the center of the upper surface 1 and the lower surface 2, and is a mounting hole for fixing. The diameter of the through hole 4 is d, and 7.7mm≤d≤10mm. Specifically, the diameter of the through hole 4 is d, which can be any one or any two of, such as, but not limited to, 7.7mm, 8.0mm, 8.5mm, 9.0mm, 9.5mm, and 10mm.

[0034] Two adjacent side surfaces 3 are connected by a cylindrical surface 5, the radius of which is R, and 1.5mm≤R≤3.3mm. Specifically, the radius of the cylindrical surface 5 is R, which can be any one or any two of, for example, but not limited to, 1.5mm, 1.8mm, 2.0mm, 2.2mm, 2.5mm, 2.7mm, 3.0mm, and 3.3mm.

[0035] like Figure 4As shown, another embodiment of the present invention provides an indexable turning insert for heavy roughing, comprising an upper surface 1, a lower surface 2, four side surfaces 3 connecting the upper surface 1 and the lower surface 2, and a through hole 4 passing through the upper surface 1 and the lower surface 2; a cutting edge 6 is formed at the intersection of the upper surface 1 and the side surfaces 3, and the upper surface 1 includes the cutting edge 6, a recessed chip breaker groove 7, and a reinforcing ring 8; the lower surface 2 is a large annular plane; adjacent side surfaces 3 are connected and transitioned by a cylindrical surface 5.

[0036] like Figure 2-4 As shown, in this embodiment, the cross-sectional shape of the reinforcing ring 8 is basically a boss shape, continuously arranged around the periphery of the through hole 4 in the middle of the cutting tool. Its function is to increase the cross-sectional area of ​​the middle section of the cutting tool, increase the strength of the cutting tool, and reduce the failure mode of the cutting tool breaking in half. The reinforcing ring 8 and the chip breaker groove 7 are connected and transitioned by an inclined plane, and the angle between the inclined plane and the lower surface 2 is 20-70°, preferably 30-60°. The top of the reinforcing ring 8 can be a plane or a curved surface of any shape. Any point on its top is L lower than the cutting edge of the cutting tool tip, and 0.1mm≤L≤1mm. The width of the reinforcing ring 8 is generally 1-4mm, which ensures the strength of the cutting tool while also ensuring the chip space of the turning tool for heavy roughing. The height of the reinforcing ring 8 is generally ≥0.4mm. Specifically, the reinforcing ring 8 and the chip breaker groove 7 are connected and transitioned by an inclined plane. The angle between the inclined plane and the lower surface 2 can be, for example, but not limited to, any one or any two of 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, and 70°. Any point on the top is L lower than the cutting edge of the tool tip. L can be, for example, but not limited to, any one or any two of 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, and 1mm. The width of the reinforcing ring 8 can be, for example, but not limited to, any one or any two of 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm; the height of the reinforcing ring 8 can be, for example, but not limited to, any one or any two of 0.4mm, 0.5mm, 0.6mm, 0.7mm; in this embodiment, less material is added to the reinforcing ring 8 at the two diagonals, which not only achieves the same effect, but also reduces the material used in the blade manufacturing, thereby reducing costs.

[0037] The purpose of the cutting edge 6 is to remove excess material from the workpiece, including multiple main cutting edges and multiple tool tip cutting edges. The cylindrical surface 5 intersects with the upper surface 1 to form the tool tip cutting edge.

[0038] The arcs of multiple cutting edges are on the same plane, and this plane is parallel to the lower surface 2. Each main cutting edge is generally composed of an arc that is concave towards the lower surface 2 and a straight line connecting each end of the arc, or it is composed of multiple straight lines, or it is composed of only one arc. The distance from any point on the main cutting edge to the lower surface 2 is not greater than the distance from the cutting edge to the lower surface 2.

[0039] In this embodiment, the chip breaker groove 7 is arranged in a ring along the cutting edge 6 to form short chips. Long chips will entangle the workpiece and the tool, preventing the machining process from continuing and potentially injuring the operator. The cross-section of the chip breaker groove 7 is basically a groove or a U-shape. The chips are first cut off by the cutting edge, flow through the rake face of the chip breaker groove 7, then reach the bottom of the groove, and are finally broken by the inclined surface between the chip breaker groove 7 and the reinforcing ring 8.

[0040] The lower surface 2 is a large annular plane, with a width typically ≥3mm. Due to the high cutting force of the cutting tool in heavy roughing operations, the lower surface 2 requires a large plane for support to achieve a relatively small pressure per unit area. The distance between the upper surface 1 and the lower surface 2 forms the cutting tool thickness H, where 5.8mm ≤ H ≤ 9.8mm. Specifically, the width of the ring can be, for example, but not limited to, any one or any two of 3mm, 3.2mm, 3.5mm, 3.7mm, 4.0mm; the distance between the upper surface 1 and the lower surface 2 forms the cutting tool thickness H, which can be, for example, but not limited to, any one or any two of 5.8mm, 6.0mm, 6.5mm, 7.0mm, 7.5mm, 8.0mm, 8.5mm, 9.0mm, 9.5mm, 9.8mm.

[0041] The upper surface 1 and the lower surface 2 are connected to form a side surface 3, which forms the flank face of the cutting tool. Side surface 3 is a large flat surface, and its function is to provide positioning during installation. Sometimes, the tip of a cutting chip hits the flank face, allowing the chip to be broken. The number of side surfaces 3 can be set as needed. In this embodiment, there are four side surfaces 3, forming a rhombus. The inscribed circle of the rhombus is D, and 18.5mm ≤ D ≤ 38.5mm. The included angle between any two adjacent side surfaces is α, and 78° ≤ α ≤ 102°. Specifically, the inscribed circle of the rhombus is D, which can be any one or a range between any two of the following: 18.5mm, 20mm, 22.5mm, 25mm, 27.5mm, 30mm, 32.5mm, 35mm, 38.5mm; the included angle between any two adjacent lateral faces is α, which can be any one or a range between any two of the following: 78°, 80°, 85°, 90°, 95°, 100°, 102°.

[0042] The through hole 4 passes through the center of the upper surface 1 and the lower surface 2, and is a mounting hole for fixing. The diameter of the through hole 4 is d, and 7.7mm≤d≤10mm. Specifically, the diameter of the through hole 4 is d, which can be any one or any two of, such as, but not limited to, 7.7mm, 8.0mm, 8.5mm, 9.0mm, 9.5mm, and 10mm.

[0043] Two adjacent side surfaces 3 are connected by a cylindrical surface 5, the radius of which is R, and 1.5mm≤R≤3.3mm. Specifically, the radius of the cylindrical surface 5 is R, which can be any one or any two of, for example, but not limited to, 1.5mm, 1.8mm, 2.0mm, 2.2mm, 2.5mm, 2.7mm, 3.0mm, and 3.3mm.

[0044] The present invention sets a reinforcing ring around the through hole in the center of the blade to increase the cross-sectional area and resist splitting. The blade of the present invention hardly occupies the chip space of the chip breaker groove, does not affect the chip breaking effect of the chip breaker groove, and increases the cross-sectional area of ​​the blade in the middle section, increases the strength of the blade, reduces the probability of the blade splitting failure mode, and improves the service life of the blade.

[0045] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A type of indexable turning tool for heavy roughing, characterized in that, The tool includes an upper surface, a lower surface, multiple side surfaces connecting the upper and lower surfaces, and a through hole passing through the upper and lower surfaces. A cutting edge is formed at the intersection of the upper surface and the side surfaces. The upper surface includes the cutting edge, a recessed chip breaker groove, and a reinforcing ring. The reinforcing ring is continuously arranged around the through hole in the middle of the tool, and its cross-sectional shape is a boss. The top of the reinforcing ring is a plane or a curved surface of any shape. The width of the reinforcing ring is 1~4mm, and the height of the reinforcing ring is ≥0.4mm. The reinforcing ring and the chip breaker groove are connected and transitioned by an inclined plane, and the angle between the inclined plane and the lower surface is 20~70°. Any point on the top of the reinforcing ring is L lower than the cutting edge of the tool tip, and 0.1mm≤L≤1mm.

2. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, The cutting edge includes multiple main cutting edges and multiple tip cutting edges, with the cylindrical surface intersecting the upper surface to form the tip cutting edge.

3. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, The arcs of multiple cutting edges are on the same plane, and this plane is parallel to the lower surface.

4. The indexable turning tool for heavy roughing as described in claim 3, characterized in that, Each main cutting edge is composed of a concave arc pointing downwards and a straight line connecting each end of the arc, or it may be composed of multiple straight lines or only a single arc. The distance from any point on the main cutting edge to the lower surface is no greater than the distance from the cutting edge at the tip to the lower surface.

5. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, The lower surface is a ring plane, and the width of the ring on the lower surface is ≥3mm.

6. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, Adjacent side surfaces are connected and transitioned by a cylindrical surface.

7. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, The distance between the upper and lower surfaces forms the thickness H of the blade, and 5.8mm≤H≤9.8mm.

8. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, The through hole passes through the center of the upper and lower surfaces, and the diameter of the through hole is d, with 7.7mm≤d≤10mm.

9. The indexable turning tool for heavy roughing as described in claim 1, characterized in that, Two adjacent sides are connected by a cylindrical surface with radius R, and 1.5mm≤R≤3.3mm.

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