flat center

Abstract

The application provides a plane top, and belongs to the technical field of top. The existing top has the problems of poor applicability and low machining precision. The plane top comprises a sleeve and a body, the front end surface of the sleeve is a pressing surface, the front end surface of the body is provided with a matching part, the sleeve is sleeved on the matching part, the rear end of the sleeve and / or the front end of the body is provided with a curved surface, the sleeve can be deflected through the curved surface when the front end surface of the sleeve is stressed, and a gap two for the deflection of the sleeve is arranged between the sleeve and the matching part. The plane top has the advantages of ensuring machining precision, wide applicability, reliable structure and the like.

Description

Technical Field

[0001] This invention belongs to the field of cutting-edge technology and relates to a planar tip. Background Technology

[0002] When machining gear blanks, such as drilling or grooving the end face, the conventional method of fixing them is to use a three-jaw hydraulic chuck on a CNC lathe to clamp the outer edge of the gear blank, and then use a center to apply force to tighten it on the end face to be machined. Generally, when the gear blank has a center hole, a conventional tapered center is used to directly press against the inner wall of the center hole port of the gear blank to form a tight contact. However, for some drive gear blanks that do not have a center hole, using a conventional tapered center to tighten them results in only point contact, which not only has a small contact area and is prone to workpiece vibration, but also easily damages the center, making it unsuitable.

[0003] A Chinese patent application (application number 201020283199.0) disclosed a driven spiral bevel gear tooling, which includes a tie rod, a main spindle sleeved outside the tie rod, a positioning spindle sleeved outside the main spindle, a support plate sleeved outside the positioning spindle, support blocks evenly distributed along the circumference of the support plate and abutting against the end face of the support plate, an elastic expansion sleeve installed in the workpiece positioning hole and having a tapered fit with the positioning spindle, and a pressure cap installed at the end of the elastic expansion sleeve. This driven spiral bevel gear tooling provides an inspiration for using the support blocks to press against the end face of the driven spiral bevel gear to ensure uniform axial force, thereby achieving the purpose of ensuring machining accuracy. However, in actual processing, it was found that because gear blanks are forgings, their end faces are often not completely flat. In this state, some support blocks often abut against the end face of the gear blank while others are suspended in the air. This makes it impossible to ensure that the end face of the gear blank is subjected to uniform axial force during processing. Processing under this condition will affect the processing accuracy, meaning that workpieces with uneven end faces are not suitable for processing.

[0004] In response to this situation, a technical approach readily conceived by those skilled in the art is to make the connection between the support block and the support plate adjustable. For example, the support block can be fixed to a positioning rod, and then the positioning rod can be inserted into the support plate and tightened using a locking screw screw screwed in from the side. When the support block is suspended, the locking screw screw can be loosened and the support block pulled outward until it rests against the end face of the gear blank. Finally, the locking screw screw can be tightened again to ensure full contact with the end face of the gear blank. Alternatively, each support block can be paired with a spring. When the end face of the gear blank is uneven, some springs will have a larger compression and some will have a smaller compression. The difference in the compression of each spring can be used to compensate for the unevenness of the end face of the gear blank. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned problems in the existing technology by proposing a planar tip, which solves the problems of poor applicability and low machining accuracy.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A planar tip includes a cylindrical pressure sleeve and a body with a connecting part on the rear end face. The front end face of the pressure sleeve is a pressing surface. The characteristic is that the front end face of the body has a mating part, the pressure sleeve is sleeved on the mating part, the rear end of the pressure sleeve and / or the front end of the body has an arc surface, and when the front end face of the pressure sleeve is subjected to force, the pressure sleeve can swing through the arc surface. A gap is provided between the pressure sleeve and the mating part to allow the pressure sleeve to swing.

[0008] In use, the entire flat tip is fixed to the tailstock of the CNC machine tool via a connecting part. The hydraulic three-jaw chuck on the CNC machine tool clamps the outer edge of one end of the drive gear blank. Then, the tailstock drives the entire flat tip to move towards the drive gear blank to achieve clamping. Under normal circumstances, the end face of the drive gear blank is flat, so the front end face of the pressure sleeve can fit perfectly against the end face of the drive gear blank, and the flatness of the two is in the optimal state. At this time, the arc surface does not play a role, that is, the pressure sleeve will not wobble. When the end face of the drive gear blank is not flat due to poor precision, part of the front end face of the pressure sleeve will abut against the end face of the drive gear blank, while the other part of the front end face of the pressure sleeve will be suspended. At this time, by the continuous clamping force applied to the drive gear blank by the tailstock, the drive gear blank will exert a reaction force on the pressure sleeve in the opposite direction. Since the body is fixed on the tailstock and remains stationary, the pressure sleeve will wobble relative to the body along the arc surface. In this way, the front end face of the pressure sleeve, which was previously suspended, can also abut against the end face of the drive gear blank.

[0009] This planar center, by setting an arc surface between the rear end of the pressure sleeve and / or the front end of the body, and by setting a second gap between the pressure sleeve and the mating part, allows the pressure sleeve and the body to float. When the end face of the drive gear blank is uneven, the pressure sleeve can automatically tilt using the arc surface, thereby compensating for the gap between the front end face of the pressure sleeve and the end face of the drive gear blank. This ensures sufficient contact between the front end face of the pressure sleeve and the end face of the drive gear blank, guaranteeing uniform axial force on the end face of the drive gear blank during machining and improving machining accuracy. Furthermore, since the pressure sleeve automatically tilts when the end face of the drive gear blank is uneven due to the interaction of the arc surface and the first gap, no manual operation is required, greatly improving convenience and clamping efficiency. Sufficient contact is achieved regardless of whether the end face of the drive gear blank is flat or not, thus increasing its applicability.

[0010] In the aforementioned planar tip, the front end face of the main body is provided with an annular flange, the arc surface is located inside the annular flange, the rear end of the pressure sleeve abuts against the arc surface of the arc flange, and there is a gap between the rear end face of the pressure sleeve and the front end face of the main body.

[0011] There is a gap between the rear end face of the pressure sleeve and the front end face of the body, which means that the two are not in contact. This allows the rear end of the pressure sleeve to automatically oscillate smoothly with the arc surface, so that the pressure sleeve and the end face of the drive gear blank can make full contact to improve the machining accuracy.

[0012] In the aforementioned planar tip, as another technical solution, the rear end face of the pressure sleeve has an annular shoulder, the arc surface is located inside the annular shoulder, the front end of the body abuts against the arc surface of the annular shoulder, and there is a gap between the rear end face of the pressure sleeve and the front end face of the body.

[0013] In the aforementioned planar tip, the arc surface is a circular arc surface.

[0014] In the aforementioned planar tip, the front end face of the main body is provided with a first cavity, which is adjacent to the arc surface, and the rear end face of the pressure sleeve is provided with a second cavity, which is adjacent to the arc surface.

[0015] Based on gap one, two additional cavities are created, forming a space between the pressure sleeve and the body that allows the pressure sleeve to swing over a wider range, reducing the obstruction when the pressure sleeve swings automatically.

[0016] In the aforementioned planar tip, the pressure sleeve includes a disc body, a connecting body that expands outward from the front end of the disc body, and a pressing body that extends straight forward from the front end of the connecting body. The disc body is fitted onto the mating part.

[0017] The pressure sleeve includes a disc body, a connecting body that expands outward from the front end of the disc body, and a clamping body that extends straight forward from the front end of the connecting body. This structure makes the pressure sleeve resemble a bowl shape, which is more conducive to the pressure sleeve automatically tilting when the end face of the drive gear blank is uneven. In other words, the pressure sleeve can tilt more easily, thereby making the axial force on the drive gear blank uniform during processing and improving the processing accuracy.

[0018] In the aforementioned planar tip, a central hole is provided through the disc body, and the inner wall of the front port of the central hole of the disc body has an annular protrusion, and the mating part is columnar and passes through the annular protrusion.

[0019] The inner wall of the front end of the center hole of the disc has an annular protrusion, and the mating part passes through the annular protrusion, meaning that the second clearance is located at the mating part and the annular protrusion. In this way, the rear end of the center hole of the disc provides a larger swing space for the pressure sleeve, allowing the pressure sleeve to automatically swing more smoothly when encountering uneven end faces of the drive gear blank, ensuring sufficient contact and improving machining accuracy.

[0020] In the aforementioned planar tip, the outer side of the mating part is provided with a blocking member that can prevent the pressure sleeve from coming off the mating part. When the pressure sleeve swings, the annular protrusion and the blocking member are in flexible contact.

[0021] Flexible contact means that there will be no rigid compression between the pressure sleeve and the blocking component when the pressure sleeve deflects, which can prevent the blocking component from deforming and affecting the reliability of the structure.

[0022] In the aforementioned planar tip, the blocking element includes a retaining ring and a deformable elastic element, with the elastic element located between the retaining ring and the annular protruding front end face.

[0023] The presence of the retaining ring prevents the pressure sleeve from disengaging from the mating part, thus limiting the axial movement of the mating part. Simultaneously, the elastic element contacts the annular protruding front end face. When the pressure sleeve wobbles, the axial force generated is applied to the elastic element, which is absorbed by the deformation of the elastic element to prevent the retaining ring from being directly stressed and deformed. This improves machining accuracy while ensuring structural reliability. Furthermore, when the tailstock drives the entire planar tip away from the drive gear blank after machining, the elastic element's own elastic force can guide the wobbleed pressure sleeve back to its correct position.

[0024] In the aforementioned planar tip, a push rod is slidably disposed within the body along the front-back direction. The front end of the push rod extends beyond the front end of the mating part. A large spring is disposed within the body, and the push rod always tends to slide forward under the elastic force of the large spring.

[0025] During use, the front end face of the pressure sleeve presses against the corresponding end face of the drive gear blank, while the end of the push rod presses against the bottom surface of the concave position on the corresponding end face of the drive gear blank, further increasing the contact area between the center point of this plane and the drive gear blank, improving the clamping effect and ensuring machining accuracy.

[0026] Compared with existing technologies, this planar tip has the following advantages:

[0027] 1. Through the cooperation of the arc surface, gap one and gap two, the pressure sleeve and the body are in a floating state. When the end face of the drive gear blank is uneven, the pressure sleeve can automatically swing to form full contact, ensuring that the axial force on the end face of the drive gear blank is uniform during processing, thus improving the processing accuracy.

[0028] 2. The wobble of the pressure sleeve is achieved automatically, without the need for manual operation by the staff, which greatly improves convenience and clamping efficiency. Moreover, it can form full contact regardless of whether the end face of the workpiece is flat, and has good applicability.

[0029] 3. The blocking component and the pressure sleeve can form a flexible contact, which realizes the axial soft fixation of the pressure sleeve. At the same time, it can generate rigid extrusion on the blocking component when the pressure sleeve swings, thereby deforming it and ensuring the reliability of the structure.

[0030] 4. While the pressure sleeve is tightened, the use of the push rod increases the contact area between the push sleeve and the drive gear blank, making the axial force on the drive gear blank more uniform during processing, and further improving the processing accuracy. Attached Figure Description

[0031] Figure 1 This is a cross-sectional view of the drive gear blank when the center of this plane is pressing against it.

[0032] Figure 2 yes Figure 1 Enlarged view of point A in the middle.

[0033] In the figure, 1. Body; 1a. Base; 1a1. Connecting part; 1a11. Protruding ring; 1b. Connecting seat; 1b1. Mating part; 1b2. Annular flange; 1b3. Cavity 1; 1b4. Connecting hole; 1b5. Assembly hole; 2. Pressure sleeve; 2a. Disc; 2a1. Cavity 2; 2a2. Annular protrusion; 2b. Connecting body; 2c. Pressing body; 3. Arc surface; 4. Blocking component; 4a. Snap ring; 4b. Retaining ring; 4c. O-ring; 5. Push rod; 6. Large spring; 7. Fastening screw; 8. Sealing ring; 9. Removal nut; 10. Drive gear blank; H1. Clearance 1; H2. Clearance 2. Detailed Implementation

[0034] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0035] Example 1

[0036] The planar top includes a body 1 and a cylindrical pressure sleeve 2. A columnar connecting portion 1a1 is located at the center of the rear end face of the body 1, and the pressure sleeve 2 is connected to the front end of the body 1. The front end face of the pressure sleeve 2 is a pressing surface, and the front end face of the body 1 has a columnar mating portion 1b1. The pressure sleeve 2 is fitted onto the mating portion 1b1. A gap H1 is provided between the rear end face of the pressure sleeve 2 and the front end face of the body 1. The rear end of the pressure sleeve 2 and / or the front end of the body 1 have an arc surface 3 that allows the pressure sleeve 2 to deflect through the gap H1 when the front end face of the pressure sleeve 2 is subjected to force. A second gap H2 is provided between the pressure sleeve 2 and the mating portion 1b1 to allow for the deflection of the pressure sleeve 2. In this embodiment, both the rear end of the pressure sleeve 2 and the front end of the body 1 have arc surfaces 3.

[0037] The front end face of the body 1 has an annular flange 1b2 along its edge. The rear end of the pressure sleeve 2 is located inside the annular flange 1b2. The arc surface 3 is located inside the annular flange 1b2 and is a concave arc surface. The arc surface 3 at the rear end of the pressure sleeve 2 is a convex arc surface connecting the rear end face and the outer side of the pressure sleeve 2, and the two arc surfaces 3 are in contact. The center of the front end face of the body 1 also has a cavity 1b3, and the inner peripheral wall of the cavity 1b3 is connected to the concave arc surface 1b21. The rear end face of the pressure sleeve 2 has a cavity 2a1, which is opposite to the cavity 1b3. The inner peripheral wall of the cavity 2a1 is adjacent to the convex arc surface 2a1. Based on the gap H1, the combination of the cavity 1b3 and the cavity 2a1 allows the pressure sleeve 2 to swing over a wider range.

[0038] Furthermore, the pressure sleeve 2 includes a disc body 2a, a connecting body 2b extending outward from the front end of the disc body 2a, and a pressing body 2c extending straight forward from the front end face of the connecting body 2b. The connecting body 2b is roughly trumpet-shaped, with an outwardly convex arc surface 2a1 and a concave cavity 2a1 on the disc body 2a. A central hole is also provided through the disc body 2a. The inner wall of the front end of the central hole of the disc body 2a has an annular protrusion 2a2. The mating part 1b1 passes through the annular protrusion 2a2, and the gap 2 H2 is located between the inner wall of the central hole of the annular protrusion 2a2 and the outer wall of the mating part 1b1. A blocking member 4 is provided on the outer side of the mating part 1b1 to prevent the pressure sleeve 2 from falling off the mating part 1b1. When the pressure sleeve 2 wobbles, there is a flexible contact between the annular protrusion 2a2 and the blocking member 4. The blocking member 4 includes a retaining spring 4a and a deformable elastic element, which is located between the retaining spring 4a and the front end face of the annular protrusion 2a2. In this embodiment, the elastic element is an O-ring 4C, and the blocking member 4 also includes a retaining ring 4b. The blocking member 4, the retaining ring 4b, and the O-ring 4C abut against each other in sequence, with the O-ring 4C abutting against the front end face of the annular protrusion 2a2. In addition to the O-ring 4C, small springs, wave washers, etc., can also be used as elastic elements, as long as they can be locally compressed and deformed when the pressure sleeve 2 sways, they can meet the requirements.

[0039] A push rod 5 is slidably mounted inside the main body 1 along the front-rear direction, with its front end extending beyond the front end of the mating part 1b1. A large spring 6 is installed inside the main body 1, and the push rod 5 always tends to slide forward under the elastic force of the large spring 6. Specifically, the main body 1 includes a base 1a and a connecting seat 1b, both of which are disc-shaped. A cavity 1b3 is located on the front end face of the connecting seat 1b, and the bottom surface of the cavity 1b3 has several connecting holes 1b4 around the center of the connecting seat 1b. The connecting seat 1b and the base 1a are fixed together by several fastening screws 7 that pass through the connecting holes 1b4 and into the front end face of the base 1a. The mating part 1b1 is located at the center of the front end face of the connecting seat 1b, and an assembly hole 1b5 is provided at the center of the rear end face of the connecting seat 1b. A guide hole is provided on the front end face of the mating part 1b1. The guide hole and the assembly hole 1b5 are aligned along the same centerline and are connected. The diameter of the assembly hole 1b5 is larger than the diameter of the guide hole. The push rod 5 passes through the guide hole via the self-assembly hole 1b5. A lug is provided on the outer rear end of the push rod 5 to prevent it from dislodging from the assembly hole 1b5. A large spring 6 is located inside the assembly hole 1b5, with its rear end abutting against the front end face of the base 1a and its front end abutting against the rear end face of the push rod 5. A sealing ring 8 is provided at the front end of the guide hole, with its inner edge contacting the outer circumferential surface of the push rod 5 to form a seal.

[0040] During assembly, first, insert the push rod 5 and the large spring 6 into the assembly hole 1b5 of the connecting seat 1b in sequence. Then, fix the connecting seat 1b to the front end face of the base 1a with the fastening screw 7, thus fixing the connecting seat 1b to the base 1a to form the body 1. In order to facilitate quick assembly, a concave-convex fitting structure can be provided between the front end of the base 1a and the rear end of the connecting seat 1b to achieve quick positioning. Next, put the pressure sleeve 2 on the mating part 1b1. The outer convex arc surface - 2a1 on the pressure sleeve 2 fits together with the inner concave arc surface - 1b21 on the body 1. Then, put the O-ring 4C and the retaining ring 4b on the mating part 1b1 in sequence. Finally, fix the retaining spring 4a in the annular groove on the outside of the mating part 1b1, so that the retaining spring 4a, the retaining ring 4b and the O-ring 4C form a blocking member 4 to prevent the pressure sleeve 2 from falling off the mating part 1b1.

[0041] In use, the entire flat tip is fixed to the tailstock of the CNC machine tool via the connecting part 1a1. The hydraulic three-jaw chuck on the CNC machine tool clamps the outer edge of one end of the drive gear blank 10. Then, the tailstock drives the entire flat tip to move towards the drive gear blank 10 to achieve clamping. The actual end face area of ​​the drive gear blank 10 is much larger than the area covered by the pressure sleeve 2, so that after the pressure sleeve 2 presses against the end face of the drive gear blank 10, it will not affect the drilling or grooving operations on that end face.

[0042] During the tightening process, the front end face of the pressure sleeve 2 presses against the other end face of the drive gear blank 10, while the front end of the push rod 5 presses against the bottom surface of the concave position at the center of that end face of the drive gear blank 10. Normally, if the end faces of the drive gear blank 10 and the pressure sleeve 2 are flat, the front end face of the pressure sleeve 2 can perfectly fit against the end face of the drive gear blank 10, and the flatness of both is at its optimal state. In this case, the arc surface 3 does not function, meaning the pressure sleeve 2 will not wobble. When the end face accuracy of the drive gear blank 10 is poor, part of the front end face of the pressure sleeve 2 will abut against the end face of the drive gear blank 10, while the remaining front end face of the pressure sleeve 2 will be suspended. In this state, the contact area between the pressure sleeve 2 and the drive gear blank 10 is small. At this time, by means of the clamping force applied to the drive gear blank 10 by the tailstock, the drive gear blank 10 will apply a reverse force to the pressure sleeve 2. Since the body 1 is fixed on the tailstock and remains stationary, the arc surface 3 on the pressure sleeve 2 moves along the arc surface 3 on the body 1, so that the pressure sleeve 2 will wobble through the gap H1 between it and the body 1. In this way, the front end face of the pressure sleeve 2, which was previously in a suspended state, can also press against the end face of the drive gear blank 10.

[0043] This planar center, by setting a gap H1 between the rear end face of the pressure sleeve 2 and the front end face of the body 1, and a gap H2 between the pressure sleeve 2 and the mating part 1b1, allows the pressure sleeve 2 and the body 1 to float. Simultaneously, with the addition of the arc surface 3, the pressure sleeve 2 can automatically tilt when the end face of the drive gear blank 10 is uneven, thereby compensating for the gap between the front end face of the pressure sleeve 2 and the end face of the drive gear blank 10. This ensures sufficient contact between the front end face of the pressure sleeve 2 and the end face of the drive gear blank 10, guaranteeing uniform axial force on the end face of the drive gear blank 10 during machining and improving machining accuracy. Furthermore, since the pressure sleeve 2 automatically tilts when the end face of the drive gear blank 10 is uneven through the interaction of the arc surface 3 and the gap H1, manual operation is unnecessary, significantly improving convenience and clamping efficiency.

[0044] The connecting part 1a1 has a threaded protruding ring 1a11 near the base 1a at its front end, and a disassembly nut 9 is threaded onto the protruding ring 1a11. When the connecting part 1a1 is fixed to the tailstock, the disassembly nut 9 is axially fixed between the end face of the tailstock and the rear end face of the body 1. When it is necessary to remove the flat tip from the tailstock to replace it with a conventional tip, simply use a tool to wedge it against the outside of the disassembly nut 9 and rotate it. Since the disassembly nut 9 is axially fixed between the end face of the tailstock and the rear end face of the body 1, the threaded connection will drive the body 1 forward, thereby gradually disengaging it from the tailstock.

[0045] Example 2

[0046] The structure and principle of this embodiment are basically the same as those of Embodiment 1. The difference is that in this embodiment, the rear end face edge of the pressure sleeve 2 has an annular shoulder, the arc surface 3 is located inside the annular shoulder 2 and the arc surface 3 is an outwardly convex arc surface, and the arc surface 3 at the front end of the body 1 is an inwardly concave arc surface connecting the front end face and the outer side of the body 1, and the two arc surfaces 3 are in contact.

[0047] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A planar tip, comprising a cylindrical pressure sleeve (2) and a body (1) with a connecting portion (1a1) on its rear end face, wherein the front end face of the pressure sleeve (2) is a pressing surface, characterized in that, The front end face of the main body (1) is provided with a mating part (1b1), and the pressure sleeve (2) is fitted onto the mating part (1b1). The rear end of the pressure sleeve (2) and / or the front end of the main body (1) are provided with an arc surface (3). When the front end face of the pressure sleeve (2) is subjected to force, the pressure sleeve (2) can be deflected through the arc surface (3). There is a gap (H2) between the pressure sleeve (2) and the mating part (1b1) to allow the pressure sleeve (2) to deflect. The pressure sleeve (2) includes a disc (2a), a connecting body (2b) that expands outward from the front end face of the disc (2a), and a pressing body (2c) that extends straight forward from the front end face of the connecting body (2b). The disc (2a) is fitted onto the mating part (1b1). A central hole is provided through the disc body (2a). The inner wall of the front end of the central hole of the disc body (2a) has an annular protrusion (2a2). The mating part (1b1) passes through the annular protrusion (2a2). A blocking member (4) is provided on the outside of the mating part (1b1) to prevent the pressure sleeve (2) from falling off the mating part (1b1). When the pressure sleeve (2) swings, there is a flexible contact between the annular protrusion (2a2) and the blocking member (4). The blocking member (4) includes a retaining spring (4a) and a spring that can generate The elastic element is deformable and located between the snap ring (4a) and the front end face of the annular protrusion (2a2). The mating part (1b1) is columnar. A push rod (5) is slidably provided in the body (1) along the front-back direction. A guide hole is provided in the mating part (1b1). The push rod (5) passes through the guide hole to the front end of the mating hole (1b1). A large spring (6) is provided in the body (1). The push rod (5) always has a tendency to slide forward under the elastic force of the large spring (6).

2. The planar tip according to claim 1, characterized in that, The front end face of the body (1) is provided with an annular flange (1b2), the arc surface (3) is located inside the annular flange (1b2), the rear end of the pressure sleeve (2) abuts against the arc surface (3) of the annular flange (1b2), and there is a gap (H1) between the rear end face of the pressure sleeve (2) and the front end face of the body (1).

3. The planar tip according to claim 1, characterized in that, The rear end face of the pressure sleeve (2) has an annular shoulder, the arc surface (3) is located inside the annular shoulder, the front end of the body (1) abuts against the arc surface (3) of the annular shoulder, and there is a gap (H1) between the rear end face of the pressure sleeve (2) and the front end face of the body (1).

4. The planar tip according to claim 2 or 3, characterized in that, The arc surface (3) is a circular arc surface.

5. The planar tip according to claim 4, characterized in that, The front end face of the main body (1) is provided with a concave cavity 1 (1b3), which is adjacent to the arc surface (3). The rear end face of the pressure sleeve (2) is provided with a concave cavity 2 (2a1), which is adjacent to the arc surface (3).

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