T-shaped rib butt joint multi-station integrated automatic composite welding method

Abstract

The invention discloses a T-shaped rib butt joint multi-station integrated automatic composite welding method which comprises the steps that a plurality of T-shaped ribs are arranged in a whole circle, panels of the T-shaped ribs are placed on an assembly machine in a vertical position mode, webs of the T-shaped ribs are placed on an assembly machine in a horizontal position mode, and X-shaped grooves are formed in butt joints of every two adjacent T-shaped ribs; and TIG backing welding and MIG filling and cosmetic welding are conducted on butt joints of every two adjacent T-shaped ribs according to the welding sequence that the thickness side of a web plate and the inner side of a panel are welded, the outer side of the panel is welded, and after the whole circle of T-shaped ribs are turned over, the other thickness side of the web plate and the inner side of the panel are welded. By the adoption of the TIG + MIG composite welding method based on the groove characteristics and the welding sequence, the welding efficiency, roundness deformation control and quality stability of unequal-thickness butt joint of the large-diameter T-shaped shell ring structure can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of automatic welding technology without root cleaning, and in particular to an integrated automatic composite welding method for multi-station T-rib butt joints. Background Technology

[0002] In the field of butt welding of high-strength steel T-rib structures, the large thickness of the T-ribs and the significant difference in thickness between the web and the face plate present challenges in construction, requiring high intensity and resulting in substantial welding deformation. Achieving high-quality welding of unequal-thickness butt joints in T-rib structures using automated welding is a problem that needs to be solved. Summary of the Invention

[0003] This application provides a multi-station integrated automatic composite welding method for T-rib butt joints, which solves the technical problem in the prior art of how to achieve high-quality welding of T-rib structures with unequal thickness butt joints through automatic welding.

[0004] This application provides a multi-station integrated automatic composite welding method for T-rib butt joints, comprising: placing multiple T-ribs in a circular arrangement, with the T-rib panels in an upright position and the webs in a horizontal position on an assembly machine; forming an X-shaped bevel at the butt joint of two adjacent T-ribs; performing TIG root pass welding and MIG filler and cover pass welding at the butt joint of two adjacent T-ribs, in the following welding sequence: welding the web thickness side to the inner side of the panel, welding the outer side of the panel, and welding the other side of the web thickness and the inner side of the panel after flipping the entire circle of T-ribs.

[0005] In some embodiments, when welding the thickness side of the web to the inner side of the panel, the welding is performed continuously in the order of web, weld corner area and panel.

[0006] In some embodiments, at the location of the X-shaped bevel, the opening size of the bevel on the web is equal to the opening size of the inner bevel on the panel.

[0007] In some implementations, when TIG root pass welding is used for flat web plates, the welding parameters include a welding current of 150A to 180A, an arc voltage of 22V to 27V, a welding speed of 220mm / min to 300mm / min, and a gas flow rate of 18L / min to 25L / min.

[0008] When using TIG root pass welding for upright panels, the welding parameters include welding current of 180A~220A, arc voltage of 22V~27V, welding speed of 260mm / min~450mm / min, and gas flow rate of 18L / min~25L / min.

[0009] When MIG filler and cover welding is used for flat webs, the welding parameters include welding current of 200A to 260A, arc voltage of 22V to 26V, welding speed of 220mm / min to 300mm / min, and gas flow rate of 18L / min to 25L / min.

[0010] When using MIG filler and cover welding for upright panels, the welding parameters include welding current of 110A to 130A, arc voltage of 20V to 27V, welding speed of 260mm / min to 450mm / min, and gas flow rate of 18L / min to 25L / min.

[0011] In some implementations, the MIG filler and capping welds are multi-layer, multi-pass welds.

[0012] In some implementations, the robot used for MIG filling and capping welding is equipped with a pendulum at its end, which is used to regulate the robot's swinging posture.

[0013] In some implementations, after the entire T-rib is turned over and the thickness of the web plate is welded on the other side and the inner side of the panel, TIG root pass welding is used to remelt and reshape the surface of the already formed root weld.

[0014] In some implementations, a machining center is used to simulate the beveling process during the testing phase. After the simulation results meet the machining accuracy and assembly requirements, the T-shaped ribs are then beveled using specialized machining equipment.

[0015] In some implementations, the X-shaped bevel has a uniform bevel depth, the assembly gap between two adjacent T-shaped ribs is uniform, the opening size of the X-shaped bevel is uniform, and the difference between the maximum and minimum values ​​of the assembly gap and the opening size on the same side is no more than 2 mm.

[0016] In some implementations, the assembly gap is controlled at 3mm-6mm, and the number of joints formed by multiple T-shaped ribs arranged in a circle is controlled at 4, 6, or 8.

[0017] The beneficial effects of this application are as follows: It provides a multi-station integrated automatic composite welding method for T-rib butt joints, used to automatically weld unequal thickness butt joints of T-rib structures. Multiple T-ribs are arranged in a circular pattern on an assembly machine, with the T-rib panels positioned vertically and the webs horizontally. An X-shaped bevel is formed at the butt joint of adjacent T-ribs, creating an unequal thickness butt joint. TIG root pass welding and MIG filler and cover pass welding are performed before and after the unequal thickness butt joint. The TIG root pass welding is followed by MIG filler and cover pass welding. The TIG root pass welding achieves single-sided welding with double-sided forming. The two layers of weld filler optimize and adjust the bevel consistency, meeting the requirements. To ensure the continuity and stability of subsequent MIG robot automated welding, the welding sequence is as follows: welding the web thickness side with the inner side of the panel, welding the outer side of the panel, and welding the web thickness side and the inner side of the panel after flipping the entire T-shaped rib. Before welding, the welding parameters for the weld between the panel and the web are set in three areas: the web area, the weld bead area, and the panel area. By maintaining consistent thickness and meeting the specified requirements for weld forming coefficient through welding parameters, the weld layer and pass are unified in the three areas. The TIG+MIG composite welding method provided in this application, based on the above-mentioned bevel characteristics and welding sequence, can effectively improve the welding efficiency, roundness deformation control, and quality stability of unequal thickness butt joints of large-diameter T-shaped shell ring structures. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention.

[0019] Figure 1 A schematic diagram showing the formation of an X-shaped bevel at the joint of two adjacent T-ribs in a multi-station integrated automatic composite welding method for T-rib butt welding provided in this application.

[0020] Figure 2 This is a schematic diagram of the blunt edge at the base of a T-shaped rib.

[0021] Figure 3 A detailed schematic diagram of the X-shaped bevel of the web;

[0022] Figure 4 This is a schematic diagram of the weld seams on the panel;

[0023] Figure 5 This is a schematic diagram of the weld seam of the web plate;

[0024] Figure 6 This is a schematic diagram illustrating the welding sequence in the method of this application;

[0025] Figure 7 This is a schematic diagram showing the arrangement of multiple T-shaped ribs in a complete circle in the method of this application.

[0026] Attached diagram labels: 10-panel, 11-inner side, 12-outer side, 20-web, 30-X-shaped bevel, 40-T-shaped rib, 50-butt joint. Detailed Implementation

[0027] This application provides a multi-station integrated automatic composite welding method for T-rib butt joints, belonging to the field of root-cleaning-free automatic composite welding technology. It is used to realize automatic welding of butt joints with unequal thickness in T-rib structures. In particular, it relates to a root-cleaning-free, stable and reliable welding method for multi-station integrated robot automatic welding of T-rib structures made of 10CrNi5MoV high-strength steel with a nominal yield strength of 785MPa and a carbon equivalent of 0.6% to 0.7%.

[0028] Overall, this application delves into the aspects of welding modeling, programming, welding torch motion trajectory planning, and robotic automatic welding for multi-station integrated construction, which can meet the welding quality requirements of unequal thickness butt joints in high-quality T-rib structures.

[0029] Please refer to Figure 1 and Figure 2 The T-rib 40 includes a face plate 10 and a web plate 20 connected to the face plate 10. This application employs an X-shaped bevel 30; please refer to [reference needed]. Figure 1 and Figure 3 An X-shaped bevel 30 is formed at the joint 50 between two adjacent T-ribs 40. The joint 50 between the panels 10 of two adjacent T-ribs 40 is also an X-shaped bevel 30, as is the joint 50 between the webs 20 of two adjacent T-ribs 40. Since the thickness of the panel 10 of the T-rib 40 is greater than the thickness of the web 20, an unequal thickness joint is formed at the X-shaped bevel 30 in the T-rib 40 structure.

[0030] Please refer to Figure 7 In this application, multiple T-shaped ribs 40 are arranged in a complete circle on an assembly machine, and as... Figure 6 The T-rib 40 is positioned so that its face plate 10 is placed upright on the assembly machine, and its web 20 is placed horizontally on the assembly machine. An X-shaped bevel 30 is formed at the butt joint 50 of adjacent T-ribs 40. TIG root pass welding and MIG filler and cover pass welding are performed at the butt joint 50 of adjacent T-ribs 40. Figure 4 and Figure 6 The shapes and locations of the welds formed by TIG root pass and MIG filler and cover passes are given respectively. This application specifies the welding sequence for TIG root pass and MIG filler and cover passes; please refer to the relevant documentation for details. Figure 6During the TIG root pass welding, first weld the thickness side of the web plate 20 to the inner side 11 of the panel 10, then weld the outer side 12 of the panel 10, and finally, after turning the T-rib 40 over, weld the other side of the thickness of the web plate 20 and the inner side 11 of the panel 10. The inner side 11 of the panel 10 is the side facing the web plate 20, and the outer side 12 is the side away from the web plate 20.

[0031] This application involves performing TIG root pass welding and MIG filler and cover pass welding before and after the unequal thickness butt joint of the T-rib 40 structure. The MIG filler and cover pass welding are performed after the TIG root pass welding. The TIG root pass welding achieves single-sided welding with double-sided forming. The bevel consistency is optimized and adjusted after two layers of weld filler to ensure the continuity and stability of subsequent MIG robotic automatic welding. The welding sequence is as follows: welding one side of the web 20 thickness to the inner side 11 of the panel 10, welding the outer side 12 of the panel 10, and welding the other side of the web 20 thickness and the inner side 11 of the panel 10 after flipping the entire T-rib 40. Before welding, the welding parameters for the weld between the panel 10 and the web 20 are set in three areas: the web 20 area, the weld bead area, and the panel 10 area. These three areas correspond to the welding sequence. By maintaining consistent thickness and meeting the specified weld forming coefficient through welding parameters, the weld layer and pass count are unified across the three areas.

[0032] The TIG+MIG composite welding method provided in this application, based on the above-mentioned bevel characteristics and welding sequence, can effectively improve the welding efficiency, roundness deformation control, and quality stability of unequal thickness butt joints in large-diameter T-shaped shell ring structures.

[0033] This application places extremely high demands on the beveling accuracy of the T-rib 40 and its assembly on the assembly machine. Therefore, it is essential to ensure both the beveling accuracy and the assembly accuracy of the T-rib 40.

[0034] Before applying the multi-station integrated automatic composite welding method for T-ribs 40 as described in this application, it is necessary to design the bevel form of the T-ribs with unequal thicknesses. Specifically, a three-dimensional modeling method is used to simulate the assembly accuracy of the components and perform contour bevel machining. It is required that, under the given bevel machining accuracy and assembly requirements, the opening size of the bevel of the web 20 of the T-rib 40 is consistent with the opening size of the bevel on the inner side 11 of the panel 10. The opening size of the bevel of the web 20 is as follows: Figure 5 As shown in b1, the opening size of the inner bevel of panel 10 is as follows: Figure 4 As shown in b2, please refer to the reference. Figure 1 , Figure 4 and Figure 5 , Figure 1 This indicates that the opening size of the bevel of the web 20 is equal to the opening size of the inner bevel of the face plate 10.

[0035] During the experimental phase, a machining center was first used to simulate the beveling process. This successfully simulated variable cross-sections and beveling patterns, achieving the goal of ensuring approximately equal beveling filler for welded structures of unequal thickness. Subsequently, specialized machining equipment needs to be procured to machine the beveling of the T-shaped unequal thickness rib ends. The weld beveling depth should be uniform, and the assembly gap between two adjacent T-shaped ribs should also be uniform. Figure 1 and Figure 3 The symbol c is used to indicate that the opening size on the same side of the X-shaped bevel 30 should also be uniform. In some embodiments, the assembly gap must meet the requirement that the difference between the maximum and minimum values ​​is not greater than 2mm, and the opening size on the same side must meet the requirement that the difference between the maximum and minimum values ​​is not greater than 2mm.

[0036] Before welding, the ribs are placed on a special automatic circular assembly machine in a circular arrangement. Based on the pre-designed CNC three-dimensional simulation layout dimensions, the assembly gap is controlled within the range of 3-6mm. The entire rib has 4 / 6 / 8 joints, and the gap difference of a single butt weld does not exceed 2mm.

[0037] During assembly of the T-shaped unequal thickness rib structure, arc-extinguishing plates must be correctly installed at three locations according to specifications: on both sides of the panel 10 of the T-shaped rib 40 and at one end of the web 20. Given the relatively small cross-sectional dimensions of the T-shaped unequal thickness rib structure, the arc-extinguishing plates are appropriately lengthened, and the locating welds are placed on the arc-extinguishing plates to ensure assembly rigidity while guaranteeing weld continuity and consistency in automatic welding.

[0038] The root pass weld is formed using TIG automated welding, achieving single-sided welding with double-sided forming. Two layers of weld filler are used to optimize and adjust the bevel consistency, ensuring the continuity and stability of subsequent MIG robotic automated welding. The single-sided welding with double-sided forming at the root avoids the need for root cleaning. During welding, a special fixture is used on the back side to form a chamber, which is filled with argon gas to provide back protection during welding.

[0039] Based on the characteristics of the T-shaped unequal thickness rib butt joint bevel, in order to achieve high-quality integrated welding from horizontal to vertical position, the welding parameters for the weld between the face plate 10 and the web plate 20 are divided into three areas before welding: the web plate 20 area, the weld corner area, and the face plate 10 area, which have already been described above. It should be noted here that the web plate 20 area represents the horizontal welding area for thin plates, the weld corner area represents the spindle-shaped transition area, and the face plate 10 area represents the vertical welding area for thick plates. Considering the above requirements for the bevel, welding parameters need to be rationally designed. Welding parameters include, but are not limited to, welding current, welding voltage, and welding speed. While maintaining consistent weld thickness and meeting the specified weld formation coefficient through welding parameters, the goal of unifying the weld layers and passes in the three areas can be achieved by increasing the welding torch oscillation. The method of increasing the welding torch oscillation will be described in detail below.

[0040] The above mentioned the uniformity of weld layers and passes in the three areas. Further explanation is provided here regarding the MIG filler and capping welds in this application, which can be found in [reference needed]. Figure 4 and Figure 5 The welds formed by MIG filler and cover welding are multi-layer, multi-pass welding. Each layer of welding is carried out in the following order: welding the thickness of the web plate 20 to the inner side 11 of the panel 10, welding the outer side 12 of the panel 10, and welding the thickness of the web plate 20 to the other side and the inner side 11 of the panel 10 after the entire T-rib 40 is turned over.

[0041] The MIG welding robot also needs to adapt to the weld consistency test in the three different areas mentioned above. By planning and specifying the welding process requirements, a pendulum device is added to the end of the MIG welding robot to regulate its swing posture. During the weld movement, the welding torch head of the MIG welding robot is kept perpendicular to the weld surface and the bevels on both sides, ensuring the stability of the weld quality for both the filler and cover welds.

[0042] Please refer to Figure 6 In the welding sequence of welding the thickness side of the web 20 to the inner side 11 of the panel 10, welding the outer side 12 of the panel 10, and then welding the other side of the thickness of the web 20 and the inner side 11 of the panel 10 after turning over the entire circle of T-ribs 40, the entire circle of T-ribs 40 needs to be turned over. This turning over operation ensures that the welding position is always horizontal and vertical, avoiding any upward tilting. The welding of the thickness side of the web 20 to the inner side 11 of the panel 10 before turning over corresponds to... Figure 6 The welding sequence shown in the figure ① and the welding of the outer side 12 of panel 10 correspond to Figure 6 The welding sequence in ② refers to the welding of the other side of the web plate 20 after turning over and the welding of the inner side 11 of the panel 10. Figure 6The welding sequence is shown in ③. In this application, after the TIG root pass is completed, the TIG root pass is only one pass. In the subsequent MIG filler and cover passes, the root weld is remelted and shaped by the TIG root pass, and then the MIG welding robot is used for welding to finally form an integrated weld.

[0043] Regarding the outer 12 welds of panel 10, the root of the inner TIG weld of panel 10 is remelted and repaired using TIG welding, and then the filler and cover are re-welded from bottom to top using a MIG welding robot.

[0044] It should be noted that, in the multi-station integrated automatic composite welding method for T-rib 40 butt joints provided in this application, the area requiring beveling and approximately 30-50mm on both sides of the beveling should be ground before welding until a metallic luster is visible. In the multi-station integrated automatic composite welding method for T-rib 40 butt joints provided in this application, preheating should be performed as required before welding, and the preheating and interpass temperatures should be controlled within the range of 80℃~130℃.

[0045] In the T-rib 40 butt joint multi-station integrated automatic composite welding method provided in this application, multi-layer and multi-pass welding is adopted. During the welding process of the welds belonging to the web 20 area and the face plate 10 area, the welding torch moves in a straight line; during the welding process of the welds belonging to the weld corner area, the welding torch posture is adjusted accordingly. The welding current, voltage, and welding speed are matched by the welding machine's own expert recommendation system, with the welding current as the main adjustment object. When the welding current is too high, although the arc penetration and arc stiffness increase, the welding speed needs to be increased accordingly in order to control the welding heat input. When the welding speed is too high, the stability of the welding arc decreases, which is not conducive to ensuring welding quality. When the welding current is too low, the arc stiffness, penetration, and width decrease, which is not conducive to the elimination of inter-pass defects and affects the welding quality. In addition, if the current is too low, the welding efficiency will be reduced, making it difficult to realize the advantages of automatic welding.

[0046] This application provides applicable welding parameters that offer advantages such as faster welding speed, better arc stability, and elimination of interpass defects in practical applications, resulting in excellent welding quality. Specifically, the welding parameters are as follows: For TIG root pass welding of the web plate 20 in a horizontal position, the welding parameters include a welding current of 150A–180A, an arc voltage of 22V–27V, a welding speed of 220mm / min–300mm / min, and a gas flow rate of 18L / min–25L / min; For TIG root pass welding of the panel 10 in a vertical position, the welding parameters include a welding current of 180A–220A, an arc voltage of 22V–27V, a welding speed of 260mm / min–450mm / min, and a gas flow rate of 18L / min–25L / min. When MIG filler and cover welding is used for flat web plates 20, the welding parameters include welding current of 200A to 260A, arc voltage of 22V to 26V, welding speed of 220mm / min to 300mm / min, and gas flow rate of 18L / min to 25L / min. When MIG filler and cover welding is used for upright panels 10, the welding parameters include welding current of 110A to 130A, arc voltage of 20V to 27V, welding speed of 260mm / min to 450mm / min, and gas flow rate of 18L / min to 25L / min.

[0047] In the multi-station integrated automatic composite welding method for T-rib 40 butt joints provided in this application, when performing multi-layer, multi-pass welding, laser cleaning is used to remove oxides between weld beads. Robot programming is used to simulate the weld path, appropriately increasing the oscillation arc in the transition area belonging to the weld corner. The preheating requirements, welding parameters, weld bead distribution principle, wire directionality, and wire angle of the subsequent weld surface are the same as those of the preceding weld surface. To effectively control welding deformation, when the thickness of panel 10 is greater than 20mm, an alternating forward and reverse welding method can be used.

[0048] In summary, this application provides a root-cleaning-free welding method for butt welds of 40mm T-shaped ribs made of 10CrNi5MoV high-strength steel. During welding, a specialized bevel is machined without allowance on the T-shaped cross-section of the ribs with unequal thickness using specialized processing equipment. The 40mm T-shaped rib structure is then welded using a TIG+MIG composite method. The TIG root pass achieves single-sided welding with double-sided forming, thus avoiding root cleaning. The MIG welding robot's automated welding improves production efficiency and manufacturing quality, while saving manufacturing costs. The technical requirements for the series of processing equipment, bevel design, welding process parameters, and welding equipment technical requirements provided in this application ensure weld quality. The TIG+MIG composite welding method provided in this application can effectively improve the welding efficiency, roundness deformation control, and quality stability of butt welds of large-diameter T-shaped shell ring structures with unequal thickness.

[0049] In actual production, the inventors welded the shell ring structure through penetration butt weld according to the technical solution provided in this application. After welding, non-destructive testing and mechanical property tests were carried out in accordance with relevant standards, and all met the relevant technical requirements.

[0050] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0051] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A T-rib butt joint multi-station integrated automatic composite welding method, characterized in that: include: Arrange a plurality of T-shaped ribs in a full circle, place the face plates of the T-shaped ribs in an upright position and the web plates in a horizontal position on an assembly machine, and form an X-shaped groove at the butt joint of two adjacent T-shaped ribs; TIG base welding, MIG filling and capping welding are performed on the butt joints of two adjacent T-shaped ribs, in the order of welding one side of the thickness of the web and the inner side of the panel, welding the outer side of the panel, and welding the other side of the thickness of the web and the inner side of the panel after turning over the entire circle of the T-shaped ribs.

2. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 1, characterized in that: When welding the thickness side of the web and the inner side of the face plate, continuous welding is performed in the order of the web, the fillet area and the face plate.

3. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 2, characterized in that: At the position of the X-shaped groove, the opening size of the groove of the web is equal to the opening size of the inner groove of the panel.

4. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 2, characterized in that: When the TIG backing welding is used for the flat web, the welding parameters include welding current 150A-180A, arc voltage 22V-27V, welding speed 220mm / min-300mm / min and gas flow 18L / min-25L / min; When the TIG base welding is used for the panel in a standing position, the welding parameters include welding current 180A-220A, arc voltage 22V-27V, welding speed 260mm / min-450mm / min and gas flow 18L / min-25L / min; When the MIG filling and capping welding is used for the flat web, the welding parameters include welding current 200A-260A, arc voltage 22V-26V, welding speed 220mm / min-300mm / min and gas flow 18L / min-25L / min; The welding parameters of the MIG filling and capping welding for the panel in a standing position include welding current 110A-130A, arc voltage 20V-27V, welding speed 260mm / min-450mm / min and gas flow 18L / min-25L / min.

5. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 2, characterized in that: The MIG filling and capping welding is multi-layer and multi-pass welding.

6. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 2, characterized in that: A pendulum is added to the end of the robot used for performing the MIG filling and cap welding, and the pendulum is used to regulate the swing posture of the robot.

7. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 2, characterized in that: After the T-shaped ribs of the entire circle are turned over and welded to the other side of the thickness of the web and the inner side of the panel, the TIG base welding is used to re-melt and reshape the weld surface of the formed root weld.

8. The T-rib butt joint multi-station integrated automatic composite welding method according to any one of claims 1 to 7, characterized in that: During the test phase, a machining center is used to simulate the processing of grooves. After the simulation results meet the processing accuracy and assembly requirements, the T-shaped ribs are processed by special processing equipment.

9. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 8, characterized in that: The groove depth of the X-shaped groove is uniform, the assembly gap between two adjacent T-shaped ribs is uniform, the opening size of the X-shaped groove is uniform, and the assembly gap and the opening size on the same side both satisfy that the difference between the maximum value and the minimum value is no more than 2 mm.

10. The T-rib butt joint multi-station integrated automatic composite welding method according to claim 9, characterized in that: The assembly gap is controlled to be 3 mm-6 mm, and the number of joints formed by the plurality of T-shaped ribs arranged in a whole circle is controlled to be 4, 6 or 8.

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