Manufacturing method of high-speed asynchronous motor rotor

Abstract

The invention discloses a manufacturing method of a high-speed asynchronous motor rotor. A plurality of grooves are formed in the steel side wall of the rotor; brazing filler metal is sprayed on the surface of the conducting bar to obtain a sprayed conducting bar; assembling the spraying conducting bar into the groove to form an assembly body, wherein the conducting bar protrudes out of the side wall of the rotor steel; an outer lantern ring with the expansion coefficient smaller than that of the guide strip is sleeved outside the assembly body; vacuum diffusion welding is carried out, and the outer lantern ring extrudes the conducting bar under the difference of expansion coefficients of the outer lantern ring and the conducting bar; after vacuum diffusion welding is finished, the outer lantern ring is taken out to obtain a welded body; end ring areas are machined at the two ends of the welding body; the end ring area is sleeved with an end ring, the end ring and the welding body are combined, and the end ring is connected with the guide strip. The outer sleeve is arranged outside, axial extrusion force is provided for the conducting bar at the high temperature of vacuum diffusion, so that contact pressure adjustment needed by diffusion welding is achieved, and the welding strength of the conducting bar and a rotor steel assembly face is guaranteed; and the grooves are formed in the side surfaces, so that the conducting bars can be assembled deeper, and the skin effect is reduced.

Description

Technical Field

[0001] The invention relates to the field of motor rotors, and in particular to a method for manufacturing a high-speed asynchronous motor rotor. Background Art

[0002] High-speed asynchronous motors have seen a surge in demand for use in drive systems for new energy vehicles (especially electric and hybrid vehicles) due to their simple structure, high cost-effectiveness and high efficiency. They are also increasingly used in industrial automation, aerospace, medical equipment and power generation equipment, showing a demand for high precision, high-speed operation and low vibration characteristics. With technological advances and the market's pursuit of energy-saving products, its market demand is expected to continue to grow.

[0003] Conductor bars and end rings are important components of asynchronous motor rotors, and they have a significant impact on the performance of the motor. Conductor bars are usually inserted into the core slots and welded to the end rings to form a closed conductive loop. The function of the end ring is to connect the individual conductor bars to form a complete current path, and it is also an important structure to withstand the centrifugal force of the conductor bars. Since the conductor bars and end rings are subject to the combined effects of electromagnetic force, thermal stress, centrifugal force and welding residual stress during motor operation, and the welding strength between the conductor bars and the core slots is not high in the conventional insertion method, it may cause the conductor bars to loosen or break, increase the vibration and noise of the rotor, and affect the stable operation of the motor. Therefore, it is very important to study the conductor bars and end rings of high-speed asynchronous motors.

[0004] Patent No. CN112792423A discloses a method for preparing a copper sleeve of a CT tube rotor. In this patent, the conductor bars and the core slots are welded only by utilizing the self-expansion of the conductor bars, which results in low welding strength of the welding surface.

[0005] If the conductor bars are inserted into the core slots from the ends, since a clamping area and an end ring connection area must be left at the ends, the core slots can only be opened near the surface of the rotor steel, and the conductor bars can only be located near the surface of the rotor steel, which will produce a skin effect, thereby increasing resistance losses and limiting the transmission capacity. Summary of the invention

[0006] The object of the present invention is to provide a method for manufacturing a high-speed asynchronous motor rotor.

[0007] The innovation of the present invention lies in that in addition to the contact pressure caused by the diffusion of the guide phase in all directions due to the difference in expansion coefficient between the guide bar and the rotor steel, an outer sleeve with an expansion coefficient smaller than that of the guide bar is arranged on the outside, so as to provide axial extrusion force on the guide bar under high temperature of vacuum diffusion, so as to achieve the contact pressure adjustment required for diffusion welding, thereby ensuring the welding strength of the assembly surface of the guide bar and the rotor steel; it is not easy to loosen in the later stage; and by providing grooves on the side, the present invention can assemble the guide bar deeper to reduce the skin effect.

[0008] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0009] A method for manufacturing a high-speed asynchronous motor rotor, characterized in that it comprises the following steps:

[0010] (1) A number of grooves are formed on the side wall of the rotor steel;

[0011] (2) spraying solder on the surface of the conductor bar to obtain a sprayed conductor bar;

[0012] (3) Assembling the sprayed guide bar into the groove to form an assembly, with the guide bar protruding from the rotor steel side wall;

[0013] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar is connected to the outer sleeve of the assembly;

[0014] (5) Vacuum diffusion welding is performed, and the outer sleeve extrudes the conductor bar due to the difference in expansion coefficient between the outer sleeve and the conductor bar;

[0015] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0016] (7) Processing end ring areas at both ends of the weld body;

[0017] (8) Put the end ring on the end ring area, combine the end ring and the welding body, and connect the end ring and the guide bar.

[0018] Furthermore, the material of the rotor steel is 25Cr2Ni4MoV steel, 30Cr2Ni4MoV steel or 30CrNi3Mo; the material of the outer sleeve ring is 3Cr13, 2Cr13, 4Cr13 or P91.

[0019] Furthermore, the assembly in step (3) is an interference fit, so that the axial and circumferential assembly surfaces of the guide bar and the rotor steel are provided with a certain contact pressure in advance to obtain a weld with higher strength.

[0020] Furthermore, the outer sleeve is heated to a temperature before being put on the assembly.

[0021] ≤300℃. Heat the assembly to provide more axial extrusion force.

[0022] Furthermore, in step (3), the rotor steel is first heated to a temperature of ≤300° C. The length and width of the groove are expanded by heating, so that the guide bar can be more easily installed in the groove. The heating temperature should not be too high to avoid oxidation on the surface that affects the quality of the weld.

[0023] Furthermore, in step (3), the guide bar is cooled before assembly, and liquid nitrogen is used for cooling. The cooling of the guide bar makes it easier to fit into the groove and closely fit with the bottom of the groove, thereby reducing the assembly gap and improving the welding reliability.

[0024] Furthermore, the outer sleeve ring and the assembly body are sleeved in an interference fit, so that the outer sleeve ring squeezes the guide bar to avoid a gap on the assembly surface of the groove bottom, and provides a certain contact pressure on the welding surface of the groove bottom to obtain a weld with higher strength.

[0025] Furthermore, the depth of the conductor bar inserted into the rotor steel is 25-30% of the radius of the rotor steel. Increasing the cross-sectional area of ​​the conductor makes the current distribution inside the conductor more uniform, thereby reducing the influence of the skin effect, and at the same time increasing the welding area between the conductor bar and the rotor steel, so that the rotor steel can obtain higher power transmission efficiency and stability during ultra-high-speed rotation, and improve the structural strength and reliability of the rotor at high-speed operation.

[0026] Furthermore, during the vacuum diffusion welding, a vacuum operation is performed in the furnace to a vacuum degree of 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3 to 5 minutes; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, the furnace is cooled to below 60℃, the pressure is released and the furnace is taken out. The brazing material is in the range of 900~950℃, and the chromium-zirconium-copper and stainless steel are diffused and welded at the same furnace and temperature for a short time, which can ensure the welding rate and the tensile strength requirements of the weld and the base material. When diffusion brazing, the vacuum degree is kept at 5×10 -2 Pa, that is, the vapor pressure of copper at 900℃

[0027] Just go on.

[0028] Furthermore, in step (8), the end ring and the weld body are combined by a cladding or surfacing process; the end ring and the conductor bar are made of chromium zirconium copper, and the cross section of the conductor bar is rectangular; after step (8) is completed, the outer surface is processed to be smooth. The end ring and the weld body are better metallurgically combined to improve the overall strength and reduce the resistance.

[0029] The beneficial effects of the present invention are:

[0030] 1. In the present invention, in addition to the contact pressure caused by the diffusion of the guide phase in all directions due to the difference in expansion coefficient between the guide bar and the rotor steel, an outer sleeve with an expansion coefficient smaller than that of the guide bar is provided on the outside to provide an axial extrusion force on the guide bar under high temperature of vacuum diffusion, so as to achieve the contact pressure adjustment required for diffusion welding, thereby ensuring the welding strength of the assembly surface of the guide bar and the rotor steel; and it is not easy to loosen in the later stage.

[0031] 2. The present invention provides a groove on the side so that the guide bar can be assembled deeper and the skin effect can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the structure in which the outer sleeve ring is sleeved on the rotor steel.

[0033] Figure 2 It is a cross-sectional view of the outer sleeve ring mounted on the rotor steel.

[0034] Figure 3 This is a schematic diagram of the structure of the finished product. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Embodiment 1: A method for manufacturing a high-speed asynchronous motor rotor comprises the following steps:

[0036] (1) A plurality of grooves 1.1 are provided on the side wall of the rotor steel 1, and the rotor steel 1 is 25Cr2Ni4MoV steel;

[0037] (2) spraying solder on the surface of the conductor bar 2 to obtain a sprayed conductor bar 2;

[0038] (3) Assemble the sprayed guide bar 2 into the groove 1.1 to form an assembly, and the guide bar 2 protrudes from the side wall of the rotor steel 1. The depth of the guide bar 2 inserted into the rotor steel 1 is 25% of the radius of the rotor steel 1.

[0039] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar 2 is connected to the outer sleeve of the assembly; the outer sleeve ring is made of 2Cr13;

[0040] (5) Perform vacuum diffusion welding. Due to the difference in expansion coefficients between the outer sleeve and the conductor bar 2, the outer sleeve squeezes the conductor bar 2. During vacuum diffusion welding, evacuate the furnace until the vacuum degree is

[0041] 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3min; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2Pa vacuum cooling, the furnace is cooled to 60℃, the pressure is released and the furnace is taken out;

[0042] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0043] (7) Processing end ring areas at both ends of the weld body;

[0044] (8) An end ring 4 is put on the end ring area, and the end ring 4 is combined with the welding body, and the end ring 4 is connected with the conductor bar 2; the end ring 4 is combined with the welding body by using a cladding process; the materials of the end ring 4 and the conductor bar 2 are both chromium-zirconium-copper.

[0045] Through testing, it was found that the transverse and longitudinal tensile strength of the rotor steel and guide bar welding points was 330 MPa, the weld shear strength was 203 MPa, the cross-sectional elongation was 10%, and the welding rate through metallographic and ultrasonic testing was >97%.

[0046] Embodiment 2: A method for manufacturing a high-speed asynchronous motor rotor, comprising the following steps:

[0047] (1) A plurality of grooves 1.1 are provided on the side wall of the rotor steel 1, and the rotor steel 1 is 30Cr2Ni4MoV steel;

[0048] (2) spraying solder on the surface of the conductor bar 2 to obtain a sprayed conductor bar 2;

[0049] (3) Assemble the sprayed guide bar 2 into the groove 1.1 to form an assembly, and the guide bar 2 protrudes from the side wall of the rotor steel 1. The depth of the guide bar 2 inserted into the rotor steel 1 is 27% of the radius of the rotor steel 1.

[0050] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar 2 is connected to the outer sleeve of the assembly; the outer sleeve ring is made of 2Cr13;

[0051] (5) Perform vacuum diffusion welding. Due to the difference in expansion coefficients between the outer sleeve and the conductor bar 2, the outer sleeve squeezes the conductor bar 2. During vacuum diffusion welding, evacuate the furnace until the vacuum degree is

[0052] 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 4min; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, when the furnace is cooled to 40℃, the pressure is released and the furnace is taken out;

[0053] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0054] (7) Processing end ring areas at both ends of the weld body;

[0055] (8) Put the end ring 4 on the end ring area, combine the end ring 4 with the welding body, and connect the end ring 4 with the conductor bar 2; use the surfacing process to combine the end ring 4 with the welding body; the material of the end ring 4 and the conductor bar 2 are both chromium zirconium copper, and the cross-section of the conductor bar 2 is rectangular; after completion, process the outer surface to be smooth.

[0056] Through testing, it was found that the transverse and longitudinal tensile strength of the rotor steel and guide bar welding points was 340Mpa, the weld shear strength was 208MPa, the cross-sectional elongation was 12%, and the welding rate through metallographic and ultrasonic flaw detection was >95%.

[0057] Embodiment 3: A method for manufacturing a high-speed asynchronous motor rotor, comprising the following steps:

[0058] (1) A plurality of grooves 1.1 are provided on the side wall of the rotor steel 1, and the rotor steel 1 is 30CrNi3Mo steel;

[0059] (2) spraying solder on the surface of the conductor bar 2 to obtain a sprayed conductor bar 2;

[0060] (3) Assembling the sprayed guide bar 2 into the groove 1.1 to form an assembly, the assembly is interference fit, and the guide bar 2 protrudes from the side wall of the rotor steel 1, and the depth of the guide bar 2 inserted into the rotor steel 1 is 30% of the radius of the rotor steel 1; before the interference fit, the rotor steel 1 is heated to a temperature of 300° C., and before the interference fit, the guide bar 2 is cooled by liquid nitrogen;

[0061] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar 2 is connected to the outer sleeve of the assembly; the outer sleeve ring and the assembly are connected by interference fit; the outer sleeve ring is made of 4Cr13; the outer sleeve ring is heated to a temperature of 300°C before being connected to the assembly;

[0062] (5) Perform vacuum diffusion welding. Due to the difference in expansion coefficients between the outer sleeve and the conductor bar 2, the outer sleeve squeezes the conductor bar 2. During vacuum diffusion welding, evacuate the furnace until the vacuum degree is

[0063] 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3 to 5 minutes; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, when the furnace is cooled to below 60℃, the pressure is released and the furnace is taken out;

[0064] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0065] (7) Processing end ring areas at both ends of the weld body;

[0066] (8) Put the end ring 4 on the end ring area, combine the end ring 4 with the weld body, and connect the end ring 4 with the conductor bar 2; combine the end ring 4 with the weld body by cladding or surfacing; the end ring 4 and the conductor bar 2 are made of chromium zirconium copper, and the cross-section of the conductor bar 2 is rectangular; after completion, process the outer surface to be smooth.

[0067] Through testing, it was found that the transverse and longitudinal tensile strength of the rotor steel and guide bar welding points was 355 MPa, the weld shear strength was 215 MPa, the cross-sectional elongation was 16%, and the welding rate through metallographic and ultrasonic testing was >98%.

[0068] Embodiment 4: A method for manufacturing a high-speed asynchronous motor rotor, comprising the following steps:

[0069] (1) A plurality of grooves 1.1 are provided on the side wall of the rotor steel 1, and the rotor steel 1 is 25Cr2Ni4MoV steel;

[0070] (2) spraying solder on the surface of the conductor bar 2 to obtain a sprayed conductor bar 2;

[0071] (3) Assembling the sprayed guide bar 2 into the groove 1.1 to form an assembly, the assembly is interference fit, and the guide bar 2 protrudes from the side wall of the rotor steel 1, and the depth of the guide bar 2 inserted into the rotor steel 1 is 30% of the radius of the rotor steel 1; before the interference fit, the rotor steel 1 is heated to a temperature of 280° C., and before the interference fit, the guide bar 2 is cooled by liquid nitrogen;

[0072] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar 2 is connected to the outer sleeve of the assembly; the outer sleeve ring and the assembly are connected by interference fit; the material of the outer sleeve ring is P91; the outer sleeve ring is heated to a temperature of 280° C. before being connected to the assembly;

[0073] (5) Perform vacuum diffusion welding. Due to the difference in expansion coefficients between the outer sleeve and the conductor bar 2, the outer sleeve squeezes the conductor bar 2. During vacuum diffusion welding, evacuate the furnace until the vacuum degree is

[0074] 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3 to 5 minutes; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, when the furnace is cooled to below 60℃, the pressure is released and the furnace is taken out;

[0075] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0076] (7) Processing end ring areas at both ends of the weld body;

[0077] (8) The end ring 4 is put on the end ring area, the end ring 4 is combined with the welding body, and the end ring 4 is connected with the conductor bar 2; the end ring 4 is combined with the welding body by a cladding or surfacing process; the end ring 4 and the conductor bar 2 are both made of chromium zirconium copper.

[0078] Through testing, it was found that the transverse and longitudinal tensile strength of the rotor steel and guide bar welding points was 340 MPa, the weld shear strength was 210 MPa, the cross-sectional elongation was 12%, and the welding rate through metallographic and ultrasonic flaw detection was >96%.

[0079] Embodiment 5: A method for manufacturing a high-speed asynchronous motor rotor, comprising the following steps:

[0080] (1) A plurality of grooves 1.1 are provided on the side wall of the rotor steel 1, and the rotor steel 1 is 25Cr2Ni4MoV steel;

[0081] (2) spraying solder on the surface of the conductor bar 2 to obtain a sprayed conductor bar 2;

[0082] (3) Assembling the sprayed guide bar 2 into the groove 1.1 to form an assembly, the assembly is interference fit, and the guide bar 2 protrudes from the side wall of the rotor steel 1, and the depth of the guide bar 2 inserted into the rotor steel 1 is 30% of the radius of the rotor steel 1; before the interference fit, the rotor steel 1 is heated to a temperature of 250° C., and before the interference fit, the guide bar 2 is cooled by liquid nitrogen;

[0083] (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar 2 is connected to the outer sleeve of the assembly; the outer sleeve ring and the assembly are connected by interference fit; the outer sleeve ring is made of 3Cr13; the outer sleeve ring is heated to a temperature of 250°C before being connected to the assembly;

[0084] (5) Perform vacuum diffusion welding. Due to the difference in expansion coefficients between the outer sleeve and the conductor bar 2, the outer sleeve squeezes the conductor bar 2. During vacuum diffusion welding, evacuate the furnace until the vacuum degree is

[0085] 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3 to 5 minutes; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, when the furnace is cooled to below 60℃, the pressure is released and the furnace is taken out;

[0086] (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body;

[0087] (7) Processing end ring areas at both ends of the weld body;

[0088] (8) An end ring 4 is put on the end ring area to combine the end ring 4 with the weld body, and the end ring 4 is connected to the conductor bar 2; the end ring 4 and the weld body are combined by a cladding or surfacing process; the end ring 4 and the conductor bar 2 are made of chromium-zirconium-copper, and the cross-section of the conductor bar 2 is rectangular.

[0089] Through testing, it was found that the transverse and longitudinal tensile strength of the rotor steel and guide bar welding points was 345 MPa, the weld shear strength was 213 MPa, the cross-sectional elongation was 15%, and the welding rate through metallographic and ultrasonic testing was >95%.

[0090] The described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-speed asynchronous motor rotor, characterized in that: The following steps are involved: (1) A number of grooves are formed on the side wall of the rotor steel; (2) spraying solder on the surface of the conductor bar to obtain a sprayed conductor bar; (3) Assembling the sprayed guide bar into the groove to form an assembly, with the guide bar protruding from the rotor steel side wall; (4) An outer sleeve ring having an expansion coefficient smaller than that of the conductor bar is connected to the outer sleeve of the assembly; (5) Vacuum diffusion welding is performed, and the outer sleeve extrudes the conductor bar due to the difference in expansion coefficient between the outer sleeve and the conductor bar; (6) After the vacuum diffusion welding is completed, the outer sleeve ring is removed to obtain the welded body; (7) Processing end ring areas at both ends of the weld body; (8) Put the end ring on the end ring area, combine the end ring with the welding body, and connect the end ring with the guide bar.

2. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: The material of the rotor steel is 25Cr2Ni4MoV steel, 30Cr2Ni4MoV steel or 30CrNi3Mo steel; the material of the outer sleeve ring is 3Cr13, 2Cr13, 4Cr13 or P91.

3. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: The assembly in step (3) is an interference fit.

4. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: The outer sleeve is heated before being sleeved on the assembly body, and the temperature is heated to ≤300°C.

5. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: In the step (3), the rotor steel is first heated before assembly to a temperature of ≤300°C.

6. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: In the step (3), the guide bar is cooled before assembly, and liquid nitrogen is used for cooling.

7. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: The outer sleeve ring and the assembly body are interference-fitted.

8. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: The depth of the guide bar inserted into the rotor steel is 25-30% of the radius of the rotor steel.

9. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: During the vacuum diffusion welding, the furnace is evacuated to a vacuum degree of 1×10 -1 Pa, heating was started, the heating rate was 5℃ / min, and the insulation platforms were set at 500℃, 700℃, 800℃, and 850℃, respectively, and each stage was kept warm for 3 to 5 minutes; then the vacuum was set at 5×10 -2 Pa to 930℃, heating rate: 3℃ / min; maintain 5×10 -2 Pa vacuum cooling, the furnace is cooled to below 60℃, the pressure is released and the furnace is taken out of the furnace.

10. The method for manufacturing a high-speed asynchronous motor rotor according to claim 1, characterized in that: In the step (8), the end ring and the weld body are combined by a cladding or overlay welding process; the end ring and the guide bar are both made of chromium zirconium copper, and the cross-section of the guide bar is rectangular; after the step (8) is completed, the outer surface is processed to be smooth.

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