Thermal field-electric field-ultrasonic multi-field compound gear rolling strengthening device and method

Abstract

The invention discloses a thermal field-electric field-ultrasonic multi-field compound gear rolling strengthening device and method, and relates to the technical field of gear machining surface modification and strengthening, the device comprises a machine tool body and a sliding box, and further comprises a gear positioning clamp, a thermal field assembly, an electric pulse strengthening assembly, an ultrasonic assembly, an in-situ monitoring assembly and a gear indexing assembly, the gear indexing assembly and the machine tool chuck are oppositely arranged, insulating parts are fixedly arranged at the two ends of the gear positioning clamp correspondingly, the insulating parts at the two ends are clamped and fixed to the machine tool chuck and the gear indexing chuck correspondingly, the thermal field assembly is used for applying a thermal field to a gear, and the electric pulse strengthening assembly is used for applying a pulse current field to the gear. The ultrasonic assembly comprises an ultrasonic generator and a rolling wheel connected with the ultrasonic generator, the rolling wheel can be meshed with the tooth surface contour of the gear, and the in-situ monitoring assembly is used for monitoring the deformation amount of the gear before and after ultrasonic rolling. The anti-fatigue performance of the gear can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of surface modification and strengthening technology for gear processing, and in particular to a multi-field composite gear rolling strengthening device and method involving thermal field, electric field, and ultrasonic field. Background Technology

[0002] Gears are crucial components in mechanical products, and gear transmission is a vital form of transmitting mechanical motion and power, widely used in aerospace, automotive, instrumentation, and other major industrial sectors. Gears operate under complex and harsh environments characterized by high speeds, heavy loads, alternating high and low temperatures, and corrosion and oxidation, while also enduring severe impact and alternating loads, making them highly susceptible to fatigue failure. During service, gears are prone to bending fatigue failures such as cracks and tooth breakage at the tooth root, and contact fatigue failures such as pitting and spalling at the tooth surface. Bending fatigue failure at the gear tooth root and contact fatigue failure at the tooth surface limit the improvement of gear fatigue life, thus seriously jeopardizing the operational safety of equipment.

[0003] In gear manufacturing, the commonly used mechanical shot peening process can introduce residual compressive stress and improve surface hardness, thereby improving the contact fatigue life of gears to some extent. However, the residual compressive stress layer introduced by shot peening is relatively shallow, and the surface roughness increases after shot peening, making it easier to induce stress concentration. These factors are not conducive to further exploring the fatigue resistance potential of gears.

[0004] In view of this, in order to improve the fatigue resistance of gears, we propose a thermal field and pulsed current field assisted ultrasonic rolling strengthening technology. Summary of the Invention

[0005] The purpose of this invention is to provide a multi-field composite gear rolling strengthening device and method based on thermal field, electric field, and ultrasonic field to solve the problems existing in the prior art and effectively improve the fatigue resistance of gears.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] This invention provides a multi-field composite gear rolling strengthening device consisting of a thermal field, an electric field, and an ultrasonic field. The device includes a machine tool bed and an apron mounted on the machine tool bed. It also includes a gear positioning fixture, a thermal field assembly, an electric pulse strengthening assembly, an ultrasonic assembly, an in-situ monitoring assembly, and a gear indexing assembly. The gear indexing assembly is fixed to the machine tool bed and positioned opposite to the machine tool chuck. The gear positioning fixture is used to fix a gear. Insulating components are fixed at both ends of the gear positioning fixture, and these insulating components are respectively clamped and fixed to the machine tool chuck and the gear indexing assembly. On the gear indexing chuck, the thermal field component is fixed to the machine tool bed and is used to apply a thermal field to the gear. The electric pulse enhancement component is fixed to the machine tool bed and is used to electrically connect the gear positioning fixture and apply a pulsed current field to the gear. The ultrasonic component includes an ultrasonic generator and a rolling wheel connected to the ultrasonic generator. The ultrasonic generator is fixed to the machine tool post, and the rolling wheel can mesh with the tooth surface profile of the gear. The in-situ monitoring component is fixed to the slide box and is used to monitor the deformation of the gear before and after ultrasonic rolling.

[0008] In one embodiment, the thermal field assembly includes a bracket and heat pipes. The bracket is fixed to the machine tool bed, and two heat pipes are fixedly mounted on the bracket. The two heat pipes are located on both sides of the gear, and the two heat pipes are electrically connected to the thermal field power supply.

[0009] In one embodiment, the thermal field assembly further includes a temperature monitoring device for real-time monitoring of the temperature at which the thermal field is applied.

[0010] In one embodiment, the electrical pulse enhancement component includes an insulating support base, two conductive metal blocks, a positive terminal piece, and a negative terminal piece. The insulating support base is fixed to the machine tool bed, and the two conductive metal blocks are fixed to the insulating support base. The two conductive metal blocks are respectively located on both sides of the gear positioning fixture and are in conductive contact with both sides of the gear positioning fixture. The two conductive metal blocks are respectively connected to the positive terminal piece and the negative terminal piece. The positive terminal piece is connected to the positive terminal of the pulse power supply, and the negative terminal piece is connected to the negative terminal of the pulse power supply.

[0011] In one embodiment, the ultrasonic component further includes a force gauge, and the ultrasonic generator is fixed to the machine tool post via the force gauge, which is used to monitor the ultrasonic rolling force in real time.

[0012] In one embodiment, the ultrasonic component is connected to an ultrasonic power source.

[0013] In one embodiment, the gear positioning fixture includes a gear positioning shaft, the gear is fixedly mounted on the gear positioning shaft, and the conductive metal block has an arc surface with the same radius as the gear positioning shaft on one side facing the gear positioning shaft, the arc surface being in close contact with the gear positioning shaft.

[0014] In one embodiment, the rolling roller is made of SiC material.

[0015] In one embodiment, the bottom of the machine tool bed is provided with a circulating oil groove.

[0016] This invention also provides a method for strengthening gears by multi-field composite rolling of thermal field, electric field, and ultrasonic field, based on the aforementioned multi-field composite rolling strengthening device for gears, comprising the following steps:

[0017] S1: The in-situ monitoring component scans the tooth root and the tooth surface contours on both sides of the gear to be strengthened, transmits the data to the computer and fits the tooth root and the tooth surface contours on both sides, and makes a preliminary decision on the process parameters required for strengthening.

[0018] S2: According to the process parameters, a thermal field is applied to the tooth root to be strengthened and its two sides of the tooth surface through the thermal field component, a pulse current is applied to the gear through the electric pulse strengthening component, and the rolling wheel is fed through the slide box to perform ultrasonic rolling strengthening of the tooth root to be strengthened and its two sides of the tooth surface.

[0019] S3: The in-situ monitoring component scans the enhanced tooth root and the tooth surface contours on both sides, transmits the data to the computer and fits the tooth root and the tooth surface contours on both sides, compares them with the contours before enhancement and the contours determined during gear design, and decides whether further enhancement is needed.

[0020] S4: If further strengthening is required, determine the process parameters needed for the next strengthening and return to step S2; if further strengthening is not required, proceed to step S5.

[0021] S5: Rotate the gear and control the gear rotation angle through the gear indexing assembly to rotate the next tooth root to be strengthened and its two sides to the strengthening position;

[0022] S6: Repeat steps S1 to S5 until all tooth roots and tooth surfaces have been strengthened.

[0023] The present invention achieves the following technical effects compared to the prior art:

[0024] The thermal-electric-ultrasonic multi-field composite gear rolling strengthening device and method provided by the present invention can provide thermal and pulsed current field assistance to strengthen the gear tooth root and tooth surface by ultrasonic rolling. This can effectively increase the residual compressive stress amplitude and influence layer depth on the gear material surface, improve the gear surface morphology and surface roughness, increase the hardness of the gear surface layer, and thus effectively improve the fatigue resistance and fatigue life of the gear.

[0025] This invention controls the strengthening effect of gear tooth roots and surfaces by changing the temperature of the thermal field, the magnitude of the pulse current, and the relevant process parameters of ultrasonic rolling. By monitoring the deformation of the gear tooth roots and surfaces before and after strengthening with an in-situ monitoring component and comparing it with the dimensions determined during gear design, it determines whether multiple strengthenings are needed and the specific process parameters required for the next strengthening, ultimately achieving precise control of gear dimensions and strengthening effect. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments 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 these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of the thermal-electric-ultrasonic multi-field composite gear rolling strengthening device in an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of a portion of the thermal-electric-ultrasonic multi-field composite gear rolling strengthening device in an embodiment of the present invention;

[0029] Figure 3 This is a top view of a portion of the structure of the thermal-electric-ultrasonic multi-field composite gear rolling strengthening device in an embodiment of the present invention;

[0030] Figure 4 This is a front view of a portion of the structure of the thermal-electric-ultrasonic multi-field composite gear rolling strengthening device in an embodiment of the present invention;

[0031] Figure 5 This is a diagram showing the connection relationship between the electrical pulse enhancement component and the gear positioning shaft in an embodiment of the present invention.

[0032] Figure 6 This is a schematic diagram of the rolling wheel and gear meshing in the ultrasonic component of the present invention.

[0033] In the diagram: 1-Machine bed, 2-Apron, 3-Gear positioning fixture, 4-Thermal field assembly, 5-Electrical pulse enhancement assembly, 6-Ultrasonic assembly, 7-In-situ monitoring assembly, 8-Gear indexing assembly, 9-Machine chuck, 10-Gear, 11-Insulating component, 12-Gear indexing chuck, 13-Ultrasonic generator, 14-Rolling roller, 15-Machine tool post, 16-Bracket, 17-Heat pipe, 18-Thermal field power supply, 19-Insulating support, 20-Conductive metal block, 21-Positive terminal piece, 22-Negative terminal piece, 23-Pulse power supply, 24-Ultrasonic power supply, 25-Gear positioning shaft, 26-Arc surface, 27-Circulating oil groove. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all 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.

[0035] The purpose of this invention is to provide a multi-field composite gear rolling strengthening device and method based on thermal field, electric field, and ultrasonic field to solve the problems existing in the prior art and effectively improve the fatigue resistance of gears.

[0036] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Example 1

[0038] like Figures 1-6As shown, this embodiment provides a multi-field composite gear rolling strengthening device consisting of a thermal field, an electric field, and an ultrasonic field. It includes a machine tool bed 1 and a slide box 2 mounted on the machine tool bed 1. It also includes a gear positioning fixture 3, a thermal field assembly 4, an electric pulse strengthening assembly 5, an ultrasonic assembly 6, an in-situ monitoring assembly 7, and a gear indexing assembly 8. The gear indexing assembly 8 is fixed to the machine tool bed 1 and is positioned opposite to the machine tool chuck 9. A gear 10 is fixedly mounted on the gear positioning fixture 3. Insulating members 11 are fixed to both ends of the gear positioning fixture 3, and the insulating members 11 at both ends are clamped and fixed to the machine tool chuck 9 and the gear indexing assembly 8, respectively. The gear indexing chuck 12 of the indexing component 8 is equipped with a thermal field component 4 fixed to the machine tool bed 1 for applying a thermal field to the gear 10. The electric pulse enhancement component 5 is fixed to the machine tool bed 1 for electrically connecting the gear positioning fixture 3 and applying a pulsed current field to the gear 10. The ultrasonic component 6 includes an ultrasonic generator 13 and a rolling wheel 14 connected to the ultrasonic generator 13. The ultrasonic generator 13 is fixed to the machine tool post 15. The rolling wheel 14 can mesh with the tooth surface profile of the gear 10. The in-situ monitoring component 7 is fixed to the slide box 2 for monitoring the deformation of the gear 10 before and after ultrasonic rolling.

[0039] While the ultrasonic component 6 performs ultrasonic rolling strengthening on the tooth root and tooth surface of the gear 10, it provides a thermal field through the thermal field component 4 and a pulsed current field through the electric pulse strengthening component 5. This can effectively increase the residual compressive stress amplitude and influence layer depth on the surface of the gear 10 material, improve the surface morphology and surface roughness of the gear 10, increase the surface hardness of the gear 10, and thus effectively improve the fatigue resistance and fatigue life of the gear 10.

[0040] By insulating the component 11, the machine tool is protected from pulse current after the electrical pulse strengthening component 5 is energized, preventing damage and improving safety. The in-situ monitoring component 7 is a displacement monitoring device. This device, along with the ultrasonic component 6, is fixed to the slide box 2 and kept parallel. The height of the displacement monitoring device is approximately at the same level as the tooth root and two sides of the tooth surface to be strengthened. Before strengthening, the displacement monitoring device scans the tooth root and tooth surface contours, transmits the data to the computer, and fits the tooth contour, initially determining the required process parameters for strengthening. After strengthening, the slide box 2 continues to feed, and the displacement monitoring device scans the tooth root and tooth surface contours after strengthening, transmits the data to the computer, and fits the tooth contour. This contour is compared with the original gear contour and the contour determined during gear design to determine whether multiple strengthenings are needed and the specific process parameters required for the next strengthening. The gear indexing component 8 adjusts the rotation angle of the gear after each strengthening according to the gear shape and number of teeth, thereby achieving sequential strengthening of each tooth root and tooth surface of the gear 10. The ultrasonic component 6 can be fed and rolled by the slide box 2, and the rolling depth can be adjusted by the feed of the machine tool post 15.

[0041] In this embodiment, the thermal field assembly 4 includes a bracket 16 and heat pipes 17. The bracket 16 is fixed to the machine tool bed 1, and two heat pipes 17 are fixedly mounted on the bracket 16. The two heat pipes 17 are located on both sides of the gear 10 and close to the tooth groove to be strengthened. The two heat pipes 17 are electrically connected to the thermal field power supply 18. The heat pipes 17 are positioned below the tooth root of the gear 10 to ensure that they do not affect the axial feed of the ultrasonic component 6 and the in-situ monitoring component 7. After being powered on, the heat pipes 17 gradually heat up to the set temperature, and the heating power can be changed by changing the current, thereby adjusting the set temperature. The thermal field assembly 4 also includes a temperature monitoring device for real-time monitoring of the applied thermal field temperature. The temperature monitoring device can monitor the temperature of the area of ​​the gear to be strengthened in real time, and adjust the set temperature of the heat pipes 17 accordingly.

[0042] The electrical pulse enhancement component 5 includes an insulating support base 19, two conductive metal blocks 20, a positive terminal piece 21, and a negative terminal piece 22. The insulating support base 19 is fixed to the machine tool bed 1, and the two conductive metal blocks 20 are fixed to the insulating support base 19. The two conductive metal blocks 20 are located on both sides of the gear positioning fixture 3, and are in conductive contact with both sides of the gear positioning fixture 3. The two conductive metal blocks 20 are connected to the positive terminal piece 21 and the negative terminal piece 22, respectively. The positive terminal piece 21 is connected to the positive terminal of the pulse power supply 23, and the negative terminal piece 22 is connected to the negative terminal of the pulse power supply 23. The good conductivity of the metal allows the pulse current to be conducted to the gear 10.

[0043] The ultrasonic component 6 also includes a force gauge. The ultrasonic generator 13 is fixed to the machine tool post 15 via the force gauge. The force gauge is used to monitor the ultrasonic rolling pressure in real time, and based on this, the magnitude of the ultrasonic rolling pressure can be precisely controlled. By controlling the magnitude of the ultrasonic rolling pressure, combined with changes in other process parameters such as the feed speed, ultrasonic amplitude, rolling cycles, and ultrasonic frequency of the ultrasonic component, the strengthened surface can be precisely controlled. In the ultrasonic rolling strengthening process parameters, the rolling pressure and feed speed are controlled by the machine tool, the ultrasonic amplitude is adjustable within 1 to 10 μm, and the initial setting is 1 μm. The ultrasonic frequency is generally greater than 19 kHz, such as 20 kHz or 28 kHz. The contour shape of the rolling wheel 14 meshes with the contour of the gear 10, but is slightly smaller than the tooth surface contour, which can achieve simultaneous ultrasonic rolling strengthening of the tooth root and both sides of the gear 10. The small initial amplitude and the shape of the rolling wheel 14 specially designed for the gear ensure that the tooth root and tooth surface of the gear 10 are strengthened simultaneously, and prevent the appearance of tool marks. The ultrasonic component 6 is connected to the ultrasonic power supply 24.

[0044] The gear positioning fixture 3 includes a gear positioning shaft 25, a gear 10 is fixedly installed on the gear positioning shaft 25, and a conductive metal block 20 has an arc surface 26 with the same radius as the gear positioning shaft 25 on the side facing the gear positioning shaft 25. The arc surface 26 is in close contact with the gear positioning shaft 25.

[0045] The rolling roller 14 is made of SiC, which has good insulation properties, and can prevent pulse current from being transmitted to the ultrasonic generator 13 and causing damage to the device. The rolling roller 14 is not limited to SiC; other materials with good insulation and high hardness can also be used.

[0046] The bottom of the machine tool bed 1 is provided with a circulating oil groove 27 for the recycling of cooling oil during ultrasonic rolling strengthening.

[0047] During the strengthening process, this device has four application states: single ultrasonic rolling strengthening, thermal field-assisted ultrasonic rolling strengthening, pulsed current field-assisted ultrasonic rolling strengthening, and thermal field and pulsed current field-assisted ultrasonic rolling strengthening. Different strengthening processes can be selected according to the different design requirements of different gears.

[0048] Example 2

[0049] This embodiment provides a multi-field composite gear rolling strengthening method based on the thermal-electric-ultrasonic multi-field composite gear rolling strengthening device in Embodiment 1, including the following steps:

[0050] S1: By scanning the tooth root and the tooth surface contours on both sides of the gear 10 to be strengthened by the in-situ monitoring component 7, the data is transmitted to the computer and the tooth root and the tooth surface contours on both sides are fitted, and the process parameters required for strengthening are initially determined.

[0051] S2: According to the process parameters, a thermal field is applied to the tooth root to be strengthened and its two sides by the thermal field component 4, a pulse current is applied to the gear 10 by the electric pulse strengthening component 5, and the rolling roller 14 is fed by the slide box 2 to perform ultrasonic rolling strengthening of the tooth root to be strengthened and its two sides.

[0052] S3: By scanning the reinforced tooth root and the tooth surface contours on both sides of the tooth after the in-situ monitoring component 7, the data is transmitted to the computer and the tooth root and the tooth surface contours on both sides of the tooth are fitted. The contours are compared with the contours before reinforcement and the contours determined during gear design to determine whether reinforcement is needed again.

[0053] S4: If further strengthening is required, determine the process parameters needed for the next strengthening and return to step S2; if further strengthening is not required, proceed to step S5.

[0054] S5: Rotate gear 10 and control the rotation angle of gear 10 through gear indexing assembly 8 to rotate the next tooth root to be strengthened and its two sides to the strengthening position.

[0055] S6: Repeat steps S1 to S5 until all tooth roots and tooth surfaces have been strengthened.

[0056] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A multi-field composite gear rolling strengthening device comprising thermal field-electric field-ultrasonic field, comprising a machine tool bed and an apron box disposed on the machine tool bed, characterized in that: The system also includes a gear positioning fixture, a thermal field assembly, an electrical pulse enhancement assembly, an ultrasonic assembly, an in-situ monitoring assembly, and a gear indexing assembly. The gear indexing assembly is fixed to the machine tool bed and is positioned opposite to the machine tool chuck. The gear positioning fixture is used to fix and install the gear. Insulating components are fixed at both ends of the gear positioning fixture, and the insulating components at both ends are clamped and fixed to the machine tool chuck and the gear indexing chuck of the gear indexing assembly, respectively. The thermal field assembly is fixed to the machine tool bed and is used to apply a thermal field to the gear. The electrical pulse enhancement assembly is fixed to the machine tool bed and is used to electrically connect to the gear positioning fixture to apply a pulsed current field to the gear. The ultrasonic assembly includes an ultrasonic generator and a rolling wheel connected to the ultrasonic generator. The ultrasonic generator is fixed to the machine tool post, and the rolling wheel can mesh with the tooth surface profile of the gear. The in-situ monitoring assembly is fixed to the slide box and is used to monitor the deformation of the gear before and after ultrasonic rolling.

2. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The thermal field assembly includes a bracket and heat pipes. The bracket is fixed to the machine tool bed, and two heat pipes are fixedly mounted on the bracket. The two heat pipes are located on both sides of the gear, and the two heat pipes are electrically connected to the thermal field power supply.

3. The multi-field composite gear rolling strengthening device according to claim 2, characterized in that: The thermal field assembly also includes a temperature monitoring device for real-time monitoring of the temperature of the applied thermal field.

4. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The electrical pulse enhancement component includes an insulating support base, two conductive metal blocks, a positive terminal piece, and a negative terminal piece. The insulating support base is fixed to the machine tool bed, and the two conductive metal blocks are fixed to the insulating support base. The two conductive metal blocks are respectively located on both sides of the gear positioning fixture and are in conductive contact with both sides of the gear positioning fixture. The two conductive metal blocks are respectively connected to the positive terminal piece and the negative terminal piece. The positive terminal piece is connected to the positive terminal of the pulse power supply, and the negative terminal piece is connected to the negative terminal of the pulse power supply.

5. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The ultrasonic component also includes a force gauge, and the ultrasonic generator is fixed to the machine tool post via the force gauge. The force gauge is used to monitor the ultrasonic rolling force in real time.

6. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The ultrasonic component is connected to an ultrasonic power source.

7. The multi-field composite gear rolling strengthening device according to claim 4, characterized in that: The gear positioning fixture includes a gear positioning shaft, the gear is fixedly mounted on the gear positioning shaft, and the conductive metal block has an arc surface with the same radius as the gear positioning shaft on one side facing the gear positioning shaft, the arc surface being in close contact with the gear positioning shaft.

8. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The rollers are made of SiC material.

9. The multi-field composite gear rolling strengthening device according to claim 1, characterized in that: The bottom of the machine tool bed is equipped with a circulating oil tank.

10. A method for strengthening gears by multi-field composite rolling of thermal field, electric field, and ultrasonic field, characterized in that, The multi-field composite gear rolling strengthening device based on any one of claims 1 to 9 includes the following steps: S1: The in-situ monitoring component scans the tooth root and the tooth surface contours on both sides of the gear to be strengthened, transmits the data to the computer and fits the tooth root and the tooth surface contours on both sides, and makes a preliminary decision on the process parameters required for strengthening. S2: According to the process parameters, a thermal field is applied to the tooth root to be strengthened and its two sides of the tooth surface through the thermal field component, a pulsed current field is applied to the gear through the electric pulse strengthening component, and the rolling wheel is fed through the slide box to perform ultrasonic rolling strengthening of the tooth root to be strengthened and its two sides of the tooth surface. S3: The in-situ monitoring component scans the enhanced tooth root and the tooth surface contours on both sides, transmits the data to the computer and fits the tooth root and the tooth surface contours on both sides, compares them with the contours before enhancement and the contours determined during gear design, and decides whether further enhancement is needed. S4: If further strengthening is required, determine the process parameters needed for the next strengthening and return to step S2; if further strengthening is not required, proceed to step S5. S5: Rotate the gear and control the gear rotation angle through the gear indexing assembly to rotate the next tooth root to be strengthened and its two sides to the strengthening position; S6: Repeat steps S1 to S5 until all tooth roots and tooth surfaces have been strengthened.

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