A Novel Polishing Method for Extending the Service Life of Magnetic Compound Fluid

: Magnetic ﬁeld-assisted magnetic compound ﬂuid (MCF) ultra-precision machining technology is regarded as an effective method to obtain a smooth surface. However, due to the evaporation and splashing of water in the polishing ﬂuid during processing, the service life of the MCF slurry is reduced. This paper presents a material removal model for MCF polishing, and a novel experimental apparatus is proposed to extend the service life by supplying MCF components into the MCF slurry. Firstly, in order to obtain the ideal polishing tool, the appearance morphologies and the formation process of the MCF slurry were observed by an industrial camera. On this basis, the optimum parameters were determined by multi-factor and multi-level orthogonal experiments. Finally, the investigation of the MCF service life was carried out under the optimal processing parameters. The main ﬁndings are summarized as follows. (1) Excellent MCF polishing tools are obtained when the eccentric distance r is 4 mm and the MCF slurry supply V is 1 mL. (2) When the eccentric distance increases from 2 mm to 4 mm, the forming time of the MCF tool decreases sharply, but when the eccentricity exceeds 4 mm, the decreasing trend becomes slow. The molding time grows steadily as the supply is increased. (3) When the machining gap ∆ , the MCF carrier speed n c , the eccentricity r , and the revolution speed of magnetic n m are 1 mm, 500 rpm, 4 mm, and 600 rpm, respectively, the ideal machining effect can be obtained. (4) It could be proven that the polishing device is feasible to extend the service time of the MCF slurry by adding MCF components.


Introduction
With the rapid development of aerospace, biomedical, electronics, new energy, and other industries, the requirements for ultra-smooth optical components increases. Therefore, the effect of grinding technology has attracted a lot of attention. The conventional grinding techniques usually remove the material with rigid polishing tools [1][2][3]. Although good surface roughness can be obtained, it has disadvantages, such as low form accuracy, uncontrollable material removal, and the easy production of a subsurface damage layer during polishing [4]. Hence, it is difficult to obtain reliable ultra-smooth optical components through conventional grinding techniques [5,6].
In order to obtain optical components with smooth surfaces, researchers have made a variety of attempts. Previous studies have found that it is difficult to effectively enhance the surface quality of optical components when only relying on the improvement and adjustment of tools, machines, and process parameters. In this case, energy field-assisted processing methods, including magnetic field-assisted polishing, electrophoresis polishing, ultrasonic vibration polishing, chemical mechanical polishing, etc., have become a relevant issue in the research of finishing technology [7]. These processing methods are characterized by using energy fields such as magnetic fields, electric fields, ultrasonic fields, and chemical initial polishing force slightly, the non-magnetic abrasive grains are more evenly distributed in the dynamic magnetic field, which improves the shape recovery ability of the MCF slurry. Thus, MCF slurry can have good polishing force and improve the polishing performance during polishing [33].
Although many studies have proven that the magnetic field-assisted MCF polishing technology can obtain a higher-quality surface, the MCF carrier plate will change the viscosity of the MCF slurry during processing and rotation, thus accelerating the loss of water in the polishing solution (evaporation and splash), and ultimately significantly affecting the polishing performance of the MCF slurry. Therefore, the MCF slurry needs to be replaced frequently to ensure the polishing quality, which leads to a large increase in time costs and processing costs [34]. Hence, ensuring the usage time of MCF slurry in the polishing process has become a key problem in the research of MCF slurry.

Polishing Principle
To ensure the service life of MCF slurry, a new processing method for MCF using an injector to add MCF components regularly was proposed. Figure 1 shows the principle of MCF polishing. The cylindrical magnet is installed on the left side of the bracket; the eccentricity r is adjusted by moving the magnet and is driven by a servo motor with coupling. The MCF slurry carrier plate is installed in front of the cylindrical magnet with a clearance of δ, which is driven by another servo motor through a belt. When the cylindrical magnet rotates around the spindle and the main shaft, it will produce a dynamic magnetic field of constant strength. The rotating speed of the magnetic field is n m , and its magnetic induction line changes regularly with the rotation of the magnet. At the same time, in order to prevent non-magnetic abrasive grains from staying in the same place for a long time during processing, the MCF carrier plate is rotated at a speed n c . A certain amount of MCF polishing solution is attached to the bottom of the liquid carrier plate. The nano-magnetic particles and micron carbonyl iron powder in the polishing solution are distributed along the magnetic induction line under the action of the dynamic magnetic field to form a chain structure, and these chain structures form magnetic clusters. Afterward, the viscoelasticity of the magnetic collections is enhanced by the α-cellulose fibers interspersed between magnetic clusters. Most of the abrasive grains are concentrated around the apex of the magnetic cluster due to the magnetic levitation force, and these particles are trapped in the magnetic cluster or distributed among carbonyl iron powder particles. Finally, in order to maintain the abrasive grains, a polishing tool that can restore the appearance is formed. When the polishing tool is used to process the parts, the abrasive grains in the polishing fluid act on the surface of the machined workpiece, and the micro-cutting effect produced by the abrasive grains is used to remove materials. After processing for a while, we use a syringe to add a certain volume of MCF components to the MCF slurry to extend the usage time of MCF slurry. The MCF components are rapidly blended with the MCF slurry due to the stimulating effect generated by the constant rotation of the magnetic line of force. Thus, the polishing performance of the MCF slurry is maintained. According to previous research, a non-magnetic abrasive moves from a high-magnetic-field area to a low-magnetic-field area under the effect of the magnetic suspension force [35,36]. Therefore, the forces of abrasive grains are mainly magnetic levitation force and gravity, and gravity does not affect the polishing process. Based on the theoretical analysis of Sidpara et al., the magnetic levitation force Fabr of wear particles is given as [37,38] (1) where Vabr is the volume of abrasive grains, μ0 is the permeability of a vacuum, Mf is the intensity of magnetization of the MCF slurry, and ▽H is the gradient of the magnetic field, which is related to the machining gap Δ.
Assuming that the magnetic particles in the MCF slurry are spherical, the intensity of magnetization M of the magnetic particles is [39] (2) where μ is the permeability of the magnetic particle, and H is the magnetic field intensity. If the volume ratio of magnetic particles in MCF slurry is φ, the magnetization intensity of MCF slurry Mf can be expressed as [40] (3) A typical material removal mechanism is shown in Figure 2 [41]. The abrasive grains are pressed into the machined surface of the workpiece under the action of magnetic levitation force. The abrasive grains are moved due to the MCF fluid plate rotation. With the increase in processing time, some peaks are removed, and the surface roughness is decreased. According to previous research, a non-magnetic abrasive moves from a high-magneticfield area to a low-magnetic-field area under the effect of the magnetic suspension force [35,36]. Therefore, the forces of abrasive grains are mainly magnetic levitation force and gravity, and gravity does not affect the polishing process. Based on the theoretical analysis of Sidpara et al., the magnetic levitation force F abr of wear particles is given as [37,38] where V abr is the volume of abrasive grains, µ 0 is the permeability of a vacuum, M f is the intensity of magnetization of the MCF slurry, and H is the gradient of the magnetic field, which is related to the machining gap ∆.
Assuming that the magnetic particles in the MCF slurry are spherical, the intensity of magnetization M of the magnetic particles is [39] where µ is the permeability of the magnetic particle, and H is the magnetic field intensity. If the volume ratio of magnetic particles in MCF slurry is ϕ, the magnetization intensity of MCF slurry M f can be expressed as [40] A typical material removal mechanism is shown in Figure 2 [41]. The abrasive grains are pressed into the machined surface of the workpiece under the action of magnetic levitation force. The abrasive grains are moved due to the MCF fluid plate rotation. With the increase in processing time, some peaks are removed, and the surface roughness is decreased. The indentation diameter of the abrasive grains pressed into the material can be determined by the hardness of the material. The Brinell hardness HB is calculated by the following formula (4) where Dabr is the diameter of the abrasive grain, and Di is the indentation diameter of the abrasive grain pressed into the material.
Thus, the indentation diameter Di can be expressed as The indentation diameter of the abrasive grains pressed into the material can be determined by the hardness of the material. The Brinell hardness H B is calculated by the following formula where D abr is the diameter of the abrasive grain, and D i is the indentation diameter of the abrasive grain pressed into the material. Thus, the indentation diameter D i can be expressed as Therefore, the projected area of indentation S i is According to the classical dynamics, the relative velocity of abrasive grains and workpiece surface V can be expressed as where R is the radius of the point where the abrasive grains are located in the polishing area, which is related to the eccentricity r, and n c is the rotational speed of the MCF fluid carrier plate. The pressure P of the abrasive grain on the surface is The Preston equation is generally used to express the relationship between the material removal rate and processing parameters. The Preston equation can be described as where K is the empirically determined Preston coefficient. According to the theory of the Preston equation, the material removal rate of a single abrasive grain in MCF polishing is expressed as The workpiece surface has irregular bumps with a random distribution. To determine the surface roughness machining model, it is assumed that the workpiece has a uniform roughness profile that is triangular, as shown in Figure 2b. Ra 0 is the initial surface roughness before polishing, and Ra i is the surface roughness after a stroke (a stroke is defined as a circle of abrasive grains around the center of the polishing area). The actual contact length L a between the abrasive grains and the work surface is proportional to the moving distance L w of the abrasive grains, and the expression is as follows [42]: Lubricants 2022, 10, 299 The depth h of the workpiece surface embedded with abrasive grains is The area of the abrasive grains pressed into the surface of the workpiece A is According to the research of Jain et al., the indentation volume of abrasive grains on the workpiece surface can be defined as material removal [43]. Hence, the material removal of a single abrasive grain in the ith stroke can also be expressed as the product of the actual contact length between the abrasive grain and the surface and the cross-sectional area of the abrasive grain embedded in the surface.
The total volume of material removed after n times of cutting is Comparing Equations (10) and (15), the surface roughness model can be simplified as follows: Thus, the final surface quality is closely related to the polishing time, the initial roughness, and the MRR, while the MRR is related to the machining gap, the speed, and the eccentricity of the MCF carrier. Therefore, we need to pay attention to the machining gap, the MCF carrier plate speed, and the influence of eccentricity on the polishing performance. In addition, the rotational speed of magnets can irritate the MCF slurry, so the influence of the rotation speed of the magnet on the polishing performance should be considered.

Experimental Setup
To achieve the processing principle, the experimental setup shown in Figure 3 was constructed in the laboratory based on a three-way milling machine. The polishing unit is composed of a non-magnetic MCF carrier and a servo motor, which drives the rotation of the MCF carrier through a belt. A cylindrical magnet (ϕ 20 mm × t 10 mm, 0.5 T) that can adjust the eccentricity distance is installed on the front side of the magnet bracket, and the rotation of the magnet base is controlled by another motor. The workpiece is fixed on the Z-axis of the three-way milling machine, and the position of the workpiece is adjusted by moving the X-axis and Z-axis. The workpiece is located in the front side of the MCF liquid carrier plate, leaving a certain machining gap. Therefore, the machining gap of the workpiece can be adjusted instantly by controlling the three-way milling machine. Because the magnet has a certain eccentric distance from the magnet base axis, the dynamic magnetic field will be generated when the magnet base rotates. rotation of the magnet base is controlled by another motor. The workpiece is fixed on the Z-axis of the three-way milling machine, and the position of the workpiece is adjusted by moving the X-axis and Z-axis. The workpiece is located in the front side of the MCF liquid carrier plate, leaving a certain machining gap. Therefore, the machining gap of the workpiece can be adjusted instantly by controlling the three-way milling machine. Because the magnet has a certain eccentric distance from the magnet base axis, the dynamic magnetic field will be generated when the magnet base rotates.

Experimental Conditions and Procedures
To show the polishing effect of this new device in more detail, there are two steps in this experiment. Firstly, the forming process and final morphology of the MCF cutter were observed under different experimental conditions to obtain the appropriate cutter. In the second step, the optimal parameters of the experimental setup were determined through orthogonal experiments. Furthermore, the influences of the given polishing parameters on the surface quality were analyzed. Finally, the service life of the MCF slurry was studied under the optimal processing parameters.
The MCF slurry compositions and its distribution are shown in Table 1. The optimum composition of MCF of 45 wt.% water-based magnetic fluid, 40 wt.% iron powder, 12 wt.% abrasive grains, and 3 wt.% α-cellulose was chosen firstly, according to the previous study. Table 2 shows the process parameters of this experiment. A polycarbonate board (PC board) was chosen as the workpiece to explore the polishing characteristics of the proposed experimental installation and effectively extend the service life of the MCF slurry. In addition, 800-mesh sandpaper was used to polish the workpiece surface so that the roughness of the workpiece surface was approximately 500 nm.

Experimental Conditions and Procedures
To show the polishing effect of this new device in more detail, there are two steps in this experiment. Firstly, the forming process and final morphology of the MCF cutter were observed under different experimental conditions to obtain the appropriate cutter. In the second step, the optimal parameters of the experimental setup were determined through orthogonal experiments. Furthermore, the influences of the given polishing parameters on the surface quality were analyzed. Finally, the service life of the MCF slurry was studied under the optimal processing parameters.
The MCF slurry compositions and its distribution are shown in Table 1. The optimum composition of MCF of 45 wt.% water-based magnetic fluid, 40 wt.% iron powder, 12 wt.% abrasive grains, and 3 wt.% α-cellulose was chosen firstly, according to the previous study. Table 2 shows the process parameters of this experiment. A polycarbonate board (PC board) was chosen as the workpiece to explore the polishing characteristics of the proposed experimental installation and effectively extend the service life of the MCF slurry. In addition, 800-mesh sandpaper was used to polish the workpiece surface so that the roughness of the workpiece surface was approximately 500 nm.

MCF Tool
The polishing performance of slurry is profoundly influenced by the appearance of MCF during the polishing process; therefore, it is necessary to observe the size of MCF appearance and its formation process in order to obtain a suitable MCF slurry.
The observation process of MCF slurry is shown in Figure 4. Initially, a certain volume of MCF polishing solution is adsorbed on the side surface of the MCF carrier plate through a syringe. Afterward, under the action of an external magnetic field, the MCF slurry is stably adsorbed on the MCF carrier plate. Subsequently, the permanent magnetic rotates with the magnetic holder at a speed of n m = 600 rpm, and the MCF carrier also rotates at a speed of n c = 500 rpm in the opposite direction. The MCF slurry with the initial state varies with polishing time, and finally forms a complete MCF polishing tool. An industrial camera (OSG130-210UM by YVSION) was used to observe the formation process with various eccentricity values and MCF supply to reveal the formation time. Finally, the cross-section of the MCF tool was measured to obtain the size of its appearance morphology using an optical photograph. nm = 300, 400, 500, 600 rpm MCF carrier nc = 200, 300, 400, 500 rpm Supplying of MCF slurry, V 1, 1.5, 2, 2.5 mL Machining gap, Δ 1, 1.5, 2, 2.5 mm Processing time, t 10 min

MCF Tool
The polishing performance of slurry is profoundly influenced by the appearance of MCF during the polishing process; therefore, it is necessary to observe the size of MCF appearance and its formation process in order to obtain a suitable MCF slurry.
The observation process of MCF slurry is shown in Figure 4. Initially, a certain volume of MCF polishing solution is adsorbed on the side surface of the MCF carrier plate through a syringe. Afterward, under the action of an external magnetic field, the MCF slurry is stably adsorbed on the MCF carrier plate. Subsequently, the permanent magnetic rotates with the magnetic holder at a speed of nm = 600 rpm, and the MCF carrier also rotates at a speed of nc = 500 rpm in the opposite direction. The MCF slurry with the initial state varies with polishing time, and finally forms a complete MCF polishing tool. An industrial camera (OSG130-210UM by YVSION) was used to observe the formation process with various eccentricity values and MCF supply to reveal the formation time. Finally, the cross-section of the MCF tool was measured to obtain the size of its appearance morphology using an optical photograph.  The formation process of a typical MCF tool under magnet eccentricity r = 4 mm and MCF supply V = 1 mL is shown in Figure 5. The poor appearance of the MCF tool was observed in the initial state (0 ms) when the MCF slurry was only injected into the MCF carrier. Once the motor drives the magnet and the magnet seat to rotate around the spindle, the morphology of the MCF slurry will immediately change with the magnet. Eventually, a complete MCF slurry is formed at t = 950 ms, which was better than previous research conclusions [44]. Moreover, it should be noted that the formation time T of the MCF slurry can be defined as the period from the initial state to the formation of an extremely regular appearance of the MCF slurry.
To further explore the polishing performance of the MCF slurry, the sizes of the MCF tool's appearance shape were quantitatively studied. The final appearances of MCF tools with different eccentricities and MCF supplies are displayed in Figures 6 and 7, respectively. It can be found that the effect of eccentricity on the MCF appearance is greater than that of the MCF supply. The bulky magnetic clusters can be notably observed when the eccentricity is smaller than 4 mm, which indicates that the MCF slurry was stirred insufficiently. Once the eccentricity is larger than 4 mm, the magnetic clusters are vimineous and uniform. With the amount of MCF slurry supply increasing, the height and diameter of the MCF tool increase. MCF supply V = 1 mL is shown in Figure 5. The poor appearance of the MCF tool was observed in the initial state (0 ms) when the MCF slurry was only injected into the MCF carrier. Once the motor drives the magnet and the magnet seat to rotate around the spindle, the morphology of the MCF slurry will immediately change with the magnet. Eventually, a complete MCF slurry is formed at t = 950 ms, which was better than previous research conclusions [44]. Moreover, it should be noted that the formation time T of the MCF slurry can be defined as the period from the initial state to the formation of an extremely regular appearance of the MCF slurry. To further explore the polishing performance of the MCF slurry, the sizes of the MCF tool's appearance shape were quantitatively studied. The final appearances of MCF tools with different eccentricities and MCF supplies are displayed in Figures 6 and 7, respectively. It can be found that the effect of eccentricity on the MCF appearance is greater than that of the MCF supply. The bulky magnetic clusters can be notably observed when the eccentricity is smaller than 4 mm, which indicates that the MCF slurry was stirred insufficiently. Once the eccentricity is larger than 4 mm, the magnetic clusters are vimineous and uniform. With the amount of MCF slurry supply increasing, the height and diameter of the MCF tool increase.     In order to investigate thoroughly, a simplified schematic, as shown in Figure 8, was obtained according to the appearances. Obviously, a lateral mountain shape with a circular base diameter of d and a maximum peak height of Hmax is formed by the MCF slurry. As can be seen, the top of the mountain deviates significantly from the center of the bottom surface and rotates around the spindle of the magnet holder with the radius of L. According to the motion analysis, the larger the diameter of the base and the maximum height of the top, the greater the possibility of MCF slurry being spilled due to the centrifugal force generated by the MCF carrier rotation, and the more pronounced the top of the mountain's deviation from the center of the bottom surface, the lower the material removal at In order to investigate thoroughly, a simplified schematic, as shown in Figure 8, was obtained according to the appearances. Obviously, a lateral mountain shape with a circular base diameter of d and a maximum peak height of H max is formed by the MCF slurry. As can be seen, the top of the mountain deviates significantly from the center of the bottom surface and rotates around the spindle of the magnet holder with the radius of L. According to the motion analysis, the larger the diameter of the base and the maximum height of the top, the greater the possibility of MCF slurry being spilled due to the centrifugal force generated by the MCF carrier rotation, and the more pronounced the top of the mountain's deviation from the center of the bottom surface, the lower the material removal at the center of the polishing region. Therefore, it is necessary to maintain the appropriate size of the MCF tool to obtain a better polishing effect.  Figure 9 analyzes the relationship between the eccentric distance r of the magnet and the morphology of the MCF slurry and the molding time T. It can be seen that the formation time T of the MCF tool decreases gradually with the increase in eccentricity r. When the eccentricity r increases from 2 to 4 mm, the formation time T of the MCF slurry decreases sharply. However, the decrease rate declines when the eccentricity r exceeds 4 mm. It is worth noting that when eccentricity r = 0 mm, eventually, a jagged MCF slurry is formed. Regarding the appearance of the MCF slurry, the MCF tool with a better appearance shape size is obtained at r = 4 mm. The value of circular base diameter d gradually increases as the eccentricity r increases, and the increasing tendency remains in a stable state. The maximum height Hmax gradually decreases with the rise in eccentricity r, and its decrease tendency is basically the same as the increase in base diameter. The reason is that the MCF slurry supply remains constant, and the maximum height and the base diameter's changing trends are opposite. Moreover, the distance L between the maximum height and the centerline decreases first and then increases with the increase in eccentricity r, and the value of L reaches the minimum at r = 4 mm. 40 4 Figure 8. Schematic diagram of MCF tool. Figure 9 analyzes the relationship between the eccentric distance r of the magnet and the morphology of the MCF slurry and the molding time T. It can be seen that the formation time T of the MCF tool decreases gradually with the increase in eccentricity r. When the eccentricity r increases from 2 to 4 mm, the formation time T of the MCF slurry decreases sharply. However, the decrease rate declines when the eccentricity r exceeds 4 mm. It is worth noting that when eccentricity r = 0 mm, eventually, a jagged MCF slurry is formed. Regarding the appearance of the MCF slurry, the MCF tool with a better appearance shape size is obtained at r = 4 mm. The value of circular base diameter d gradually increases as the eccentricity r increases, and the increasing tendency remains in a stable state. The maximum height H max gradually decreases with the rise in eccentricity r, and its decrease tendency is basically the same as the increase in base diameter. The reason is that the MCF slurry supply remains constant, and the maximum height and the base diameter's changing trends are opposite. Moreover, the distance L between the maximum height and the centerline decreases first and then increases with the increase in eccentricity r, and the value of L reaches the minimum at r = 4 mm. mm. It is worth noting that when eccentricity r = 0 mm, eventually, a jagged MCF slurry is formed. Regarding the appearance of the MCF slurry, the MCF tool with a better appearance shape size is obtained at r = 4 mm. The value of circular base diameter d gradually increases as the eccentricity r increases, and the increasing tendency remains in a stable state. The maximum height Hmax gradually decreases with the rise in eccentricity r, and its decrease tendency is basically the same as the increase in base diameter. The reason is that the MCF slurry supply remains constant, and the maximum height and the base diameter's changing trends are opposite. Moreover, the distance L between the maximum height and the centerline decreases first and then increases with the increase in eccentricity r, and the value of L reaches the minimum at r = 4 mm. The influence of the MCF slurry supply on the geometric size and formation time T of the MCF tool are shown in Figure 10. The values of the base diameter d, the maximum height Hmax, the distance L between the maximum height and the centerline, and the formation time T all grow with the increase in the MCF slurry supply, and the growth rate is The influence of the MCF slurry supply on the geometric size and formation time T of the MCF tool are shown in Figure 10. The values of the base diameter d, the maximum height H max , the distance L between the maximum height and the centerline, and the formation time T all grow with the increase in the MCF slurry supply, and the growth rate is consistent. The reason is that the MCF slurry supply has increased, resulting in an increasing number of magnetic clusters. However, the MCF slurry becomes easier to dislodge from the polishing area as the slurry driven by the rotary carrier is uncontrolled with the increase in MCF supply. Therefore, when the volume of MCF slurry is 1 mL, the best morphology is obtained. consistent. The reason is that the MCF slurry supply has increased, resulting in an increasing number of magnetic clusters. However, the MCF slurry becomes easier to dislodge from the polishing area as the slurry driven by the rotary carrier is uncontrolled with the increase in MCF supply. Therefore, when the volume of MCF slurry is 1 mL, the best morphology is obtained. In addition, according to previous studies, the effects of the rotation speed of the magnet nm and the rotation speed of the MCF carrier nc on the appearance of the MCF tool are negligible [41]. In conclusion, when the eccentric distance r is 4 mm and the supply of MCF slurry is 1 mL, the MCF polishing tool is the best.

Optimal Processing Parameters
Before formally using the device for MCF polishing, it is necessary to decide the optimal parameters. Therefore, a multi-factor and multi-level orthogonal experiment is designed and carried out to explore the optimal parameters of the device. In addition, by comparing the best surface roughness Ra and MRR, the relationship between process parameters and polishing effect is analyzed. It is found during the measurements that the optimal surface roughness is primarily found in the shaded area shown in Figure 11. This part will be further explored in future studies. Meanwhile, the previous study found that In addition, according to previous studies, the effects of the rotation speed of the magnet n m and the rotation speed of the MCF carrier n c on the appearance of the MCF tool are negligible [41]. In conclusion, when the eccentric distance r is 4 mm and the supply of MCF slurry is 1 mL, the MCF polishing tool is the best.

Optimal Processing Parameters
Before formally using the device for MCF polishing, it is necessary to decide the optimal parameters. Therefore, a multi-factor and multi-level orthogonal experiment is designed and carried out to explore the optimal parameters of the device. In addition, by comparing the best surface roughness Ra and MRR, the relationship between process parameters and polishing effect is analyzed. It is found during the measurements that the optimal surface roughness is primarily found in the shaded area shown in Figure 11. This part will be further explored in future studies. Meanwhile, the previous study found that an annular polishing area with a W-shaped cross-section was generated, shown in Figure 12a [28]. Therefore, the maximum material removal depth MR max was used to characterize the material removal during MCF polishing.
are negligible [41]. In conclusion, when the eccentric distance r is 4 mm and the supply of MCF slurry is 1 mL, the MCF polishing tool is the best.

Optimal Processing Parameters
Before formally using the device for MCF polishing, it is necessary to decide the optimal parameters. Therefore, a multi-factor and multi-level orthogonal experiment is designed and carried out to explore the optimal parameters of the device. In addition, by comparing the best surface roughness Ra and MRR, the relationship between process parameters and polishing effect is analyzed. It is found during the measurements that the optimal surface roughness is primarily found in the shaded area shown in Figure 11. This part will be further explored in future studies. Meanwhile, the previous study found that an annular polishing area with a W-shaped cross-section was generated, shown in Figure  12a [28]. Therefore, the maximum material removal depth MRmax was used to characterize the material removal during MCF polishing.

P1 P2
P3 R/2 R Figure 11. Schematic diagram of measuring points in the polishing area. Figure 11. Schematic diagram of measuring points in the polishing area. According to the study described in Section 2.1, the eccentricity, machining gap, MCF carrier speed, and magnet speed have great influences on the polishing effect. It was found that if the machining gap was too small or the carrier speed was too large, the liquid within the MCF slurry would splash during the polishing process. Therefore, the levels of process parameters were designed as shown in Table 3. The results of the orthogonal experiment are shown in Table 4. In this study, 16 groups of process tests were completed. The surface roughness Ra before and after MCF polishing was measured, and the maximum material removal depth of each sample was recorded. Since the surface roughness of each workpiece before polishing is different, in order to better represent the polishing effect of the workpiece, the surface roughness reduction rate Ra% is used to explain. (17) where Rai is the surface roughness before polishing, and Rap is the surface roughness after polishing.  According to the study described in Section 2.1, the eccentricity, machining gap, MCF carrier speed, and magnet speed have great influences on the polishing effect. It was found that if the machining gap was too small or the carrier speed was too large, the liquid within the MCF slurry would splash during the polishing process. Therefore, the levels of process parameters were designed as shown in Table 3. The results of the orthogonal experiment are shown in Table 4. In this study, 16 groups of process tests were completed. The surface roughness Ra before and after MCF polishing was measured, and the maximum material removal depth of each sample was recorded. Since the surface roughness of each workpiece before polishing is different, in order to better represent the polishing effect of the workpiece, the surface roughness reduction rate Ra% is used to explain. where Ra i is the surface roughness before polishing, and Ra p is the surface roughness after polishing. The results of the orthogonal test are analyzed as shown in Table 5. K i (i = 1, 2, 3, 4) is the mean value of the surface roughness decrease rate for each parameter at different levels, and F i (i = 1, 2, 3, 4) is the mean value of the maximum material removal depth for each parameter at different levels. R and T are the range; if the range of a certain parameter is more prominent, it means that the influence of this parameter on the experimental results is more remarkable. The mean value of the surface roughness decrease rate Ra% at different levels of various parameters is shown in Figure 13. The decrease rate reaches the maximum value when the eccentricity r, machining gap ∆, the rotation speed of the magnet n m , and the rotation speed of the MCF carrier n c are 4 mm, 1 mL, 600 rpm, and 500 rpm, respectively. With the increase in eccentric distance, the decline first increases and then decreases, and reaches the maximum when the eccentricity is 4 mm, which is consistent with the result that the apparent shape of the MCF tool is the most ideal when the eccentricity is 4 mm. In addition, the decrease rate also declines with the increase in the machining gap, and grows with the growth of the rotational speed of the MCF carrier plate and the magnet. According to the definition of decrease rate Ra%, the Ra% can represent the overall polishing effect of the workpiece. Therefore, when the eccentric distance r, machining gap ∆, magnet speed n m , and MCF carrier speed n c are 4 mm, 1 mL, 600 rpm, and 500 rpm, the resulting surface quality is the best. Meanwhile, it should be noted from Figure 13 that the machining gap has the greatest impact on surface roughness, followed by the speed of the MCF carrier. The machining gap affects the magnetic field intensity in the machining area, which affects the polishing force of the abrasive grains in the polishing fluid. The rotational speed of the MCF carrier affects the velocity of the abrasive grains relative to the workpiece surface. The larger the relative speed is, the more the abrasive grains participate in cutting at the same time.
The mean value of the maximum material removal depth MRmax at different levels of various parameters is shown in Figure 14. It has a trend consistent with the decline rate of surface roughness. However, MRmax reaches the maximum value when the rotation speed of the magnet is 500 rpm, which is different from Figure 13. The reason is that the stirring effect of the magnetic lines of force is too strong for the MCF slurry due to the excessive rotation of the magnet. The contact time of the abrasive grains with the workpiece surface is too short to utilize the micro-cutting effect of the abrasive grain fully. Therefore, the maximum amount of material removal was obtained when the eccentricity r, the machining gap Δ, the magnet speed nm, and the MCF carrier nc speed were 4 mm, 1 mL, 500 rpm, and 500 rpm, respectively. At the same time, comparing the effects of various process parameters on the maximum material removal depth, it can be found that the influence of the magnet speed on material removal is minimal. Moreover, the maximum material removal depth when the Meanwhile, it should be noted from Figure 13 that the machining gap has the greatest impact on surface roughness, followed by the speed of the MCF carrier. The machining gap affects the magnetic field intensity in the machining area, which affects the polishing force of the abrasive grains in the polishing fluid. The rotational speed of the MCF carrier affects the velocity of the abrasive grains relative to the workpiece surface. The larger the relative speed is, the more the abrasive grains participate in cutting at the same time.
The mean value of the maximum material removal depth MR max at different levels of various parameters is shown in Figure 14. It has a trend consistent with the decline rate of surface roughness. However, MR max reaches the maximum value when the rotation speed of the magnet is 500 rpm, which is different from Figure 13. The reason is that the stirring effect of the magnetic lines of force is too strong for the MCF slurry due to the excessive rotation of the magnet. The contact time of the abrasive grains with the workpiece surface is too short to utilize the micro-cutting effect of the abrasive grain fully. Therefore, the maximum amount of material removal was obtained when the eccentricity r, the machining gap ∆, the magnet speed n m , and the MCF carrier n c speed were 4 mm, 1 mL, 500 rpm, and 500 rpm, respectively. Meanwhile, it should be noted from Figure 13 that the machining gap has the greatest impact on surface roughness, followed by the speed of the MCF carrier. The machining gap affects the magnetic field intensity in the machining area, which affects the polishing force of the abrasive grains in the polishing fluid. The rotational speed of the MCF carrier affects the velocity of the abrasive grains relative to the workpiece surface. The larger the relative speed is, the more the abrasive grains participate in cutting at the same time.
The mean value of the maximum material removal depth MRmax at different levels of various parameters is shown in Figure 14. It has a trend consistent with the decline rate of surface roughness. However, MRmax reaches the maximum value when the rotation speed of the magnet is 500 rpm, which is different from Figure 13. The reason is that the stirring effect of the magnetic lines of force is too strong for the MCF slurry due to the excessive rotation of the magnet. The contact time of the abrasive grains with the workpiece surface is too short to utilize the micro-cutting effect of the abrasive grain fully. Therefore, the maximum amount of material removal was obtained when the eccentricity r, the machining gap Δ, the magnet speed nm, and the MCF carrier nc speed were 4 mm, 1 mL, 500 rpm, and 500 rpm, respectively. At the same time, comparing the effects of various process parameters on the maximum material removal depth, it can be found that the influence of the magnet speed on material removal is minimal. Moreover, the maximum material removal depth when the At the same time, comparing the effects of various process parameters on the maximum material removal depth, it can be found that the influence of the magnet speed on material removal is minimal. Moreover, the maximum material removal depth when the rotation speed of the magnet is 600 rpm is second only to that when the rotation speed of the magnet is 500 rpm.
In addition, considering Figures 13 and 14 comprehensively, the influence of the processing parameters on surface roughness is more significant than that on material removal. Therefore, when the eccentricity distance r is 4 mm, the machining gap ∆ is 1 mm, the magnet speed n m is 600 rpm, and the MCF carrier speed n c is 500 rpm, the best polishing effect is obtained.

MCF Service Life
In order to inhibit the drying of the MCF slurry and prolong its service time, the following experiments were designed. The workpiece was polished for 10 min under the condition of no addition, and then the workpiece was replaced without changing the MCF slurry, and the process was continued for 10 min. The above steps were repeated until the accumulated processing time was 60 min. The weight change of the MCF slurry before and after the polishing was also measured to determine the water loss during the polishing process. At the same time, in order to explore whether there were other losses besides water in the polishing process, MF was also added as the control (using a syringe for addition). It was determined that the weight of the MCF slurry decreased by 0.724 g after polishing for 60 min. Therefore, 0.1 mL water or MF should be added to the MCF slurry every 10 min.
The optimal surface roughness of the polishing area under different addition conditions and its decrease rate are shown in Figure 15. Regardless of the addition conditions, the decrease rate shows a declining trend. The decrease rate with no addition was reduced more dramatically, while the decrease rate when adding water and MF recovered slightly after a sharp decline. The decrease rate is more significant when adding MF compared with adding water. The reason is that due to the addition of MF, the viscosity of the MCF slurry becomes larger, which leads to a more significant polishing force. rotation speed of the magnet is 600 rpm is second only to that when the rotation speed of the magnet is 500 rpm.
In addition, considering Figures 13 and 14 comprehensively, the influence of the processing parameters on surface roughness is more significant than that on material removal. Therefore, when the eccentricity distance r is 4 mm, the machining gap Δ is 1 mm, the magnet speed nm is 600 rpm, and the MCF carrier speed nc is 500 rpm, the best polishing effect is obtained.

MCF Service Life
In order to inhibit the drying of the MCF slurry and prolong its service time, the following experiments were designed. The workpiece was polished for 10 min under the condition of no addition, and then the workpiece was replaced without changing the MCF slurry, and the process was continued for 10 min. The above steps were repeated until the accumulated processing time was 60 min. The weight change of the MCF slurry before and after the polishing was also measured to determine the water loss during the polishing process. At the same time, in order to explore whether there were other losses besides water in the polishing process, MF was also added as the control (using a syringe for addition). It was determined that the weight of the MCF slurry decreased by 0.724 g after polishing for 60 min. Therefore, 0.1 mL water or MF should be added to the MCF slurry every 10 min.
The optimal surface roughness of the polishing area under different addition conditions and its decrease rate are shown in Figure 15. Regardless of the addition conditions, the decrease rate shows a declining trend. The decrease rate with no addition was reduced more dramatically, while the decrease rate when adding water and MF recovered slightly after a sharp decline. The decrease rate is more significant when adding MF compared with adding water. The reason is that due to the addition of MF, the viscosity of the MCF slurry becomes larger, which leads to a more significant polishing force. In order to better represent the surface roughness of the polishing area, different positions of the polishing area (P1, P2, P3) were measured, as shown in Figure 11. Furthermore, the obtained data samples were statistically analyzed, and the overall surface roughness decrease rate of the polishing area was expressed in the form of error bars, as shown in Figure 16. In this experiment, the values of material removal at point P1 and P3 are far lower than that of P2, which result in large differences in the surface roughness in different areas. Consequently, the range of error bars in the decrease rate of surface roughness is wide, which is shown in the figure. Overall, the decrease rate is the largest when In order to better represent the surface roughness of the polishing area, different positions of the polishing area (P1, P2, P3) were measured, as shown in Figure 11. Furthermore, the obtained data samples were statistically analyzed, and the overall surface roughness decrease rate of the polishing area was expressed in the form of error bars, as shown in Figure 16. In this experiment, the values of material removal at point P1 and P3 are far lower than that of P2, which result in large differences in the surface roughness in different areas. Consequently, the range of error bars in the decrease rate of surface roughness is wide, which is shown in the figure. Overall, the decrease rate is the largest when MF is added, followed by the decrease rate when water is added, and the decrease rate is the smallest with no addition. MF is added, followed by the decrease rate when water is added, and the decrease rate is the smallest with no addition. The material removal rate of the polishing area under different addition conditions is shown in Figure 17. When no water and MF are supplied ("None"), the material removal ability is significantly reduced after 60 min of polishing. However, when water and MF are provided, the material removal rate is significantly improved compared to the former condition. The material removal rate is the largest when MF is added, and the processing performance of the MCF slurry can best be maintained in this case.

Conclusions
In order to solve the water loss problem of MCF slurry during the experiment, an experimental setup was built to add water or MF into the MCF slurry conveniently. The appearance morphology and forming time of the MCF tool were observed. The orthogonal experiments were designed to determine the optimal process parameters of the polishing device. Finally, the polishing experiment of adding water or MF to the MCF slurry was carried out under the optimal processing parameters, which confirmed the feasibility of this polishing device to extend the service life of the MCF slurry and reduce the production and processing costs by adding water or MF, which is of great significance in production and application. The results are analyzed as follows. The material removal rate of the polishing area under different addition conditions is shown in Figure 17. When no water and MF are supplied ("None"), the material removal ability is significantly reduced after 60 min of polishing. However, when water and MF are provided, the material removal rate is significantly improved compared to the former condition. The material removal rate is the largest when MF is added, and the processing performance of the MCF slurry can best be maintained in this case. MF is added, followed by the decrease rate when water is added, and the decrease rate is the smallest with no addition. The material removal rate of the polishing area under different addition conditions is shown in Figure 17. When no water and MF are supplied ("None"), the material removal ability is significantly reduced after 60 min of polishing. However, when water and MF are provided, the material removal rate is significantly improved compared to the former condition. The material removal rate is the largest when MF is added, and the processing performance of the MCF slurry can best be maintained in this case.

Conclusions
In order to solve the water loss problem of MCF slurry during the experiment, an experimental setup was built to add water or MF into the MCF slurry conveniently. The appearance morphology and forming time of the MCF tool were observed. The orthogonal experiments were designed to determine the optimal process parameters of the polishing device. Finally, the polishing experiment of adding water or MF to the MCF slurry was carried out under the optimal processing parameters, which confirmed the feasibility of this polishing device to extend the service life of the MCF slurry and reduce the production and processing costs by adding water or MF, which is of great significance in production and application. The results are analyzed as follows.

Conclusions
In order to solve the water loss problem of MCF slurry during the experiment, an experimental setup was built to add water or MF into the MCF slurry conveniently. The appearance morphology and forming time of the MCF tool were observed. The orthogonal experiments were designed to determine the optimal process parameters of the polishing device. Finally, the polishing experiment of adding water or MF to the MCF slurry was carried out under the optimal processing parameters, which confirmed the feasibility of this polishing device to extend the service life of the MCF slurry and reduce the production and processing costs by adding water or MF, which is of great significance in production and application. The results are analyzed as follows.
(1) The MCF polishing tool has the best polishing effect when the eccentricity r and MCF slurry supply V are 4 mm and 1 mL. (2) The formation time of the MCF tool decreased sharply when the eccentricity increased from 2 to 4 mm, but once the eccentricity exceeded 4 mm, the trend of decline became reversed. Moreover, with the increase in supply, the formation time grew steadily.
(3) An ideal processing result could be obtained when the machining gap ∆, the revolution speed of the MCF carrier n c , the eccentricity r, and the revolution speed of the magnet n m were 1 mm, 500 rpm, 4 mm, and 600 rpm, respectively. Moreover, the influence of the processing parameters on surface roughness is more significant than that on material removal. (4) Adding water and MF could extend the service life of MCF, and, compared with adding water, adding MF could obtain a better polishing effect.

Data Availability Statement:
The study did not report any data.

Conflicts of Interest:
The authors declare that they have no conflict of interest.