Novel Batch Polishing Method of Ceramic Cutting Inserts for Reducing Tool Wear

Ceramic cutting inserts are a type of cutting tool commonly used in high-speed metal cutting applications. However, the wear of these inserts caused by friction between the workpiece and cutting inserts limits their overall effectiveness. In order to improve the tool life and reduce wear, this study introduces an emerging method called magnetic field-assisted batch polishing (MABP) for simultaneously polishing multiple ceramic cutting inserts. Several polishing experiments were conducted under different conditions, and the wear characteristics were clarified by cutting S136H steel. The results showed that after 15 min of polishing, the surface roughness at the flank face, edge, and nose of the inserts was reduced to below 2.5 nm, 6.25 nm, and 45.8 nm, respectively. Furthermore, the nose radii of the inserts did not change significantly, and there were no significant changes in the weight percentage of elements before and after polishing. Additionally, the tool life of the batch polished inserts was found to be up to 1.75 times longer than that of unpolished inserts. These findings suggest that the MABP method is an effective way to mass polish ceramic cutting inserts, resulting in significantly reduced tool wear. Furthermore, this novel method offers new possibilities for polishing other tools


Introduction
Nowadays, a machining tool is essential for shaping processes in a wide range of industrial applications [1].Since it contacts with the workpiece and influences the machining quality and efficiency of the workpiece directly, the cutting insert plays a vital role in the machining tool [2,3].In recent years, various cutting insert materials, including cemented carbide [4], cermet [5], ceramic [6], polycrystalline diamond (PCD) [7], and cubic-boron-nitride (CBN) [8], have been developed and successfully implemented concerning different machining conditions.Among them, ceramic cutting inserts have been widely applied for the finishing of cast iron and hardened materials due to their mechanical properties and chemical stability at high temperatures [9,10].However, recent pieces of evidence suggest that some undesirable characteristics, such as the friction at the insert-chip interface [11,12] and machining marks on the insert surface [13], can accelerate the wear of ceramic cutting inserts.As a result, an effective and efficient approach to achieve a notable extension of the tool life during machining is urgently needed.
Up to present, several studies have reported that cryogenic coolant [14], coating [15], changing insert shapes [16], and process parameters [17] are employed for improving the cutting insert life.The cryogenic coolant is carried out at lower the machining temperature to improve insert life [18].However, conventional cutting oil is considered harmful to the environment [19] and shows low cooling efficiency [20].Moreover, cryogenic coolants using liquid nitrogen are not suitable for longterm processing because they are normally used in industrial applications [21].In the coating process, chemical vapor deposition (CVD) [22] and physical vapor deposition (PVD) [23] methods are now frequently applied for cutting inserts corresponding to different applications.Due to the extremely high requirement on the cutting insert surface quality, the coating materials and equipment are relatively expensive [24].Although the technique to alter insert shapes and process parameters is constantly advancing, there are still several steps for preparing a micro-texture cutting insert [25].In addition, the grinding marks and micro-defects on the insert surface remain unresolved [26].
Compared with the aforementioned technologies, some researchers have proven that polishing does not require expensive machine equipment and can smooth the insert surface, reduce the friction on the insert-chip interface and eliminate machining marks simultaneously [27,28].In 1997, Yoshikawa et al. [29] reported a hot-iron-metal method (HIMM) as the polishing method for the coated cutting insert.They also developed an apparatus to polish cutting insert flank by HIMM.To polish inserts flank with complex shapes, Lyu et al. [30] found that the shear thickening polishing (STP) method achieves precision polishing of cutting insert, and the optimal combination of parameters was obtained as well.In a follow-up study, brush tool-assisted shear thickening polishing (B-STP) was proposed to break the thickened agglomerates and improve the slurry flow ability, and the surface roughness of cutting insert flank could reach 7.9 nm after 15 min of polishing [13].Yamaguchi et al. [31] proposed a polishing method, named magnetic abrasive finishing (MAF), to smooth cutting insert nose and flank, and the treated insert life was extended by 150%.On the other hand, the edges of the as-received cutting inserts are uneven [32], which increases friction and accelerates cutting insert wear.Tanaka et al. [33] provided a polishing approach of cutting insert edge using an AC electric field which was applied between the polishing pad and the cutting insert.A smooth edge of the inserts was obtained, and the tool life of polished inserts tended to be longer than unpolished ones.But the whole process consisted of four steps because of the equipment limitations.Ishimarua et al. [34] and Izumi et al. [35] studied the polishing performance of the cutting insert edge by ultraviolet-ray irradiation-assisted polishing.After treatment, the radii of the insert edge decreased slightly.However, the surface roughness of the cutting inserts is very dependent on the quartz glass polishing plate [32].For improving the polishing efficiency of individual cutting inserts, the plasma electrolytic polishing (PEP) method was employed by An et al. [1].A better-polished surface of the coated cutting insert was obtained with the voltage of 110 V, but the surface roughness (Ra) only stayed at a deep sub-micron level with 0.096 μm [36].Although some research has been carried out on polishing cutting inserts, no single study exists which provides a batch polishing machine for smoothing flank, nose, and edge simultaneously with nanometer scale surface roughness.Meanwhile, other mainstream polishing methods such as bonnet polishing [37,38], fluid jet polishing [39,40] and magnetorheological finishing [41,42] are not suitable for the polishing of cutting inserts due to the high equipment cost and polishing cost.
This paper presents a novel method, called 'magnetic field-assisted batch polishing (MABP)' , for automatic batch polishing cutting inserts simultaneously, together with nanometer scale surface finish.Section two of this paper will examine the working principle of the MABP method.Hence, a series of polishing and turning experiments are carried out.The fourth section presents the findings of the research, focusing on the four key themes including polishing performance, surface roughness, element composition, and nose radius.Meanwhile, the possibility of batch polishing by MABP will be examined through multiple insert polishing in Section 4 as well.Section 5 analyses the effects of MABP processing on cutting insert life by cutting S136H steel.

Sample Preparation
Ceramics inserts are capable of running at high speeds, thus reducing expensive machining time [43].In addition, ceramic maintains good surface finishes due to its low affinity to workpiece materials [44].Accordingly, one commercially available uncoated triangular ceramic cutting inserts (TNGA160404), manufactured by Kyocera corporation, is chosen in this paper.The insert material is aluminum oxide and titanium carbide (Al 2 O 3 +TiC) and it is mainly used in semi-roughing to finishing cast iron and hardened materials.Other features of this cutting insert are listed in Table 1.

Methodology
The graphical illustration of the working principle of the MABP method is shown in Figure 1.In this system, two pairs of permanent magnets are mounted on the rotary table which is driven by a servo motor.During the polishing, the magnetic abrasives inside the chamber generate the continuous rotating magnetic abrasive brush under the effect of a rotating magnetic field while the chamber is fixed on the metal frame and does not rotate.
As for the magnetic abrasive brush, it is a type of loose magnetic abrasives brush which are the magnetic particles mixing with the polishing abrasives in the carrier fluid [45].
The workpiece held by the fixture is fixed in the annular chamber.Moreover, the polishing abrasives inside the continuously rotating magnetic abrasive brushes impinge the workpiece and remove material from the surface.

MABP Experiments
Schematic diagram of the MABP and corresponding experimental set-up is presented in Figure 2. The apparatus was mainly composed of a lid, chamber, magnets, and rotary table.As shown in Figure 2(d), the ceramic inserts were fixed by the fixtures which were secured to the lid with a stainless screw.Two pairs of magnets were installed on the rotary table which was driven by the motor.The chamber was mounted above the magnets and did not move during the polishing.Under the magnetic field, two pairs of magnetic abrasive brushes were formed inside the chamber as shown in Figure 2(e).Moreover, these magnetic abrasive brushes which were a loose type of magnetic abrasive included carbonyl iron powder (CIP) (average size is 3 μm, BASF Co. Ltd., Germany) and diamond polishing slurry (average size is 125 nm, Universal Photonics Inc., USA) as shown in Figure 2(f ).In addition, the concentration of the diamond polishing slurry was 25%.To verify the repeatability of the experiments, two inserts were polished simultaneously in this experiment.Based on our previous investigation [46][47][48], other experimental conditions are listed in Table 2.
After polishing experiments, the samples were cleaned with an ultrasonic cleaning machine for 5 min.The measurement positions of the cutting inserts involved the edge, flank, and nose, and each side of the insert was designated by letters A and B respectively, as shown in Figure 2(g).3D topography images and surface roughness were measured by the Zygo Nexview white light interferometer, and the magnification of the objective lens was 40.The surface roughness of each position was measured at four different points.The micro-topography of the cutting inserts was observed by a Tescan MAIA3 field emission scanning electron microscope (SEM).The element compositions of the cutting insert were analyzed by energy disperses spectroscopy (EDX).In addition, a 3D optical measuring system (Alicona IFM G4) was employed for measuring nose radius.

Cutting Experiments
To evaluate the effect of MABP polishing on cutting inserts, cutting experiments were performed on a 25 mm diameter S136H steel with hardness of 48 HRC using unpolished and polished inserts as shown in Figure 3.The cut length per pass was 80 mm in the feed direction.Other cutting experiment conditions are summarized in Table 3.
After cutting experiments, the images of flank wear were captured by the Keyence VH-6000 digital microscope, and the maximum flank wear was used to evaluate the tool life.For the experiments, the cutting operation was terminated once the insert was broken.

Polishing Performance of MABP on Cutting Insert
The photograph of the ceramic cutting inserts before and after polishing are presented in Figure 4, which are polished for 15 min.For both samples, the cutting insert surface successfully achieved a mirror effect after polishing as shown in Figure 4(b).It demonstrates that MABP can achieve precision polishing of ceramic cutting inserts.
Furthermore, the 3D topography images of the cutting insert (sample 1) at flank-A, nose and edge-A are demonstrated in Figure 5.As for the flank, there are lots of machining marks because of finishing by grinding as shown in Figure 5(a).After polishing, the machining marks have been eliminated effectively, and smooth and flat surfaces are achieved, as seen in Figure 5(b).In addition, the surface roughness in regard to mean height (Sa) and maximum height (Sz) decreased rapidly from 61 nm and 687 nm to 2 nm and 29 nm, respectively.The same phenomenon can also be observed in the edge as shown in Figure 5(d)−(f ).From initial 94 nm and 1031 nm, the surface roughness and maximum height of the edge dramatically decreased to 4 nm and 90 nm, respectively.In the nose case, the surface roughness showed a moderate reduction after 15 min polishing, while the maximum height went

Parameters (unit) Values
Polishing time (min) 15 Rotational speed (r/min) 1500 Weight of magnetic abrasive (g) 60 Weight of CIP (g) 45 Weight of diamond polishing slurry (g) 15 Concentration of the polishing slurry (wt.%) 25 down slightly.The reason is that the impinging angle of the brush is 90°, and the magnetic abrasive particles impact the nose vertically, leading to a decrease in material removal rates [49].On the other hand, when the brush impinges on the flank and edge, the angle is less than 90°, resulting in an increase in material removal rates due to the shear force applied.As shown in Figure 6, the SEM micro-surface topography of the cutting insert (sample 2) at flank-B and edge-B is measured by the Tescan MAIA3 SEM.In terms of the initial surface, numerous clear scratches and pits were observed on the flank and nose after machining as shown in Figure 6(a), (b), formed during the shaping of the cutting insert by the diamond grinding wheel.After 15 min MABP, it is obviously seen in Figure 6(c), (d) that a smooth surface without any micro-defects could be achieved.Overall, the surface roughness of two cutting inserts was sharply reduced in all cases as compared with the as-received conditions.To be more specific, from initial 62.3±3.2 nm and 79±12.7 nm, the Sa of flank-A and edge-A for sample 1 steeply dropped to 2.5±1.0 nm and 4.3±0.7 nm, respectively.The same trend could also be observed in flank-B and edge-B of sample 1.However, the effect of MABP on the surface roughness of the nose was not as obvious as that on the flank and edge.These results are likely to be related to the nose with a complex shape and much higher initial surface roughness.In addition, sample 2 and sample 1 were more equally represented in the value of average surface roughness of the flank, nose and edge.It indicates that MABP exhibits great potential for batch polishing, which can achieve the polishing of multiple cutting inserts to improve processing efficiency.

Element Composition
EDX tests were conducted to analyze any differences in the material before and after the polishing process.Figure 8 provides the experimental data on the wt.% range of the element composition before and after MABP.The EDX results showed that the cutting insert (including flank, edge, and nose) consisted of C, O, Al, and Ti.After polishing, insert surfaces showed an elemental composition of C, O, Al, and Ti as well.To be more specific, the wt.% of Al increased slightly after MABP, increasing by about 2.0%, 0.2%, and 4.3% corresponding to flank, edge, and nose, respectively.In the contrast, the wt.% of O declined around 3.1%, 0.2%, and 3.0% corresponding to flank, edge, and nose, respectively.As for other elements, a moderate fluctuation could be observed in wt.%.In summary, the element composition does not significantly change in ceramics cutting insert after MABP.This indicates that there is no grain refinement due to mechanical load

Nose Radius
Figure 9(a) and (b) present the 3D nose profile before and after MABP, respectively.It should be noted that the nose is rounded after polishing.Moreover, Figure 9(c) gives information about the average nose radius before and after polishing.For the as-received insert, the average nose radius was 403 μm.After 15 min MABP, the average nose radii of sample 1 and sample 2 were 391 μm and 388 respectively, and both of these two samples were characterized by small standard deviations (SD).It indicates that a sharp nose can be obtained, and the nose radius only decreases narrowly after MABP.

Application of MABP for Multiple Cutting Inserts
If a batch system is realized in MABP, the time needed for MABP of inserts can be further reduced.A total of six inserts were fixed on the lid through fixtures for simultaneous batch polishing.The photograph of the ceramic cutting inserts after batch polishing is shown in Figure 10(a).In terms of all the inserts, the mirror effect could be successfully obtained after 15 min polishing.It indicates that MABP can achieve the precision polishing of a batch of ceramic cutting inserts simultaneously in a short time.Furthermore, the SEM micro-images of the inserts edge and flank measured by Tescan MAIA3 SEM are shown in Figure 10(b)−(e).After batch polishing, the machining marks (e.g., scratches and pits) on the edge and flank were efficiently removed.In addition, the height roughness parameters of the flank and edge after batch polishing is presented in Figure 11.In general, the average Sa were all improved after batch polishing.From initial 57.8±7.3 nm and 73.7±13.1 nm, the average surface roughness at flank-A and edge-A deceased sharply to 9.1±2.6 nm and 30.0±17.9 nm, respectively.Meanwhile, the same trend could also be observed for flank-B and edge-B.As for the surface roughness in terms of the Sz, it was also reduced moderately from 946.5±173.9nm to 586.2±129.2nm at flank-A and from 1142.3±210.3nm to 711.2±254.3nm at edge-A, respectively.
Figure 12 shows the experimental data on nose height roughness parameters and radius before and after batch polishing.From the figure, it could be seen that the height roughness parameters of decreases gradually from Sa 112.3±6.5 nm to Sa 65.5±11.9nm and Sz 2355.0±379.8nm to Sz 1856.5±587.8nm after batch polishing, respectively.In the contrast, a slight decrease in nose radius could be observed after batch polishing.Taken together, the MABP method enables batch polishing, which can allow the polishing of multiple ceramic cutting inserts to improve the polishing efficiency and accuracy in a short time.However, compared with two cutting inserts, surface roughness increases gradually for the polishing batch of inserts.It appears possible that these findings are attributed to the decreased mobility of the polishing slurry, as the velocity differential between the inserts and the polishing fluid diminishes, leading to a reduction in shear force [50].

Cutting Performance and Insert Wear
Figure 13 presents the changes in the maximum flank wear of polished and unpolished inserts with cutting time.At the early cutting stage, there was no clear difference in the maximum flank among MABP inserts and batch-polished inserts.The value of unpolished inserts was slightly larger than theirs.Subsequently, the maximum flank wear of unpolished inserts increased rapidly, while a moderate growth could be observed in the maximum flank wear of polished inserts.It should be noted that the unpolished insert was broken after cutting for 8 min, and the maximum flank wear of the MABP insert and batch polished insert was 332 μm and 358 μm, respectively as shown in Figure 13.With the increasing cutting time, the batch polished insert was also broken, and the batch polished inserts have the tool life of up to 1.75 times as long as unpolished inserts.These relationships can be explained by the insert surface condition.The polished inserts with a smooth surface can reduce the friction on the chip-insert interface, and result in extended tool life.

Conclusions
This paper presents a novel magnetic field-assisted batch polishing (MABP) method for polishing ceramics cutting inserts and evaluates tool life after polishing by MABP.The specific conclusions of this study can be summarized as follows: (1) The MABP method enables batch polishing, which can allow the polishing of multiple ceramic cutting inserts to improve the polishing efficiency and accuracy in a short time.
(2) The surface roughness in regard to Sa less than 2.5 nm on the flank and less than 6.25 nm on the edge and less than 45.8 nm on the nose could be achieved after 15 min of MABP.(3) The nose radii of inserts decrease slightly after MABP, indicating that MABP has a superior performance on form maintainability during polishing.(4) The MABP-processed inserts can extend the tool life.In the S136H steel cutting, batch-polished inserts have a tool life of up to 1.75 times as long as unpolished inserts.

Figure 1 Figure 2
Figure 1 Graphical illustration of the working principle of the MABP system

Figure 3 3 Figure 4
Figure 3 Schematic of the cutting experiment

Figure 9
Figure 9 3D nose profile (a) before and (b) after MABP, (c) Average nose radius of cutting insert comparison between before and after MABP

Figure 10 Figure 11
Figure 10 Batch polishing performance: (a) Photograph of the ceramic cutting inserts after batch polishing, SEM micro-images of cutting insert (b), (c) edge and (d), (e) flank after batch polishing

Figure 12
Figure 12 Height roughness parameters and radius of inserts nose before and after batch polishing

Figure 13
Figure 13 Changes in the maximum flank wear with cutting time

Table 1
Features of the ceramic cutting insert