Effect of AC Electric Field Polishing with Abrasives on cBN and Cermet Cutting Tool Wear

This paper describes the effect of AC electric field polishing (EFP) on the cutting tool wear. The authors have been studying the novel polishing method for wear reduction of the tool edge. The polishing apparatus with the free-abrasives control technology with AC electric field was developed. It is found that the increase of the flank wear using an AC electric field was suppressed. In this work for a detail investigation of the EFP mechanism, we investigated the effect of AC electric field on the cBN or Cermet tool edge surface after EFP. Then, the motion of abrasives around a tool edge in AC electric field was confirmed. In order to clarify the effect of AC electric field, we measured the surface roughness of the edge and the radius of curvature of the cBN and Cermet tool after polishing with and without AC electric field. The surface roughness with AC electric field was smaller than that without AC electric field, and the amount of the increase was the almost the same on the condition of both with AC electric field and without it. In addition, the motion of abrasives around a tool edge in AC electric field was evaluated. Results showed that diamond abrasives gathered around the tip of the cutting tool edge even when a tool edge and a polishing pad were away from each other in AC electric field. Finally, hypothesis of the effect of EFP on the edge surface was showed. It was that abrasives gathered by AC electric field at the portion of the tool edge not plunged in the polishing pad is able to remove the weak portion of the surface of the sintered body and only the strong bonded portion is likely to remain during polishing.


Improving a cutting tool life
Cutting has been one of the basic technologies of "Monozukuri". Generally, it is cut using a cutting tool which is four times harder than the work material. However, a cutting tool edge wears necessarily (1) . In order to improve a cutting tool life, a lot of researchers or engineers have been studied analysis or methods for a long cutting tool life (2)(3)(4)(5)(6)(7) . As typical methods, use of proper cutting conditions, cutting fluids or shape of the tool edge, a coating on the tool edge, a formation of micro texture on the tool edge and so on have been reported.
Kiyota et. al. (8) reported in the case of Inconel 718 that a notch wear on the cBN tool edge caused by the Built-Up Edge (BUE) extrusion. The behavior depended on the tool geometry. They showed that Optimization of the tool geometry needed to obtain a stable BUE extrusion. Usuki et. al. (9) showed in the case of Inconel 718 that the life of the cutting tool coated with a TiBON film was longer than that with a TiAlN film widely used on difficult-to-cut metals. Sakai et al. (10) investigated the effect of oil-immersion treatment on a carbide cutting tool wear reduction and its mechanism. As a result, the oil-immersion treatment carbide tool exhibited less wear than the non-treated tool. It was showed using surface analysis that sulfide or inorganic carbon was produced on the oil-immersion-treated surface, and it contributed to the reduction of friction on the tool surface. Sugihara et al. (11) developed a cBN cutting tool with the textured flank face, which showed that the textured flank face improved the wear resistance in the high-speed machining of Inconel 718. Chou et al. (12) showed that the wear resistance increases monotonically with decreasing cBN grain size. Chryssolouris (13) showed that the relation between tool life and cutting speed follows the Taylor tool life equation when flank wear is used as the life criterion.
Although a lot of method for improving a tool life have already been proposed, it has still been necessary to reduce the cost and improve workability. In addition, it may be needed to develop a lower cost and small processing device which can set into cutting production line.
We have been studying the AC electric field polishing (EFP) with abrasives on the cutting tool edge. Akagami et al. developed a free-abrasives control method by which the motion of abrasives can be controlled using Coulomb forces generated at low frequency in an AC electric field (14)(15)(16) .
By using the free-abrasives control principle, the polishing apparatus with AC electric field for a cutting tool edge smoothly and uniformly was developed. We showed that the flank wear width of cBN tool polished uniformly and smoothly by EFP method before cutting could reduce. Although the flank wear width in the case of the polished tool without an AC electric field increased suddenly during the cutting, the increase of the flank wear using an AC electric field was suppressed under the same condition. A smoother surface and a higher rate of crack removal were inferred to derive from the AC electric field effect (17,18) . Moreover, it will be needed the detail study about the wear reduction mechanism using EFP. Our goal is to clarify the mechanism of ACEFP on the tool edge.
In this work, in order to confirm the effect of AC electric field on the polishing, we investigated the difference of cBN or Cermet tool edge surface after polishing with and without AC electric field. In addition, in order to consider why the edge surface condition differed with and without AC electric field, the motion of abrasives around a tool edge in AC electric field was evaluated. Finally, the model of the effect of ACEFP on the tool edge surface was discussed.

AC Electric Field Polishing (ACEFP)
The ACEFP apparatus is showed in Fig. 1. In the apparatus, polishing can be accomplished uniformly using a numerical control (NC) system.
Alternating voltage is applied between the polishing plate and the insert holder during polishing treatment. Applying an AC electric field during polishing suppressed the scattering of abrasives and abrasives concentrates towards the tool cutting edge.
The holder is insulated and connected to the NC system drive shaft. Moreover, it is possible to drive the X, Z and theta axes simultaneously. An insert is fixed to the holder so that the edge to be polished faces the polishing pad.
The cutting edge is polished in a state plunged slightly in the polishing pad. The amount of the plunge is the order of tens to hundreds of micrometers. The tool holder can rotate at a constant amount of the plunge by the NC system.
As presented in Fig. 2, because the tool rotates from 0 to 120 degrees and from 120 to 0 degrees back as one reciprocation, the entire edge of the tool can be polished uniformly and smoothly. In this experiment, the reciprocation for one polishing is defined as one polishing repetition number, and the polishing is performed repeatedly. Fig. 3 shows a schematic diagram of ACEFP. When a polishing slurry containing abrasives dispersed in insulating oil is used and an AC voltage is applied between the tool and the polishing plate, the electric field concentrates at the edge of the tool, abrasives gather around the tool edge. Polishing is performed by rotating the tool holder and changing the height of the tool holder so that the tool cutting edge intrudes the polishing pad to a constant depth.

Investigation of tool edge surface after polishing 2.1 Experimental method
We reported in our previous work (17) that there was the difference with the tool edge after cutting between when the polishing with AC electric field and when without it, even if the same polishing treatment is performed. In order to confirm the effect of the AC electric field on the tool edge before cutting, we investigated the surface of the tool edge after polishing with AC electric field or without it. cBN (Sumitomo Electric Hard Metal; TNGA160408-BN250) and Cermet (Sumitomo Electric Hard Metal; TNGA160408-T1500A) tool were used. By using two types of tool materials, we thought that it would be possible to consider how the effect of the electric field differs depending on the material. Using the apparatus shown in Fig. 1, the tool edge was polished with AC electric field or without it. Table  1 shows polishing conditions. The shape of the cutting tool was an equilateral triangular. Abrasives were # 2000 diamond grains, and the polishing oil was silicone (Shinetsu silicone KF-96).
After the polishing treatment, the tool edge was observed using a laser microscope (KEYENCE VX-200) or a scanning electron microscope (JEOL JSM-6700F). We used arithmetic average roughness Ra as an index to compare surface roughness. As shown in Fig. 4, the surface roughness was measured at 5 points in the range of 50μm on the rake face side of the boundary between the flank and the rake face of the cutting edge. In addition, the radius of curvature of the tool edge at each polishing times was measured using the laser microscope.

The difference of tool edge surface after polishing with AC electric field and without it
Laser micrographs of the cBN tool edge at each polishing times are showed in Fig. 5. The cBN tool cutting edge before polishing, that is, the tool edge as received from the manufacturer, was confirmed to be finished by grinding. The edge has small scratches because of finishing by grinding. After polishing, the tool edge was smooth and scratches on the surface of the edge reduced. In addition, it was confirmed that the surface tended to become smoother and the polishing amount of the edge increased with applying AC voltage. Fig. 6 shows the laser micrograph of the cermet tool edge at each polishing times. After polishing, the tool edge was also smooth and scratches on the surface of the edge reduced. In the case of cermet tool after polishing, the difference of the edge surface between with AC electric field and without AC electric field was clearly observed.
Comparing the tool edges of cBN and cermet, the cermet has a larger roundness. This could be due to the difference in hardness between the tool materials Boron nitride and Titanium nitride. This was clearly seen from SEM micrographs of two polishing cycles (Figs.7 (a) and (b) for Cermet tool edge, ref. (17) for cBN tool edge).
Moreover, in order to clarify quantitatively the effect of AC electric field, we measured the surface roughness of the edge and the radius of curvature of the tool edge after polishing with and without AC electric field at each polishing times.  The surface roughness of the cBN and Cermet tool edge are showed in Fig. 8 and Fig. 9 show Comparing with the same polishing times, it was found that the surface roughness of both cBN and Cermet with AC electric field was smaller than that without AC electric field. Fig. 10 and Fig. 11 show the radius of curvature of the cBN and Cermet tool edge at each polishing times. Comparing the same polishing times, the radius of both cBN with AC electric field tended to be slightly larger than that without AC electric field. In the case of Cermet, the radius of curvature increased on the both with AC electric field and without it. The amount of the increase was the almost the same on the condition of both with AC electric field and without it.
From these measurement results of the surface roughness and the radius of curvature, it was found that the AC electric field was effective on the surface roughness, but not on the shape change of the cutting edge.

Observation method of abrasives motion
In order to consider why the electric field is effective on the surface roughness, we devised a method to quantitatively understand how much the abrasive grains gather during electric field abrasive grain controlled polishing, and to what position of the blade the abrasive grains come into contact.
As next step in this work, we confirmed the motion of abrasives with and without AC electric field between the tool and the polishing plate. Due to make it easier to observe the motion, experimental setup without rotating the tool holder was used as shown in Fig. 12. The situation of abrasives was observed when the tool edge is plunged into or when it is positioned above a polishing pad. The cutting tool used was a cBN cutting tool (Sumitomo Electric Hard Metal TNGA160408-BN250). The amount of the tool edge plunge is plus (+) 100μm to minus (-) 1000μm, with + as the amount of the plunge into the polishing pad and with -as when the polishing pad and the tool edge are positioned separately. The applied voltage was 1.2 kV to 4.0 kV and the applied frequency was 8 Hz. The voltage application time was 60 seconds. Abrasives gathering width shown in Fig. 13 was used as an index for evaluating the motion of abrasives.

Gathering of abrasives on the polishing pad using AC electric field
The amount of the intrusion dependence on the abrasive concentration under various AC voltage application conditions is showed in Fig. 14. Figs. 14(a) is photos of abrasive concentration situation under various AC voltage application. It was confirmed that abrasives gathered around the triangular shape of the cutting tool edge when the tool edge plunge amount was plus.
Abrasives were concentrated at the tip of the tool edge even when the cutting edge plunge amount was 0μm, that is, when the tool edge and the polishing pad were in slight contact. In addition, the abrasives were concentrated under the tip of the cutting edge even when the amount of the plunge was minus.
For example, when the applied voltage is 4kV and the distance between the cutting edge and the pad is away from 500μm, the polishing slurry began to bridge between the tool edge and the pad and abrasives were concentrated around the tool edge when the voltage was started.
Figs. 14(b) is The amount of the intrusion dependence on the abrasive concentration width under various AC voltage application. The abrasive concentration width was from 1.8mm to 3.0mm.
From above observation results, it was found that the cutting edge and the polishing pad were separated by more than 200μm. This means that when performing electric field abrasive grain polishing, the tool edge is close to the polishing pad by about 100μm, however the abrasive grains also gather and contact around the blade tip that is not intruded in the polishing pad.

Discussion: Effect of AC Electric Field on the Polishing
One of the purpose of ACEFP is to improve the process efficiency. It was reported that when the polishing was performed with applying AC voltage, compared with no AC electric field, the polishing rate is improved by more than 20% using the effect of abrasives concentration (19) . On the other hand, we found that the AC electric field has a great influence on the surface roughness, but not on the shape change of the cutting edge. We would like to consider that what kind of the action does the AC electric field caused the tool edge surface during polishing.
In this study, we confirmed that the gathering phenomena around the tool edge under an AC electric field by changing the amount of the plunge between the polishing pad and the tool edge. As shown in Fig. 14, it was found that the abrasives gathered around the tool edge even if the tool edge and the polishing pad were not in contact with each other. In the case of EFP, as shown in the schematic model of Fig. 15, when the tool edge is being polished, the abrasives repeatedly can contact even at the portion of the tool edge not submerged in the polishing pad. It is considered that this motion of abrasives may be affected on the tool edge surface.
In our previous research, it was found that the amount of the flank wear on the tool edge suddenly increased during cutting in the case of without AC electric field. In the case of with AC electric field, even if polished under the same conditions as with an electric field, the amount of the flank wear did not increase. This suggests that the surface polished with applying an AC electric field is a more stable. Fig. 16 shows a schematic model of the tool edge surface with and without AC electric field. The cBN and cermet tool are a sintered body, and the main component is harder abrasives such as cBN, TiN or TiC, which held by a binder (20) . It is considered that the abrasives gathered by AC electric field at the portion of the tool edge not submerged in the polishing pad is able to remove the weak portion of the surface of the sintered body and only the strong bonded portion is likely to remain. Direct evidence for the model shown in Fig. 16 has not yet been obtained. However, in our previous work, we confirmed that the tool edge surface after cutting was different due to the difference between with and without AC electric field. We have previously shown in Ref. (18) that in cutting SUJ2 with a cBN tool, there were adherents and almost no visible chips on the tool edge surface applying AC electric field. Although this experimental result may not be a direct evidence data, we consider that the deference of the edge surface between with AC electric field and without it shows the effect of AC electric field. We plan to proceed with micro surface analysis of the cutting edge in order to obtain direct evidence.

Conclusions
In this work, in order to confirm the effect of AC electric field on the polishing, we investigated the difference of cBN or Cermet tool edge surface after polishing with and without AC electric field. From these measurement results of the surface roughness and the radius of curvature, it was found that the AC electric field was effective on the surface roughness, but not on the shape change of the cutting edge.
In addition, in order to consider why the edge surface condition differed with and without AC electric field, the motion of abrasives around a tool edge in AC electric field was evaluated. Diamond abrasives gathered around the tip of the cutting tool edge even when a tool edge and a polishing pad were away from each other in AC electric field. Finally, the model of the effect of ACEFP on the tool edge surface was proposed. It was considered that abrasives gathered by AC electric field at the portion of the tool edge not plunged in the polishing pad is able to remove the weak portion of the surface of the sintered body and only the strong bonded portion is likely to remain during polishing.
In the future, in order to further verify this model, the confirmation of the bonding state by the tool edge surface analysis will be performed.