EDM Wire Cutting of Polycrystalline Diamond (PCD) for Customized Cutting Tool Fabrication

LIU Kui1, a *, WU Hu1, b, NG Seow Tong1, c, KUMAR A Senthil2, d 1 Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, Singapore, 637662

2 Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575

a *kliu@simtech.a-star.edu.sg, bhwu@simtech.a-star.edu.sg, cstng@simtech.a-star.edu.sg, dmpeask@nus.edu.sg

Keywords: EDM, wire cutting, PCD.

Abstract. Polycrystalline diamond (PCD) has been widely used as cutting tools, moulds and dies, and tooling in the industry due to its excellent wear resistance, extremely high hardness, good chemical and thermal stability, good toughness and thermal conductivity, etc. However, machining of PCD is very challenge especially due to its high wear resistance and extremely high hardness. Since the binder materials of PCD are metals like cobalt and nickel, it is likely to machine PCD by electrical discharge machining (EDM). In this paper, wire cutting EDM was used to cut PCD blanks into small pieces for making customized PCD cutting tools. The wire-cut PCD was examined using an optical 3D non-contact profilometer and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscope (EDX). It was found that at the EDM wire-cut PCD edge there is a damaged layer,of which its thickness is around 100µm. The binder metal of PCD was evaporated by electricalsparking left some voids on the blank. Meanwhile, the chemical content percentages near the wire-cutPCD edge were also changed within the damaged layer. Both factors would largely degrade PCDperformance as a cutting tool. As such, the damaged layer needs to be totally removed by grinding inthe PCD tool fabrication. And cutting performance tests ware also conducted to evaluate its tool lifeand benchmarking with the commercial PCD tools.

Introduction

Polycrystalline diamond (PCD) has been widely used as cutting tools, moulds and dies, and tooling in the industry due to its excellent wear resistance, extremely high hardness, good chemical and thermal stability, good toughness and thermal conductivity, etc. PCD can be processed into very sharp cutting edge and very smooth tool geometry. This property makes its application of manufacturing components in high precision and very good surface quality. PCD cutting tools have very significant advantages compared with traditional cutting tools such as cemented tungsten carbide and high speed steel tools. Super hardness and high wear resistance allow it to be applied for tough materials’ removal such as nonferrous super alloys [1] and composite materials [2-3] used in aerospace and automobile industries. The durability of PCD tools also bring them in other industries like wood product manufacturing [4-5].

Many research works were done on applications of PCD tools. However, technology related to fabrication of PCD cutting tools is less introduced. Although machining of PCD is very challenge especially due to its high wear resistance and extremely high hardness. Since the binder materials of PCD are metals like cobalt and nickel, theoretically it can be machined by electrical discharge machining (EDM). Currently, electrical discharge machining (EDM) and grinding are the dominated manufacturing processes to create the tool geometries and cutting edges for PCD tools fabrication [6-9]. The cutting edge quality and surface integrity of various grades of PCD cutting tool blanks

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generated by electrical discharge grinding (EDG) was investigated [6]. PCD grain size dominated the EDG material removal rate and surface/edge roughness. EDM machinability of PCD was studied with the electrode vibrated by ultrasonic transducer [7]. The EDM efficiency was largely improved compared with the original no vibration EDM. EDM machinability and carrion suppressing of PCD was also investigated in water [8]. Ultrapure water with high resistivity can suppress the electrochemical corrosion of PCD binder metal. Grinding is also commonly used for manufacturing PCD tools. The diamond wheels for grinding of PCD tools was optimized through experimental studies on cutting edge quality and process efficiency in grinding PCD tools [9]. Some other methods for PCD tool fabrication were also investigated such as abrasive waterjet machining and laser machining [10-12]. However, these methods are not widely applied in tool industry for cutting of PCD blanks due to high erosion rate to the focusing nozzle and cost of abrasives in abrasive waterjet machining, as well as not cost effective in laser machining.

However, the fundamental studies on machining behaviors and characteristics for wire cutting EDM of PCD materials is not sufficient to guide the industry. In this paper, an experimental study will be carried out on EDM wire cutting of PCD blanks to investigate their cutting behaviors and characteristics. The EDM wire-cut PCD samples was examined using an optical 3D non-contact profilometer and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscope (EDX).

Experimental Details

Traditionally, polycrystalline diamond (PCD) tool blanks consist of a layer of micro sized man-made diamond particles, sintered and integrally bonded to a cemented tungsten carbide substrate using a high temperature high pressure process. In PCD cutting tool fabrication, PCD tool blanks are machined or diced into small PCD pieces mostly by wire-cut electrical discharge machining (EDM) or laser cutting, following by the brazing process, which is a critical process to join the PCD small pieces onto carbide (WC) inserts or steel tool holders together to form a complete cutting tool. The last process is the finishing process by EDM and grinding to generate final very smooth geometries and sharp cutting edges.

Work Material. Work material used in this study is a sintered PCD blank, Compax 1500, from Diamond Innovations, of which the average diamond grit size is 25µm and the volume of diamond is 94%. Compax 1500 diamond 360R58.0/1.6-15P tool blank is a disc with the diameter of 58mm, diamond layer thickness of 0.5mm and overall thickness of 1.6mm. Its mechanical properties are listed in Table 1.

Table 1 Material properties of Compax 1500 PCD.

Property Compax 1500

Compressive Strength [GPA]

7.5

Elastic Modulus [GPa] 1100 Transverse Rupture Strength [GPA] 0.85 Thermal Conductivity [W/mk] 600 Electrical Resistivity [Ω⋅cm] 4.0 Density [g/cm3] 3.9 Knoop Hardness – 3kg [kg/mm2] 4000

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Fig. 1 PCD tool blank and EDM wire-cut shape.

Wire-cut EDM. A Sodic A500 wire-cut electrical discharge machining (EDM) machine was used in this study. As shown in Fig. 1, PCD tool blank was mounted on the working table of the machine, of wich a ϕ200µm copper wire was used as the electrode and tapper water was used as the dielectric liquid. Fig. 2 (a) shows the wire-cut EDMed small PCD pieces and then brazed to a carbide insert blank as shown in Fig. 2 (b). The brazed PCD tool inserts were used as the workpiece for grinding to produce the final PCD cutting tools.

(a) Wire-cut small pieces (b) Carbide insert blanks

Fig. 2 Wire-cut EDMed PCD small pieces and carbide insert blanks.

Grinding Process. Brazed PCD tool inserts were firstly machined using EDM for roughing to remove extra material, error generated by blank preparation and damaged layer caused by wire-cut EDM. Fine grinding was applied to produce expected tool geometry and cutting edge quality. A 6-Axis CNC Ewag Ewamatic Line cutter grinder was used for brazed PCD tool inserts fine grinding as shown in Fig. 3. Diamond abrasive wheel 6A2 from Ultrawheels was used for the PCD tool inserts fine grinding, which is verified bond D5C125V-400 with a diameter of Ø150x5x8x30x40mm, super fine diamond grit size of 5µm and 125% abrasive concentration.

Fig. 3 6-Axis CNC Ewag Ewamatic Line cutter grinder.

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Results and Discussions

Fig. 4 shows the EDM wire-cutting PCD blanks: (a) top view and (b) side view. It is clearly indicated that there is a damaged layer, which shows aooarently distinct color from the center substrate material. This damaged layer’s thickness is appromaxely around 100µm. Due to extremely high temperture generated during EDM process, the PCD binding metal was melted and evaprated, and also material microstructures changed, such that the material properties have been changed and degraded. Thus the damaged layer must be totally removed by the subsequent process.

(a) Top view (b) Side view

Fig. 4 EDM wire-cutting damaged layer of the PCD small pieces.

During EDM wire cutting, the presence of water as the dielectric liquid brought electrolysis effects, which dissolves the metal cobalt, bonding material in PCD blanks. Fig. 5 shows the voids generated on the PCD pieces after EDM wire cutting. Diamond particles lost the connection strength and pulled out after the bonding material cobalt dissolved. Fig. 5 (a) shows the overview of PCD small pieces near the EDM wire cutting edge. Some voids were found on PCD small pieces near the cutting edge. Fig. 5 (b) shows the close view of one void under high magnification. The void formed following diamond particles pulled out.

(a) Overview (b) Close view

Fig. 5 Void created by wire cutting EDM at PCD small piece edge.

Fig. 6 shows the energy dispersive X-ray spectroscope (EDX) analysis results of PCD chemical element distribution around the EDM wire cutting edge on the PCD small piece. Horizontal axis is the distance (µm) from the analyzed point to the EDM wire cutting edge. Vertical axis is the atomic contents (%) of chemical elements found by EDX analysis. Three major chemical elements such as C, O and Co, dominate the PCD chemical contents near the EDM wire cutting edge. Some other chemical elements like Cu, Zn, W, Fe and Al are negligible that may be found due to the contamination during process by the substrate of PCD blank, EDM wire and fixture. The atomic content of oxygen drops

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tremendously from the EDM wire cutting edge up to away the edge of 100µm. Therefore the process affected zone by the EDM wire cutting was confirmed being around 100µm under the process conditions. It must be removed in the following process to ensure the tool quality. Else, the edge chipping is likely to happen.

Fig.6 PCD chemical elements distribution around the EDM wire cutting edge .

Summary

EDM wire cutting behaviors and characteristics has been studied on PCD tool blanks for customized PCD cutting tools. The EDM wire-cutting PCD was examined using an optical 3D non-contact profilometer and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscope (EDX). It was found that at the EDM wire-cutting PCD edge there is a damaged layer, of which its thickness is around 100µm. The binder metal of PCD was evaporated by electrical sparking left some voids on the blank. Meanwhile, the chemical content percentages near the EDM wire-cutting PCD edge were also changed within the damaged layer. Both factors would largely degrade PCD performance as a cutting tool. As such, the damaged layer must be totally removed by subsequent EDM and grinding processes in the PCD tool fabrication.

Acknowledgements

The authors would like to thank SIMTech funding the project and Mr. Kanno Shigeyuki for his advice on PCD material grinding.

References

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