Cutting performance of micro-textured PCBN tool

Lin Fan,Zilong Deng, Xingjun Gao,and Yan He

AFFILIATIONS School of Mechanical Engineering,Liaoning Petrochemical University,Fushun 113001,China

ABSTRACT To study the effect of micro-texture on the cutting performance of polycrystalline cubic boron nitride (PCBN) tools,five types of microtextures (circular pits,elliptical grooves,transverse grooves,composite grooves,and wavy grooves) were applied to the rake surface of PCBN tools by an optical fber laser marking machine.Through a combination of three-dimensional cutting simulations and experiments,the influences of micro-texture on chip-tool contact area,cutting force,chip morphology,shear angle,and surface roughness during the cutting process were analyzed.The results indicated that the chip-tool contact area and cutting force of both non-textured and micro-textured tools increased with increasing cutting speed,while the shear angle decreased with increasing cutting speed.The chip-tool contact area and cutting force of the five types of micro-textured tools were smaller than those of the non-textured tool.The chip-tool contact area and cutting force obtained by the wavy -groove micro-textured tool were the smallest.The chip radius produced by the five types of micro-textured tools was smaller than that produced by the non-textured tool,and the chip morphology was more stable.The transverse groove micro-textured tool had a better chip breaking effect.The chip radius generated by the elliptical groove micro-textured tool was 0.96 cm,while that generated by the wavy-groove tool varied from 0.55 cm to 1.26 cm.The presence of a micro-texture reduced the surface roughness of the workpiece by 11.73%-56.7%.Under the same cutting conditions,the five types of micro-textured tools gave a smaller chip-tool contact area,cutting force,chip radius,and surface roughness and a larger shear angle than the non-textured tool.In addition,the elliptical groove and wavy-groove micro-textured tools had better cutting performance.

KEYWORDS Micro-textured tool,Chip-tool contact area,Cutting force,Chip radius

I.INTRODUCTION

A micro-textured tool is one in which the rake face has a pattern of micro-pits,micro-grooves,or other micro-structures.Many studies have shown that micro-textured tools can improve cutting performance and extend tool life.2-6Among the various geometrical parameters and distribution characteristics of such tools,it has been found that the type of texture is the main factor affecting cutting performance.

Zhang et al.7used micro-textured tools with linear grooves,sinusoidal grooves,or rhombic grooves to dry-cut Ti-6Al-4V alloy,and their results indicated that the greatest reductions in cutting force and tool wear and the best chip morphology were achieved with linear-groove micro-textured tools.Zheng et al.8performed cutting experiments with linear,rhombic,and sinusoidal groove textures on the rake face of cemented carbide tools,and the results under the same cutting lubrication conditions also showed that the tool with the linear texture had the best cutting performance.Yang et al.9compared the machining performances of tools with four types of micro-textures (pits,longitudinal grooves,transverse grooves,and cross grooves) in combination with friction experiments.They found that the presence of a micro-texture changed the stress distribution of the workpiece and improved the wear resistance of the tool.Moreover,the pit micro-texture had the best effect in reducing stress concentration.Orra and Choudhuryl0investigated ceramic tools with three different micro-textures:horizontal,vertical,and elliptical.They found that a vertical micro-texture impregnated with solid MoS2 lubricant gave the best cutting performance.Yu et al.11carried out a high-speed cutting experiment on SUS304 stainless steel using cemented carbide micro-textured tools with pit or groove micro-textures.Their results showed that a pit micro-texture and a groove micro-texture parallel to themain cutting edge gave the best cutting performance.Duanet al.12designed groove-type micro-textured tools with different parameters by means of orthogonal experiments.The results of cutting experiments with these tools showed that a groove-type microtexture with appropriate parameters could effectively reduce the cutting force and friction coefficient Furthermore,it has been found that the use of finit element analysis (FEA) techniques to study the cutting performance of micro-textured tools13–15can improve analysis efficiency reduce the cost of testing,and shorten test cycles.

Texturing technology has also been applied to polycrystalline cubic boron nitride(PCBN)tools,and it has been found that microtexturing can improve wear resistance,16reduce cutting force17and cutting temperature,18and improve surface quality.19However,there have been relatively few studies of the influenc of texture shape on the cutting performance of PCBN tools.

In this paper,with the aim of further improving the cutting performance of micro-textured PCBN tools,pit-shaped and transversegroove textures are optimized,and the influenc of texture shape on cutting performance is studied.FEA simulations and cutting experiments are carried out to study the influence of different micro-texture shapes on chip–tool contact area,cutting force,chip morphology,shear angle,and surface roughness of a PCBN tool in dry cutting of Cr12MoV.

II.EXPERIMENTAL MATERIALS

A.Tools and workpiece

The tools used in this experiment were all FBS9500 PCBN tools,the tool material hardness was 2700–2900 HV,the binder was a ceramic bond,the tool rake angle was 0° ,the back angle was 3° ,and the tool nose radius was 0.4 mm.The workpiece material was a heat-treated Cr12MoV round bar with a hardness of 52 HRC and a workpiece diameter of 50 mm.

B.Design and preparation of micro-textured tools

Five types of micro-texture were applied to the rake face of each PCBN tool:circular pits,elliptical grooves,transverse grooves,composite grooves,and wavy grooves.An AGXCX-2001 fibe laser marking machine was used to apply these micro-textures.The processing parameters were as shown in Table I.

The pit diameter and pitch of the circular-pit texture were both 70μm.The groove width of the elliptical-groove texture was 70μm,the length was 150μm,and the groove spacing was 70μm.The groove width of the transverse-groove texture was 40μm,and the groove pitch was 70μm.The groove width and groove spacing of the composite-groove texture were 40μm and 70μm,respectively.The groove width of the wavy grooves was 40μm,and the groove pitch was 70μm.A Hitachi S-3400N scanning electron microscope(SEM)was used to observe the micro-morphology of the fiv types of micro-texture,as shown in Fig.1.

FIG.1.SEM images of fiv types of micro-textured tools:(a)circular pits;(b)elliptical grooves;(c)transverse grooves;(d)composite grooves;(e)wavy grooves.

FIG.2.Cross section of each layer as the tool cuts into the workpiece along the feed direction.

III.CUTTING SIMULATIONS AND TESTS

A.Cutting simulations

1.Model establishment

A 1 mm × 1 mm area of the tool tip was selected for simulation.The tool model was drawn using UG three-dimensional design software and subsequently imported into ABAQUS software,and the workpiece model was then drawn.The initial temperatures of the tool and the workpiece were set to 20° C,and the parameters selected for the cutting simulation were cutting speeds of 110 m/min,130 m/min,150 m/min,and 170 m/min,a feed rate of 0.1 mm/rev,and a depth of cut of 0.3 mm.

The PCBN tool tip had a 0.4 mm rounded corner,as shown in Fig.2,and when the tool was cutting into the workpiece,this rounded corner gave a uniform cross-sectional shape to the chips from the firs to the fift layer.Since the chip shape after the fift layer could be kept consistent,the process of the tool cutting the fift layer of the workpiece was simulated,giving results consistent with those of an actual cutting experiment.The cutting simulation model is shown in Fig.3.The material performance parameters of the Cr12MoV and PCBN are listed in Table II.

2.Material constitutive model

The Johnson–Cook constitutive model is valid from room temperature to the melting temperature of a material and gives a comprehensive description of the effects of strain hardening,strain rate hardening,and temperature softening on the flo stress of the material.It is especially suitable for metallic materials under high strain rates.

FIG.3.Cutting simulation model.

The Johnson–Cook constitutive equation is

whereεis the equivalent plastic strain,is the plastic strain rate,is the reference strain rate,Tris the ambient temperature,Tmis the melting temperature of the material,Ais the yield strength of the material under quasi-static conditions,Bis the hardening strength of the material,nis the strain strengthening coefficien of the material,Cis the strain rate strength coefficient andmis the temperature sensitivity coefficient Table III shows the parameters of the Johnson–Cook model for Cr12MoV.

3.Fracture criterion

The Johnson–Cook failure criterion takes account of strain,strain rate,pressure,and temperature,with the material failure parameter being define as

where Δεplis the equivalent plastic strain increment in an integration period andis the equivalent fracture strain.Whenωis greater than or equal to 1,the material fails completely,and the equivalent fracture strain can be expressed as

whereσmis the three-directional normal stress,is the equivalent stress,andd1,...,d5are the failure parameters.The material failure parameters of Cr12MoV are shown in Table IV.

TABLE II.Material performance parameters of Cr12MoV20 and PCBN.21

TABLE III.Parameters of Johnson–Cook model for Cr12MoV.20

TABLE IV.Material failure parameters of Cr12MoV.22

FIG.4.Device for cutting experiment.

B.Cutting test conditions

The cutting experiment was performed using a CAK6136 CNC lathe from the Shenyang No.1 Machine Tool Factory.The processing method was continuous dry turning with a cutting speed of 130 m/min,a feed rate of 0.1 mm/rev,and a depth of cut of 0.3 mm.The cutting length of each group of tests was 100 mm,and each group of cutting experiments was carried out three times to ensure consistency of the experimental results.The experimental device is shown in Fig.4.A YDC-III89A piezoelectric turning dynamometer was used to detect the three-directional cutting force during the cutting process.A Time 3110 Surface Roughometer was used to measure the machined surface roughness.

IV.ANALYSIS OF SIMULATION AND TEST RESULTS

A.Real chip–tool contact area

Figure 5 shows the real chip–tool contact areaAwat different cutting speeds.With increasing cutting speed,Awshows a decreasing trend for all the tools.At the same cutting speed,Awof the non-textured tool is the largest and that of the wavy-groove tool is the smallest.

FIG.5.Real chip–tool contact area vs cutting speed.

whereawis the cutting width,is the real chip contact length,lfis the nominal chip contact length,kis the number of micro-textures in the chip contact area,andl0is the micro-textured groove width.

It can be seen from Eq.(4)that under the same cutting conditions,the presence of a micro-texture makes the real chip–tool contact lengthof a micro-textured tool smaller than that of the nontextured tool.Compared with the real chip–tool contact areaAwof the non-textured tool,those of the micro-textured tools with circular pits,elliptical grooves,transverse grooves,composite grooves,and wavy grooves are reduced by 3.6%–4.1%,4.9%–8.1%,2.1%–3.7%,0.1%–1.9%,and 5.7%–11.3%,respectively.

B.Cutting force

FIG.6.Cutting model of micro-textured tool.

FIG.7.Total cutting force vs cutting speed.

Figure 7 shows the total cutting force on the rake face of the tools obtained by cutting simulations at different cutting speeds.When the cutting speed increases,the total cutting force on the tool decreases.The total cutting force for each of the fiv types of microtextured tool is less than that of the non-textured tool at the same cutting speed.The wavy-groove micro-textured tool has the smallest total cutting force,followed by the elliptical-groove tool.Compared with the non-textured tool,the total cutting forces of the microtextured tools with circular pits,elliptic grooves,transverse grooves,composite grooves,and wavy grooves are decreased by 6.1%–12.2%,7.1%–14.4%,3.5%–9.9%,2.1%–7.2%,and 9.7%–16.1%,respectively.

As shown in Fig.8,the three-directional cutting forces obtained from the cutting experiments indicate that the presence of a microtexture has a significan influenc on the main cutting forceF(Z)and the radial cutting forceF(Y),but no obvious influenc on the axial forceF(X).For all fiv types of micro-textured tool,F(Z)andF(Y) are smaller than those of the non-textured tool.The cutting force behavior of each tool is consistent with the cutting simulation results.The main cutting forceF(Z)and the radial cutting forceF(Y) of the wavy-groove micro-textured tool are both the smallest among all the tools.Figure 9 shows a comparison of the total cutting force at a cutting speed of 130 m/min between the cutting simulation and cutting test results.It can be seen that the total cutting force error is 11%–18%,which is within the acceptable range,and thus the simulation model is validated.

FIG.8.Experimental results for three-directional cutting force.

During the cutting process,the frictional forceFfand total cutting forceFrcan be expressed as

whereτcis the frictional shear strength andβis the friction angle.It can be seen from Eq.(6)thatτcandβremain basically unchanged,and the total cutting force is proportional to the real chip–tool contact area.The presence of a micro-texture makes the real chip–tool contact area smaller,and so the total cutting force of a microtextured tool is smaller than that of a non-textured tool.At the same time,during the cutting process,it is easier for chips to enter the textural grooves,resulting in a secondary cutting phenomenon,17which increases the total cutting force.The shape of a wavy groove parallel to the cutting edge is more conducive to chip outflow and also provides the minimum real chip–tool contact area,and so the total cutting force of the wavy-groove micro-textured tool is the smallest.It is not easy for chips to enter a circular-pit micro-texture,and the real chip–tool contact area of the transverse-groove micro-texture is small.Surprisingly,the elliptical-groove micro-texture combines the advantages of the others,and so its total cutting force is smaller.

C.Chip morphology

FIG.9. Comparison of total cutting force between cutting simulation and cutting test at a cutting speed of 130 m/min.

FIG.10.Chip morphology:(a)non-textured tool;(b)circular-pit micro-textured tool;(c)elliptical-groove micro-textured tool;(d)transverse-groove micro-textured tool;(e)composite-groove micro-textured tool;(f)wavy-groove micro-textured tool.

In the actual cutting process,the factors affecting chip shape are relatively complex,and the chip geometry cannot be completely consistent.Therefore,typical chips with stable shape parameters are selected from each group of chips for study,as shown in Fig.10.Owing to the obvious winding phenomenon in the cutting process,the chip from the non-textured tool is irregular in shape and its edges have a spiral burr.The chips from the micro-textured tools are continuous spirals,with the transverse-groove tool producing a short spiral chip and exhibiting the best chip breaking effect.The chip from the non-textured tool has the largest radius of 1.56 cm.The chips from the circular-pit,elliptical-groove,transverse-groove,and composite-groove micro-textured tools have uniform chip crimp radii,among which the chip from the elliptical-groove tool has the smallest radius of 0.96 cm.The chip from the wavy-groove tool has a wide range of radii from 0.55 cm to 1.29 cm.

As shown in Fig.6,during the cutting process,the total cutting forceFron the rake face and the cutting resistanceFwon the chip shear face are equal in magnitude,opposite in direction,but not collinear,and this leads to the creation of a bending moment and consequent chip curling.The chip radius from the wavy-groove micro-textured tool varies along the chip,indicating that the cutting force of the tool fluctuate during the cutting process.According to the cutting simulation and experimental results,the total cutting forceFrof the elliptical-groove micro-textured tool is also reduced,which causes the bending moment responsible for chip curling to increase.Therefore,the chip becomes more curled,reducing the chip radius.It should be noted that the analytical method used here can be easily transferred to other micro-textured tools.

D.Shear angle

As shown in Fig.6,the chip deformation coefficienξcan be expressed as

whereacis the cutting thickness andahis the chip thickness.The shear angleϕcan be expressed as

whereγis the rake angle of the tool.In the present study,γ=0° ,and so the shear angle can be expressed as

According to the selected cutting amount,the cutting thicknessacis 0.1 mm.After the cutting simulation,the chip thickness of each tool was measured at three places,and the average value was taken to calculate the shear angle.Figure 11 shows the variation of the shear angle with cutting speed.It can be observed that the shear angle of each tool increases with increasing cutting speed.Under the same cutting conditions,the shear angle of the nontextured tool is smaller than those of all the micro-textured tools.The shear angles of the wavy-groove and elliptical-groove microtextured tools are the largest,followed by those of the transversegroove and circular-groove tools,and that of the composite-groove tool is the smallest.

E.Surface roughness

FIG.11.Shear angle vs cutting speed.

FIG.12.Surface roughness Ra of the machined surfaces obtained by the different tools.

As shown in Fig.12,the surface roughnessRaof the machined surface obtained by the non-textured tool is the largest.A microtexture changes the direction of chip flo to a certain extent,improves the bonding phenomenon on the machined surface,and reduces the roughness of the machined surface.The surface roughness obtained by the elliptical-groove micro-textured tool is the smallest,being reduced by 56.7%compared with that from the nontextured tool.The wavy-groove micro-textured tool reduces the surface roughness by 54.6%,the transverse-groove tool by 36.1%,the circular-pit tool by 29.8%,and the composite-groove tool by 11.3%.

V.CONCLUSIONS

Five types of micro-textures were applied to PCBN tools by a fibe laser marking machine.Using FEA simulations and cutting experiments,the influenc of micro-texture on chip–tool contact area,cutting force,and chip morphology was studied,and the following conclusions were drawn:

1.The chip–tool contact area of the non-textured tool and the fiv types of micro-textured tools increased with increasing cutting speed,while the cutting force decreased with increasing cutting speed.

2.The presence of a micro-texture reduced the chip–tool contact area of the tool during the cutting process,thereby reducing the cutting force.Among the fiv types of micro-textures,the cutting force of the transverse wavy-groove micro-textured tool was the smallest,followed by the elliptical-groove tool.

3.The micro-textured tools reduced the chip radius.The transverse-groove micro-textured tool had a better chip breaking effect.The chip radius generated by the elliptical-groove micro-textured tool was 0.96 cm,while that generated by the wavy-groove tool varied from 0.55 cm to 1.26 cm.

4.The presence of a micro-texture increased the shear angle,with the influenc of wavy grooves and elliptical pits on shear angle being the most significant

5.The presence of a micro-texture reduced the surface roughnessRaof the machined surface,with the elliptical-pit and wavy-groove micro-textured tools reducingRaby 56.7%and 54.6%,respectively,compared with the non-textured tool.

ACKNOWLEDGMENTS

This work was supported by the Basic Scientifi Research Program of the Educational Commission of Liaoning Province,China(Grant No.L2017LQN024).