Influence of Coolant on Chip Forming in Gun?Drill Based on Fluid?Solid Coupling Method

，，，，

1.College of Mechanical Engineering，Yancheng Institute of Technology，Yancheng 224051，P.R.China；

2.College of Mechanical and Electrical Engineering，Nanjing University of Aeronautics and Astronautics，Nanjing 210016，P.R.China；

3.College of Mechanical and Electrical Engineering，Jiangsu University，Zhenjiang 212013，P.R.China；

4.College of Mechanical and Electrical Engineering，Nanjing College of Information Technology，Nanjing 210046，P.R.China

（Received 10 March 2020；revised 12 May 2020；accepted 20 May 2020）

Abstract: Chip shape is one of the important factors that affect the processing quality of the deep hole. The flow field of 17 mm standard gun-drill is simulated by taking the coolant pressure as a single factor variable，and the influence of coolant pressure on chip forming is discussed by combining with experiments in this paper. The results show that at the initial stage of chip forming，the flow of cutting fluid will intensify the lateral crimp of chips，and then affect the crimp radius of the chip and the number of turns of the crimp screw. The lateral crimp degree increases first and then decreases with the increase of coolant pressure，and the crimp degree is the smallest at 3 MPa. In addition，during the chip removal process，the stream shrinking in the flow field is the main influencing factor that drive and force the chip to break again，and their influence on the chip removal and chip breaking is proportional to the coolant pressure.

Key words：gun drill；chip deformation；Ti6Al4V titanium alloy；fluid-solid coupling

0 Introduction

Ti6Al4V is a（α+β）type of titanium alloy with excellent comprehensive properties，which is a typical difficult-to-process material with high specif?ic strength，low density and good corrosion resis?tance［1］. It is widely used in aerospace，health care，petrochemical and other fields［2］.At present，domes?tic and foreign scholars mainly focus on turning and milling of titanium alloy，while there are relatively few studies on drilling，especially deep hole drilling.However，in the machining field，deep hole process?ing accounts for about 1/8 of the total workload［3］.With the special needs of equipment manufacturing，the demand for drilling small-diameter deep holes in difficult-cut-to materials becomes increasingly prom?inent.

Deep hole machining has a typical closed or semi-closed cutting environment. During cutting，there are some problems such as blocked chips，ex?truded chips，cutting heat accumulation and torsion?al load increase caused by longer broken chips and poor chip removal. The deterioration of cutting per?formance will not only affect the machining quality of hole wall surface，but also distort the drill shaft and bit in serious cases［4］. The above problems are especially obvious in the drilling of small diameter deep hole with single lip gun-drill［5］，because the overall structure of chip removal space is narrow and long，which is composed of V-channel and hole wall. To solve the problem of chip blocking in the deep hole，Ref.［6］analyzed the critical condition of chip blocking by establishing the mathematical mod?el of chip force，and found that increasing the peak pressure of flow field can enhance the impact force on chip，and then enhance the chip removal ability.Meanwhile，Ref.［7］ determined that the coolant pressure in the V-channel was proportional to the in?let pressure based on the prediction model of flow rate and pressure. The establishment of the mathe?matical model is based on the assumption of model simplification，which cannot specifically describe the flow velocity behaviors and rheological property of the fluid during the chip movement. In addition，because the machining environment of gun drill is in?accessible，it is impossible to measure the flow and pressure distribution of cutting fluid. Therefore，some scholars use CFD simulation to study the fac?tors affecting cutting quality. For example，Refs.［8-9］used CFD simulation to quantify the fluid rheo?logical characteristics and chip transport behavior of high-pressure coolant，and determined the effects of the nose grind contour，coolant hole configuration and shoulder dub-off angle on the application effi?ciency of coolant and the service life of gun-drill.Ref.［10］found that the change of flow dead zone near the cutting edge of the drill has nothing to do with the flow rate of coolant. Combined with the test，it is found that the tool life and machining qual?ity are significantly improved with the increase of in?let pressure.

These results enrich the research foundation of the drilling flow field，but the influence of coolant oil pressure on chip forming has not been analyzed in detail. Therefore，based on the study of chip spi?ral forming，this paper combined the fluid-solid sim?ulation of coolant-chip and the inlet pressure test to carry out further research，in order to study the ef?fect of the change of coolant pressure on chip form?ing. In addition，by reconstructing the flow path of coolant in the V-channel，the source of coolant that dominates chip movement and“secondary fracture”is determined，and the influence of coolant pressure change on it is analyzed. The relevant research re?sults can provide some reference for the reasonable selection of coolant pressure in the deep hole gundrilling，further improving the chip breaking and chip removal ability of gun-drill.

1 Analysis of Chip Forming Process

Gun-drill is a typical double-edged tool with a theoretical rake angle of 0°. According to the previ?ous research，the chip forming process can be divid?ed into four stages：lateral curling，spiral curling，chip forming，and root fracture.

（1）During the gun drilling process，the chips produced by the inner and outer edges will move along the direction perpendicular to the cutting edge on the front face，and generate shear slip at the junc?tion of the chip and the edge，which causes the chips up?curl natually，as shown in Fig.1. Due to the different cutting speeds of the inner and outer edges，the lateral curl of the outer chip is larger，and it has a certain squeezing effect on the inner chip.

Fig.1 Chip forming process of inner and outer edges

（2）As the chip forms，it will touch pointC1on the V-channel wall，and then it will curl up for the first time after receiving resistance from the wall.The second curl is caused by the contact between the chip and the hole wall，andC2is the contact point.rcis the theoretical curling radius of a chip，which is related to the machined aperture and tool size，as shown in Fig.2.

Fig.2 Sketch of up-curling of chip

（3）As the drilling process goes on，the chips continue to grow. When it reaches the rake face of gun-drill bit again，the first round of curling of the spiral chips is completed，and the chips can form multiple spiral curl cycles before it fractures.

（4）According to the existing research［7，10］，the chip up-curl radius isru1in the initial stage of machin?ing. Under the combined action of the normal force of the hole wall，V-channel and coolant pressure，the curling radius increases fromru1to the critical value of fractureru2，as shown in Fig.3.

Fig.3 Increase in up-curl radius due to coolant pressure leading to chip breaking[3]

At this time，chip root will crack first. After that，with the cyclic crimping of chips，Ftwill con?tinue to increase until the ultimate tensile forceFbreached，and the crack will gradually expand until the complete fracture. Eq.（1）shows that the ulti?mate tensile force is mainly associated with the feed，cutting width and material characteristics.

whereαis the tensile strength of chip，s1the cutting width，andfthe feed.

During the process of chip forming，the spiral chip exerts normal forces at pointsC1andC2asFnc1andFnc2for each loop of the spiral chip due to elastic recovery inside the hole. In this paper，the chip force is simplified along the axial and radial direc?tions of the gun drill，and the component forceFtis represented byFZandFY，as shown in Fig.4.

Fig.4 Impact force of coolant on spiral chip surface[6]

According to Eqs.（2）—（3），it is determined that the impact force of the cutting fluid is related to the factors such as the chip curl radiusrc，the spiral leadL，the number of turnsn，the impact force of the fluid ΔΡ，the angle of the jetα，and the angle of the chip cross-sectionθ，among which the impact force ΔΡis a controllable parameter for machining.Therefore，this paper chooses the inlet pressure as a variable to explore its influence on chip forming.

2 Experiment Design

2.1 Test setup

Through analyzing the spiral forming process and fracture mechanism of chip during the gun drill process，we find that the shape of chip is closely re?lated to the material of the workpiece and the tech?nological parameters. In order to analyze the influ?ence of coolant inlet pressure on the morphology of Ti6Al4V titanium alloy chip，the following experi?ments are designed.

The drilling machine is NCS1600 CNC，as shown in Fig.5，with drilling depth up to 1 600 mm and rotating speed up to 6 000 r/min. SANDWIK standard single-edged gun drill is selected for the ex?periment，as shown in Fig.6. The material of drill tip is cemented carbide-P20. Ti6Al4V titanium al?loy is a typical difficulty processing materials，and its processing range is relatively narrow. Therefore，we design the following experimental parameters，as shown in Table 1［11］.

Fig.5 NCS1600 CNC deep-hole drilling machine

Fig.6 Φ17 mm standard gun drill

Table 1 Test parameters[11]

2.2 Analysis of test results

During the drilling process，the coolant not on?ly plays the role of cooling lubrication，but also af?fects the chip breaking and chip movement. The coolant pressure has an important influence on the chip size and the machining quality. According to the traditional view，the greater the import pres?sure，the better the chip crushing effect and the bet?ter the processing quality，but there are some differ?ences between the results and the conventional view. When the inlet pressure is 1 MPa，the chip will be blocked quickly，resulting in the processing cannot continue. When the pressure is 5 MPa，the chip length is the shortest，the increase of coolant pressure leads to the aggravation of chips separation between inner and outer edges，and the chip frag?mentation is obvious. At the same time，the vibra?tion of drill pipe increases and the stability decreases obviously. Therefore，we take the chip collected in the stable processing interval as the research object.

Fig.7 Effect of coolant pressure on chip morphology

Fig.7 shows the chip morphology under differ?ent inlet pressure conditions. The comparison shows that the order of the chip curl radius isr3MPa>r4MPa>r2MPa，and the length is inversely proportional to the curl radius. The crimp radius of the chip under 4 MPa oil pressure is slightly smaller than that un?der 3 MPa oil pressure，but the chip length is basi?cally the same. In addition，with the increases of in?let pressure，the fluid impact and shear force on the chips increase，the tearing between chip units inten?sifies，and the proportion of short chips produced by the inner and outer edges separately gradually in?creases. Analysis from the chip morphology shows that the chip length of the Ti6Al4V decreases gradu?ally with the increase of the pressure. When the coolant pressure is in the range of 2—4 MPa，the chip curl radius is first increased and then decreased.

3 Simulation Analysis

In order to analyze the influence of coolant on chip forming more clearly，the simulation analysis of flow field is carried out based on CFD in this pa?per. The model is designed referring to a real ce?mented carbide gun-drill used in processing. The steps of the simulation include generation of a CAD model，discretization of the CAD model，definition of boundary conditions and material properties.

3.1 CAD model

Based on the assumption that cutting fluid is an incompressible fluid and has no free surface，the ge?ometry of gun drill and fluid are simplified as shown in Fig.8. This paper focuses on the chip forming and chip removal near the drill bit，and only the bit is se?lected to participate in the simulation.

Fig.8 CAD model of gun-drill and cutting fluid

3.2 Selection of turbulence model

CFD methods can be used to calculate approxi?mate solutions of physical quantities such as pres?sure and velocity of fluids［12］. ANSYS Fluent is built on the Navier-Stokes theory，which describes the properties of viscous flows. Theoretically the Navier-Stokes equations are capable to solve turbu?lent flows. Unfortunately，this would require ex?tremely fine discretization levels so that the compu?tation time would increase indefinitely［13］. There?fore，the modelling of turbulent flow problems is handled by the use of Reynolds-averaged Navier-Stokes equations（RANS）turbulence models. A ba?sic requirement of a reliable turbulence model for in?dustrial tasks，such as the coolant flow，is accurate and robust near-wall processing. Therefore，we choose thek-ωshear stress transport（SST）model to describe the turbulent motion in the near-wall re?gion and the central region. In which，kis the turbu?lent kinetic energy，andωthe specific rate of dissi?pation of turbulent kinetic energy. Thek?ωSST model is one of the most common turbulence mod?els，which combines the advantageous near-wall be?havior of thek?ωmodel with the more robust proper?ties of thek?ωmodel at free flow regions（e.g. inlet and all regions far away from the walls）［14-19］.

3.3 Meshing and setting of boundary condi?tions

The accuracy of the simulation analysis de?pends on the quality of the mesh. Due to the compli?cated structure of gun-drill bit，it is difficult to carry out accurate topology，so the tetrahedral mesh is used in the calculation models in this paper. In order to capture the flow and rheological properties of near-wall layer and the cutting edge accurately，the inflation layer mesh is divided on the wall and the lo?cal refinement of the flow field near the cutting edge is carried out，as shown in Fig.9. The boundary con?dition setting and machining parameter selection of CFD simulation are based on the practical parame?ters of drilling Ti6Al4V with gun-drill，as shown in Table 2.

Fig.9 Meshing for fluid field

Table 2 Boundary condition settings

3.4 Analysis of simulation results

The cutting fluid flow in the deep hole during gun drilling belongs to a complex rotating turbulent jet in the finite space. As a typical semi-closed cutter with chip removal from the outside，the coolant hole of the gun-drill is not on the rotary axis. The first rotating jet will formed when the coolant is ejected from the hole. When the jet has pressure loss（mainly shock loss）in the bottom clearance，the high rotating turbulent jet will form again and en?ter the V-channel.

Under the action of shear stress and centrifugal force on the end face of the gun-drill，the coolant flows along the outer side of the end face after jet?ting out from Hole 1，then flows through the outer edge of Hole 2，and finally forms an inclined up?ward jet at the shoulder dub-off，as shown in Fig.10. The inclination of the jet entering V-channel is related to the flank angle of the shoulder dub-off.After impacting the chip，the jet will diverge at the“chip ridge”，and a part flows down along the chip surface of the outer edge，forming a small diameter swirl. In the other part，a vortex with high turbu?lence intensity is formed near the inner chip，and gradually evolves into a large-diameter swirl after leaving the area. Hole 2 is located on the shoulder dub-off，which is far from the cutting layer. There?fore，compared with Hole 1，its jet boundary layer has a high degree of diffusion，so most of those jets enter the V-channel above the stream exiting from Hole 1. After entering the V-channel，the jet radius gradually shrinks and the flow velocity increases.The remaining coolant ejected from Hole 2 first col?lides with the cutting layer and then collides with the shoulder dub-off，changing the flow direction twice，finally enters the V-channel along the outer edge of the shoulder dub-off，and merges with the cutting fluid of Hole 1.

Fig.10 Streamline distribution maps in flow field

（1）Effect of inlet pressure on chip formation

Coolant mainly causes chip deformation and even fracture through the shear and impact effects.Because the flow field on the gun-drill head is very complex［20-21］and the distribution on chip surface load is uneven，it is difficult to describe the impact of cutting fluid on chip forming intuitively and con?cretely. To analyze the influence of coolant pressure on chip formation，this paper uses the software Flu?ent and Static Structural to carry out a single fluidsolid coupling simulation analysis with the inlet pres?sure as the boundary condition.

According to the simulation results，in the ear?ly stage of chip forming，the chip appears“cantile?ver type”bending deformation under the action of coolant，as shown in Fig.11. The chip bends in?wards and forwards，the peak value of curling defor?mation appearing at the outer edge of the chip，and the peak value of equivalent stress appearing at the root of the outer edge of the chip. The deformation of the chip decreases gradually from the outside to the inside along the radial direction，and the defor?mation at the drill point is minimal. The results show that the flow of coolant can enhance the lateral curling of chips. According to the analysis of the chip forming mechanism，the increase of lateral curl?ing will change the position of contact pointC2be?tween chip and hole wall，making the curl degree in?verse to the curl radius.

Fig.11 Deformation of lateral curl of chips under different inlet oil pressures

As shown in Fig.12，with the increase of the in?let pressure，the chip curl deformation decreases first and then increases. The maximum deformation of the chip is 0.53 mm when the oil pressure is 1 MPa，0.38 mm at 3 MPa，and 0.495 mm at 6 MPa. The order of the curl radius isr3MPa>r4MPa>r5MPa>r2MPa>r6MPa>r1MPa，and the largest radius is at 3 MPa.

Fig.12 Curve of peak value of equivalent stress and curl deformation

According to the chip forming mechanism of gun drill，the external force of chip spiral forming mainly comes from the friction force，extrusion pres?sure and coolant impact force on the wall of chip re?moval space. The cutting fluid mainly consists of the radial force and the axial tension，which produc?es the bending deformation of“cantilever beam”and the tensile deformation of“pull rod”. After the first spiral curling，the bending deformation ampli?tude of the chip is limited by the slender space of the V-channel. At the same time，the influence of the radial force of the cutting fluid acting on the spiral surface on the root strain of the chip is greatly weak?ened，and the axial stress area of the chip increases with chip forming. Therefore，the axial tension of the chip will increase until the chip breaks. When the coolant pressure increases from 1 MPa to 3 MPa，the radius of chip curl increases，the inlet pressure increases，and the axial length of the chip becomes shorter，because the number of helical turns needed to reach the ultimate tension is re?duced. When the coolant pressure increases from 3 MPa to 6 MPa，although the coolant pressure in?creases，the crimp radius and the stressed area de?crease. Therefore，there is no significant increase in the axial tension of chip and no significant change in the axial length of chip.

（2）Effect of coolant pressure on chip removal

As shown in Fig.13，there are different de?grees of tearing on the outer edge of the spiral chip，which divides the chip into several units. In addi?tion，there is a certain proportion of short chips in the chips during the processing of gun-drill，so it can be inferred that there is a“secondary fracture”phenomenon in the chip removal movement. The analysis of the flow field shows that the movement of chip separated from the cutting area in the flow field can be divided into the overturning movement after contraction flow impact and the torsional move?ment under the influence of the spiral flow.The forc?es applied to the chip include：the normal extrusion pressureFnc1andFnc2of the hole wall and the Vchannel，the axial friction forceFfa1andFfa2，the cir?cumferential friction forceFfr1andFfr2，the impact force of contraction flowFwand the torsionTsfrom the spiral flow. During the process of chip flow，when the axial friction force of the chip is larger than the axial impact force from the coolant，the chip movement speed will be reduced in the V-groove and chip clogging is easy to occur.

According to Eq.（5），it can be seen that the load distribution of chips is not uniform due to the different flow velocity of coolant in different regions.The coolant flows at the highest velocity in the con?traction flow zone，so the chip is subjected to the greatest impact in this area. This shows that the con?traction stream is the main external force source to promote chip movement，chip breaking，and frac?ture.

whereDis the flow resistance of the chip，Cdthe di?mensionless resistance coefficient，Athe projected area of the chip，υ0the coolant speed before interfer?ence，andρthe Fluid density.

According to the point cloud，as shown in Fig.14，with the increase of the inlet pressure，the area of the contraction stream remains around 3.2 mm2.Fig.15 shows the change of the average ve?locity diagram in the region of the contraction stream.With the increase of the inlet oil pressure，the aver?age velocity presents a linear increasing trend. The average velocity of the coolant is 27.37 m/s at 1 MPa，50.113 m/s at 3 MPa，and 71.96 m/s at 6 MPa. Therefore，it can be inferred that with the increase of the inlet pressure，the coolant flows fast?er and the degree of chip breakage will be increased.

Fig.14 Schematic diagram of uniform point cloud distribution

Fig.15 Average flow velocity of contracted stream area

In addition to the above forces，the chip is af?fected by the shear stress of the fluid because of the uneven distribution of the velocity in the flow field.U，VandWare the velocity gradient components in the three coordinate directions ofX，Y，Z，respec?tively. It can be seen from Fig.16 that under differ?ent pressure conditions，the velocity gradient near the V-channel and hole wall is larger and increases with the increase of the coolant pressure，while the gradient value of the swirl cavitation zone in the form of“sag”is generally smaller. Moreover，the change of coolant pressure has no obvious effect on the gradient distribution in this region. The velocity gradientWis larger than the componentsUandVin the distribution area and peak value，which indi?cates that the velocity gradientWis the main influ?encing factor of fluid shear force，and the high gradi?ent value is mainly distributed near the contraction stream. The above analysis shows that the fluid shear stress of the chip mainly comes from the con?traction stream during the chip flow process and with the increase of coolant pressure，the larger the shear force，the more obvious the secondary frac?ture of chip.

Fig.16 3?D morphologies of velocity gradient

4 Conclusions

The single fluid-solid coupling simulation ad?opted in this study ignores the influence of chip de?formation on the flow field，and the simplification of the model also strengthens the limitations of the sim?ulation. Therefore，the relevant analysis needs to be combined with the experimental research，and the research results are as follows：

（1）In this paper，the spiral chip forming pro?cess of the gun?drill is divided into four stages from the theoretical point of view，and the concrete influ?ence of cutting fluid flow on-chip forming is dis?cussed in combination with simulation and experi?mental research. In the early stage of chip forming，the flow of cutting fluid will affect the curl radius of the chip by strengthening the lateral curl of the chip，and then affect the length of chip breaking at the first time. The degree of lateral crimp tends to de?crease at first and then increases with the increase of inlet pressure of cutting fluid，and the maximum crimp radius is at 3 MPa.

（2） Through the deconstruction of the flow path of cutting fluid，the shrinkage flow in the flow field is the main factor to push the chip and force the chip to produce“secondary fracture”. With the in?crease of inlet pressure，the area of contraction flow unit has little change，and the average flow rate and velocity gradient increase. Therefore，the coolant oil pressure increases，and the impact force and shear stress of the fluid also increase，which is more favorable for the chip’s secondary crushing and chip removal.

（3）In summary，3—4 MPa is a better choice for deep hole gun drilling of Ti6Al4V titanium alloy with 17 mm diameter.