Coated tools’ wear description in down and up milling based on the cutting edge entry impact...

COATED TOOL’S WEAR DESCRIPTION IN DOWN AND UP MILLING AT VARIOUS CHIP LENGTHS BASED

ON THE CUTTING EDGE ENTRY IMPACT DURATION

K.-D. Bouzakis1,2, G. Katirtzoglou1,2, E. Bouzakis1,2, S. Makrimallakis1,2, G. Maliaris1,2 1. Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering

Department, Aristotle University of Thessaloniki, Greece 2. Fraunhofer Project Center for Coatings in Manufacturing PCCM, Centre for Research

and Technology Hellas (CERTH), Thessaloniki, Greece and Fraunhofer Institute for Production Technology (IPT), Aachen, Germany

ABSTRACT Milling operations for manufacturing dies and molds are commonly linked to complicated chip geometry and contact conditions between tool and workpiece. These parameters render the optimization of the cutting conditions and the description of the tool wear difficult. In the described experiments, coated cemented carbide inserts fixed on a milling cutter were applied in down and up milling for monitoring the wear behavior at various cutting edge entry conditions, which result in various chip lengths. The corresponding developed strain rates cause different film-substrate deformations and resulting loads. These phenomena were investigated with the aid of a new impact tester with adjustable impact force characteristics. The effective tool life up to a certain flank wear land width versus the cutting edge entry duration was explained and analytically described.

KEYWORDS: Milling, optimization, tool contact

1. INTRODUCTION In milling processes with coated tools, the wear mechanisms may vary, depending, among others, on the cutting speed, tool kinematics etc., thus complicating the optimization of the cutting conditions /1,2,3,4,5,6/. The investigation of the related wear phenomena is of crucial importance towards explaining the coating failure and adjusting appropriately the cutting conditions. For investigating the effect of the strain rate on the surface response and film fatigue fracture of coated specimens subjected to cyclic impact loads, an impact tester was developed, facilitating the impact force modulation concerning its signal pattern. The surface response depends among others, on the substrate and coating strain as well as strain rate properties. Repetitive impact loads with variable duration, are exercised on the coated cutting edge in interrupted cutting, depending on the applied kinematics and cutting conditions. The tool life under certain cutting conditions can be correlated to the film strain rate dependent fatigue behavior. In this way, the coated inserts’ cutting performance at various conditions can be explained, considering impact test results at variously modulated repetitive forces. 2. EXPERIMENTAL DETAILS The geometry of the employed cutter and cutting inserts in the described investigations is exhibited in Figure 1. These cutters are commonly used in NC-milling of dies and molds. In the conducted investigations, a cutter of 63 mm external diameter with the possibility of employing four coated cutting inserts was applied. In the experiments only one cutting insert was used. This fact was considered in the adjustment of the cutting conditions. The cemented carbide

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3 - 5 October 2011,Thessaloniki, Greece

Proceedings of the 9th International Conference THE-“A” Coatings in Manufacturing EngineeringEdited by: K.-D. Bouzakis, K. Bobzin, B. Denkena, M. Merklein

Published by: Laboratory for Machine Tools and Manufacturing Engineering (EEΔΜ) of theAristoteles University of Thessaloniki and of the Fraunhofer Project Center Coatings in Manufacturing (PCCM),

a joint initiative by Fraunhofer-Gesellschaft and Centre for Research and Technology Hellas

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Figure 2: Coating, substrate and workpiece material properties.

Figure 3: Effect of the total impact signal duration on the critical impact force.

inserts were coated by a TiAlN PVD film of ca. 3 m thickness. The cutting edge chamfer of ca. 280 m and radius 20 m respectively (see Figure 1) contributes to an augmentation of its strength especially against dynamic loads. This may lead to an effective avoidance of cutting edge micro breakages, especially when the chip formation is not stable, as for example at the cutting edge entry into the workpiece material during up milling.

Figure 1: The employed milling cutter and cemented carbide inserts. The mechanical properties of the applied coating and substrate materials were detected by nanoindentations and a FEM-based algorithm, facilitating the determination of related stress-strain curves /7,8/. The elastoplastic film material laws and moreover, the specifications of the applied workpiece material are displayed in Figure 2. Milling investigations were conducted by a 3-axis numerically controlled milling center. The selected material was a modified hot work tool steel AISI P20, which features a low thermal conductivity compared to other hardened steels, as for instance 42CrMo4. This characteristic affects the chip generation during milling, since due to the comparably increased heat transfer into the tool, the cutting temperature at the endangered position of the cutting edge in the transient region between the tool rake and flank grows. In this way the coating material can withstand more effective the cutting loads, even at the start of up milling where the cutting temperatures are lower compared to down milling /9/, due to the coating strength properties improvement of this film up to ca. 200 ºC /10/. 3. RESULTS AND DISCUSSION For investigating the effect of the entry conditions of the coated cutting insert into the workpiece material on the tool’s wear behavior, impact tests with modulated force signal were carried out on coated inserts. These experiments demonstrate the effect of the total impact signal duration as well as its growth duration in the case of constant impact force amplitude. The effect of the impact force duration on its critical value, which induces a coating failure depth (CDF) at maximum 0.5 m, after one million impacts, is monitored in Figure 3. The applied triangular force signal durations were 10, 20 and 25 ms with constant signal growth duration of 5 ms. According to the obtained results, the critical impact force amplitude (CIFA) remains practically invariable versus the total impact force duration.

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Figure 2: Coating, substrate and workpiece material properties.

Figure 3: Effect of the total impact signal duration on the critical impact force.

inserts were coated by a TiAlN PVD film of ca. 3 m thickness. The cutting edge chamfer of ca. 280 m and radius 20 m respectively (see Figure 1) contributes to an augmentation of its strength especially against dynamic loads. This may lead to an effective avoidance of cutting edge micro breakages, especially when the chip formation is not stable, as for example at the cutting edge entry into the workpiece material during up milling.

Figure 1: The employed milling cutter and cemented carbide inserts. The mechanical properties of the applied coating and substrate materials were detected by nanoindentations and a FEM-based algorithm, facilitating the determination of related stress-strain curves /7,8/. The elastoplastic film material laws and moreover, the specifications of the applied workpiece material are displayed in Figure 2. Milling investigations were conducted by a 3-axis numerically controlled milling center. The selected material was a modified hot work tool steel AISI P20, which features a low thermal conductivity compared to other hardened steels, as for instance 42CrMo4. This characteristic affects the chip generation during milling, since due to the comparably increased heat transfer into the tool, the cutting temperature at the endangered position of the cutting edge in the transient region between the tool rake and flank grows. In this way the coating material can withstand more effective the cutting loads, even at the start of up milling where the cutting temperatures are lower compared to down milling /9/, due to the coating strength properties improvement of this film up to ca. 200 ºC /10/. 3. RESULTS AND DISCUSSION For investigating the effect of the entry conditions of the coated cutting insert into the workpiece material on the tool’s wear behavior, impact tests with modulated force signal were carried out on coated inserts. These experiments demonstrate the effect of the total impact signal duration as well as its growth duration in the case of constant impact force amplitude. The effect of the impact force duration on its critical value, which induces a coating failure depth (CDF) at maximum 0.5 m, after one million impacts, is monitored in Figure 3. The applied triangular force signal durations were 10, 20 and 25 ms with constant signal growth duration of 5 ms. According to the obtained results, the critical impact force amplitude (CIFA) remains practically invariable versus the total impact force duration.

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The contact conditions at the tool entry into the material in milling are pivotal for the tool wear /3,4,5,6/. The impact load on the cutting edge at the tool entrance into the workpiece material depends on the tool rake entry angle, since this angle is associated with various chip cross sections and thus cutting force development versus the tool rotation, at the same maximum undeformed chip thickness. For describing these dependencies, the contact between tool and workpiece was simulated by solid modeling techniques /11/. Related results are exhibited in Figure 6. The sketches in this figure demonstrate the cutting insert contact with the workpiece, at various chip formation stages versus the cutting time at the tool entry in the cases of down and up milling. The related chip cross sections on the cutting edge are also displayed.

Figure 6: Simulation of the cutter – workpiece contact at the tool rake entry into the material.

On the other hand, the applied force signal growth duration (impact entry) affects significantly the film fatigue behavior, as it is exhibited in Figure 4. An increase of the impact entry duration from 5 to 15 ms at constant total force duration of 20 ms results in a significant critical impact force augmentation from ca. 150 up to 220 daN respectively. At signal force entry durations less than 1 ms, a steeper diminution of the critical impact force develops. Figure 5 shows the growth of the critical impact force in relation to a signal duration of 15 ms versus the entry impact duration. The increase of the signal growth duration over 15 ms leads to a significant deceleration of the critical impact force growth, as the cyclic impact load approaches static conditions.

Figure 4: Effect of the force signal growth duration on the critical impact force.

Figure 5: Alteration of the critical impact force amplitude versus the cutting edge entry impact

duration.


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The contact conditions at the tool entry into the material in milling are pivotal for the tool wear /3,4,5,6/. The impact load on the cutting edge at the tool entrance into the workpiece material depends on the tool rake entry angle, since this angle is associated with various chip cross sections and thus cutting force development versus the tool rotation, at the same maximum undeformed chip thickness. For describing these dependencies, the contact between tool and workpiece was simulated by solid modeling techniques /11/. Related results are exhibited in Figure 6. The sketches in this figure demonstrate the cutting insert contact with the workpiece, at various chip formation stages versus the cutting time at the tool entry in the cases of down and up milling. The related chip cross sections on the cutting edge are also displayed.

Figure 6: Simulation of the cutter – workpiece contact at the tool rake entry into the material.

On the other hand, the applied force signal growth duration (impact entry) affects significantly the film fatigue behavior, as it is exhibited in Figure 4. An increase of the impact entry duration from 5 to 15 ms at constant total force duration of 20 ms results in a significant critical impact force augmentation from ca. 150 up to 220 daN respectively. At signal force entry durations less than 1 ms, a steeper diminution of the critical impact force develops. Figure 5 shows the growth of the critical impact force in relation to a signal duration of 15 ms versus the entry impact duration. The increase of the signal growth duration over 15 ms leads to a significant deceleration of the critical impact force growth, as the cyclic impact load approaches static conditions.

Figure 4: Effect of the force signal growth duration on the critical impact force.

Figure 5: Alteration of the critical impact force amplitude versus the cutting edge entry impact

duration.


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Depending on the undeformed chip length, the chip generation time amounts up to few decades of milliseconds. Since the chip cross section is approximately proportional to the developed cutting force, the chip generation time can be used for estimating the expected fatigue resistance of the applied coating. Figure 8 illustrates the entrance impact duration versus the undeformed chip length. In the case of down milling the entry impact duration rises at chip lengths higher than 40 mm, whereas below this value practically constant low durations can be observed. In contrast, during up milling a linear increase of the corresponding impact duration is visible up to lcu =40 mm as well as a stabilization up to lcu =78 mm.

Figure 8: Entrance duration time versus the undeformed chip length in down and up milling. The chip length influences variously the flank wear development. As it can be observed in Figure 9, in the case of down milling the achieved tool life up to a flank wear land width of 0.15 mm amounts 60000 to 75000 cuts within the range of investigated undeformed chip lengths. Thus, the chip length shows a minor influence on the wear development. On the other hand, the reduction of the chip length from 78 to 14 mm results in the case of up milling in significant improvements regarding the wear behavior of the coated tools (see bottom chart). Chip lengths of 14 mm led to a tool life of 230000 cuts compared to approximately 65000 cuts in the case of lcu = 78 mm. The upper chart in Figure 10 summarizes the described results. The increase of the undeformed chip length leads in the case of up milling to a reduction of the achieved number of cuts, whereas in down milling the tool life remains unaffected at a low level. However, the corresponding accumulative tool lives, which depend on the removed chip volumes, show contradictory courses. The bottom chart of Figure 10 illustrates the accumulative tool life and the related entry impact duration of the cutting edge versus the undeformed chip length. At high entry impact durations of approximately 15 ms the accumulative tool life rises. The latter is due to the increased critical impact force amplitude (see also Figure 4), which leads to an improved fatigue behavior. On the contrary, lower entrance durations cause lower accumulative tool lives in both down and up milling.

By this milling simulation, various process parameters can be quantified, as for example the chip cross section versus the cutting time. The diagrams of Figure 7 illustrate the corresponding results.

Figure 7: Chip cross section versus the cutting time at various chip lengths.


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Depending on the undeformed chip length, the chip generation time amounts up to few decades of milliseconds. Since the chip cross section is approximately proportional to the developed cutting force, the chip generation time can be used for estimating the expected fatigue resistance of the applied coating. Figure 8 illustrates the entrance impact duration versus the undeformed chip length. In the case of down milling the entry impact duration rises at chip lengths higher than 40 mm, whereas below this value practically constant low durations can be observed. In contrast, during up milling a linear increase of the corresponding impact duration is visible up to lcu =40 mm as well as a stabilization up to lcu =78 mm.

Figure 8: Entrance duration time versus the undeformed chip length in down and up milling. The chip length influences variously the flank wear development. As it can be observed in Figure 9, in the case of down milling the achieved tool life up to a flank wear land width of 0.15 mm amounts 60000 to 75000 cuts within the range of investigated undeformed chip lengths. Thus, the chip length shows a minor influence on the wear development. On the other hand, the reduction of the chip length from 78 to 14 mm results in the case of up milling in significant improvements regarding the wear behavior of the coated tools (see bottom chart). Chip lengths of 14 mm led to a tool life of 230000 cuts compared to approximately 65000 cuts in the case of lcu = 78 mm. The upper chart in Figure 10 summarizes the described results. The increase of the undeformed chip length leads in the case of up milling to a reduction of the achieved number of cuts, whereas in down milling the tool life remains unaffected at a low level. However, the corresponding accumulative tool lives, which depend on the removed chip volumes, show contradictory courses. The bottom chart of Figure 10 illustrates the accumulative tool life and the related entry impact duration of the cutting edge versus the undeformed chip length. At high entry impact durations of approximately 15 ms the accumulative tool life rises. The latter is due to the increased critical impact force amplitude (see also Figure 4), which leads to an improved fatigue behavior. On the contrary, lower entrance durations cause lower accumulative tool lives in both down and up milling.

By this milling simulation, various process parameters can be quantified, as for example the chip cross section versus the cutting time. The diagrams of Figure 7 illustrate the corresponding results.

Figure 7: Chip cross section versus the cutting time at various chip lengths.


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Figure 10: Number of cuts up to flank wear of 0.15 mm and obtained effective tool life T0.15 versus the undeformed chip length in down and up milling.

Figure 11 introduces the cutting edge entry impact factor k(te) as well as the corresponding equation, which describes the influence of the entry impact duration on the wear behavior of the coated cutting tool. Low entry impact durations correspond to comparably reduced critical impact force amplitudes i.e. to lower fatigue strength. In contrast, entry durations higher than 10 ms lead to a saturation of the fatigue strength improvement. In this case the cutting edge entry impact factor approaches to k(te) = 1. Over an entry impact duration of ca. 10 ms the tool life is associated with the maximum accumulated chip length, which can be obtained up to a flank wear land width of VB = 0.15 mm and which, at constant cutting speed and feed rate, is practically not affected by the cutting kinematic and conditions. Applying the equation of Figure 11, the tool life of coated tools in milling at various kinematics and cutting conditions can be determined, considering the occurring cutting edge entry impact duration and the accumulative tool life at cutting edge entry impact durations larger than ca. 10 ms. 4. CONCLUSIONS The presented experimental results demonstrate the effect of significant process parameters, on the wear behavior of coated tools in down and up milling. The experiments were conducted by employing characteristic tool geometries for manufacturing of dies and molds. The effect of the cutting edge entry impact duration on the tool wear is pivotal and can be investigated by the

Figure 9: Wear development versus the number of cuts at various undeformed chip lengths in down and in up milling.


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Figure 10: Number of cuts up to flank wear of 0.15 mm and obtained effective tool life T0.15 versus the undeformed chip length in down and up milling.

Figure 11 introduces the cutting edge entry impact factor k(te) as well as the corresponding equation, which describes the influence of the entry impact duration on the wear behavior of the coated cutting tool. Low entry impact durations correspond to comparably reduced critical impact force amplitudes i.e. to lower fatigue strength. In contrast, entry durations higher than 10 ms lead to a saturation of the fatigue strength improvement. In this case the cutting edge entry impact factor approaches to k(te) = 1. Over an entry impact duration of ca. 10 ms the tool life is associated with the maximum accumulated chip length, which can be obtained up to a flank wear land width of VB = 0.15 mm and which, at constant cutting speed and feed rate, is practically not affected by the cutting kinematic and conditions. Applying the equation of Figure 11, the tool life of coated tools in milling at various kinematics and cutting conditions can be determined, considering the occurring cutting edge entry impact duration and the accumulative tool life at cutting edge entry impact durations larger than ca. 10 ms. 4. CONCLUSIONS The presented experimental results demonstrate the effect of significant process parameters, on the wear behavior of coated tools in down and up milling. The experiments were conducted by employing characteristic tool geometries for manufacturing of dies and molds. The effect of the cutting edge entry impact duration on the tool wear is pivotal and can be investigated by the

Figure 9: Wear development versus the number of cuts at various undeformed chip lengths in down and in up milling.


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PERFORMANCE CHARACTERISTICS OF COATED MICRO MILLING TOOLS

E. Uhlmann1,2, F. Mahr1, A. Loewenstein1, N. Raue1, D. Oberschmidt2 1. Institute for Machine Tools and Factory Management,

Technische Universität Berlin, Germany 2. Fraunhofer-Institute for Production Systems and Design Technology (IPK),

Berlin, Germany

ABSTRACT The condition of cutting tool edges is a great influential factor on part quality and productivity in micro scale manufacturing. Hard coatings, such as titanium aluminum nitride TiAlN, can significantly extend the tool life and improve the wear characteristics of cutting tools. Also, in order to assess the condition of a cutting tool, means of measurement and monitoring have to be available. Strategies for tool condition assessment include direct methods, such as camera-based measurement of the flank wear as well as indirect methods, e. g. sensor-based acquisition of force and acoustic emission signals. This article presents the effects of tool coatings on tool life and surface quality as well as examines different methods for tool condition assessment. KEYWORDS: Coatings, micro milling, tool wear, condition assessment

1. INTRODUCTION Micro milling using cemented carbide end mills with nominal diameters of 0.1 mm < D < 1 mm is well-established, e. g. for die and mould fabrication. An increased utilization for medical and biotechnical components and applications can also be stated. Compared to other micro manufacturing processes such as laser ablation or electrical discharge machining (EDM) it is characterized by low processing time and high flexibility of work piece shapes. The process performance is mainly limited by the work piece’s hardness and decreased tool life as a result of tool wear. Size effects at very small chip thickness have negative effects on the process result in the form of ploughing and burr formation. Also process machine interactions (PMI) such as regenerative chatter that may occur as a result of the interrupted cutting conditions negatively influence the cutting process in terms of bad surface quality and loss of accuracy /1/2/. Due to dynamic loads and different wear mechanisms during the tool life of micro milling tools the cutting edge geometry changes. In order to reduce the tool wear and extend the tool life micro end mills with a cemented carbide substrate can be coated, e. g. with TiAlN. As the tool wear cannot be avoided completely, at least steady-going changes of the cutting edges are preferable. The intention of this article is to make a contribution on improving the process reliability of micro milling and to deduce options for process and tool optimization. At first the performance characteristics of TiAlN coated end mills are analyzed qualitatively. End mills with different diameters are applied for a finishing process on a hardened steel work piece (90 CrMnV 8). With regard to the tool life and the achievable surface roughness an adequate coating type is determined which is used for the following detailed analyses. Subsequently, in order to analyze the effects on the coated cutting edges experiments with different process strategies are carried out. Direct and indirect condition assessments are applied on full immersion cutting and finishing processes in copper, common steel (C45E) and powder metallurgic mold steel (PM X 190 CrVMo 20). A machine tool integrated camera is used to capture the cutting edge status after certain points of the tool life. Analysing the captured images allows the determination of the flank wear (VB). To analyze the surface characteristics

Figure 11: Cutting edge entry impact factor versus the cutting edge impact duration in down and up milling.

developed impact test with modulated repetitive force. In this way, the fatigue behavior of the cutting edge during milling can be correlated to characteristic data of the chip geometry, affecting the cutting edge impact load duration. 5. REFERENCES 1. Bouzakis K.-D., Mirisidis I., Michailidis N., Skordaris G., Lili E., Sampris A., Pavlidou E.,

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Coated tools’ wear description in down and up milling based on the cutting edge entry impact...

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