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Effects of the cutting edge microgeometry on tool wear and its thermo-mechanical load
CIRP Annals - Manufacturing Technology 60 (2011) 73–76
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Effects of the cutting edge microgeometry on tool wear and its thermomechanical load B. Denkena (1)*, A. Lucas, E. Bassett Institute of Production Engineering and Machine Tools (IFW), Leibniz Universita¨t Hannover, An der Universita¨t 2, 30823 Garbsen, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Tool life Honed cutting edge
Tailored cutting edge micro geometries lead to a significant enhancement of the cutting tool performance and increase its tool life. This paper presents the influence of honed cutting edge geometries on the tool wear behavior, process forces and thermal load of the inserts during turning operations. Tool life maps, which show the influence of the honed cutting edge on the wear behavior, are developed for different thermomechanical load profiles of the cutting tool. Furthermore, an approach for space resolved temperature measurements near the cutting edge via two-color ratio pyrometer is presented. ß 2011 CIRP.
1. Introduction
2. Definition of the cutting edge microgeometry
The need of high performance cutting tools is driven by continuously improved mechanical properties of new workpiece materials, which often results in a difficult machinability. Cutting tools represent the interface between machine tool and workpiece. Therefore cutting tools must perform a reliable and stable cutting process with high productivity and maximum tool life. Cutting edge microgeometries influence the tool wear behavior and therefore the tool life significantly [1,2]. Furthermore, the process stability can be influenced by honed cutting edges especially in micro machining processes [3]. The chip formation mechanism is also influenced by changes of the ratio between undeformed chip thickness and the radius of the honed cutting edge [4]. Furthermore the surface roughness during micro machining becomes independent of feed and proportional to the radius of the honed cutting edge [5]. The thermal load on cutting tools is one of the most important factors influencing the tool life. FEM-simulations with symmetrical honed cutting edges reveal a shift of the temperature field in the wedge influenced by the ratio rb/h. One of the relevant works in the field of temperature measurement from M’Saoubi et al. presents temperature maps using a IR-CDD measurement system in orthogonal turning of stainless steel with sharp and small honed cutting edges [6]. However, the effect of the size and the form of honed cutting edges on the temperature field in wedge of the cutting tool is unknown. This paper presents the influence of different forms and sizes of honed cutting edges on the performance of indexable inserts during turning operations. Changes of tool life will be explained by analyzing the wear mechanisms, cutting forces and thermal loads on the cutting edge.
Different sizes and forms of cutting edge microgeometries can be produced by means of microblasting, brushing, magnet- or drag-finishing or by laser ablation techniques [7]. The characterization of the cutting tool microgeometry is a fundamental requirement in order to investigate its specific influence on machining processes. The hone radius rb is not sufficient to characterize different cutting edge microgeometries precisely. This can be achieved by the definition of four fundamental parameters shown in Fig. 1. Applying this characterization of the honed cutting edge it is possible to distinguish three cases. A symmetrical cutting edge microgeometry is defined by a form factor K = 1, while K > 1 indicates a slope towards the rake face and K < 1 describes a slope towards the flank face. The size of the asymmetrical honed cutting edges is described by the parameters Sg, Sa and K. Symmetrical ¯ honed cutting edges will be described through their mean size S.
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.098
3. Influence of the cutting edge geometry on tool life 3.1. Experimental setup Cemented carbide indexable inserts without chip breaker (SNMA190612) were used as cutting tools during orthogonal turning of AISI1045. The inserts are coated with a PVD multilayer of TiN–TiAlN. All prepared cutting edges were measured with an optical measurement system. Fig. 2 shows the methodical variation of the cutting edge microgeometries. Process forces during cutting have been measured using a three-component force dynamometer. Wear measurements are performed by means of a digital video microscope. Following criteria have been considered to determine the end of tool life: edge chipping, width of flank wear land VB = 200 mm and crater width KB = 800 mm. The tool life tests have been repeated once.
[()TD$FIG]
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
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Fig. 4. Changing of the wear behavior due to cutting edge preparation.
[()TD$FIG]
Fig. 1. Characterization of the honed cutting edge.
Fig. 2. Methodical variation of the honed cutting edges.
3.2. Tool life maps Novel tool life maps have been developed in order to illustrate the relationship between the tool life and the micro geometry. It is the top view of a three dimensional surface regression comprising the results of 18 tool life tests of differently honed cutting edges. In addition the tool life of unprepared cutting tools has been also implemented as a reference value to demonstrate the general benefit of cutting edge preparation. Through this method it is also possible to predict dimensions of honed cutting edges, which could lead to further increased tool life. Fig. 3 shows the tool life map for orthogonal turning of AISI1045. For unprepared cutting edges chipping appears after a short tool life travel path of lc = 0.49 km (Fig. 4—left). No edge chippings were detected for prepared cutting edges. These tools reach their tool life criterion either through crater or flank wear (Fig. 4—right). All prepared cutting tools have a higher tool life compared to the unprepared ones. The highest tool life travel path of lc = 1.9 km was detected for cutting tools with a honed cutting edge of Sg = 50 mm/Sa = 30 mm (Fig. 3F). The lowest path of lc = 1.3 km was reached by cutting tools with Sg = 100 mm/Sa = 100 mm and Sg = 50 mm/Sa = 100 mm.
[()TD$FIG]
Fig. 3. Tool life map for free orthogonal turning of AISI1045.
In between there is an area of significant tool life gradient which appears at the middle of the tool map. Changes of the wear behavior of the cutting tool cause this gradient area. The form factor K = Sg/Sa influences the dominating mechanism of wear. Cutting tools with constant Sg and higher values of Sa show an increased flank wear. The initiation of crater wear is mainly influenced by Sg. The time delay of the crater wear initiation determines the achieved tool life dominantly. For example, the prepared cutting tools with cutting edge microgeometry of Sg = 100 mm/Sa = 50 mm show the first crater wear at a tool life travel path of lc = 0.5 km and reaches an average lc = 1.7 km, while cutting tools with Sg = 30 mm and Sa = 50 shows the first crater wear at a tool life travel path of lc = 0.7 km and reaches an average lc = 1.8 km. An influence of the honed cutting edge on the position of crater wear could not be detected. 3.3. Process forces The cutting edge microgeometry influences the distribution of the mechanical loads on the cutting edge and therefore the process forces. The resultant force Fz can be determined by measuring the cutting force Fc and feed force Ff. An undeformed chip width of b = 5 mm has been applied with an undeformed chip thickness of h = f = 0.2 mm. A normalized resultant force F 0z is defined by: F 0z ¼
Fz ðN=mmÞ b
(1)
Although the mean size of all honed cutting edges S¯ does not exceed the applied undeformed chip thickness h, there is an increase of the normalized cutting forces up to 40% compared to unprepared cutting edges (Fig. 5). Thereby Sg shows no significant influence. The forces are mainly influenced by the symmetrical honed cutting edge S¯ and the parameter Sa. The high accordance between the two curves of S¯ and Sa indicates that Sa is the dominant factor of influence. Furthermore, based on the approach of Merchant [8] and Dohmen [9] the normalized resultant force F 0z is transferred to a normalized tangential cutting force on rake face F 0Tg and a normalized normal cutting force on rake face F 0Ng . Fig. 6 illustrates the influence of the normalized forces and thereby the determined coefficient of friction on tool life of unprepared as well as differently honed cutting edges. This diagram indicates that honed cutting tools have a minor influence on F 0Ng compared to the normalized tangential force F 0Tg . As a result the coefficient of friction ¼ F 0Tg =F 0Ng increases. The lowest coefficient of friction of m = 0.38 appears by unprepared cutting edges with S¯ ¼ 1215 mm. The highest one of m = 0.57 is determined for Sa = 100 mm. This leads to an increase of tool life [()TD$FIG]from an average tool life travel path of lc = 1.3 km for Sa = 100 mm
Fig. 5. Influence of honed cutting edge on normalized process forces.
[()TD$FIG]
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
75
Fig. 8. Thermal load of the cutting tool (vc = 300 m/min, f = 0.2 mm).
Fig. 6. Influence of the coefficient of friction on tool life.
to lc = 2 km for Sa = 30 mm. According to that, a compromise between a low coefficient of friction and a stabilized cutting edge via edge preparation is required. 3.4. Thermal load of the cutting tool Based on the presented results, it is essential to measure the temperature near the cutting edge. A two-color ratio pyrometer has been applied to generate a temperature mapping in the wedge. This measurement device, developed by Mu¨ller and Renz, exhibits a high temporal resolution [10]. The electromagnetic IR-radiation will be collected and transmitted to the two IR-detectors of the pyrometer by a graded-index-multimode fiber optic. The concept of the temperature measurement in the wedge of the cutting tool is presented in Fig. 7. Basically 12 measuring points on three levels are obtained for each temperature map of the indexable inserts used in the tool life tests. The first measurement level starts at a distance of 350 mm from the rake face of the cutting edge. The fiber optic is positioned in a groove in the insert and fixed down with a spring to a precise stable position. All measurement points for one temperature field were generated with one prepared cutting edge for each microgeometry. Through the geometrical predefined positioning of the measurement points the temperature gradient in the wedge can be determined. By means of a polynomial surface fitting a function of the temperature field is calculated between the obtained 12 measurement points. This surface function is used for an extrapolation towards the flank and the rake face. Fig. 8 shows the resulting temperature maps of one unprepared and four differently honed cutting edges. The interpretation is focused on changes in the thermal load profile in the wedge of the cutting tool due to changes in the micro geometry. The temperatures are higher at the honed cutting edges than at [()TD$FIG] unprepared one. A rotation of the isotherms is occurring in the the
Fig. 7. Experimental setup for the temperature measurement in the insert.
direction of the flank face. The temperature maps of 2 and 3 show almost the same isotherm orientation. These cutting tools have the same Sa = 100 mm and different Sg. In conclusion, Sg has no significant influence on the isotherms in the wedge. This correlates with the presented results of forces and friction coefficient (Fig. 6). 4. Tailored cutting edge geometry In order to transfer the findings to different thermomechanical loads of the cutting edge, investigations during external turning process have been carried out with varied workpiece materials. Tool life tests are carried out using CNGG120408 inserts of cemented carbide and tool life maps are generated. All inserts are coated with PVD-TiN. The tool life maps are scaled on the tool life achieved without cutting edge preparation. Fig. 9 shows the influence of cutting edge microgeometry on the tool life in turning of AISI1045. The left map shows the tool life during continuous cut. Crater wear dominated the tool life in these tests. Therefore Sg has to be reduced in order to achieve the highest tool life. Furthermore, a high precision in cutting edge preparation is not necessary for this load profile. This is indicated through the low gradient of tool life between the different honed cutting edges. By machining workpieces with longitudinal grooves a dynamical mechanical load profile is applied. The dominant wear mechanism then is flank wear and tool breakage (Fig. 9—right). Setting a higher Sg leads to the highest stability of the cutting edge and therefore to the longest tool life. The influence of material properties on the tool life is presented in Fig. 10. Due to changes of the material properties and therefore the thermomechanical load of the cutting edge, the area of highest tool life is shifted specifically for each load profile. Furthermore, the specific potential of the cutting edge preparation to increase tool life is illustrated.
[()TD$FIG]
Fig. 9. Influence of the cutting edge microgeometry on the tool life by different load profiles of the inserts.
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
76
Depending on the expected thermo-mechanical load profile respectively the dominating wear behavior of the cutting tool, a specific microgeometry can be designed. The presented tool life maps provide an efficient tool, to determine a suitable form and size of the cutting edge hone. Further studies will focus on the verification and extension of the existing force models, which assist the prediction of process forces for sharp or symmetrical honed cutting edges, to consider the influences of tailored sloped cutting edge microgeometries, defined by the presented parameters Sg and Sa. Acknowledgment
Fig. 10. Tailored cutting edge micro geometry for specific load profiles.
During cylindrical turning of AISI1045 with coolant, no significant changes in the wear behavior or tool life due to changed cutting edge microgeometry occur. During cylindrical turning of 42CrMo4 both, crater and flank wear are the decisive criteria for the tool life. Therefore both, Sg and Sa have to be set symmetrically to a minimum that still provides a sufficient mechanical stability of the cutting edge. The machining of TiAl6V4 leads to a significant increase of the static thermomechanical load on the cutting edge. Process forces as well as the induced temperature are kept to a minimum by applying sharp cutting edges with a minimum hone, in order to achieve the longest tool life. An effective coolant strategy during machining of TiAl6V4 reduces the thermal loads on the cutting edge. As a result, larger hone radii are applicable in order to achieve a higher mechanical stability of the cutting edge. 5. Conclusions and outlook The effect of the cutting edge microgeometry on wear behavior and tool life plays an important role in cutting processes. Overall the parameter Sa exhibits the major influence on the thermal load on the wedge of the cutting tool. This is a result of the increased contact area and higher friction of the flank face with the workpiece. An ideal cutting tool has the highest mechanical stability, lowest wear and therefore the maximum tool life.
The authors thank the German Research Foundation (DFG) for the financial support within the project ‘‘DE447/71-1’’ and the Research Association for Machine Tools and Manufacturing Technology (FWF) as well as Seco Tools, Walter AG, Sulzer Metaplas, Deckel Maho Pfronten and Alfred H. Schu¨tte for the financial support.
References [1] Rech J, et al, (2005) Influence of Cutting Edge Radius on the Wear Resistance of PM-HSS Milling Inserts. Wear 259(7–12):1168–1178. [2] Denkena B, Reichstein M, Brodehl J, de Leo´n-Garcı´a L (2005) Surface Preparation, Coating and Wear Performance of Geometrically defined Cutting Edges. Proceedings of the 5th International Conference ‘‘THe Coatings’’ in Manufacturing Engineering, . [3] Biermann D, Baschin A (2009) Influence of Cutting Edge Geometry and Cutting Edge Radius on the Stability of Micromilling Processes. Production Engineering 3(4–5):375–380. [4] Jivishov V (2008) Mikrogeometrische Einflu¨sse beim Weich- und Hartspanen, Dr. -Ing. Dissertation, Leibniz Universita¨t Hannover. Manufacturing Systems, Universita¨t des Saarlandes. pp. 425–432. [5] Childs THC, et al, (2008) Surface Finishes from Turning and Facing with Round Nosed Tools. CIRP Annals—Manufacturing Technology 57:89–92. [6] M’Saoubi R, Nordh LG (2004) Comparative Temperature Study in Turning Using IR-CCD Technique and Tool-Work Thermocouple Method. Report from the Corrosion and Metals Research Institute (KIMAB), Report No. IM-2004-556. [7] Aurich JC, Zimmermann M, Leitz L (2010) The Preparation of Cutting Edges Using a Marking Laser. Production Engineering . 10.1007/s11740-010-0275-9. [8] Merchant E (1945) Mechanics of the Metal Cutting Process. I. Orthogonal Cutting and a Type 2 Chip. Journal of Applied Physics 16(5):267–275. [9] Dohmen HG (1974) Vergleich der Zerspanungsmechanischen Theorien unter Anwendung der Plastizita¨tsmechanik auf den Zerspanungsvorgang, Habilitation, Ruhruniversita¨t Bochum. [10] Mu¨ller B, Renz U (2001) Development of a Fast Fiber-Optic Two-Color Pyrometer for the Temperature Measurement of Surfaces with Varying Emissivities. Review of Scientific Instruments 72(8):3366–3374.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Effects of the cutting edge microgeometry on tool wear and its thermomechanical load B. Denkena (1)*, A. Lucas, E. Bassett Institute of Production Engineering and Machine Tools (IFW), Leibniz Universita¨t Hannover, An der Universita¨t 2, 30823 Garbsen, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Tool life Honed cutting edge
Tailored cutting edge micro geometries lead to a significant enhancement of the cutting tool performance and increase its tool life. This paper presents the influence of honed cutting edge geometries on the tool wear behavior, process forces and thermal load of the inserts during turning operations. Tool life maps, which show the influence of the honed cutting edge on the wear behavior, are developed for different thermomechanical load profiles of the cutting tool. Furthermore, an approach for space resolved temperature measurements near the cutting edge via two-color ratio pyrometer is presented. ß 2011 CIRP.
1. Introduction
2. Definition of the cutting edge microgeometry
The need of high performance cutting tools is driven by continuously improved mechanical properties of new workpiece materials, which often results in a difficult machinability. Cutting tools represent the interface between machine tool and workpiece. Therefore cutting tools must perform a reliable and stable cutting process with high productivity and maximum tool life. Cutting edge microgeometries influence the tool wear behavior and therefore the tool life significantly [1,2]. Furthermore, the process stability can be influenced by honed cutting edges especially in micro machining processes [3]. The chip formation mechanism is also influenced by changes of the ratio between undeformed chip thickness and the radius of the honed cutting edge [4]. Furthermore the surface roughness during micro machining becomes independent of feed and proportional to the radius of the honed cutting edge [5]. The thermal load on cutting tools is one of the most important factors influencing the tool life. FEM-simulations with symmetrical honed cutting edges reveal a shift of the temperature field in the wedge influenced by the ratio rb/h. One of the relevant works in the field of temperature measurement from M’Saoubi et al. presents temperature maps using a IR-CDD measurement system in orthogonal turning of stainless steel with sharp and small honed cutting edges [6]. However, the effect of the size and the form of honed cutting edges on the temperature field in wedge of the cutting tool is unknown. This paper presents the influence of different forms and sizes of honed cutting edges on the performance of indexable inserts during turning operations. Changes of tool life will be explained by analyzing the wear mechanisms, cutting forces and thermal loads on the cutting edge.
Different sizes and forms of cutting edge microgeometries can be produced by means of microblasting, brushing, magnet- or drag-finishing or by laser ablation techniques [7]. The characterization of the cutting tool microgeometry is a fundamental requirement in order to investigate its specific influence on machining processes. The hone radius rb is not sufficient to characterize different cutting edge microgeometries precisely. This can be achieved by the definition of four fundamental parameters shown in Fig. 1. Applying this characterization of the honed cutting edge it is possible to distinguish three cases. A symmetrical cutting edge microgeometry is defined by a form factor K = 1, while K > 1 indicates a slope towards the rake face and K < 1 describes a slope towards the flank face. The size of the asymmetrical honed cutting edges is described by the parameters Sg, Sa and K. Symmetrical ¯ honed cutting edges will be described through their mean size S.
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.098
3. Influence of the cutting edge geometry on tool life 3.1. Experimental setup Cemented carbide indexable inserts without chip breaker (SNMA190612) were used as cutting tools during orthogonal turning of AISI1045. The inserts are coated with a PVD multilayer of TiN–TiAlN. All prepared cutting edges were measured with an optical measurement system. Fig. 2 shows the methodical variation of the cutting edge microgeometries. Process forces during cutting have been measured using a three-component force dynamometer. Wear measurements are performed by means of a digital video microscope. Following criteria have been considered to determine the end of tool life: edge chipping, width of flank wear land VB = 200 mm and crater width KB = 800 mm. The tool life tests have been repeated once.
[()TD$FIG]
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
74
Fig. 4. Changing of the wear behavior due to cutting edge preparation.
[()TD$FIG]
Fig. 1. Characterization of the honed cutting edge.
Fig. 2. Methodical variation of the honed cutting edges.
3.2. Tool life maps Novel tool life maps have been developed in order to illustrate the relationship between the tool life and the micro geometry. It is the top view of a three dimensional surface regression comprising the results of 18 tool life tests of differently honed cutting edges. In addition the tool life of unprepared cutting tools has been also implemented as a reference value to demonstrate the general benefit of cutting edge preparation. Through this method it is also possible to predict dimensions of honed cutting edges, which could lead to further increased tool life. Fig. 3 shows the tool life map for orthogonal turning of AISI1045. For unprepared cutting edges chipping appears after a short tool life travel path of lc = 0.49 km (Fig. 4—left). No edge chippings were detected for prepared cutting edges. These tools reach their tool life criterion either through crater or flank wear (Fig. 4—right). All prepared cutting tools have a higher tool life compared to the unprepared ones. The highest tool life travel path of lc = 1.9 km was detected for cutting tools with a honed cutting edge of Sg = 50 mm/Sa = 30 mm (Fig. 3F). The lowest path of lc = 1.3 km was reached by cutting tools with Sg = 100 mm/Sa = 100 mm and Sg = 50 mm/Sa = 100 mm.
[()TD$FIG]
Fig. 3. Tool life map for free orthogonal turning of AISI1045.
In between there is an area of significant tool life gradient which appears at the middle of the tool map. Changes of the wear behavior of the cutting tool cause this gradient area. The form factor K = Sg/Sa influences the dominating mechanism of wear. Cutting tools with constant Sg and higher values of Sa show an increased flank wear. The initiation of crater wear is mainly influenced by Sg. The time delay of the crater wear initiation determines the achieved tool life dominantly. For example, the prepared cutting tools with cutting edge microgeometry of Sg = 100 mm/Sa = 50 mm show the first crater wear at a tool life travel path of lc = 0.5 km and reaches an average lc = 1.7 km, while cutting tools with Sg = 30 mm and Sa = 50 shows the first crater wear at a tool life travel path of lc = 0.7 km and reaches an average lc = 1.8 km. An influence of the honed cutting edge on the position of crater wear could not be detected. 3.3. Process forces The cutting edge microgeometry influences the distribution of the mechanical loads on the cutting edge and therefore the process forces. The resultant force Fz can be determined by measuring the cutting force Fc and feed force Ff. An undeformed chip width of b = 5 mm has been applied with an undeformed chip thickness of h = f = 0.2 mm. A normalized resultant force F 0z is defined by: F 0z ¼
Fz ðN=mmÞ b
(1)
Although the mean size of all honed cutting edges S¯ does not exceed the applied undeformed chip thickness h, there is an increase of the normalized cutting forces up to 40% compared to unprepared cutting edges (Fig. 5). Thereby Sg shows no significant influence. The forces are mainly influenced by the symmetrical honed cutting edge S¯ and the parameter Sa. The high accordance between the two curves of S¯ and Sa indicates that Sa is the dominant factor of influence. Furthermore, based on the approach of Merchant [8] and Dohmen [9] the normalized resultant force F 0z is transferred to a normalized tangential cutting force on rake face F 0Tg and a normalized normal cutting force on rake face F 0Ng . Fig. 6 illustrates the influence of the normalized forces and thereby the determined coefficient of friction on tool life of unprepared as well as differently honed cutting edges. This diagram indicates that honed cutting tools have a minor influence on F 0Ng compared to the normalized tangential force F 0Tg . As a result the coefficient of friction ¼ F 0Tg =F 0Ng increases. The lowest coefficient of friction of m = 0.38 appears by unprepared cutting edges with S¯ ¼ 1215 mm. The highest one of m = 0.57 is determined for Sa = 100 mm. This leads to an increase of tool life [()TD$FIG]from an average tool life travel path of lc = 1.3 km for Sa = 100 mm
Fig. 5. Influence of honed cutting edge on normalized process forces.
[()TD$FIG]
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
75
Fig. 8. Thermal load of the cutting tool (vc = 300 m/min, f = 0.2 mm).
Fig. 6. Influence of the coefficient of friction on tool life.
to lc = 2 km for Sa = 30 mm. According to that, a compromise between a low coefficient of friction and a stabilized cutting edge via edge preparation is required. 3.4. Thermal load of the cutting tool Based on the presented results, it is essential to measure the temperature near the cutting edge. A two-color ratio pyrometer has been applied to generate a temperature mapping in the wedge. This measurement device, developed by Mu¨ller and Renz, exhibits a high temporal resolution [10]. The electromagnetic IR-radiation will be collected and transmitted to the two IR-detectors of the pyrometer by a graded-index-multimode fiber optic. The concept of the temperature measurement in the wedge of the cutting tool is presented in Fig. 7. Basically 12 measuring points on three levels are obtained for each temperature map of the indexable inserts used in the tool life tests. The first measurement level starts at a distance of 350 mm from the rake face of the cutting edge. The fiber optic is positioned in a groove in the insert and fixed down with a spring to a precise stable position. All measurement points for one temperature field were generated with one prepared cutting edge for each microgeometry. Through the geometrical predefined positioning of the measurement points the temperature gradient in the wedge can be determined. By means of a polynomial surface fitting a function of the temperature field is calculated between the obtained 12 measurement points. This surface function is used for an extrapolation towards the flank and the rake face. Fig. 8 shows the resulting temperature maps of one unprepared and four differently honed cutting edges. The interpretation is focused on changes in the thermal load profile in the wedge of the cutting tool due to changes in the micro geometry. The temperatures are higher at the honed cutting edges than at [()TD$FIG] unprepared one. A rotation of the isotherms is occurring in the the
Fig. 7. Experimental setup for the temperature measurement in the insert.
direction of the flank face. The temperature maps of 2 and 3 show almost the same isotherm orientation. These cutting tools have the same Sa = 100 mm and different Sg. In conclusion, Sg has no significant influence on the isotherms in the wedge. This correlates with the presented results of forces and friction coefficient (Fig. 6). 4. Tailored cutting edge geometry In order to transfer the findings to different thermomechanical loads of the cutting edge, investigations during external turning process have been carried out with varied workpiece materials. Tool life tests are carried out using CNGG120408 inserts of cemented carbide and tool life maps are generated. All inserts are coated with PVD-TiN. The tool life maps are scaled on the tool life achieved without cutting edge preparation. Fig. 9 shows the influence of cutting edge microgeometry on the tool life in turning of AISI1045. The left map shows the tool life during continuous cut. Crater wear dominated the tool life in these tests. Therefore Sg has to be reduced in order to achieve the highest tool life. Furthermore, a high precision in cutting edge preparation is not necessary for this load profile. This is indicated through the low gradient of tool life between the different honed cutting edges. By machining workpieces with longitudinal grooves a dynamical mechanical load profile is applied. The dominant wear mechanism then is flank wear and tool breakage (Fig. 9—right). Setting a higher Sg leads to the highest stability of the cutting edge and therefore to the longest tool life. The influence of material properties on the tool life is presented in Fig. 10. Due to changes of the material properties and therefore the thermomechanical load of the cutting edge, the area of highest tool life is shifted specifically for each load profile. Furthermore, the specific potential of the cutting edge preparation to increase tool life is illustrated.
[()TD$FIG]
Fig. 9. Influence of the cutting edge microgeometry on the tool life by different load profiles of the inserts.
[()TD$FIG]
B. Denkena et al. / CIRP Annals - Manufacturing Technology 60 (2011) 73–76
76
Depending on the expected thermo-mechanical load profile respectively the dominating wear behavior of the cutting tool, a specific microgeometry can be designed. The presented tool life maps provide an efficient tool, to determine a suitable form and size of the cutting edge hone. Further studies will focus on the verification and extension of the existing force models, which assist the prediction of process forces for sharp or symmetrical honed cutting edges, to consider the influences of tailored sloped cutting edge microgeometries, defined by the presented parameters Sg and Sa. Acknowledgment
Fig. 10. Tailored cutting edge micro geometry for specific load profiles.
During cylindrical turning of AISI1045 with coolant, no significant changes in the wear behavior or tool life due to changed cutting edge microgeometry occur. During cylindrical turning of 42CrMo4 both, crater and flank wear are the decisive criteria for the tool life. Therefore both, Sg and Sa have to be set symmetrically to a minimum that still provides a sufficient mechanical stability of the cutting edge. The machining of TiAl6V4 leads to a significant increase of the static thermomechanical load on the cutting edge. Process forces as well as the induced temperature are kept to a minimum by applying sharp cutting edges with a minimum hone, in order to achieve the longest tool life. An effective coolant strategy during machining of TiAl6V4 reduces the thermal loads on the cutting edge. As a result, larger hone radii are applicable in order to achieve a higher mechanical stability of the cutting edge. 5. Conclusions and outlook The effect of the cutting edge microgeometry on wear behavior and tool life plays an important role in cutting processes. Overall the parameter Sa exhibits the major influence on the thermal load on the wedge of the cutting tool. This is a result of the increased contact area and higher friction of the flank face with the workpiece. An ideal cutting tool has the highest mechanical stability, lowest wear and therefore the maximum tool life.
The authors thank the German Research Foundation (DFG) for the financial support within the project ‘‘DE447/71-1’’ and the Research Association for Machine Tools and Manufacturing Technology (FWF) as well as Seco Tools, Walter AG, Sulzer Metaplas, Deckel Maho Pfronten and Alfred H. Schu¨tte for the financial support.
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