- Email: [email protected]
Micro crack formation in hardmetal milling tools
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Micro crack formation in hardmetal milling tools Berend Denkena, Thilo Grove, Mirko Theuer
MARK
⁎
Institute of Production Engineering and Machine Tools, An der Universität 2, 30823 Garbsen, Germany
A B S T R A C T Hardmetal milling tools are frequently used for machining of hard materials, e.g. titanium alloys. Such tools are often reground after the end of their lifetime. This is intended to increase the resource and the economical efficiency of the required but expensive hardmetal blanks. However, end users often notice a decreased lifetime when using reground tools. Subsurface damages that are not removed during regrinding probably cause this effect. This paper investigates the formation of cracks in hardmetal milling tools and consequently suggests grinding strategies that consider the removal of all present micro cracks. The results show a correlation between the required amount of material removal and the size of the optically measurable breakouts at the cutting edge.
1. Introduction
decreases.
Titanium alloys are used in many different fields of applications, e.g. the aerospace industry. Their comparatively low density, high hardness, corrosion resistance, and high temperature properties allow their utilization in very hot environments, where Aluminum alloys are not suitable [1]. However, the occurring removal rates of up to 95% of the titanium alloy workpieces as well as the high tool load during the corresponding milling process demand the utilization of hardmetal tools [2]. The main properties of the hardmetals, i.e. high hardness and toughness, cause a market share of up to 50% of all produced milling tools [3,4]. However, the comparatively low lifetimes of 30 to 60 min and the high costs for the hardmetal blanks make regrinding operations for an economical recycling process of those tools essential (see Fig. 1). The main challenge for the recycling process is the precise evaluation of the present defects of a worn milling tool. The maximum depth of the biggest defect of each individual tool has to be identified in order to determine the necessary allowance during the subsequent regrinding operation. Currently, the responsible worker for this job detects the damage depth only using magnifying glasses or without any help of measuring equipment. Therefore, the process is very tedious and a detection of microscopic damages, e.g. micro cracks, cannot be performed. However, the remaining micro cracks in the reground milling tool can result in a significant reduction of the tools lifetime in the consecutive milling process. Furthermore, wrong estimations of the worker may lead to allowances that are too high and thus reduce the resource efficiency since more material than necessary is removed. In consequence, the amount of possible regrinding operations is reduced and the economability of the tool and its related milling operation
2. Experimental setup
⁎
This paper investigates the wear behavior of hardmetal milling tools in order to estimate economically and environmentally friendly grinding allowances that ensure constant tool lifetimes. The investigated end mills have a diameter of about 23 mm. As material, the specification Extramet EMT 210 is used. Each milling tool has been used differently and therefore experienced divergent process loads. This leads to a high variation of different forms and sizes of tool wear in the investigated sample. Different measuring devices have been investigated during preliminary research work in order to identify viable methods to quantitatively measure the tool wear of hardmetal end mills. For the detection and evaluation of external defects, e.g. breakouts, optical measurement devices based on stripe light projection or focus variation have been identified as the best choice. Both methods can determine the depth of breakouts with a resolution of under 5 μm in a timely manner [5]. Consequently, a Keyence VR 3000 Stripe light projection microscope has been used for the determination of external defects. However, this device only determines the defect depth orthogonal to the cutting edge. Therefore, it does not give any information about the defect size at the flank and rake face that might be of larger extend. Thus, cross sections of the milling tools were produced and investigated using a Keyence VHX 600 digital microscope. The resulting pictures have been evaluated in regards of the defect size at the rake and flank face. Furthermore, a topcon SM 510 W scanning electron microscope was utilized for the creation of high resolution pictures of the end mills cross section. X-
Corresponding author. E-mail address: [email protected] (M. Theuer).
http://dx.doi.org/10.1016/j.ijrmhm.2017.10.008 Received 16 August 2017; Received in revised form 2 October 2017; Accepted 16 October 2017 Available online 18 October 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 1. Life cycle of milling tools.
time of up to several days for the creation of a single picture. Furthermore, a detection of micro cracks on those pictures must be done manually since the cracks are hard to distinguish from grain boundaries and the diameter of the crack exit is below 1 μm. In that sense the manual detection can lead to identification errors. The crack exits also do not give any information about the depth of identified cracks. Consequently, surface pictures would not lead to any conclusions regarding the necessary allowance during the subsequent grinding operation. The measurement by SEM is the only viable option for the evaluation of micro cracks, but requires the destruction of the investigated tools. Therefore it is not transferable to the industrial application. However, the method can be used to investigate the statistical relationship between the optically measurable defect depth and the depth of the present micro cracks. The developed model can consequently be considered during the determination of an individual allowance for worn milling tools. Three different states of micro cracks have been detected: Macroscopic cracks (see Fig. 3) have a width of several micrometer and occur close to macroscopic defects of the cutting edge. Those cracks are the main reason for the local spalling of the cutting edge. The second state is micro cracks that occur due to the high strain at the edges of macroscopic cracks (see Fig. 3) and damage the tool into a higher depth. These cracks crop up between the tungsten carbide grains and weaken the stability of the milling tool. They have a length of up to 400 μm and may lead to tool failure if they are not properly removed during the regrinding process. Broken tools may damage the workpiece and thus can lead to high follow-up costs. Furthermore, remaining cracks increase the variation of the lifetime of reground tools. Thus, the
ray examinations were considered for the detection of internal defects of the hardmetal. However, the high atomic number of the tungsten leads to a high density of the hardmetal, which makes x-ray investigations unsuitable. 3. Investigation of worn milling tools The results show that the defect length on rake and flank face increase with increasing defects at the cutting edge (see Fig. 2). The data scatters strongly and thus does not show a direct correlation of the defects at the cutting edge and the defects on the flanks. Contrary, for optical defects of above 300 μm, defects on the rake face tend to be greater than the corresponding defects on the flank face. This effect is probably caused by the higher process load of the rake face during the previous milling operation. Further, scanning electron microscope (SEM) pictures of the cross section of worn milling tools were investigated. As a result multiple micro cracks could be detected near the macroscopic defects. Consequently, the sample of 30 worn milling tools has been evaluated in consideration of micro cracks and their size. It has been shown that all identified micro cracks have a width of below 500 nm. Therefore, they cannot be detected by dye penetrant inspection or the investigated optical systems other than the SEM. The resolution of dye penetration inspection methods is too low and can only be used reliably for cracks wider than 1,5 μm. Therefore, it is not suitable for the identification of micro cracks in hardmetal tools. Contrary, common optical methods can reach the required resolutions when using a fitting wavelength and numerical aperture of the utilized lenses. However, the disadvantage of those optical approaches is the required measurement and processing
Fig. 2. Size of the defects on the rake and flank face in dependency of the defect size at the cutting edge.
211
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 3. Different states of cracks in worn hardmetal milling tools.
the maximum distance of the crack to the nearest rake or flank face (see Fig. 4). Therefore, it describes the necessary allowance for the removal of the corresponding micro crack. A direct correlation of the depth and the length of cracks has not been found since their size can vary strongly (see Fig. 5 a and b). Neither the position of the cracks is clearly predictable. About 40% of the cracks are closer to the rake than the flank face (Fig. 5 c) and 35% are closer to the flank face. The remaining 25% of the detected micro cracks lie between the rake and flank face and cannot be clearly assigned to either flank. The micro cracks at the rake face occur due to its high load during the milling process. The engagement with the generated chips and the process friction lead to crack initiation at this flank. Contrary, the chamfer at the flank face continuously absorbs process vibrations and thus is permanently under oscillating load. This eventually leads to micro cracks in the hardmetals. In contrast to the ratio of crack length to depth, a correlation of the necessary allowance for the crack removal with the optically measured defect size VO has been identified. The optical defect is measured orthogonal to the cutting edge and therefore represents the angle bisector of the cutting wedge (see Fig. 6). The depth of micro cracks and thus the necessary allowance increases with increasing defect values VO. This
service time of all milling tools regardless of the presence of micro cracks is reduced in industrial processes in order to avoid damages of the workpiece. This leads to higher tooling costs and reduces the economical efficiency of the milling operations. The third identified state of micro cracks is the crack initiation that is characterized by microscopic structural changes of the material. Those structural changes will presumably become micro cracks if they are not removed but further stressed, e.g. in a milling operation. However, those crack initiators are in general smaller than 10 μm and thus will be removed automatically when removing the present micro cracks. In consequence, this paper will only consider the removal of the macroscopic and microscopic defects and recommend an allowance strategy for individual defect sizes. For the determination of the maximum defect depth, all investigated milling tools were evaluated in regards of the optical defect depth (VO), the defect length along the flank (VF) and rake face (VS) as well as the length (RL) and depth (RT) of the identified micro cracks (see Fig. 4). The characteristic values for the external defect VO, VS and VF were determined a stripe light projection microscope and a digital Keyence microscope (see experimental setup). The micro cracks are evaluated by their absolute length RL and depth RT, whereas the depth is defined by
Fig. 4. Exemplary micro crack and defect notation.
212
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 5. Different positions and orientations of exemplary micro cracks.
means that bigger breakouts and spallings are more likely to have caused longer and deeper micro cracks. Consequently, a higher allowance regarding the micro crack removal is required for bigger defects. This leads to two different strategies for the approximation of the necessary allowance during regrinding (see Fig. 6). Those strategies are recommended for regrinding operations in order to remove all micro cracks with reasonable assurance. The first strategy is optimized in regards of the reground tools' lifetime. It suggests a minimum allowance of 100 μm independent of the optically measured defect size. That means that a regrinding process is recommended even for tools without visible damage in order to remove potentially present micro cracks. Therefore, a reground tool without damages and thus maximized lifetimes is assured. For defects that are optically measured and above 200 μm of size, one half of the measured defect depth should be used as allowance on the rake and flank face each (see Table 1). Exemplarily, for a defect depth of 460 μm, an allowance of 230 μm is recommended for each flank. It is very important to regrind both flanks independent of visible signs to tool wear in order to remove all present cracks and thus reduce the variation of the lifetime of the reground tools.
Table 1 Recommended allowances for different strategies and defect sizes. Optically measured defect depth VO
Up to 200 μm 200 μm to 500 μm 500 μm to 1250 μm Above 1250 μm
Allowance on flank and rake face Strategy 1
Strategy 2
100 μm 50% of VO 50% of VO 50% of VO
100 μm 100 μm 200 μm 250 μm
The second strategy was developed in order to maximize the economical efficiency and sustainability of the investigated hardmetal mills. This strategy reduces the utilized allowance and therefore decreases the diameter loss per regrinding operation. Consequently, the amount of possible regrinding operations is increased, until the critical minimum diameter of the tool is reached. However, this goes along with a small risk of remaining micro cracks in the reground tool. For the investigated sample of 30 tools, only a single micro crack would not have been removed. The main advantage of this strategy is the higher Fig. 6. Relationship between the optically measurable defect and the length and depth of present micro cracks.
213
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
resource efficiency due to significantly reduced allowances during the regrinding of milling tools that show optical defects of above 200 μm (see Table 1). The hereby increased amount of regrinding operations leads to reduced costs per tool lifetime, even though some micro cracks may remain in the tool and therefore increase the variability of the lifetime overall.
milling tool quality. For further research work, it is suggested that the properties of the ground hardmetal using the conventional and the newly developed grinding strategy are compared. The edge fracture test [6] shows potential regarding the cost efficient evaluation of the edge toughness. Therefore, it should be further investigated in order to estimate the cutting tool performance.
4. Summary and conclusion
Acknowledgements
The investigation of worn hardmetal end mills has shown that different states of micro cracks may occur in such tools. Those cracks have to be removed during regrinding in order to reduce the variation of the lifetime of reground tools. Two different allowance strategies, focusing on economic efficiency and tool lifetime, have been developed for this purpose. It is shown that the optically measured defect of a tool may be used as an indicator for the size of the present micro cracks at the cutting edge defects. Consequently, the suggested strategy optimized in regards of the reground tools lifetime or the strategy optimized regarding the resource efficiency of the hardmetal blanks is recommended for future regrinding operations of tungsten carbide end mills. Future investigations will focus on the optimization of the regrinding process in regards of the process parameters, grinding wheel specification and grinding strategies. The investigation of the grinding strategies will exemplarily include a distribution of the determined allowance in multiple strokes and the effect of the stroke size on the
The authors would like to thank the Federal Ministry for Economic Affairs and Energy (BMWi) Germany for their organizational and financial support within the project “Autoregrind” (IGF Nr. 19121 N). References [1] E.O. Ezugwu, et al., Titanium alloys and their machinability a review, J. Mater. Process. Technol. 68 (1997) 262–274. [2] P.A. Dearnley, et al., Wear mechanisms of cemented carbides and ceramics used for machining titanium alloys, High Technol. Ceram. 38 (1987) 2699–2712. [3] G.S. Upadhyaya, Cemented Tungsten Carbides, Production, Properties and Testing, Noyes Publications, USA, 1998. [4] A. Sawka, et al., Cemented carbide cutting tools life with nanocrystalline Al203 layer deposited by MOCVD, Arch. Civ. Mech. Eng. 16 (2016) 351–364. [5] B. Denkena, T. Grove, M. Theuer, Ressourceneffizient Nachschleifen, VDI-Z 159 (3) (2017) 43–45 (2017, März). [6] R. Morrell, A.J. Gant, Edge chipping of hard materials, Int. J. Refract. Met. Hard Mater. 19 (2001) 293–301.
214
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Micro crack formation in hardmetal milling tools Berend Denkena, Thilo Grove, Mirko Theuer
MARK
⁎
Institute of Production Engineering and Machine Tools, An der Universität 2, 30823 Garbsen, Germany
A B S T R A C T Hardmetal milling tools are frequently used for machining of hard materials, e.g. titanium alloys. Such tools are often reground after the end of their lifetime. This is intended to increase the resource and the economical efficiency of the required but expensive hardmetal blanks. However, end users often notice a decreased lifetime when using reground tools. Subsurface damages that are not removed during regrinding probably cause this effect. This paper investigates the formation of cracks in hardmetal milling tools and consequently suggests grinding strategies that consider the removal of all present micro cracks. The results show a correlation between the required amount of material removal and the size of the optically measurable breakouts at the cutting edge.
1. Introduction
decreases.
Titanium alloys are used in many different fields of applications, e.g. the aerospace industry. Their comparatively low density, high hardness, corrosion resistance, and high temperature properties allow their utilization in very hot environments, where Aluminum alloys are not suitable [1]. However, the occurring removal rates of up to 95% of the titanium alloy workpieces as well as the high tool load during the corresponding milling process demand the utilization of hardmetal tools [2]. The main properties of the hardmetals, i.e. high hardness and toughness, cause a market share of up to 50% of all produced milling tools [3,4]. However, the comparatively low lifetimes of 30 to 60 min and the high costs for the hardmetal blanks make regrinding operations for an economical recycling process of those tools essential (see Fig. 1). The main challenge for the recycling process is the precise evaluation of the present defects of a worn milling tool. The maximum depth of the biggest defect of each individual tool has to be identified in order to determine the necessary allowance during the subsequent regrinding operation. Currently, the responsible worker for this job detects the damage depth only using magnifying glasses or without any help of measuring equipment. Therefore, the process is very tedious and a detection of microscopic damages, e.g. micro cracks, cannot be performed. However, the remaining micro cracks in the reground milling tool can result in a significant reduction of the tools lifetime in the consecutive milling process. Furthermore, wrong estimations of the worker may lead to allowances that are too high and thus reduce the resource efficiency since more material than necessary is removed. In consequence, the amount of possible regrinding operations is reduced and the economability of the tool and its related milling operation
2. Experimental setup
⁎
This paper investigates the wear behavior of hardmetal milling tools in order to estimate economically and environmentally friendly grinding allowances that ensure constant tool lifetimes. The investigated end mills have a diameter of about 23 mm. As material, the specification Extramet EMT 210 is used. Each milling tool has been used differently and therefore experienced divergent process loads. This leads to a high variation of different forms and sizes of tool wear in the investigated sample. Different measuring devices have been investigated during preliminary research work in order to identify viable methods to quantitatively measure the tool wear of hardmetal end mills. For the detection and evaluation of external defects, e.g. breakouts, optical measurement devices based on stripe light projection or focus variation have been identified as the best choice. Both methods can determine the depth of breakouts with a resolution of under 5 μm in a timely manner [5]. Consequently, a Keyence VR 3000 Stripe light projection microscope has been used for the determination of external defects. However, this device only determines the defect depth orthogonal to the cutting edge. Therefore, it does not give any information about the defect size at the flank and rake face that might be of larger extend. Thus, cross sections of the milling tools were produced and investigated using a Keyence VHX 600 digital microscope. The resulting pictures have been evaluated in regards of the defect size at the rake and flank face. Furthermore, a topcon SM 510 W scanning electron microscope was utilized for the creation of high resolution pictures of the end mills cross section. X-
Corresponding author. E-mail address: [email protected] (M. Theuer).
http://dx.doi.org/10.1016/j.ijrmhm.2017.10.008 Received 16 August 2017; Received in revised form 2 October 2017; Accepted 16 October 2017 Available online 18 October 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 1. Life cycle of milling tools.
time of up to several days for the creation of a single picture. Furthermore, a detection of micro cracks on those pictures must be done manually since the cracks are hard to distinguish from grain boundaries and the diameter of the crack exit is below 1 μm. In that sense the manual detection can lead to identification errors. The crack exits also do not give any information about the depth of identified cracks. Consequently, surface pictures would not lead to any conclusions regarding the necessary allowance during the subsequent grinding operation. The measurement by SEM is the only viable option for the evaluation of micro cracks, but requires the destruction of the investigated tools. Therefore it is not transferable to the industrial application. However, the method can be used to investigate the statistical relationship between the optically measurable defect depth and the depth of the present micro cracks. The developed model can consequently be considered during the determination of an individual allowance for worn milling tools. Three different states of micro cracks have been detected: Macroscopic cracks (see Fig. 3) have a width of several micrometer and occur close to macroscopic defects of the cutting edge. Those cracks are the main reason for the local spalling of the cutting edge. The second state is micro cracks that occur due to the high strain at the edges of macroscopic cracks (see Fig. 3) and damage the tool into a higher depth. These cracks crop up between the tungsten carbide grains and weaken the stability of the milling tool. They have a length of up to 400 μm and may lead to tool failure if they are not properly removed during the regrinding process. Broken tools may damage the workpiece and thus can lead to high follow-up costs. Furthermore, remaining cracks increase the variation of the lifetime of reground tools. Thus, the
ray examinations were considered for the detection of internal defects of the hardmetal. However, the high atomic number of the tungsten leads to a high density of the hardmetal, which makes x-ray investigations unsuitable. 3. Investigation of worn milling tools The results show that the defect length on rake and flank face increase with increasing defects at the cutting edge (see Fig. 2). The data scatters strongly and thus does not show a direct correlation of the defects at the cutting edge and the defects on the flanks. Contrary, for optical defects of above 300 μm, defects on the rake face tend to be greater than the corresponding defects on the flank face. This effect is probably caused by the higher process load of the rake face during the previous milling operation. Further, scanning electron microscope (SEM) pictures of the cross section of worn milling tools were investigated. As a result multiple micro cracks could be detected near the macroscopic defects. Consequently, the sample of 30 worn milling tools has been evaluated in consideration of micro cracks and their size. It has been shown that all identified micro cracks have a width of below 500 nm. Therefore, they cannot be detected by dye penetrant inspection or the investigated optical systems other than the SEM. The resolution of dye penetration inspection methods is too low and can only be used reliably for cracks wider than 1,5 μm. Therefore, it is not suitable for the identification of micro cracks in hardmetal tools. Contrary, common optical methods can reach the required resolutions when using a fitting wavelength and numerical aperture of the utilized lenses. However, the disadvantage of those optical approaches is the required measurement and processing
Fig. 2. Size of the defects on the rake and flank face in dependency of the defect size at the cutting edge.
211
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 3. Different states of cracks in worn hardmetal milling tools.
the maximum distance of the crack to the nearest rake or flank face (see Fig. 4). Therefore, it describes the necessary allowance for the removal of the corresponding micro crack. A direct correlation of the depth and the length of cracks has not been found since their size can vary strongly (see Fig. 5 a and b). Neither the position of the cracks is clearly predictable. About 40% of the cracks are closer to the rake than the flank face (Fig. 5 c) and 35% are closer to the flank face. The remaining 25% of the detected micro cracks lie between the rake and flank face and cannot be clearly assigned to either flank. The micro cracks at the rake face occur due to its high load during the milling process. The engagement with the generated chips and the process friction lead to crack initiation at this flank. Contrary, the chamfer at the flank face continuously absorbs process vibrations and thus is permanently under oscillating load. This eventually leads to micro cracks in the hardmetals. In contrast to the ratio of crack length to depth, a correlation of the necessary allowance for the crack removal with the optically measured defect size VO has been identified. The optical defect is measured orthogonal to the cutting edge and therefore represents the angle bisector of the cutting wedge (see Fig. 6). The depth of micro cracks and thus the necessary allowance increases with increasing defect values VO. This
service time of all milling tools regardless of the presence of micro cracks is reduced in industrial processes in order to avoid damages of the workpiece. This leads to higher tooling costs and reduces the economical efficiency of the milling operations. The third identified state of micro cracks is the crack initiation that is characterized by microscopic structural changes of the material. Those structural changes will presumably become micro cracks if they are not removed but further stressed, e.g. in a milling operation. However, those crack initiators are in general smaller than 10 μm and thus will be removed automatically when removing the present micro cracks. In consequence, this paper will only consider the removal of the macroscopic and microscopic defects and recommend an allowance strategy for individual defect sizes. For the determination of the maximum defect depth, all investigated milling tools were evaluated in regards of the optical defect depth (VO), the defect length along the flank (VF) and rake face (VS) as well as the length (RL) and depth (RT) of the identified micro cracks (see Fig. 4). The characteristic values for the external defect VO, VS and VF were determined a stripe light projection microscope and a digital Keyence microscope (see experimental setup). The micro cracks are evaluated by their absolute length RL and depth RT, whereas the depth is defined by
Fig. 4. Exemplary micro crack and defect notation.
212
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
Fig. 5. Different positions and orientations of exemplary micro cracks.
means that bigger breakouts and spallings are more likely to have caused longer and deeper micro cracks. Consequently, a higher allowance regarding the micro crack removal is required for bigger defects. This leads to two different strategies for the approximation of the necessary allowance during regrinding (see Fig. 6). Those strategies are recommended for regrinding operations in order to remove all micro cracks with reasonable assurance. The first strategy is optimized in regards of the reground tools' lifetime. It suggests a minimum allowance of 100 μm independent of the optically measured defect size. That means that a regrinding process is recommended even for tools without visible damage in order to remove potentially present micro cracks. Therefore, a reground tool without damages and thus maximized lifetimes is assured. For defects that are optically measured and above 200 μm of size, one half of the measured defect depth should be used as allowance on the rake and flank face each (see Table 1). Exemplarily, for a defect depth of 460 μm, an allowance of 230 μm is recommended for each flank. It is very important to regrind both flanks independent of visible signs to tool wear in order to remove all present cracks and thus reduce the variation of the lifetime of the reground tools.
Table 1 Recommended allowances for different strategies and defect sizes. Optically measured defect depth VO
Up to 200 μm 200 μm to 500 μm 500 μm to 1250 μm Above 1250 μm
Allowance on flank and rake face Strategy 1
Strategy 2
100 μm 50% of VO 50% of VO 50% of VO
100 μm 100 μm 200 μm 250 μm
The second strategy was developed in order to maximize the economical efficiency and sustainability of the investigated hardmetal mills. This strategy reduces the utilized allowance and therefore decreases the diameter loss per regrinding operation. Consequently, the amount of possible regrinding operations is increased, until the critical minimum diameter of the tool is reached. However, this goes along with a small risk of remaining micro cracks in the reground tool. For the investigated sample of 30 tools, only a single micro crack would not have been removed. The main advantage of this strategy is the higher Fig. 6. Relationship between the optically measurable defect and the length and depth of present micro cracks.
213
International Journal of Refractory Metals & Hard Materials 70 (2018) 210–214
B. Denkena et al.
resource efficiency due to significantly reduced allowances during the regrinding of milling tools that show optical defects of above 200 μm (see Table 1). The hereby increased amount of regrinding operations leads to reduced costs per tool lifetime, even though some micro cracks may remain in the tool and therefore increase the variability of the lifetime overall.
milling tool quality. For further research work, it is suggested that the properties of the ground hardmetal using the conventional and the newly developed grinding strategy are compared. The edge fracture test [6] shows potential regarding the cost efficient evaluation of the edge toughness. Therefore, it should be further investigated in order to estimate the cutting tool performance.
4. Summary and conclusion
Acknowledgements
The investigation of worn hardmetal end mills has shown that different states of micro cracks may occur in such tools. Those cracks have to be removed during regrinding in order to reduce the variation of the lifetime of reground tools. Two different allowance strategies, focusing on economic efficiency and tool lifetime, have been developed for this purpose. It is shown that the optically measured defect of a tool may be used as an indicator for the size of the present micro cracks at the cutting edge defects. Consequently, the suggested strategy optimized in regards of the reground tools lifetime or the strategy optimized regarding the resource efficiency of the hardmetal blanks is recommended for future regrinding operations of tungsten carbide end mills. Future investigations will focus on the optimization of the regrinding process in regards of the process parameters, grinding wheel specification and grinding strategies. The investigation of the grinding strategies will exemplarily include a distribution of the determined allowance in multiple strokes and the effect of the stroke size on the
The authors would like to thank the Federal Ministry for Economic Affairs and Energy (BMWi) Germany for their organizational and financial support within the project “Autoregrind” (IGF Nr. 19121 N). References [1] E.O. Ezugwu, et al., Titanium alloys and their machinability a review, J. Mater. Process. Technol. 68 (1997) 262–274. [2] P.A. Dearnley, et al., Wear mechanisms of cemented carbides and ceramics used for machining titanium alloys, High Technol. Ceram. 38 (1987) 2699–2712. [3] G.S. Upadhyaya, Cemented Tungsten Carbides, Production, Properties and Testing, Noyes Publications, USA, 1998. [4] A. Sawka, et al., Cemented carbide cutting tools life with nanocrystalline Al203 layer deposited by MOCVD, Arch. Civ. Mech. Eng. 16 (2016) 351–364. [5] B. Denkena, T. Grove, M. Theuer, Ressourceneffizient Nachschleifen, VDI-Z 159 (3) (2017) 43–45 (2017, März). [6] R. Morrell, A.J. Gant, Edge chipping of hard materials, Int. J. Refract. Met. Hard Mater. 19 (2001) 293–301.
214