- Email: [email protected]
High performance grinding of zirconium oxide (ZrO2) using hybrid bond diamond tools
CIRP Annals - Manufacturing Technology 62 (2013) 343–346
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
High performance grinding of zirconium oxide (ZrO2) using hybrid bond diamond tools Mohammad Rabiey (3)a,*, Nicolas Jochum b, Fredy Kuster (3)c a
ABB Turbosystems Ltd, Baden, Switzerland Blaser Swisslube AG, Hasle-Ru¨egsau, Switzerland c Institute of Machine Tools and Manufacturing, ETH Zurich, Switzerland b
Sponsored by Rainer Zu¨st (1), Seegra¨ben, Switzerland.
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Diamond tool Hybrid bond
Hybrid bond (metal–ceramic) diamond tools are proposed for grinding zirconium oxide used in medical implants. Compared to conventional grinding tools, material removal rates and tool life time are drastically increased without deterioration in mechanical properties of the workpiece. This is achieved within a selected process window in combination with an elaborate oil cooling system, where material removal is mainly occurring within the ductile cutting mode. Self-sharpening effect of the tool can be observed and the dressability of the tool further improves the grinding performance. ß 2013 CIRP.
1. Introduction In the past decade there have been major advances in production and usage of zirconium oxide, ZrO2, for dental applications because of its superior physical properties, including great strength and hardness at high temperature, low thermal expansion and good wear resistance [1,2]. Due to the precision and high quality necessary for the functionality of these components, a secondary machining process after sintering must often be performed. Furthermore, complete grinding of hard sintered ZrO2 has recently become one of the most common processes used in dental applications. However, ceramic like ZrO2 generally requires considerable time to be machined [3]. In the dental industry, the economic aspects of the machining process as well as the resulting product quality play key roles for the applications of ZrO2 to achieve good competitiveness compared to the other dental materials on the market. Therefore, an efficient grinding process of ZrO2 is a crucial challenge for medical engineering. 2. Grinding of ceramics, state of the art Diamond tools with electroplated, brazed and vitrified bonding are usually used to grind ceramics. The bond strength of resin bond wheels constraints their usage when grinding ceramics because of bond breakage. A similar situation occurs in case of vitrified bonds since grinding forces increase at higher material removal rates. Grinding ceramic materials with diamond tools, even if they contain electroplated or metal bonds, results in large great wear [4]. This causes tool costs to rise as tool life time is reduced. Costs are yet higher when the tool is not dressable. Metal bond wheels
* Corresponding author. Tel.: +41 786238293. E-mail addresses: [email protected], [email protected] (M. Rabiey). 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.073
however are generally faced with difficulties when dressed with conventional methods. This is due to the lack of porosity in the bonds which makes new dressing methods like ELID, laser dressing or EDM necessary in order to generate adequate chip pockets in the grinding tool surface [5]. Such modern dressing technologies require further equipment installed on the grinding machine, which incurs additional costs. Hybrid bond grinding wheels, as a newly developed bonding system, have great potential in industrial applications since they combine the advantages of both metal bonds (low wear and high strength) and vitrified bonds (porosity and reasonable chip pockets) [6]. Through proper selection of the grinding parameters it is possible to achieve a self-sharpening effect. This can result in a decrease of the dressing intervals. When grinding ZrO2 for dental applications, small tool dimensions are obviously necessary (hence the grinding speed is also limited). This is due to the complex 3D shape of dental bridges or implants which is to be produced by a 5-axis CNC machine using CAD/CAM software [7]. When grinding ceramics, the uncut chip thickness is one the most prominent parameters influencing the grinding process. Decreasing the uncut chip thickness below a critical value leads the grinding regime to the ductile mode [8]. The critical chip thickness depends mainly on material properties [8]. Generally, combinations of both brittle and ductile modes occur when grinding ceramics [9]. The ductile mode results superior quality in terms of accuracy and surface integrity. 3. Experiment conditions 3.1. Experimental setup Grinding experiments were carried out with a series of hybrid bond diamond tools on a 5-axis CNC machine tool (Mikron HSM400U). It was equipped with a high speed spindle (Fischer MFM-10120/11, electromotor speed 12,000–120,000 rpm, power
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
344
Table 3 Grinding parameters. Grinding parameters Grinding speed Depth of cut Width of cut Feed rate Specific material removal rate Strategy
vc = 5–35 m/s ae = 0.004–0.64 mm ap = 1–4 mm vf = 100–12,000 mm/min Q0 W = 2–8 mm3/mm s Up grinding
Table 4 shows the selected dressing parameters applied during the experiments. Negative () signs refer to up-dressing. The total depth of dressing was adjusted by the machine for each dressing experiment and set to 50 mm. Table 4 Dressing parameters for experiments. Fig. 1. Experimental setup and tools. Dressing parameters
3.5 kW) to grind ZrO2 (Fig. 1). The used coolant was oil (Blaser, Blasogrind HC5). Two different types of coolant nozzle systems were applied in the experiments: a conventional flexible coolant nozzle as well as a nozzle jet system. Both were implemented to ensure a high volumetric coolant flow rate and precise orientation of jet flow towards the contact zone (Fig. 1). The tests were conducted on workpieces with a dimension of 100 mm 10 mm 2 mm. By precisely measuring material thickness, the material removal rate was determined. The workpieces were ground to 100 mm length. The material properties of ZrO2 were constant for all tests during this investigation and are described in Table 1. Table 1 ZrO2 properties. Ceramic properties Components Composition Density Hardness Compressive strength Flexural strength Young’s-modulus Fracture toughness K1C
ZrO2/Y2O3/Al2O3 95/52/0.252 (%) 6.05 g/cm3 1200 Hv 2000 MPa 1200 MPa 210 GPa 8 MN/m2/3
Different tool shapes were used to manufacture the dental bridge. As an exception, for the systematic comparison tests only cylindrically shaped tools with a diameter of 6 mm were applied (Table 2).
Dressing wheel speed Dressing ratio Dressing feed rate (radial)
vcd = 60 m/s qd = 25 vfed = 50 mm/min
3.3. Measurement and analysis The wheel topography was measured by means of a 3D optical microscope (Alicona Infinite Focus) as well as SEM. The surface roughness of ground workpieces was measured optically by an Alicona Infinite Focus as well as by a Taylor–Hobson Form-Talysurf roughness profiler. Investigation of workpiece damage was carried out using four point bending tests and X-ray diffraction. A dynamometer platform was designed and developed using Kistler piezo-sensors, type: 9256C1 mounted under the workpiece clamping system in order to measure tangential forces Ft as well as normal forces Fn during grinding operation. For long-term grinding tests, the grinding forces were analyzed at different stages of specific material removal. To measure the change in tool radius and also to determine wear, an integrated commercial laser tool measuring system (BLUM Micro Compact NT) was applied. 4. Experimental results and discussion To study the ductile and/or brittle material removal mechanisms when grinding ZrO2, the uncut chip thickness was varied by changing depth of cut, feed rate and grinding speed. Fig. 2 shows SEM micrographs of the ground workpiece surface which exhibits three different material removal mechanisms. Fig. 2 (left) shows
Table 2 Tool properties. Tool properties Diameter Geometry Grain size Concentration Bond
1–6 mm Cylinder, ball or taper D64, D91 C100 Hybrid: metal–ceramic Fig. 2. Different material removal mechanisms when grinding ZrO2; brittle mode (left), brittle and ductile mode (middle) and ductile mode (right).
3.2. Grinding and dressing condition The grinding parameters used for all tests are summarized in Table 3. Grinding speed was limited to 35 m/s due to the rather small grinding wheel diameter as well as the maximum revolution constraint of the machine spindle (120,000 rpm). New diamond tools were dressed by a vitrified bond silicon carbide wheel (Meister 31C-80-H-10-190-V137-1) prior to each grinding process. Dressing parameters were kept constant. The initial dressing tool diameter was 100 mm with a wheel width of 5.25 mm.
the brittle mechanism with piled up material displacement. In Fig. 2 (middle) a combination of brittle and ductile mechanisms can be observed. In contrast, Fig. 2 (right) demonstrates the ductile mechanism. It can be seen that by increasing uncut chip thickness, the ductile mode slowly converts to the brittle mode. In order to detect the percentage of ductile mode, the surface of the ground workpiece was investigated. This percentage which is an important item of surface integrity was correlated with the mean uncut chip thickness (Fig. 3). The maximum uncut chip
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
Fig. 3. Ductile workpiece surface versus calculated chip thickness hcu.
345
Fig. 5. Specific grinding forces versus grinding speed.
thickness was calculated by [10]: hcu ¼
vf 1 ae vc l C max lg
1=2
(1)
where vf is the feed rate, vc is the grinding speed, ae is the depth of cut and lg is the geometric contact length. The two parameters, Cmax (kinematic cutting edges) and l (form factor or sharpness of grain) were measured based on surface topography of the grinding tool and amounts to 25 mm2 and 6.3 respectively. Measurement values and calculation results are shown in Fig. 3. It can be seen that increasing mean uncut chip thickness entails a decrease in percentage of the ductile mode. It was observed that even if a defined calculated mean uncut chip thickness is applied, there were some areas on the workpiece surface that contained a mixture of all three modes: brittle, brittle–ductile and ductile (Fig. 4). This is due to the stochastic nature of grinding tool surface topography and the fact that active cutting edges are not all situated at the same protrusion level.
thickness and smaller forces per grain as material removal rate is kept constant. A reduction of uncut chip thickness leads the grinding regime of ceramics towards a more ductile mode resulting in lower grinding forces, higher productivity (higher material removal rate) and better quality (surface roughness and surface integrity). Therefore, it is estimated that an ultra-high speed spindle with dynamic stability (low vibration and run-out) can significantly improve efficiency of the grinding process of ceramics. This particularly holds true for ZrO2 in dental applications since the tool size is small and restricts cutting speeds, especially when form grinding cavities. Another subject to be analyzed is the influence of depth of cut and feed rate at constant SMRR and grinding speed. Fig. 6 shows grinding forces versus depth of cut and feed rate at a constant material removal rate of 8 mm3/mms and a grinding speed of 30 m/s for two different grain sizes. It can be observed that an increased feed rate causes even lower grinding forces than an increased depth of cut. However, it should be taken into consideration that an increase in feed rate also has a greater impact on the increasing of uncut chip thickness than an increase in depth of cut if operating at a constant material removal rate. This may cause the brittle mode to become predominant when grinding ceramics and result in lower surface quality and integrity.
Fig. 4. Brittle mode, transition zone (brittle–ductile) and ductile mode after grinding (hcu = 0.84 mm) with inter-crystalline and trans-crystalline breaks.
In the experimental setup the possibilities to increase specific material removal rate (SMRR), as the most important parameter of productivity, are to increase either feed rate or depth of cut or both. Increasing the material removal rate must be in such a way that the favourable workpiece quality can be achieved in terms of surface integrity, dimensional tolerances and roughness. The maximum SMRR reached during investigation without deteriorating quality amounted to 8 mm3/mm s which is in itself a very high SMRR when grinding ZrO2 at a grinding speed of 30 and 35 m/s. The small tool size (diameter of 6 mm) contributes to a lower cutting speed so that even when applying a high speed spindle a maximum grinding speed of 35 m/s could be achieved. Fig. 5 shows that grinding speed has a significant influence when grinding ZrO2. Thereby, the reduction of grinding forces is achieved through increasing grinding speed at a constant material removal rate. This is due to fact that a higher grinding speed causes lower uncut chip
Fig. 6. Specific grinding forces versus grinding speed.
The characteristics of the grinding process depend on many different factors among which the topography of the wheel in terms of active grains, grain spacing, chip pockets and grain sharpness is greatly affected by wheel wear. Grinding wheel wear with respect to attrition, grain fracture (splitting) and grain breakout can affect grinding forces, roughness and integrity of the ground surface as well as greatly influence tool life. The grinding ratio (G-ratio) and the ratio of tangential grinding force over specific material removal were investigated. In addition, the arrangement of the coolant nozzle, i.e. a conventional flexible nozzle and a jet nozzle, on the tangential grinding forces and grinding ratio were also examined (Fig. 7).
346
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
roughness by increasing the specific material removal rate as well as changing of the grinding wheel topography after first dressing and after grinding with a specific material removal of 7800 mm3/mm in the case of using a jet nozzle. Both grain splitting and grain break-out were observed (Fig. 8). 5. Conclusion
Fig. 7. Specific grinding forces and G-ratio versus specific material removal.
The experiments are carried out without any dressing intervals up to a specific material removal of 7800 mm3/mm. It was noticed that the tool was still sharp enough to continue grinding without any negative effects on surface quality and surface integrity. A G-ratio of 37,500 at a specific material removal of 5300 mm3/ mm is in general an enormous value which shows the low wear of the hybrid bond wheel when grinding ZrO2. This behaviour was observed using a jet nozzle. The G-ratio is about two times smaller when employing a conventional nozzle. On the contrary, using a jet nozzle, a specific tangential force of 23 N/mm is considerably greater than the specific tangential forces of 12 N/mm in case of conventional nozzle. The lower tangential force when using conventional nozzle can be interpreted because of grain fracture and grain break-out. Thus, a greater wear accompanied by a sharper cutting edge was observed by conventional nozzle compared to jet nozzle. The second phenomenon that can be seen in both curves is a decrease in grinding forces along with a decreasing G-ratio (i.e. increasing wear) if a specific removal of 5000–6000 mm3/mm is exceeded. This can be interpreted as a self-sharpening effect. The self-sharpening effect usually generates new cutting edges due to splitting or pulling out of worn grains, which increases the contact probability of sharp grains with the workpiece along with surface roughness increasing. Fig. 8 shows the variation in surface
Fig. 8. Surface roughness Ra/Rz of workpiece and tool surface topography after dressing (left) and after high material removal (right) when using jet nozzle cooling.
This paper introduces novel hybrid bonded diamond tools used to grind zirconium oxide. The high speed spindle integrated in the machine is a key factor to process ceramics using small-sized tools. These tools, in combination with proper grinding parameters and coolant system, ensure the ductile mode of grinding and guarantee the quality of the ground workpiece in terms of surface integrity. Thereby, the uncut chip thickness plays a major role. Increasing grinding speed at a constant material removal rate decreases the uncut chip thickness and leads the process to the ductile mode. The results show that it is possible to increase the material removal up to 8 mm3/mm s with an acceptable surface roughness when operating at a grinding speed of 35 m/s. Furthermore, the life time of hybrid bond diamond tools is extremely persistent without any need for a sharpening process. The radial wear of the tool is so low that a G-ratio of 35,000 can be achieved. A specific material removal of 7800 mm3/mm and more can be reached without any dressing interval. Acknowledgements The authors wish to gratefully acknowledge the financial support which was granted by the Swiss Innovation Promotion Agency (CTI) as well as the technical support provided by Meister Abrasives AG (Andelfingen), Mikron GF AC (Biel), Metoxit AG (Thayngen), Kistler AG (Winterthur) and Fischer Precise Group AG (Herzogenbuchsee), all located in Switzerland.
References [1] Richerson D (2005) Modern Ceramic Engineering: Properties, Processing, and Use in Design, CRC Press, New York. [2] Silva N, Sailer I, Zhang Y, Coelho P, Guess P, Zembic A, Kohal R (2010) Performance of Zirconia for Dental Healthcare. Materials 3:863–896. [3] Marinescu ID, Hitchinger M, Uhlmann E, Rowe WB, Inasak I (2007) Handbook of Machining with Grinding Wheels, CRC Press, New York. [4] Wenda, A, 2002. Schleifen von Mikrostrukturen in spro¨dharten Werkstoffe, Dissertation, TU-Bruanschweig. [5] Wegener K, Hoffmeister H-W, Karpuschewski B, Kuster F, Hahmann W-C, Rabiey M (2011) Conditioning and monitoring of grinding wheels. Annals of the CIRP 60/2:757–777. [6] Beyer P (2005) High-productive grinding with vitrified bond superabrasives— HPB technology for vitrified bond CBN wheels. Industrial Diamond Review 1/ 05:39–41. [7] Vera v, Sedano j, Corchado e, Redondo r, Hernando b, Camara M, Laham A, Garcia A (2011) A Hybrid System for Dental Milling Parameters Optimisation, Hybrid Artificial Intelligent Systems, Springer, Berlin Heidelberg437–446. [8] Bifano T, Dow T, Scattergood R (1991) Ductile-regime grinding: a new technology for machining brittle materials. Journal of Engineering for Industry 113:184–189. [9] Uhlmann. E., 1993. Tiefschleifen hochfester keramischer Werkstoffe, Dissertation, TU Berlin [10] Denkena B, To¨nshoff H (2011) Spanen, Springer-Verlag/VDI Buch.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
High performance grinding of zirconium oxide (ZrO2) using hybrid bond diamond tools Mohammad Rabiey (3)a,*, Nicolas Jochum b, Fredy Kuster (3)c a
ABB Turbosystems Ltd, Baden, Switzerland Blaser Swisslube AG, Hasle-Ru¨egsau, Switzerland c Institute of Machine Tools and Manufacturing, ETH Zurich, Switzerland b
Sponsored by Rainer Zu¨st (1), Seegra¨ben, Switzerland.
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Diamond tool Hybrid bond
Hybrid bond (metal–ceramic) diamond tools are proposed for grinding zirconium oxide used in medical implants. Compared to conventional grinding tools, material removal rates and tool life time are drastically increased without deterioration in mechanical properties of the workpiece. This is achieved within a selected process window in combination with an elaborate oil cooling system, where material removal is mainly occurring within the ductile cutting mode. Self-sharpening effect of the tool can be observed and the dressability of the tool further improves the grinding performance. ß 2013 CIRP.
1. Introduction In the past decade there have been major advances in production and usage of zirconium oxide, ZrO2, for dental applications because of its superior physical properties, including great strength and hardness at high temperature, low thermal expansion and good wear resistance [1,2]. Due to the precision and high quality necessary for the functionality of these components, a secondary machining process after sintering must often be performed. Furthermore, complete grinding of hard sintered ZrO2 has recently become one of the most common processes used in dental applications. However, ceramic like ZrO2 generally requires considerable time to be machined [3]. In the dental industry, the economic aspects of the machining process as well as the resulting product quality play key roles for the applications of ZrO2 to achieve good competitiveness compared to the other dental materials on the market. Therefore, an efficient grinding process of ZrO2 is a crucial challenge for medical engineering. 2. Grinding of ceramics, state of the art Diamond tools with electroplated, brazed and vitrified bonding are usually used to grind ceramics. The bond strength of resin bond wheels constraints their usage when grinding ceramics because of bond breakage. A similar situation occurs in case of vitrified bonds since grinding forces increase at higher material removal rates. Grinding ceramic materials with diamond tools, even if they contain electroplated or metal bonds, results in large great wear [4]. This causes tool costs to rise as tool life time is reduced. Costs are yet higher when the tool is not dressable. Metal bond wheels
* Corresponding author. Tel.: +41 786238293. E-mail addresses: [email protected], [email protected] (M. Rabiey). 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.073
however are generally faced with difficulties when dressed with conventional methods. This is due to the lack of porosity in the bonds which makes new dressing methods like ELID, laser dressing or EDM necessary in order to generate adequate chip pockets in the grinding tool surface [5]. Such modern dressing technologies require further equipment installed on the grinding machine, which incurs additional costs. Hybrid bond grinding wheels, as a newly developed bonding system, have great potential in industrial applications since they combine the advantages of both metal bonds (low wear and high strength) and vitrified bonds (porosity and reasonable chip pockets) [6]. Through proper selection of the grinding parameters it is possible to achieve a self-sharpening effect. This can result in a decrease of the dressing intervals. When grinding ZrO2 for dental applications, small tool dimensions are obviously necessary (hence the grinding speed is also limited). This is due to the complex 3D shape of dental bridges or implants which is to be produced by a 5-axis CNC machine using CAD/CAM software [7]. When grinding ceramics, the uncut chip thickness is one the most prominent parameters influencing the grinding process. Decreasing the uncut chip thickness below a critical value leads the grinding regime to the ductile mode [8]. The critical chip thickness depends mainly on material properties [8]. Generally, combinations of both brittle and ductile modes occur when grinding ceramics [9]. The ductile mode results superior quality in terms of accuracy and surface integrity. 3. Experiment conditions 3.1. Experimental setup Grinding experiments were carried out with a series of hybrid bond diamond tools on a 5-axis CNC machine tool (Mikron HSM400U). It was equipped with a high speed spindle (Fischer MFM-10120/11, electromotor speed 12,000–120,000 rpm, power
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
344
Table 3 Grinding parameters. Grinding parameters Grinding speed Depth of cut Width of cut Feed rate Specific material removal rate Strategy
vc = 5–35 m/s ae = 0.004–0.64 mm ap = 1–4 mm vf = 100–12,000 mm/min Q0 W = 2–8 mm3/mm s Up grinding
Table 4 shows the selected dressing parameters applied during the experiments. Negative () signs refer to up-dressing. The total depth of dressing was adjusted by the machine for each dressing experiment and set to 50 mm. Table 4 Dressing parameters for experiments. Fig. 1. Experimental setup and tools. Dressing parameters
3.5 kW) to grind ZrO2 (Fig. 1). The used coolant was oil (Blaser, Blasogrind HC5). Two different types of coolant nozzle systems were applied in the experiments: a conventional flexible coolant nozzle as well as a nozzle jet system. Both were implemented to ensure a high volumetric coolant flow rate and precise orientation of jet flow towards the contact zone (Fig. 1). The tests were conducted on workpieces with a dimension of 100 mm 10 mm 2 mm. By precisely measuring material thickness, the material removal rate was determined. The workpieces were ground to 100 mm length. The material properties of ZrO2 were constant for all tests during this investigation and are described in Table 1. Table 1 ZrO2 properties. Ceramic properties Components Composition Density Hardness Compressive strength Flexural strength Young’s-modulus Fracture toughness K1C
ZrO2/Y2O3/Al2O3 95/52/0.252 (%) 6.05 g/cm3 1200 Hv 2000 MPa 1200 MPa 210 GPa 8 MN/m2/3
Different tool shapes were used to manufacture the dental bridge. As an exception, for the systematic comparison tests only cylindrically shaped tools with a diameter of 6 mm were applied (Table 2).
Dressing wheel speed Dressing ratio Dressing feed rate (radial)
vcd = 60 m/s qd = 25 vfed = 50 mm/min
3.3. Measurement and analysis The wheel topography was measured by means of a 3D optical microscope (Alicona Infinite Focus) as well as SEM. The surface roughness of ground workpieces was measured optically by an Alicona Infinite Focus as well as by a Taylor–Hobson Form-Talysurf roughness profiler. Investigation of workpiece damage was carried out using four point bending tests and X-ray diffraction. A dynamometer platform was designed and developed using Kistler piezo-sensors, type: 9256C1 mounted under the workpiece clamping system in order to measure tangential forces Ft as well as normal forces Fn during grinding operation. For long-term grinding tests, the grinding forces were analyzed at different stages of specific material removal. To measure the change in tool radius and also to determine wear, an integrated commercial laser tool measuring system (BLUM Micro Compact NT) was applied. 4. Experimental results and discussion To study the ductile and/or brittle material removal mechanisms when grinding ZrO2, the uncut chip thickness was varied by changing depth of cut, feed rate and grinding speed. Fig. 2 shows SEM micrographs of the ground workpiece surface which exhibits three different material removal mechanisms. Fig. 2 (left) shows
Table 2 Tool properties. Tool properties Diameter Geometry Grain size Concentration Bond
1–6 mm Cylinder, ball or taper D64, D91 C100 Hybrid: metal–ceramic Fig. 2. Different material removal mechanisms when grinding ZrO2; brittle mode (left), brittle and ductile mode (middle) and ductile mode (right).
3.2. Grinding and dressing condition The grinding parameters used for all tests are summarized in Table 3. Grinding speed was limited to 35 m/s due to the rather small grinding wheel diameter as well as the maximum revolution constraint of the machine spindle (120,000 rpm). New diamond tools were dressed by a vitrified bond silicon carbide wheel (Meister 31C-80-H-10-190-V137-1) prior to each grinding process. Dressing parameters were kept constant. The initial dressing tool diameter was 100 mm with a wheel width of 5.25 mm.
the brittle mechanism with piled up material displacement. In Fig. 2 (middle) a combination of brittle and ductile mechanisms can be observed. In contrast, Fig. 2 (right) demonstrates the ductile mechanism. It can be seen that by increasing uncut chip thickness, the ductile mode slowly converts to the brittle mode. In order to detect the percentage of ductile mode, the surface of the ground workpiece was investigated. This percentage which is an important item of surface integrity was correlated with the mean uncut chip thickness (Fig. 3). The maximum uncut chip
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
Fig. 3. Ductile workpiece surface versus calculated chip thickness hcu.
345
Fig. 5. Specific grinding forces versus grinding speed.
thickness was calculated by [10]: hcu ¼
vf 1 ae vc l C max lg
1=2
(1)
where vf is the feed rate, vc is the grinding speed, ae is the depth of cut and lg is the geometric contact length. The two parameters, Cmax (kinematic cutting edges) and l (form factor or sharpness of grain) were measured based on surface topography of the grinding tool and amounts to 25 mm2 and 6.3 respectively. Measurement values and calculation results are shown in Fig. 3. It can be seen that increasing mean uncut chip thickness entails a decrease in percentage of the ductile mode. It was observed that even if a defined calculated mean uncut chip thickness is applied, there were some areas on the workpiece surface that contained a mixture of all three modes: brittle, brittle–ductile and ductile (Fig. 4). This is due to the stochastic nature of grinding tool surface topography and the fact that active cutting edges are not all situated at the same protrusion level.
thickness and smaller forces per grain as material removal rate is kept constant. A reduction of uncut chip thickness leads the grinding regime of ceramics towards a more ductile mode resulting in lower grinding forces, higher productivity (higher material removal rate) and better quality (surface roughness and surface integrity). Therefore, it is estimated that an ultra-high speed spindle with dynamic stability (low vibration and run-out) can significantly improve efficiency of the grinding process of ceramics. This particularly holds true for ZrO2 in dental applications since the tool size is small and restricts cutting speeds, especially when form grinding cavities. Another subject to be analyzed is the influence of depth of cut and feed rate at constant SMRR and grinding speed. Fig. 6 shows grinding forces versus depth of cut and feed rate at a constant material removal rate of 8 mm3/mms and a grinding speed of 30 m/s for two different grain sizes. It can be observed that an increased feed rate causes even lower grinding forces than an increased depth of cut. However, it should be taken into consideration that an increase in feed rate also has a greater impact on the increasing of uncut chip thickness than an increase in depth of cut if operating at a constant material removal rate. This may cause the brittle mode to become predominant when grinding ceramics and result in lower surface quality and integrity.
Fig. 4. Brittle mode, transition zone (brittle–ductile) and ductile mode after grinding (hcu = 0.84 mm) with inter-crystalline and trans-crystalline breaks.
In the experimental setup the possibilities to increase specific material removal rate (SMRR), as the most important parameter of productivity, are to increase either feed rate or depth of cut or both. Increasing the material removal rate must be in such a way that the favourable workpiece quality can be achieved in terms of surface integrity, dimensional tolerances and roughness. The maximum SMRR reached during investigation without deteriorating quality amounted to 8 mm3/mm s which is in itself a very high SMRR when grinding ZrO2 at a grinding speed of 30 and 35 m/s. The small tool size (diameter of 6 mm) contributes to a lower cutting speed so that even when applying a high speed spindle a maximum grinding speed of 35 m/s could be achieved. Fig. 5 shows that grinding speed has a significant influence when grinding ZrO2. Thereby, the reduction of grinding forces is achieved through increasing grinding speed at a constant material removal rate. This is due to fact that a higher grinding speed causes lower uncut chip
Fig. 6. Specific grinding forces versus grinding speed.
The characteristics of the grinding process depend on many different factors among which the topography of the wheel in terms of active grains, grain spacing, chip pockets and grain sharpness is greatly affected by wheel wear. Grinding wheel wear with respect to attrition, grain fracture (splitting) and grain breakout can affect grinding forces, roughness and integrity of the ground surface as well as greatly influence tool life. The grinding ratio (G-ratio) and the ratio of tangential grinding force over specific material removal were investigated. In addition, the arrangement of the coolant nozzle, i.e. a conventional flexible nozzle and a jet nozzle, on the tangential grinding forces and grinding ratio were also examined (Fig. 7).
346
M. Rabiey et al. / CIRP Annals - Manufacturing Technology 62 (2013) 343–346
roughness by increasing the specific material removal rate as well as changing of the grinding wheel topography after first dressing and after grinding with a specific material removal of 7800 mm3/mm in the case of using a jet nozzle. Both grain splitting and grain break-out were observed (Fig. 8). 5. Conclusion
Fig. 7. Specific grinding forces and G-ratio versus specific material removal.
The experiments are carried out without any dressing intervals up to a specific material removal of 7800 mm3/mm. It was noticed that the tool was still sharp enough to continue grinding without any negative effects on surface quality and surface integrity. A G-ratio of 37,500 at a specific material removal of 5300 mm3/ mm is in general an enormous value which shows the low wear of the hybrid bond wheel when grinding ZrO2. This behaviour was observed using a jet nozzle. The G-ratio is about two times smaller when employing a conventional nozzle. On the contrary, using a jet nozzle, a specific tangential force of 23 N/mm is considerably greater than the specific tangential forces of 12 N/mm in case of conventional nozzle. The lower tangential force when using conventional nozzle can be interpreted because of grain fracture and grain break-out. Thus, a greater wear accompanied by a sharper cutting edge was observed by conventional nozzle compared to jet nozzle. The second phenomenon that can be seen in both curves is a decrease in grinding forces along with a decreasing G-ratio (i.e. increasing wear) if a specific removal of 5000–6000 mm3/mm is exceeded. This can be interpreted as a self-sharpening effect. The self-sharpening effect usually generates new cutting edges due to splitting or pulling out of worn grains, which increases the contact probability of sharp grains with the workpiece along with surface roughness increasing. Fig. 8 shows the variation in surface
Fig. 8. Surface roughness Ra/Rz of workpiece and tool surface topography after dressing (left) and after high material removal (right) when using jet nozzle cooling.
This paper introduces novel hybrid bonded diamond tools used to grind zirconium oxide. The high speed spindle integrated in the machine is a key factor to process ceramics using small-sized tools. These tools, in combination with proper grinding parameters and coolant system, ensure the ductile mode of grinding and guarantee the quality of the ground workpiece in terms of surface integrity. Thereby, the uncut chip thickness plays a major role. Increasing grinding speed at a constant material removal rate decreases the uncut chip thickness and leads the process to the ductile mode. The results show that it is possible to increase the material removal up to 8 mm3/mm s with an acceptable surface roughness when operating at a grinding speed of 35 m/s. Furthermore, the life time of hybrid bond diamond tools is extremely persistent without any need for a sharpening process. The radial wear of the tool is so low that a G-ratio of 35,000 can be achieved. A specific material removal of 7800 mm3/mm and more can be reached without any dressing interval. Acknowledgements The authors wish to gratefully acknowledge the financial support which was granted by the Swiss Innovation Promotion Agency (CTI) as well as the technical support provided by Meister Abrasives AG (Andelfingen), Mikron GF AC (Biel), Metoxit AG (Thayngen), Kistler AG (Winterthur) and Fischer Precise Group AG (Herzogenbuchsee), all located in Switzerland.
References [1] Richerson D (2005) Modern Ceramic Engineering: Properties, Processing, and Use in Design, CRC Press, New York. [2] Silva N, Sailer I, Zhang Y, Coelho P, Guess P, Zembic A, Kohal R (2010) Performance of Zirconia for Dental Healthcare. Materials 3:863–896. [3] Marinescu ID, Hitchinger M, Uhlmann E, Rowe WB, Inasak I (2007) Handbook of Machining with Grinding Wheels, CRC Press, New York. [4] Wenda, A, 2002. Schleifen von Mikrostrukturen in spro¨dharten Werkstoffe, Dissertation, TU-Bruanschweig. [5] Wegener K, Hoffmeister H-W, Karpuschewski B, Kuster F, Hahmann W-C, Rabiey M (2011) Conditioning and monitoring of grinding wheels. Annals of the CIRP 60/2:757–777. [6] Beyer P (2005) High-productive grinding with vitrified bond superabrasives— HPB technology for vitrified bond CBN wheels. Industrial Diamond Review 1/ 05:39–41. [7] Vera v, Sedano j, Corchado e, Redondo r, Hernando b, Camara M, Laham A, Garcia A (2011) A Hybrid System for Dental Milling Parameters Optimisation, Hybrid Artificial Intelligent Systems, Springer, Berlin Heidelberg437–446. [8] Bifano T, Dow T, Scattergood R (1991) Ductile-regime grinding: a new technology for machining brittle materials. Journal of Engineering for Industry 113:184–189. [9] Uhlmann. E., 1993. Tiefschleifen hochfester keramischer Werkstoffe, Dissertation, TU Berlin [10] Denkena B, To¨nshoff H (2011) Spanen, Springer-Verlag/VDI Buch.