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Investigations on the thermal workpiece distortion in MQL deep hole drilling of an aluminium cast alloy
CIRP Annals - Manufacturing Technology 64 (2015) 85–88
Contents lists available at 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
Investigations on the thermal workpiece distortion in MQL deep hole drilling of an aluminium cast alloy D. Biermann (2)*, I. Iovkov Institute of Machining Technology, TU Dortmund University, 44227 Dortmund, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep hole drilling Thermal effects Compensation
Dry machining is frequently applied in cutting operations, in order to reduce the energy consumption and the production costs. In deep hole drilling operations minimum quantity lubrication (MQL) is used to obtain a reliable chip evacuation, since completely dry machining is not feasible. Due to the low cooling effect of MQL, the drilling process generates a high thermal load on the workpiece, which leads to thermally induced workpiece deformations. This paper presents fundamental experimental investigations on the workpiece temperature, the resulting in-process deformations and the achievable straightness accuracy of the borehole. The investigations focus on two different strategies for enhancing the deep hole drilling using MQL. Initially, a high-feed process guiding is introduced, in order to obtain a higher productivity and to reduce the heat input into the workpiece. The second approach is a novel radial spindle compensation, which performs a directional control of the straightness deviation of the deep hole. ß 2015 CIRP.
1. Introduction Deep hole drilling methods are applied in the metalworking industry when the length-to-diameter ratio of a hole to be machined is higher than l/D 8,. . .,10 [1]. For these processes, twist drills provide advantages with regard to the achievable productivity. The symmetrical double cutting edge design allows higher feed rates in comparison to the conventional single-lip deep hole drilling tools. Furthermore, the positive helix angle of the flutes assists the chip evacuation by facilitating a chip motion opposite to the feed direction, according to the principle of the Archimedes’ screw. Contrariwise, the double-edged set-up leads to a reduced cross section of the flutes compared to the single-lip drilling tool [2,3]. Due to the increasing energy costs, modern machining processes require a continuous improvement with respect to their energy consumption. The MQL method has the potential for decreasing the total energy consumption of the production process [3–5], considering the savings of procurement, service and recycling costs of the conventional coolant. Furthermore, the complete cooling lubricant unit and the corresponding relatively high energy consumption can be reduced [6–8]. However, MQL provides not only improvements, but yields a set of requirements and technological challenges for the cutting process [9,10]. Drilling of aluminium under MQL conditions, particularly deep hole drilling, is challenging because of the high adhesion affinity of this material and the reduced chip removal performance of compressed air used as carrier medium in MQL [11,12]. Furthermore, the increased heat input into the workpiece generates thermally induced deformations and
* Corresponding author. E-mail address: [email protected] (D. Biermann). http://dx.doi.org/10.1016/j.cirp.2015.04.072 0007-8506/ß 2015 CIRP.
deviations of the machined component [13]. Former investigations indicated the major influence of the feed on the resulting temperature of the machined component [14,15]. Hence, the presented research aims at the reducing of the thermal workpiece load using an extremely high feed. Furthermore, a new technique for directional guiding of the deep hole drilling tool is introduced, in order to realise a directed straightness deviation control. 2. Experimental set-up The material used in the investigations is the aluminium alloy EN AC-46000, which is applied in the automotive industry for engine and gearbox housing components. The experiments were carried out on a 4-axis machining centre GROB BZ600 using a special internal three-channel MQL supply device with an increased operating pressure of pMQL = 15 bar. The experimental set-up is shown in Fig. 1, including the measurement equipment for the determination of the process forces and the deformations of
Fig. 1. Experimental set-up including the measurement equipment.
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D. Biermann, I. Iovkov / CIRP Annals - Manufacturing Technology 64 (2015) 85–88
the machined component. The workpiece temperature was measured by an infrared (IR) camera on its upper surface, which was pre-treated with a camera interior painting spray to provide a thin-film coating with a known emission coefficient of e 0.98. An additional shield tube (not shown in the figure) was applied to exclude the environmental radiation [16]. A long slot in the workpiece allows the thermal expansion of the upper, machined component area, independent from the clamping zone and provides symmetric thermal boundary conditions around the drilled hole. To machine the centrically applied drill holes, uncoated solid carbide twist drills with a diameter of d = 10 mm and a total length of lt = 360 mm were used. The length of the polished flutes of lf 320 mm allows a maximum drilling depth of lmax 300 mm, which was realised in the investigations. A suitable pilot drilling tool was used to generate a pilot hole with a depth of l = 30 mm. The cutting speed of vc = 175 m/min was kept constant within the investigations. 3. High-feed deep hole drilling 3.1. Fundamental investigations The experiments under high-feed conditions were carried out in the feed range between f = 1 mm and f = 4 mm using optimised tools with an adapted cutting edge and web thickness design. Due to the expected extremely high mechanical load, the final feed was not applied from the beginning of the drilling process. According to a reference experiment with a feed of fref = 0.3 mm, an initial feed of finit = 0.3 mm is used within the initial feeding length of linit = 50 mm behind the pilot hole in order to ensure a good guidance of the deep hole drilling tool. In addition, a ramped increase of the feed over a length of lramp = 50 mm was implemented to reduce the impact load on the drill. Fig. 2a shows the resulting feed increase curves over the drilling depth and the great enhancement of the material removal rate compared to conventional drilling. This is also illustrated by the resulting decrease of the machining time th (Fig. 2b). In case of the highest final feed fend = 4 mm the corresponding drilling depth of lend = 170 mm could be machined within th < 0.5 s.
Fig. 3. Mechanical (a) and thermal load (b) for different final feeds.
the machining time and the resulting total energy consumption of the process decrease. The reference process with f = 0.3 mm requires an active energy of Ea 19.3 kJ and a corresponding feed energy of only Ef 0.22 kJ. The higher amount of heat flowing into the chips, the reduced energy consumption and the shorter contact time of the high-feed process lead to a generally decreased heat input into the workpiece in comparison to the reference experiment (Fig. 3b). Due to the balance between the higher heat flux and the shorter machining time when increasing the feed, the thermal load on the workpiece remains for all high feed values nearly constant. The in-process workpiece deformations and the resulting straightness deviation of the drilled holes are shown in Fig. 4. Due to the defined workpiece geometry, the thermomechanical deformations are negligible in x-direction. Thus, the significant deflection in y-direction is analysed with regard to both the thermal and the mechanical loads (Fig. 4a). The remaining thermal deformation uth after all high-feed drilling processes is nearly on the same level of uth 70 mm. The non-thermal elastic component of the workpiece deflection umech is represented by the spring-back of the machined part after the process ends. It correlates with the increasing feed force with higher feed rates.
Fig. 2. Investigated feeds over the drilling depth and resulting material removal rate (a); calculated machining time (b).
Fig. 3 summarises the mechanical loads and the resulting workpiece temperatures for the investigated feed rates. The feed force and the drilling torque are increasing distinctly with higher feed rates. They reach mean values of Ff 8 kN and Md = 30 N m when drilling with a feed of fend = 4 mm. The calculated machining time (cf. Fig. 2b) equates not exactly the real time measured in the experiments. The discrepancy of approximately t = 0.1 s can be explained by the acceleration behaviour of the feed drive. In contrast to conventional drilling processes the feed energy in highfeed drilling is not negligible compared to the cutting energy. For example, the highest feed of f = 4 mm generates a feed energy of Ef 1.7 kJ and a cutting energy of Ec 14 kJ. Generally, the required feed and cutting power correlate with the feed force and the drilling torque. Hence, the active power, required by the process, increases when drilling with a higher feed. However, both
Fig. 4. In-process workpiece deformation (a); resulting straightness deviation of the drilled hole (b) for different final feed rates.
The straightness deviation is defined as the location error of the drilled hole exit, relative to the entry position. Generally, it decreases for the feeds of fend = 1 mm and fend = 2 mm because of the reduced thermal load and the moderate force level. When applying the highest feed of fend = 4 mm, the resulting straightness deviation increases remarkably to a value of my 0.6 mm. With regard to the measured temperature (Fig. 3b) and the in-process deformation of the workpiece, this high deviation results from the
D. Biermann, I. Iovkov / CIRP Annals - Manufacturing Technology 64 (2015) 85–88
elastic distortion of the workpiece and is not caused by thermal expansion effects of the component. 3.2. Influence of the feed increase strategy According to the feed increase method used in the feed variation presented in Section 3.1 (strategy A), two additional strategies B and C were developed in order to analyse the influence of the feed increase ramp on the deep-hole drilling process. These three strategies (A, B and C) start with a constant feed of finit = 0.3 mm/rev to a depth of l = 80 mm and differ only in the length of the feed ramp used to increase the feed to the final value of fend = 2 mm/rev (Fig. 5a). Furthermore, a fourth strategy excluding the low feed section at the beginning of the deep-hole drilling process (strategy D) was developed to investigate the influence of the initial guiding of the tool. Thereby the feed increase ramp starts at the beginning of the deep-hole drilling process, directly behind the pilot hole. The resulting machining time depends on the strategy, but even the longest process using strategy C of tC = 3.86 s is more than 2.5 times shorter than the reference main time of tref = 9.7 s (Fig. 5b).
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comparison to the reference process, the long-ramp strategy C generates a reduced heat input into the workpiece, but the highest thermal impact compared to the other approaches. This effect corresponds to the lower feed of strategy C, which generates also a different isotherm shape compared to the strategies A, B and D. The centre of the thermogram image is located at a drilling depth of l = 212.5 mm, where the process referred to strategy C attains a feed of f 1.35 mm. The strategies A, B and D have already reached their final feed of fend = 2 mm when passing the measuring field of the IR-camera. Nevertheless, there is a significant difference between the strategies with an initial low-feed zone (A/B) and the approach D without this sector. The high thermal load accumulated in the initial low feed stage of the strategies A and B (drilling depth l = 30,. . .,80 mm) is conducted through the workpiece and can be detected even in the measuring drilling depth of l = 195,. . .,230 mm (Fig. 6b). Due to the missing low feed range and hence the low initial heat input, strategy D generates the lowest workpiece temperature. The resulting workpiece deflection is presented in Fig. 7a and reaches nearly equal final values for all strategies. In contrast, the resulting straightness deviation corresponds to the guiding and the ramp length of the applied approach (Fig. 7b). The direct comparison between strategies A and B shows that the feed increase ramp has a significant influence on the magnitude of the straightness deviation in high-feed deep hole drilling. The lowest deviation could be achieved by strategy C, where the smooth increase of the feed produces a low thermal load and a low force level as well, particularly in the front of the machined hole, where the straightness deviation is initialised.
Fig. 5. Feed increase strategies (a); calculated machining time (b).
Generally, the mechanical load correlates with the actual feed in the corresponding drilling depth and attains mean values of Ff 3.8 kN and Md 17.3 N m after reaching the final feed (Fig. 6a). The energy consumption of the process corresponds to the machining time. Thus, the long ramp strategy C requires the highest active work, and strategy D consumes the lowest amount of energy. Due to the long high-feed sector of strategy D and the missing initial low-feed range in this process, the cutting work decreases compared to the other strategies (Fig. 6a). The behaviour of the workpiece temperature (Fig. 6b) is analogue to the observations of the energy consumption. In
Fig. 7. In-process workpiece deformation (a); resulting straightness deviation of the drilled hole (b) for different feed increase strategies.
4. Directional control of the straightness deviation In conventional drilling (Fig. 8a) the tool axis is coincident to the axis of the drilled hole. In order to realise a controlled correction of the drilling direction and thus to compensate systematic straightness deviations, a novel approach using an additional radial feeding path of the machine tool’s spindle was developed (Fig. 8b). Due to the tapered guiding chamfers, which are present only in the front area of the drilling tool, it is possible to perform a small directional correction of the tool by a radial displacement of the tool holder, leaving the original drilling axis. The bending
Fig. 6. Mechanical (a) and thermal load (b) depending on the strategy.
Fig. 8. Conventional drilling (a) and concept for directional control of the straightness deviation using a radial spindle compensation path (b).
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identical surface areas of AB AC 40 mm2 and produce repeatable straightness deviations of my 0.4,. . .,0.5 mm. 5. Conclusions and outlook
Fig. 9. Resulting straightness deviation my of drilled holes using varied trapezoidal functions of the radial compensation path uy.
flexibility of the standard solid carbide twist drill used for this investigation of d 0.2175 mm/N was determined at the cutting edge by a measurement of the deflection when applying a radial force of Frad = 10 N. Due to the helix angle of the tool, the bending stiffness does not depend on the measuring direction in the transverse plane. In contrast, the single-lip drill has a different bending stiffness, which can cause undesired vibration effects. In addition to the taper angle of the guiding chamfers, the radial clearance between the drilling tool and the machined hole has to be considered. The difference between the hole with d = 10 mm and the tool shaft behind the guiding chamfers with d = 9.5 mm results in a constant radial clearance of uc = 0.25 mm, but the limit for the maximal radial displacement of the spindle urad decreases by higher drilling depth due to the risk of collision. Considering the existent radial clearance uc, the maximum radial spindle respectively tool holder displacement for a tool with an exposed length lexp can be calculated by the rule of proportion umaxl = ucldrill. Applying the introduced approach and considering the presented limitations, different strategies for the radial compensation path of the spindle were analysed. Due to the given experimental set-up, the thermal expansion and the resulting straightness deviations occur particularly in positive y-direction. Therefore, the compensation strategies concentrate on radial paths directed in positive y-direction as well. The generated tool deflection works in an inverse manner and directs the straightness deviation in the opposite y-direction. The initially investigated triangle functions for the radial path along the drilling depth achieved a good compensation effect, but relatively poor reproducibility of the obtained straightness deviations. However, a further improvement could be realised by modifying the initial triangle function to a strategy with a linear compensation length. The results for the resulting trapezoidal function of the radial compensation path are presented in Fig. 9. The straightness deviations could be guided reliably to the required direction and the amount of the resulting effect correlates with the surface area of the compensation function. The trapezium strategy A with an area of AA = 25 mm2 generates a straightness deviation of my 0.3 mm. The increased amplitude of strategy C and the longer maximum value range of strategy B result in approximately
The presented fundamental investigations demonstrate the feasibility of the high-feed deep hole drilling of aluminium die cast alloys using MQL. The heat input into the workpiece could be significantly reduced using increased feed values of f = 1,. . .,4 mm, which corresponds to the higher heat removal by the chips and the generally decreased energy consumption of the cutting process. The best performance, considering the thermal load, the time saving and the resulting straightness of the machined holes, could be achieved using a long-ramped feed increase strategy with an initial low-feed guiding drilling depth. Furthermore, a novel deep-hole drilling strategy for the reproducible compensation of straightness deviation was introduced. The superimposed radial feed motion path leads to a repeatable directional control of the drilled hole straightness. Further research work will focus on the simulative prediction of the workpiece deviations and the compensation of the straightness deviations occurring in deep hole drilling situations of thin-walled components. Acknowledgements The authors acknowledge the support of the German Research Foundation (DFG) for funding this research within the project BI498/24-3 in the framework of the Priority Programme 1480.
References [1] Tai BL, Stephenson DA, Furness RJ, Shih AJ (2014) Minimum Quantity Lubrication (MQL) in Automotive Powertrain Machining. Procedia CIRP 14:523–528. [2] Weinert K, Inasaki I, Sutherland JW, Wakabayashi T (2004) Dry Machining and Minimum Quantity Lubrication. CIRP Annals 53(2):511–537. [3] Zabel A, Heilmann M (2012) Deep Hole Drilling Using Tools with Small Diameters – Process Analysis and Process Design. CIRP Annals 61(1):111–114. [4] Klocke F, Eisenbla¨tter G (1997) Dry Cutting. CIRP Annals 46(2):519–526. [5] Byrne G, Dornfeld D, Denkena B (2003) Advancing Cutting Technology. CIRP Annals 52(2):483–507. [6] Denkena B, Helmecke P, Hu¨lsemeyer L (2014) Energy Efficient Machining with Optimized Coolant Lubrication Flow Rates. Procedia CIRP 24:25–31. [7] Duflou JR, Sutherland JW, Dornfeld D, Herrmann C, Jeswiet J, Kara S, Hauschild M, Kellens K (2012) Towards Energy and Resource Efficient Manufacturing: A Processes and Systems Approach. CIRP Annals 61(2):587–609. [8] Newman ST, Nassehi A, Imani-Asrai R, Dhokia V (2012) Energy Efficient Process Planning for CNC Machining. CIRP JMST 5(2):127–136. [9] Wakabayashi T, Suda S, Inasaki I, Terasaka K, Musha Y, Toda Y (2007) Tribological Action and Cutting Performance of MQL Media in Machining of Aluminum. CIRP Annals 56(1):97–100. [10] Kelly JF, Cotterell MG (2002) Minimal Lubrication Machining of Aluminium Alloys. Journal of Materials Processing Technology 120(1–3):327–334. [11] Braga DU, Diniz AE, Miranda GWA, Coppini NL (2002) Using a Minimum Quantity of Lubricant (MQL) and a Diamond Coated Tool in the Drilling of Aluminum–Silicon Alloys. Journal of Materials Processing Technology 122(1):127–138. [12] Nouari M, List G, Girot F, Ge´hin D (2005) Effect of Machining Parameters and Coating on Wear Mechanisms in Dry Drilling of Aluminium Alloys. International Journal of Machine Tools and Manufacture 45(12–13):1436–1442. [13] Neugebauer R, Drossel WG, Ihlenfeldt S, Richter C (2012) Thermal Interactions Between the Process and Workpiece. Procedia CIRP 4:63–66. [14] Biermann D, Iovkov I, Blum H, Rademacher A, Taebi K, Suttmeier FT, Klein N (2012) Thermal Aspects in Deep Hole Drilling of Aluminium Cast Alloy Using Twist Drills and MQL. Procedia CIRP 3:245–250. [15] Biermann D, Iovkov I (2013) Modeling and Simulation of Heat Input in DeepHole Drilling with Twist Drills and MQL. Procedia CIRP 8:88–93. [16] Davies MA, Ueda T, M’Saoubi R, Mullany B, Cooke AL (2007) On the Measurement of Temperature in Material Removal Processes. CIRP Annals 56(2): 581–604.
Contents lists available at 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
Investigations on the thermal workpiece distortion in MQL deep hole drilling of an aluminium cast alloy D. Biermann (2)*, I. Iovkov Institute of Machining Technology, TU Dortmund University, 44227 Dortmund, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep hole drilling Thermal effects Compensation
Dry machining is frequently applied in cutting operations, in order to reduce the energy consumption and the production costs. In deep hole drilling operations minimum quantity lubrication (MQL) is used to obtain a reliable chip evacuation, since completely dry machining is not feasible. Due to the low cooling effect of MQL, the drilling process generates a high thermal load on the workpiece, which leads to thermally induced workpiece deformations. This paper presents fundamental experimental investigations on the workpiece temperature, the resulting in-process deformations and the achievable straightness accuracy of the borehole. The investigations focus on two different strategies for enhancing the deep hole drilling using MQL. Initially, a high-feed process guiding is introduced, in order to obtain a higher productivity and to reduce the heat input into the workpiece. The second approach is a novel radial spindle compensation, which performs a directional control of the straightness deviation of the deep hole. ß 2015 CIRP.
1. Introduction Deep hole drilling methods are applied in the metalworking industry when the length-to-diameter ratio of a hole to be machined is higher than l/D 8,. . .,10 [1]. For these processes, twist drills provide advantages with regard to the achievable productivity. The symmetrical double cutting edge design allows higher feed rates in comparison to the conventional single-lip deep hole drilling tools. Furthermore, the positive helix angle of the flutes assists the chip evacuation by facilitating a chip motion opposite to the feed direction, according to the principle of the Archimedes’ screw. Contrariwise, the double-edged set-up leads to a reduced cross section of the flutes compared to the single-lip drilling tool [2,3]. Due to the increasing energy costs, modern machining processes require a continuous improvement with respect to their energy consumption. The MQL method has the potential for decreasing the total energy consumption of the production process [3–5], considering the savings of procurement, service and recycling costs of the conventional coolant. Furthermore, the complete cooling lubricant unit and the corresponding relatively high energy consumption can be reduced [6–8]. However, MQL provides not only improvements, but yields a set of requirements and technological challenges for the cutting process [9,10]. Drilling of aluminium under MQL conditions, particularly deep hole drilling, is challenging because of the high adhesion affinity of this material and the reduced chip removal performance of compressed air used as carrier medium in MQL [11,12]. Furthermore, the increased heat input into the workpiece generates thermally induced deformations and
* Corresponding author. E-mail address: [email protected] (D. Biermann). http://dx.doi.org/10.1016/j.cirp.2015.04.072 0007-8506/ß 2015 CIRP.
deviations of the machined component [13]. Former investigations indicated the major influence of the feed on the resulting temperature of the machined component [14,15]. Hence, the presented research aims at the reducing of the thermal workpiece load using an extremely high feed. Furthermore, a new technique for directional guiding of the deep hole drilling tool is introduced, in order to realise a directed straightness deviation control. 2. Experimental set-up The material used in the investigations is the aluminium alloy EN AC-46000, which is applied in the automotive industry for engine and gearbox housing components. The experiments were carried out on a 4-axis machining centre GROB BZ600 using a special internal three-channel MQL supply device with an increased operating pressure of pMQL = 15 bar. The experimental set-up is shown in Fig. 1, including the measurement equipment for the determination of the process forces and the deformations of
Fig. 1. Experimental set-up including the measurement equipment.
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D. Biermann, I. Iovkov / CIRP Annals - Manufacturing Technology 64 (2015) 85–88
the machined component. The workpiece temperature was measured by an infrared (IR) camera on its upper surface, which was pre-treated with a camera interior painting spray to provide a thin-film coating with a known emission coefficient of e 0.98. An additional shield tube (not shown in the figure) was applied to exclude the environmental radiation [16]. A long slot in the workpiece allows the thermal expansion of the upper, machined component area, independent from the clamping zone and provides symmetric thermal boundary conditions around the drilled hole. To machine the centrically applied drill holes, uncoated solid carbide twist drills with a diameter of d = 10 mm and a total length of lt = 360 mm were used. The length of the polished flutes of lf 320 mm allows a maximum drilling depth of lmax 300 mm, which was realised in the investigations. A suitable pilot drilling tool was used to generate a pilot hole with a depth of l = 30 mm. The cutting speed of vc = 175 m/min was kept constant within the investigations. 3. High-feed deep hole drilling 3.1. Fundamental investigations The experiments under high-feed conditions were carried out in the feed range between f = 1 mm and f = 4 mm using optimised tools with an adapted cutting edge and web thickness design. Due to the expected extremely high mechanical load, the final feed was not applied from the beginning of the drilling process. According to a reference experiment with a feed of fref = 0.3 mm, an initial feed of finit = 0.3 mm is used within the initial feeding length of linit = 50 mm behind the pilot hole in order to ensure a good guidance of the deep hole drilling tool. In addition, a ramped increase of the feed over a length of lramp = 50 mm was implemented to reduce the impact load on the drill. Fig. 2a shows the resulting feed increase curves over the drilling depth and the great enhancement of the material removal rate compared to conventional drilling. This is also illustrated by the resulting decrease of the machining time th (Fig. 2b). In case of the highest final feed fend = 4 mm the corresponding drilling depth of lend = 170 mm could be machined within th < 0.5 s.
Fig. 3. Mechanical (a) and thermal load (b) for different final feeds.
the machining time and the resulting total energy consumption of the process decrease. The reference process with f = 0.3 mm requires an active energy of Ea 19.3 kJ and a corresponding feed energy of only Ef 0.22 kJ. The higher amount of heat flowing into the chips, the reduced energy consumption and the shorter contact time of the high-feed process lead to a generally decreased heat input into the workpiece in comparison to the reference experiment (Fig. 3b). Due to the balance between the higher heat flux and the shorter machining time when increasing the feed, the thermal load on the workpiece remains for all high feed values nearly constant. The in-process workpiece deformations and the resulting straightness deviation of the drilled holes are shown in Fig. 4. Due to the defined workpiece geometry, the thermomechanical deformations are negligible in x-direction. Thus, the significant deflection in y-direction is analysed with regard to both the thermal and the mechanical loads (Fig. 4a). The remaining thermal deformation uth after all high-feed drilling processes is nearly on the same level of uth 70 mm. The non-thermal elastic component of the workpiece deflection umech is represented by the spring-back of the machined part after the process ends. It correlates with the increasing feed force with higher feed rates.
Fig. 2. Investigated feeds over the drilling depth and resulting material removal rate (a); calculated machining time (b).
Fig. 3 summarises the mechanical loads and the resulting workpiece temperatures for the investigated feed rates. The feed force and the drilling torque are increasing distinctly with higher feed rates. They reach mean values of Ff 8 kN and Md = 30 N m when drilling with a feed of fend = 4 mm. The calculated machining time (cf. Fig. 2b) equates not exactly the real time measured in the experiments. The discrepancy of approximately t = 0.1 s can be explained by the acceleration behaviour of the feed drive. In contrast to conventional drilling processes the feed energy in highfeed drilling is not negligible compared to the cutting energy. For example, the highest feed of f = 4 mm generates a feed energy of Ef 1.7 kJ and a cutting energy of Ec 14 kJ. Generally, the required feed and cutting power correlate with the feed force and the drilling torque. Hence, the active power, required by the process, increases when drilling with a higher feed. However, both
Fig. 4. In-process workpiece deformation (a); resulting straightness deviation of the drilled hole (b) for different final feed rates.
The straightness deviation is defined as the location error of the drilled hole exit, relative to the entry position. Generally, it decreases for the feeds of fend = 1 mm and fend = 2 mm because of the reduced thermal load and the moderate force level. When applying the highest feed of fend = 4 mm, the resulting straightness deviation increases remarkably to a value of my 0.6 mm. With regard to the measured temperature (Fig. 3b) and the in-process deformation of the workpiece, this high deviation results from the
D. Biermann, I. Iovkov / CIRP Annals - Manufacturing Technology 64 (2015) 85–88
elastic distortion of the workpiece and is not caused by thermal expansion effects of the component. 3.2. Influence of the feed increase strategy According to the feed increase method used in the feed variation presented in Section 3.1 (strategy A), two additional strategies B and C were developed in order to analyse the influence of the feed increase ramp on the deep-hole drilling process. These three strategies (A, B and C) start with a constant feed of finit = 0.3 mm/rev to a depth of l = 80 mm and differ only in the length of the feed ramp used to increase the feed to the final value of fend = 2 mm/rev (Fig. 5a). Furthermore, a fourth strategy excluding the low feed section at the beginning of the deep-hole drilling process (strategy D) was developed to investigate the influence of the initial guiding of the tool. Thereby the feed increase ramp starts at the beginning of the deep-hole drilling process, directly behind the pilot hole. The resulting machining time depends on the strategy, but even the longest process using strategy C of tC = 3.86 s is more than 2.5 times shorter than the reference main time of tref = 9.7 s (Fig. 5b).
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comparison to the reference process, the long-ramp strategy C generates a reduced heat input into the workpiece, but the highest thermal impact compared to the other approaches. This effect corresponds to the lower feed of strategy C, which generates also a different isotherm shape compared to the strategies A, B and D. The centre of the thermogram image is located at a drilling depth of l = 212.5 mm, where the process referred to strategy C attains a feed of f 1.35 mm. The strategies A, B and D have already reached their final feed of fend = 2 mm when passing the measuring field of the IR-camera. Nevertheless, there is a significant difference between the strategies with an initial low-feed zone (A/B) and the approach D without this sector. The high thermal load accumulated in the initial low feed stage of the strategies A and B (drilling depth l = 30,. . .,80 mm) is conducted through the workpiece and can be detected even in the measuring drilling depth of l = 195,. . .,230 mm (Fig. 6b). Due to the missing low feed range and hence the low initial heat input, strategy D generates the lowest workpiece temperature. The resulting workpiece deflection is presented in Fig. 7a and reaches nearly equal final values for all strategies. In contrast, the resulting straightness deviation corresponds to the guiding and the ramp length of the applied approach (Fig. 7b). The direct comparison between strategies A and B shows that the feed increase ramp has a significant influence on the magnitude of the straightness deviation in high-feed deep hole drilling. The lowest deviation could be achieved by strategy C, where the smooth increase of the feed produces a low thermal load and a low force level as well, particularly in the front of the machined hole, where the straightness deviation is initialised.
Fig. 5. Feed increase strategies (a); calculated machining time (b).
Generally, the mechanical load correlates with the actual feed in the corresponding drilling depth and attains mean values of Ff 3.8 kN and Md 17.3 N m after reaching the final feed (Fig. 6a). The energy consumption of the process corresponds to the machining time. Thus, the long ramp strategy C requires the highest active work, and strategy D consumes the lowest amount of energy. Due to the long high-feed sector of strategy D and the missing initial low-feed range in this process, the cutting work decreases compared to the other strategies (Fig. 6a). The behaviour of the workpiece temperature (Fig. 6b) is analogue to the observations of the energy consumption. In
Fig. 7. In-process workpiece deformation (a); resulting straightness deviation of the drilled hole (b) for different feed increase strategies.
4. Directional control of the straightness deviation In conventional drilling (Fig. 8a) the tool axis is coincident to the axis of the drilled hole. In order to realise a controlled correction of the drilling direction and thus to compensate systematic straightness deviations, a novel approach using an additional radial feeding path of the machine tool’s spindle was developed (Fig. 8b). Due to the tapered guiding chamfers, which are present only in the front area of the drilling tool, it is possible to perform a small directional correction of the tool by a radial displacement of the tool holder, leaving the original drilling axis. The bending
Fig. 6. Mechanical (a) and thermal load (b) depending on the strategy.
Fig. 8. Conventional drilling (a) and concept for directional control of the straightness deviation using a radial spindle compensation path (b).
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identical surface areas of AB AC 40 mm2 and produce repeatable straightness deviations of my 0.4,. . .,0.5 mm. 5. Conclusions and outlook
Fig. 9. Resulting straightness deviation my of drilled holes using varied trapezoidal functions of the radial compensation path uy.
flexibility of the standard solid carbide twist drill used for this investigation of d 0.2175 mm/N was determined at the cutting edge by a measurement of the deflection when applying a radial force of Frad = 10 N. Due to the helix angle of the tool, the bending stiffness does not depend on the measuring direction in the transverse plane. In contrast, the single-lip drill has a different bending stiffness, which can cause undesired vibration effects. In addition to the taper angle of the guiding chamfers, the radial clearance between the drilling tool and the machined hole has to be considered. The difference between the hole with d = 10 mm and the tool shaft behind the guiding chamfers with d = 9.5 mm results in a constant radial clearance of uc = 0.25 mm, but the limit for the maximal radial displacement of the spindle urad decreases by higher drilling depth due to the risk of collision. Considering the existent radial clearance uc, the maximum radial spindle respectively tool holder displacement for a tool with an exposed length lexp can be calculated by the rule of proportion umaxl = ucldrill. Applying the introduced approach and considering the presented limitations, different strategies for the radial compensation path of the spindle were analysed. Due to the given experimental set-up, the thermal expansion and the resulting straightness deviations occur particularly in positive y-direction. Therefore, the compensation strategies concentrate on radial paths directed in positive y-direction as well. The generated tool deflection works in an inverse manner and directs the straightness deviation in the opposite y-direction. The initially investigated triangle functions for the radial path along the drilling depth achieved a good compensation effect, but relatively poor reproducibility of the obtained straightness deviations. However, a further improvement could be realised by modifying the initial triangle function to a strategy with a linear compensation length. The results for the resulting trapezoidal function of the radial compensation path are presented in Fig. 9. The straightness deviations could be guided reliably to the required direction and the amount of the resulting effect correlates with the surface area of the compensation function. The trapezium strategy A with an area of AA = 25 mm2 generates a straightness deviation of my 0.3 mm. The increased amplitude of strategy C and the longer maximum value range of strategy B result in approximately
The presented fundamental investigations demonstrate the feasibility of the high-feed deep hole drilling of aluminium die cast alloys using MQL. The heat input into the workpiece could be significantly reduced using increased feed values of f = 1,. . .,4 mm, which corresponds to the higher heat removal by the chips and the generally decreased energy consumption of the cutting process. The best performance, considering the thermal load, the time saving and the resulting straightness of the machined holes, could be achieved using a long-ramped feed increase strategy with an initial low-feed guiding drilling depth. Furthermore, a novel deep-hole drilling strategy for the reproducible compensation of straightness deviation was introduced. The superimposed radial feed motion path leads to a repeatable directional control of the drilled hole straightness. Further research work will focus on the simulative prediction of the workpiece deviations and the compensation of the straightness deviations occurring in deep hole drilling situations of thin-walled components. Acknowledgements The authors acknowledge the support of the German Research Foundation (DFG) for funding this research within the project BI498/24-3 in the framework of the Priority Programme 1480.
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