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Comparative burr heights formed on S50C and SS400 steel in drilling process
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 9424–9430
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Comparative burr heights formed on S50C and SS400 steel in drilling process Pensiri Tongpadungroda, Saisunee Laosuwana, Surangsee Dechjarerna, Chantaraporn Phalakornkuleb,c,* a
Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand c Research and Technology Center for Renewable Products and Energy, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand
Abstract Burr formation is a problem in drilling process as it not only affects quality of products but also increases processing time and operating cost. At the current, burr formation is still unavoidable, and more studies should be performed in order to gain better understandings. As the drilling progresses, the drill is continuously deteriorated, leading to increasing drilling forces and burr heights. In this study, burr heights formed on S50C and SS400 steel at the exit of 4 mm thick workpieces were compared as well as peak forces during drilling process using 8 mm HSS drills. The feed rates were set at 0.01 and 0.03 mm/rev, and the cutting speeds at 25 and 30 m/min. In each condition, 100 workpieces were drilled consecutively and was repeated twice. It was found that the steel type affected the drilling force and burr height. The steel with higher hardness, i.e. S50C, was associated with the higher drilling force especially at later workpieces. The peak force during the drilling process was between 400-650 N and 380510 N for S50C and SS400, respectively. However, the steel with higher hardness had shorter burr heights; i.e., the burr height occurred on S50C and SS400 was between 0.49-1.31 mm and 0.44-1.97 mm, respectively. For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Drilling, Burr; Drilling force; S50C; SS400
* Corresponding author. Tel.: +66 2 555 2000 ext. 8232 ; fax: +66 2 587 0024. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
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1. Introduction Burr formation on a metal workpiece during drilling process is a common problem in industry. Burr formation is an obstacle for process automation and increases processing time and operating cost. As the drilling progresses, the drill is continuously deteriorated, leading to increasing drilling forces and burr heights. Burr formation is typically associated with subsurface damage and deformation of the drilled material [1]. A number of studies have been devoted to better understand the phenomena of burr formation. The better understanding will lead to the ability to predict the size and shape of burrs and finally may lead to strategies for preventing and minimizing burr [1]. For example, Kundu et al. [2] proposed an optimal drilling strategy to minimise burr by providing back-up support on aluminum alloy using high speed steel (HSS) drill. In their study, minimising height of burr formed on aluminum alloy using a back-up support was found to be effective especially for the drilling process with moderate cutting velocity, low feed rate under wet condition. There are works describing burr minimization scheme through development of a new-concept drill [3-5] and by specifying drilling conditions using burr control charts [6]. Another parameter that is unspecified as the drilling condition but advances as the drilling operation progresses is the tool wear. Serious wear eventually causes failure and workpiece damage. A direct measurement of tool wear during a machining process is complex. It is common to measure tool wear indirectly through physical changes during machining such as cutting forces, sound and vibration and motor current [7]. Drilling forces are normally measured with a dynamometer which is commercially available. Tool wear directly affects force and surface quality such as burr produced by a machining operation [8]. Mathematical modeling and artificial intelligence have been employed to find optimal process variables for minimizing burr height. For example, Gaitonde and Karnik [9] employed artificial neural network (ANN) with drilling experiments to find the most suitable feedrate and point angle for a specified drill diameter in order to simultaneously minimise burr height and burr thickness during drilling of AISI 316L stainless steel. Kumar et al. [10] developed an analytical model for the prediction of the exit burr height at high cutting speed (ca. 100 – 150 m/min) in high speed micro-milling. The analytical model was found to be able to predict burr height formed on TiGA14V with <10% error. In addition, the model was used to study the effect of cutting speed on burr height, leading to a minimized burr height. Observation of drilling burr and finding out suitable process parameters and machining environment have been performed on various metals, e.g. low alloy steel [11], TiGAL4V [9], AISI 316L stainless steel [9], and aluminum alloy [2]. In this study, drilling of S50C and SS400 steel was performed at varying feeding speeds and cutting speeds. S50C and SS400 were of interest because SS400 is commonly used in basic structural works, while S50C as materials for forming mold. Drilling forces and burr heights were recorded and compared between the two materials. In addition, an attempt to find a relationship between the wear of the drill bit and the drilling force was illustrated. This study provided a qualitative observation of drilling burr, leading to a mathematical model for the prediction of burr height, and wear of drill bit (mm) at a given drilling force on S50C. 2. Material and method 2.1. Material Two types of materials, S50C and SS400, were used to form workpieces of size 38 mm × 38 mm with the thickness of 4 mm. The drill bits used in this study were HSS twist drills with the diameter of 8 mm. 2.2. Drilling condition The workpieces were drilled with the feed rates between 0.01 – 0.03 mm/rev and the cutting speeds between 25 – 30 m/min. In order to promote wear on the drill bit and to make the drilling condition harsh, drilling was conducted without coolant. For each drilling condition, a drill bit was used to drill 100 workpieces and repeated twice.
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2.3. Instrument The thrust force was measured with two types of actuators: an in-house developed ring type load cell (CST-100, CM, China) with strain gauges (US Type G/120ALY11, HBM, Germany) and a commercial load cell. The signal obtained from both force measuring units was amplified by an instrumentation amplifier (WGA-170 A series, Kyowa, Japan). The voltage signal was recorded with an analogue to digital converter (PCI-1710, Advantech, USA). The schematic diagram of the equipment is shown in Fig. 1.
Fig. 1. Schematic diagram of the drilling set-up.
2.4. Analysis Wear on HSS drill and burr height were recorded for every 10th workpieces. Both wear depth (mm) and burr height (mm) were inspected with a profile measurement instrument (OMIS II 6×9, RAM Optical Instrumentation, USA). 3. Results and discussion 3.1. The appearance of burrs for S50C and SS400 Fig. 2 shows the appearance of burr formed on S50C (Fig. 2a and Fig. 2b) and SS400 (Fig. 2c and Fig. 2d) for the 10th and the 100th workpieces drilled. The shape of burr formed on SS400 (Fig. 2c and Fig. 2d) appeared to be irregular with several pieces of torn caps that looked similar to Type C burr [3]. Type C burr is associated with the occurrence of fracture which begins at the drill point. The shape of burr formed on S50C (Fig. 2a and Fig. 2b) appeared to be rather uniform that was similar to Type B burr [3]. Type B burr is associated with the occurrence of fracture from the exit edge of the hole. However, for the 10th workpiece drilled of S50C (Fig. 2a) the appearance of the burr was a mix between Type B and Type C burrs.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
(a)
(b)
(c)
(d)
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Fig. 2. Burr formed on S50C and SS400 (a) the 10th workpiece of S50C, (b) the 100th workpiece S50C (c) the 10th workpiece of SS400, (d) the 100th workpiece of SS400.
It is known that besides the drilling conditions, the appearance of burr is influenced by the plastic deformation of the drilled material. As the drill approaches the exit hole, plastic deformation of the material increases and finally fracture occurs. For a highly brittle material, the fracture is initiated as the drill approaches the exit hole and exerts thrust force on the remaining material causing the material to crack. Accordingly, there will be no burr left on the workpiece. For a more ductile material, plastic deformation proceeds even after the drill breakthrough the drilled material. Eventually fracture occurs at the edge of the hole or at the drill point [3-5]. SS400 has a higher ductility than S50C and thus tends to better endure plastic deformation before experiencing fracture. As a result, the shape of the burr formed on SS400 was more irregular and larger in size. 3.2. Relationship between peak force and burr height formed on S50C When using the cutting speed of 25 m/min and the HSS drill diameter of 8 mm on S50C workpieces at the feed rates of 0.01, 0.015 and 0.02 mm/rev, a linear relationship between peak force (N) and burr height (mm) was observed with the values of R2 between 0.607-0.708 (Fig. 3a). The linear fitting between peak force and burr height for data from all three feed rates was shown in Fig. 3b with the R2 value of 0.6995. It should be noted that the drilling was made in a workshop environment with limited controlled drilling conditions and the measurement of burr heights was carried out at randomly selected locations on the exit edge of the drilled hole which may result in discrepancy. Although the R2 value was not satisfactorily prominent, the results suggested that the peak force increased linearly with the burr height regardless of the investigated feed rate. However, at the fixed cutting speed of 25 m/min, the slope of the linear relationship between the peak force and the burr height tend to increase with the increasing feed rate. At the feed rates of 0.015 and 0.02 mm/rev, the slopes of the linear fit were of a similar range and noticeably elevated compared to that at the feed rate of 0.01 mm/rev. This suggested that the burr height increased much faster when using a higher feed rate. At a higher feed rate the drill exerts a higher drilling force and thus higher peak forces were detected.
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Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
(a)
(b)
Fig. 3. Linear fitting of peak force and burr height for S50C at the feed rates of 0.01 – 0.02 mm/rev and cutting speed of 25 m/min (a) linear fitting for individual feed rates and (b) overall linear fitting.
3.3. Relationship between wear and number of workpieces drilled for S50C The development of wear on the HSS drill was further investigated for the drilling on S50C. It was found that the progress of wear on the HSS drill bit (in terms of wear depth) increased linearly with the number of workpieces drilled as shown in Fig. 4. The result shown in Fig. 4 also indicated that the cutting speed has a strong effect on the progress of wear. That is, the higher cutting speed of 30 m/min, even with a lower feed rate of 0.01 mm/rev, resulted in a notably higher rate of wear development on the HSS drill bit.
Fig. 4. Progress of wear on HSS drill bit with the increasing number of workpieces.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
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The appearances of the wear developed on the HSS drill bit after being employed for drilling S50C for the 10th and 100th workpieces are shown in Fig. 5. The wear can be observed under a microscope and was found to be deeper with increasing numbers of workpiece drilled.
(a)
(b)
Fig. 5. Comparison of wear developed on HSS drill bit for (a) the 10th and (b) the 100th of the S50C workpieces at the feed rate of 0.01 mm/rev and cutting speed of 30 m/min.
3.4. Relationship between wear and drilling forces on S50C For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738 (Table 1). An exponential curve fitting between the average drilling forces on S50C and the wear developed on the HSS drill bit gave an R2 of 0.9738 (Fig. 6), while a linear fitting gave an R2 of 0.9346 (Table 1). However, the values of R2 were slightly less (0.82300.9161) in the case of exponential and linear curve fitting between the peak drilling forces and the wear on HSS drill bit (Table 1). In the case of SS400, such mathematical relationships were not clear (R2 = 0.3665-0.7274). A possible explanation may be related to the higher hardness of S50C, which leads to higher drilling forces and higher depth of wear.
Fig. 6. Exponential curve fitting for the average drilling force on S50C and wear on HSS drill bit at the feed rates of 0.01 mm/rev and cutting speed of 30 m/min.
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Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430 Table 1. R2 values for linear and exponential curve fitting of average and peak forces with the wear developed on HSS drill bit of S50C and SS400.
Type of relationship function R2 S50C Feed rate = 0.01 mm/rev, cutting speed=30 m/min Exponential 0.9738 Average force-wear Linear 0.9346 Exponential 0.9161 Peak force-wear Linear 0.8230 SS400 Feed rate = 0.03 mm/rev, cutting speed=25 m/min Exponential 0.7274 Average force-wear Linear 0.6319 Exponential 0.3665 Peak force-wear Linear 0.3719 Parameters
4. Conclusions The appearance of burrs formed on S50C and SS400 at the exit hole of drilling has been examined. The shape of the burr formed on SS400, a more ductile material, was more irregular and larger in size. The linear relationship between peak force and burr height have been demonstrated. It was found that the progress of wear on the HSS drill bit (in terms of wear depth) increased linearly with the number of workpieces drilled and the cutting speed has a strong effect on the progress of wear than the feed rate. For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738. Hence, the measurement of drilling force had a potential for prediction of wear on the drill bit as well as the burr height. Acknowledgements The authors are grateful to King Mongkut’s University of Technology North Bangkok for the research funding (Grant number: KMUTNB-GEN-54-01). References [1] D.A. Dornfeld, Strategies for preventing and minimizing burr formation, LMA Research Reports, University of California, Berkeley, 2003. [2] S. Kundu, S. Das, P.P. Saha, Optimization of drilling parameters to minimize burr by providing back-up support on aluminium alloy, Procedia. Engineer. 97 (2014) 230 – 240. [3] S.L. Ko, J.K. Lee, Analysis of burr formation in drilling with a new-concept drill, J. Mater. Process. Tech. 113 (2001) 392-398. [4] S.L. Ko, J.E. Chang, S. Kaipakjian, Development of Drill Geometry for Burr Minimization In Drilling, CIRP Ann. Manuf. Techn. 52 (1) (2003) 45-48. [5] S.L. Ko, J.E. Chang, G.E Yang, Burr minimizing scheme in drilling, J. Mater. Process. Tech. 140 (2003) 237-242. [6] J. Kim, S. Min, D.A. Dornfeld, Optimization and control of drilling burr formation of AISI 304L and AISI 4118 based on drilling burr control charts, Int. J. Mach. Tool. Manu. 41(7) (2001) 923-936. [7] H. Y. Kim, J. H. Ahn, S. H. Kim, S. Takata, Real-time drill wear estimation based on spindle motor power, J. Mater. Process. Tech.124 (2002) 267-273. [8] D.A. Axinte, N. Gindy, K. Fox, I. Unanue, Process monitoring to assist the workpiece surface quality in machining Int. J. Mach. Tool. Manu. 44 (2004) 1091-1108. [9] V.N. Gaitonde, S.R. Karnik, Minimizing burr size in drilling using artificial neural network (ANN)- particle swarm optimization (PSO) approach, J. Intell. Manuf. 23(1) (2012) 1783–1793. [10] P. Kumar, V. Bajpai, R. Singh, Burr height prediction of Ti6Al4V in high speed micro-milling by mathematical modeling, Manufacturing Letters. 11 (2017) 12–16. [11] N. Mondal, B.S. Sardar, R.N. Halder and S. Das, Observation of drilling burr and finding out the condition for minimum burr formation, International Journal of Manufacturing Engineering. 2014(1) (2014) 1-12.
ScienceDirect Materials Today: Proceedings 5 (2018) 9424–9430
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Comparative burr heights formed on S50C and SS400 steel in drilling process Pensiri Tongpadungroda, Saisunee Laosuwana, Surangsee Dechjarerna, Chantaraporn Phalakornkuleb,c,* a
Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand c Research and Technology Center for Renewable Products and Energy, King Mongkut’s University of Technology North Bangkok, Pracharat 1 Road, Bangsue, Bangkok, Thailand
Abstract Burr formation is a problem in drilling process as it not only affects quality of products but also increases processing time and operating cost. At the current, burr formation is still unavoidable, and more studies should be performed in order to gain better understandings. As the drilling progresses, the drill is continuously deteriorated, leading to increasing drilling forces and burr heights. In this study, burr heights formed on S50C and SS400 steel at the exit of 4 mm thick workpieces were compared as well as peak forces during drilling process using 8 mm HSS drills. The feed rates were set at 0.01 and 0.03 mm/rev, and the cutting speeds at 25 and 30 m/min. In each condition, 100 workpieces were drilled consecutively and was repeated twice. It was found that the steel type affected the drilling force and burr height. The steel with higher hardness, i.e. S50C, was associated with the higher drilling force especially at later workpieces. The peak force during the drilling process was between 400-650 N and 380510 N for S50C and SS400, respectively. However, the steel with higher hardness had shorter burr heights; i.e., the burr height occurred on S50C and SS400 was between 0.49-1.31 mm and 0.44-1.97 mm, respectively. For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Drilling, Burr; Drilling force; S50C; SS400
* Corresponding author. Tel.: +66 2 555 2000 ext. 8232 ; fax: +66 2 587 0024. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
9425
1. Introduction Burr formation on a metal workpiece during drilling process is a common problem in industry. Burr formation is an obstacle for process automation and increases processing time and operating cost. As the drilling progresses, the drill is continuously deteriorated, leading to increasing drilling forces and burr heights. Burr formation is typically associated with subsurface damage and deformation of the drilled material [1]. A number of studies have been devoted to better understand the phenomena of burr formation. The better understanding will lead to the ability to predict the size and shape of burrs and finally may lead to strategies for preventing and minimizing burr [1]. For example, Kundu et al. [2] proposed an optimal drilling strategy to minimise burr by providing back-up support on aluminum alloy using high speed steel (HSS) drill. In their study, minimising height of burr formed on aluminum alloy using a back-up support was found to be effective especially for the drilling process with moderate cutting velocity, low feed rate under wet condition. There are works describing burr minimization scheme through development of a new-concept drill [3-5] and by specifying drilling conditions using burr control charts [6]. Another parameter that is unspecified as the drilling condition but advances as the drilling operation progresses is the tool wear. Serious wear eventually causes failure and workpiece damage. A direct measurement of tool wear during a machining process is complex. It is common to measure tool wear indirectly through physical changes during machining such as cutting forces, sound and vibration and motor current [7]. Drilling forces are normally measured with a dynamometer which is commercially available. Tool wear directly affects force and surface quality such as burr produced by a machining operation [8]. Mathematical modeling and artificial intelligence have been employed to find optimal process variables for minimizing burr height. For example, Gaitonde and Karnik [9] employed artificial neural network (ANN) with drilling experiments to find the most suitable feedrate and point angle for a specified drill diameter in order to simultaneously minimise burr height and burr thickness during drilling of AISI 316L stainless steel. Kumar et al. [10] developed an analytical model for the prediction of the exit burr height at high cutting speed (ca. 100 – 150 m/min) in high speed micro-milling. The analytical model was found to be able to predict burr height formed on TiGA14V with <10% error. In addition, the model was used to study the effect of cutting speed on burr height, leading to a minimized burr height. Observation of drilling burr and finding out suitable process parameters and machining environment have been performed on various metals, e.g. low alloy steel [11], TiGAL4V [9], AISI 316L stainless steel [9], and aluminum alloy [2]. In this study, drilling of S50C and SS400 steel was performed at varying feeding speeds and cutting speeds. S50C and SS400 were of interest because SS400 is commonly used in basic structural works, while S50C as materials for forming mold. Drilling forces and burr heights were recorded and compared between the two materials. In addition, an attempt to find a relationship between the wear of the drill bit and the drilling force was illustrated. This study provided a qualitative observation of drilling burr, leading to a mathematical model for the prediction of burr height, and wear of drill bit (mm) at a given drilling force on S50C. 2. Material and method 2.1. Material Two types of materials, S50C and SS400, were used to form workpieces of size 38 mm × 38 mm with the thickness of 4 mm. The drill bits used in this study were HSS twist drills with the diameter of 8 mm. 2.2. Drilling condition The workpieces were drilled with the feed rates between 0.01 – 0.03 mm/rev and the cutting speeds between 25 – 30 m/min. In order to promote wear on the drill bit and to make the drilling condition harsh, drilling was conducted without coolant. For each drilling condition, a drill bit was used to drill 100 workpieces and repeated twice.
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Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
2.3. Instrument The thrust force was measured with two types of actuators: an in-house developed ring type load cell (CST-100, CM, China) with strain gauges (US Type G/120ALY11, HBM, Germany) and a commercial load cell. The signal obtained from both force measuring units was amplified by an instrumentation amplifier (WGA-170 A series, Kyowa, Japan). The voltage signal was recorded with an analogue to digital converter (PCI-1710, Advantech, USA). The schematic diagram of the equipment is shown in Fig. 1.
Fig. 1. Schematic diagram of the drilling set-up.
2.4. Analysis Wear on HSS drill and burr height were recorded for every 10th workpieces. Both wear depth (mm) and burr height (mm) were inspected with a profile measurement instrument (OMIS II 6×9, RAM Optical Instrumentation, USA). 3. Results and discussion 3.1. The appearance of burrs for S50C and SS400 Fig. 2 shows the appearance of burr formed on S50C (Fig. 2a and Fig. 2b) and SS400 (Fig. 2c and Fig. 2d) for the 10th and the 100th workpieces drilled. The shape of burr formed on SS400 (Fig. 2c and Fig. 2d) appeared to be irregular with several pieces of torn caps that looked similar to Type C burr [3]. Type C burr is associated with the occurrence of fracture which begins at the drill point. The shape of burr formed on S50C (Fig. 2a and Fig. 2b) appeared to be rather uniform that was similar to Type B burr [3]. Type B burr is associated with the occurrence of fracture from the exit edge of the hole. However, for the 10th workpiece drilled of S50C (Fig. 2a) the appearance of the burr was a mix between Type B and Type C burrs.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
(a)
(b)
(c)
(d)
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Fig. 2. Burr formed on S50C and SS400 (a) the 10th workpiece of S50C, (b) the 100th workpiece S50C (c) the 10th workpiece of SS400, (d) the 100th workpiece of SS400.
It is known that besides the drilling conditions, the appearance of burr is influenced by the plastic deformation of the drilled material. As the drill approaches the exit hole, plastic deformation of the material increases and finally fracture occurs. For a highly brittle material, the fracture is initiated as the drill approaches the exit hole and exerts thrust force on the remaining material causing the material to crack. Accordingly, there will be no burr left on the workpiece. For a more ductile material, plastic deformation proceeds even after the drill breakthrough the drilled material. Eventually fracture occurs at the edge of the hole or at the drill point [3-5]. SS400 has a higher ductility than S50C and thus tends to better endure plastic deformation before experiencing fracture. As a result, the shape of the burr formed on SS400 was more irregular and larger in size. 3.2. Relationship between peak force and burr height formed on S50C When using the cutting speed of 25 m/min and the HSS drill diameter of 8 mm on S50C workpieces at the feed rates of 0.01, 0.015 and 0.02 mm/rev, a linear relationship between peak force (N) and burr height (mm) was observed with the values of R2 between 0.607-0.708 (Fig. 3a). The linear fitting between peak force and burr height for data from all three feed rates was shown in Fig. 3b with the R2 value of 0.6995. It should be noted that the drilling was made in a workshop environment with limited controlled drilling conditions and the measurement of burr heights was carried out at randomly selected locations on the exit edge of the drilled hole which may result in discrepancy. Although the R2 value was not satisfactorily prominent, the results suggested that the peak force increased linearly with the burr height regardless of the investigated feed rate. However, at the fixed cutting speed of 25 m/min, the slope of the linear relationship between the peak force and the burr height tend to increase with the increasing feed rate. At the feed rates of 0.015 and 0.02 mm/rev, the slopes of the linear fit were of a similar range and noticeably elevated compared to that at the feed rate of 0.01 mm/rev. This suggested that the burr height increased much faster when using a higher feed rate. At a higher feed rate the drill exerts a higher drilling force and thus higher peak forces were detected.
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Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
(a)
(b)
Fig. 3. Linear fitting of peak force and burr height for S50C at the feed rates of 0.01 – 0.02 mm/rev and cutting speed of 25 m/min (a) linear fitting for individual feed rates and (b) overall linear fitting.
3.3. Relationship between wear and number of workpieces drilled for S50C The development of wear on the HSS drill was further investigated for the drilling on S50C. It was found that the progress of wear on the HSS drill bit (in terms of wear depth) increased linearly with the number of workpieces drilled as shown in Fig. 4. The result shown in Fig. 4 also indicated that the cutting speed has a strong effect on the progress of wear. That is, the higher cutting speed of 30 m/min, even with a lower feed rate of 0.01 mm/rev, resulted in a notably higher rate of wear development on the HSS drill bit.
Fig. 4. Progress of wear on HSS drill bit with the increasing number of workpieces.
Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430
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The appearances of the wear developed on the HSS drill bit after being employed for drilling S50C for the 10th and 100th workpieces are shown in Fig. 5. The wear can be observed under a microscope and was found to be deeper with increasing numbers of workpiece drilled.
(a)
(b)
Fig. 5. Comparison of wear developed on HSS drill bit for (a) the 10th and (b) the 100th of the S50C workpieces at the feed rate of 0.01 mm/rev and cutting speed of 30 m/min.
3.4. Relationship between wear and drilling forces on S50C For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738 (Table 1). An exponential curve fitting between the average drilling forces on S50C and the wear developed on the HSS drill bit gave an R2 of 0.9738 (Fig. 6), while a linear fitting gave an R2 of 0.9346 (Table 1). However, the values of R2 were slightly less (0.82300.9161) in the case of exponential and linear curve fitting between the peak drilling forces and the wear on HSS drill bit (Table 1). In the case of SS400, such mathematical relationships were not clear (R2 = 0.3665-0.7274). A possible explanation may be related to the higher hardness of S50C, which leads to higher drilling forces and higher depth of wear.
Fig. 6. Exponential curve fitting for the average drilling force on S50C and wear on HSS drill bit at the feed rates of 0.01 mm/rev and cutting speed of 30 m/min.
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Pensiri Tongpadungrod et al./ Materials Today: Proceedings 5 (2018) 9424–9430 Table 1. R2 values for linear and exponential curve fitting of average and peak forces with the wear developed on HSS drill bit of S50C and SS400.
Type of relationship function R2 S50C Feed rate = 0.01 mm/rev, cutting speed=30 m/min Exponential 0.9738 Average force-wear Linear 0.9346 Exponential 0.9161 Peak force-wear Linear 0.8230 SS400 Feed rate = 0.03 mm/rev, cutting speed=25 m/min Exponential 0.7274 Average force-wear Linear 0.6319 Exponential 0.3665 Peak force-wear Linear 0.3719 Parameters
4. Conclusions The appearance of burrs formed on S50C and SS400 at the exit hole of drilling has been examined. The shape of the burr formed on SS400, a more ductile material, was more irregular and larger in size. The linear relationship between peak force and burr height have been demonstrated. It was found that the progress of wear on the HSS drill bit (in terms of wear depth) increased linearly with the number of workpieces drilled and the cutting speed has a strong effect on the progress of wear than the feed rate. For S50C, mathematical relationships can be found between the wear developed on the HSS drill bit and the peak and average drilling forces with promising R2 values between 0.8230-0.9738. Hence, the measurement of drilling force had a potential for prediction of wear on the drill bit as well as the burr height. Acknowledgements The authors are grateful to King Mongkut’s University of Technology North Bangkok for the research funding (Grant number: KMUTNB-GEN-54-01). References [1] D.A. Dornfeld, Strategies for preventing and minimizing burr formation, LMA Research Reports, University of California, Berkeley, 2003. [2] S. Kundu, S. Das, P.P. Saha, Optimization of drilling parameters to minimize burr by providing back-up support on aluminium alloy, Procedia. Engineer. 97 (2014) 230 – 240. [3] S.L. Ko, J.K. Lee, Analysis of burr formation in drilling with a new-concept drill, J. Mater. Process. Tech. 113 (2001) 392-398. [4] S.L. Ko, J.E. Chang, S. Kaipakjian, Development of Drill Geometry for Burr Minimization In Drilling, CIRP Ann. Manuf. Techn. 52 (1) (2003) 45-48. [5] S.L. Ko, J.E. Chang, G.E Yang, Burr minimizing scheme in drilling, J. Mater. Process. Tech. 140 (2003) 237-242. [6] J. Kim, S. Min, D.A. Dornfeld, Optimization and control of drilling burr formation of AISI 304L and AISI 4118 based on drilling burr control charts, Int. J. Mach. Tool. Manu. 41(7) (2001) 923-936. [7] H. Y. Kim, J. H. Ahn, S. H. Kim, S. Takata, Real-time drill wear estimation based on spindle motor power, J. Mater. Process. Tech.124 (2002) 267-273. [8] D.A. Axinte, N. Gindy, K. Fox, I. Unanue, Process monitoring to assist the workpiece surface quality in machining Int. J. Mach. Tool. Manu. 44 (2004) 1091-1108. [9] V.N. Gaitonde, S.R. Karnik, Minimizing burr size in drilling using artificial neural network (ANN)- particle swarm optimization (PSO) approach, J. Intell. Manuf. 23(1) (2012) 1783–1793. [10] P. Kumar, V. Bajpai, R. Singh, Burr height prediction of Ti6Al4V in high speed micro-milling by mathematical modeling, Manufacturing Letters. 11 (2017) 12–16. [11] N. Mondal, B.S. Sardar, R.N. Halder and S. Das, Observation of drilling burr and finding out the condition for minimum burr formation, International Journal of Manufacturing Engineering. 2014(1) (2014) 1-12.