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ScienceDirect Procedia Manufacturing 10 (2017) 208 – 217

45th SME North American Manufacturing Research Conference, NAMRC 45, LA, USA

Fine Surface Finish of a Hardened Stainless Steel Using a New Burnishing Tool Fang-Jung Shioua, *, Shih-Ju Huanga, Albert J. Shihb, Jiang Zhuc and Masahiko Yoshinoc a

Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan b Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, 48109, USA c Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, 152-8552, Japan

Abstract The objective of this research is to develop a new ball-burnishing tool embedded with a load cell integrated with a CNC lathe, to improve the surface roughness and hardness of the fine turned SUS420J2 (equivalent to AISI420) stainless steel. The adequate ball burnish process parameters of the cylindrical part have been determined by conducting the Taguchi’s L18 matrix experiments, the ANOVA analysis, and the verification tests. Based on the experimental results, the adequate combination of the process parameters is as follows: the ball material of WC, the burnishing force of 650 N, the feed rate of 0.05 mm/rev, the velocity of 25 m/min, the lubricant of the cutting fluid (oil/water concentration of 1/20) and the number of pass of 3. The fine turned surface roughness of the test specimen could be improved from Ra ȝP WR 5D ȝP 5PD[ ȝP DQG WKH surface hardness could be increased from HRc 51 to HRc 52.5 on average using the adequate process parameters. Applying the DGHTXDWHSURFHVVSDUDPHWHUVWRDZRUNSLHFHZLWKGLIIHUHQWWDSHUVWKHQWKHVXUIDFHURXJKQHVVFRXOGEHLPSURYHGIURP5DȝP to Ra 0.04-ȝPRQDYHUDJH © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of the Scientific Committee NAMRI/SME. Peer-review responsibility of the organizing committee of theof45th SME North American Manufacturing Research Conference Keywords: Ball burnishing tool; Load cell; CNC lathe; Taguchi’s method; Surface roughness; Hardness

1. Introduction The burnishing process, which is one of the surface finishing processes that results in a local plastic deformation on the workpiece surface by using a hardened ball or a roller, as shown in Fig. 1, has been applied to improve the surface roughness, surface hardness, and fatigue resistance, etc. in recent years. Some commercial available ball burnishing and roller burnishing tools have been developed to do the surface finish or modification [1]. Burnishing process has been combined with one of the different fabrication processes, such as ball milling [2],

2351-9789 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 45th SME North American Manufacturing Research Conference doi:10.1016/j.promfg.2017.07.048

Fang-Jung Shiou et al. / Procedia Manufacturing 10 (2017) 208 – 217

superfinishing [3], ball polishing [4], and laser cladding [5], ultrasonic machining [6],cryogenic machining [7-8], and EDM [9], etc. to improve the surface roughness of the workpiece sequentially. The improvement of the surface roughness due to the burnishing process generally ranged between 40% and 90% [2-9]. The machine tools that the burnishing process applied to are conventional lathe, CNC lathe, and CNC machining center. The surface roughness of a fine turned part on a CNC lathe, in general, ranges from Ra 0.8 ȝm to Ra 6.3 ȝm. The commonly used ball or roller materials for burnishing are tungsten carbide (WC), Silicone carbide (Si 3 N 4 ), and polycrystalline coated diamond (PCD). Conventional materials used for burnishing process are aluminum alloy, copper alloy, tool steel, stainless steel, etc. In recent years, the burnishing process has been applied to the surface modification of biomedical material such as the phase transformation of Nitinol (Ni 50.8 Ti 49.2 ) (at %) [10]. Some dominant flat surface ball burnishing parameters for the mold steel were the burnishing force, the ball material, the burnishing speed, the lubricant, and the feed. Among these parameters, the burnishing force or pressure played an important role on the surface roughness improvement, based on the analysis of variance (ANOVA) [1112]. Most of the burnishing force of the proposed burnishing tools was generated by either a mold spring or a hydraulic pressure device, and monitored by a specific dial indicator, or a load cell, or an expensive dynamometer. The general trend curve of the relationship between the normal burnishing force and the surface roughness Ra or Rz, has been reviewed in [13]. As the burnishing force increased, the roughness parameter value decreased to a point, and then started to increase. The force at that point is denoted as the optimal force. An innovative ball burnishing tool embedded with a load cell has been developed and employed on a machining center, to improve the surface roughness of a PDS5 tool steel [14]. However, a ball burnishing tool embedded with a load cell used on a CNC lathe is still to be investigated. The SUS420J2 (equivalent to AISI420) steel is classified as a martensitic stainless steel, and its most notable feature is that they can be quenched [15]. The adequate ball burnishing parameters of a cylindrical part of SUS420 J2 have not been investigated based on the literature survey.

Fig. 1. Schematic diagram of the ball burnishing process.

The aim of this study was mainly to develop a new ball burnishing tool embedded with a load cell mounted on the turret of a CNC lathe, to improve the surface roughness of the fine turned part on a CNC lathe efficiently and economically, and to determine the adequate process parameters for the developed tool. The embedded load cell can be used to monitor the burnishing force without using an expensive dynamometer system. The design, manufacture, and calibration of the innovative burnishing tool are introduced in section 2. Detailed determination of the adequate ball burnishing parameters of the rolling-contact ball burnishing tool using the Taguchi’s experimental method for the hardened SUS420J2 (equivalent to AISI420) stainless steel, is then presented in section 3. The improvements on the hardness and the residual stress and the application of the adequate burnishing parameters to the taper surface of a fine turned surface have also been discussed in section 4. 2. Design and fabrication of a new burnishing tool embedded with a load cell for a CNC lathe A new ball burnishing tool has been designed and manufactured, to simplify the control of a burnishing force without using an expensive dynamometer system. The following factors have been considered in designing the novel ball-burnishing tool [14, 16]: z

The burnishing thrust force can be measured precisely by a load cell embedded within the burnishing tool.

209

210


z z z z z

Wireless transmission of the force signal is required so that no cable binding will happen. Ball burnishing of a straight cylindrical surface or a taper surface with taper angle smaller than 30 degrees is possible. Safety device is required to protect the load cell. Pre-loading adjustment is possible for the burnishing of a taper surface. The burnishing ball and the helical spring are changeable to meet the need for burnishing different materials, such as steel, copper, aluminum, etc.

To meet the requirements of the above mentioned considerations, an innovative ball burnishing tool embedded with a load cell has been designed, as shown in figure 2 [16]. This tool mainly consists of a tool holder, a load cell, a mold helical spring, two spring retainers with cylindrical guides, a safety device, a cover, a pre-load adjustment screw, a collect chuck, a ball holder, and a burnishing ball. The burnishing tool holder can be mounted on the turret of a CNC lathe. A mold helical spring with the spring constant of 18.75 kgf/mm was used to absorb any possible vibration of the machine bed and the positioning error of the machine tool. Two spring guides were used to guide the movement of the mold spring and transmit the burnishing force to the load cell. A U-slot was milled in the cover body to limit the movement of the spring, to protect the load cell, and to serve as a safety device. The pre-load of the spring can be adjusted by an adjustment screw, so that the burnishing of an inclined surface is possible. By changing the ball holder with a tungsten carbide rod with a ground polished sphere-shaped end mounted in a collect chuck, the ball burnishing of a smoothed surface using the rolling-contact type or a surface with relatively large slope using the sliding-contact type, is possible. The diameter of the polished tungsten carbide burnishing ball is 8.0 mm. Figure 3 shows the photos of the fabricated ball burnishing tool of rolling-contact type and the integration of the developed tool with the CNC lathe, respectively. The rolling-contact type burnishing tool is appropriate for applying to the smoothed surface with the inclination angle within + 30 degrees. For surfaces with large slopes, the sliding-contact type burnishing tool is available.

Fig. 2. Design of a burnishing tool embedded with a load cell [16].

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Fang-Jung Shiou et al. / Procedia Manufacturing 10 (2017) 208 – 217 Fig. 3. (a) Photo of the fabricated burnishing tool; (b) Photo of the developed ball burnishing tool integrated with the CNC lathe.

The load cell embedded in the burnishing tool is a product of the Honeywell Sensotec Company, model 53 [17]. The maximum loading for the adopted load cell was about 1,100 N. The induced burnishing force signal is amplified by an amplifier and emitted by a wireless transmitter, type T24-ACMn-SA. The amplified signal is then received by a receiver embedded with an A/D converter, type T24-BSU, connected to a PC via a USB interface. A set of interface software programmed with the Visual Basic language has been developed to measure the burnishing force of the burnishing tool. The calibration result of the measured load compared with the Kistler dynamometer, type 9129AA, in terms of the deformation of the spring for the developed burnishing tool, is shown in figure 4. The linearity was good and the maximum error was about 0.5% of the maximum loading.

Fig. 4. Calibration result of the used load cell compared with a dynamometer in terms of the deformation of the helical mould spring.

3. Determination of the adequate ball burnishing parameters of the developed tool using Taguchi’s experimental method 3.1. Definition of the data analysis of Taguchi’s matrix experiment The effects of several parameters can be determined efficiently by conducting matrix experiments using Taguchi’s orthogonal array [18]. Some parameters for the ball burnishing process, having significant effects on surface roughness, were investigated using Taguchi’s method in this work. The L 18 orthogonal array was selected to conduct the matrix experiments to determine the adequate ball burnishing process parameters. Engineering design problems can be divided into the smaller-the-better type, the nominal–the-best type, the larger-the-better type, the signed-target type, etc. [18]. The signal-to-noise (S/N) ratio is used as the objective function for optimizing a product or process design. The surface roughness value of the ball burnished surface via adequate combination of the burnishing parameters should be smaller than that of the original fine turned surface. Consequently, the ball burnishing process is an example of a smaller-the-better type problem. The S/N ratio, K , is defined by the following equation [18], K

10 log10 ( mean square quality characteristic)

ª1 n 2º 10 log10 « ¦ yi » ¬n i 1 ¼

(1)

Where, y i : observations of the quality characteristic under different noise conditions n: number of experiment. The optimization strategy of the smaller-the-better problem is to maximize K defined by equation (1). The burnished surface roughness is taken as the quality characteristic y i for the calculation of the S/N ratio K . Levels that maximize K will be selected for the factors that have a significant effect on K . The adequate conditions for ball burnishing can then be determined.

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3.2. Configuration of the factors and their levels for the experiments The adequate ball burnishing parameters for a cylindrical surface of SUS420J2 stainless steel were determined by the Taguchi’s L 18 method. As mentioned, the main ball burnishing parameters having significant effects on surface roughness were the ball material, the burnishing force, the burnishing speed, and the feed, the lubricant, etc. Consequently, the ball material, the burnishing force, the feed, the velocity, the lubricant, and the number of pass, were selected as the six factors (parameters) and designated as factor A to F (Table 1) for ball burnishing experiments in this research. Appropriate levels (settings) for each factor were configured to cover the range of interest. The numerical values of each factor were determined based on the pre-study results. The L 18 (21 x 37) orthogonal array, which can be applied to one factor with two levels and seven factors with three levels [18], was then selected to conduct the matrix experiments for the 21 x 35 six factors with mixed levels of the ball burnishing process. All tables should be numbered with Arabic numerals. Every table should have a caption. Headings should be placed above tables, left justified. Only horizontal lines should be used within a table, to distinguish the column headings from the body of the table, and immediately above and below the table. Tables must be embedded into the text and not supplied separately. Below is an example which the authors may find useful. Table 1. Factors and their levels for ball burnishing process matrix experiments. Factors

Levels

A. Ball material B. Force (N) C. Feed (mm/rev) D. Velocity (m/min) E. Lubricant F. No. of passes

1

2

3

WC 250 0.02 25 Cutting fluid 1

Si 3 N 4 450 0.05 50 SAE 30 2

650 0.08 75 SAE 50 3

3.3. Material of the test specimens The material used in this study was the SUS420J2 (equivalent to AISI420) stainless steel with a unique combination of corrosion resistance, surface polishing, toughness, and through-hardening properties [15]. It can be applied to all types of molds, especially suited for larger tools where corrosion in production is unacceptable and where high surface finish is required. Table 2 shows the chemical composition of the used SUS420J2 stainless mold steel. The hardness of this material is about HRC50 after hardened and tempered heat treatment. Table 2. Chemical composition of SUS420J2 stainless steel. C (%)

Mn (%)

Si (%)

P (%)

S (%)

Cr (%)

Ni (%)

0.26-0.4

൒ 1.0

൒ 1.0

൒ 0.04

൒ 0.03

12.0-14.0

൒ 0.6

The surface to be burnished was divided into eighteen zones for the matrix experiments. Two specimens have been tested so that the average burnished surface roughness values were calculated from that of the eighteen zones. The pre-machined (fine turned) surface roughness of the test specimens, measured by Hommelwerke T8000 surface roughness measuring equipment made in Germany, was about 1.0 um on average. The used surface roughness measuring equipment was put in the metrology laboratory with temperature control at 20 + 0.5oC. 3.4. Setup for the experiments Figure 5 shows the setup of the ball burnishing process for the experiments. The SUS420J2 specimens have been designed and fine turned directly on the CNC lathe so that the Taguchi’s L 18 experiments could be carried out. The developed burnishing tool was mounted on the turret of a CNC lathe made by Yang-Iron Co., Taiwan, type ML-

213


15A. The CNC lathe was equipped with the NC-controller of FANUC Co., type OTC [19]. The working volume of this medium-sized machine tool is 300 mm (rotating radius) x 800 mm (axial length). This machine tool was equipped with two linear encoders for x-axis and z-axis, respectively, to increase the positioning accuracy. The positioning errors of x-axis and z-axis were within 6 microns measured with a HP5529A laser interferometer system. The NC codes needed for turning and ball burnishing paths were simulated and generated by the Master CAD/CAM software. After machining path simulation, these generated NC codes can then be transmitted to the CNC controller of the machine tool via RS232 serial interface.

Fig. 5. Experimental setup to determine the adequate ball burnishing parameters.

3.5. Results of experiments Table 3 summarizes the measured burnished surface roughness value R a , R max , and the calculated S/N ratio of R a of each L 18 orthogonal array using equation (1) after the 18 matrix experiments had been executed. The average S/N ratio for each level of the six factors with mixed levels can be obtained by taking the numerical values listed in table 3. The average S/N ratio for each level of the six factors is shown graphically in figure 6. They are separate effects of each factor and commonly denoted as main effects. The burnishing force and the ball material have obvious effects on the roughness. With the increase of the burnishing force, the mean S/N ratio also increased (surface roughness improved). Table 3. Measured surface roughness and calculated S/N ratio of the burnished specimens. No. of Expt. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 0.25 0.21 0.25 0.03 0.04 0.09 0.02 0.03 0.02 0.5 0.2 0.4 0.22 0.21 0.21 0.05

R a (um) 2 0.21 0.17 0.18 0.03 0.03 0.09 0.02 0.04 0.02 0.42 0.19 0.3 0.19 0.17 0.16 0.08

Mean 0.23 0.19 0.215 0.03 0.035 0.09 0.02 0.035 0.02 0.46 0.195 0.35 0.205 0.19 0.185 0.065

1 1.61 1.19 1.37 0.28 0.3 0.63 0.31 0.27 0.16 2.09 1.51 1.9 1.43 1.26 1.24 0.63

R max (um) 2 1.23 1.08 1.13 0.25 0.24 0.58 0.23 0.35 0.15 2.06 1.3 2.15 1.04 0.95 0.93 0.55

S/N Ratio Mean 1.42 1.135 1.25 0.265 0.27 0.605 0.27 0.31 0.155 2.075 1.405 2.025 1.235 1.105 1.085 0.59

12.733 14.377 13.238 30.458 29.031 20.915 33.979 29.031 33.979 6.7121 14.196 9.031 13.742 14.377 14.578 23.516

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Fang-Jung Shiou et al. / Procedia Manufacturing 10 (2017) 208 – 217 17 18

0.09 0.05

0.09 0.06

0.09 0.055

0.54 0.35

0.73 0.47

0.635 0.41

20.915 25.157

Fig. 6. Plots of control factor effects.

3.6. Combination of the adequate level for each factor and confirmation tests The goal in the ball burnishing process is to minimize the surface roughness value of the milled specimen by determining the adequate level of each factor. Since –log is a monotone decreasing function, it implies that we should maximize the S/N ratio. Consequently, we can determine the adequate level for each factor as being the level that has the highest value of K . According to the figure 6, the combination of the adequate level for each factor is A1B3C2D1E1F3. As a result, the adequate parameters for ball burnishing were as follows: the ball material of WC, the burnishing force of 650 N; the feed of 0.05 mm/rev., the velocity of 25 m/min., the cutting fluid as lubricant (emulsion concentration of 1:20), and the number of pass of 3, as listed in table 4. Three verification experiments have been carried out to observe the repeatability of using the adequate combination of ball burnishing parameters. The surface roughness of Ra = 0.025 m on average can be obtained based on the confirmation test results. The mean of the burnished surface roughness of the L18 specimens was 0.150 m in Table 3 without using the adequate combination of ball burnishing parameters. The improvement was about 83%. The experimental mean surface roughness was smaller than the predicted surface roughness Ra = 0.039 m with the confidence interval of 90%. It could then be confirmed that the combination of the level for each factor was the set of adequate parameters and the interactions among the factors were probably not important for the ball burnishing of SUS420J2 stainless steel. Figure 7 shows the surface roughness improvement on the fine turned test specimen from about Ra 1.0 um to about Ra 0.02 um observed by a toolmaker’s microscope. Table 4. Adequate combination of the ball burnishing parameters. Factors A. Ball material B. Force (N) C. Feed (mm/rev) D. Velocity (m/min) E. Lubricant F. No. of passes

Value WC 650 0.05 25 Cutting fluid (Emulsion concentration1:20) 3


Fig. 7. Surface roughness improvement on the fine turned test specimen from about Ra 1.06 um to Ra 0.02 um observed by a toolmaker’s microscope (30X).

3.7. Analysis of variance (ANOVA) The main effect of each factor was further analysed by using the analysis of variance (ANOVA) technique and the F ratio test in order to find out the significant factors (Table 5). The ratio value of F 0.01, 2, 14 is 6.51 for a level of risk equal to 0.01 (or 99% confidence level); the factor’s degrees of freedom is 2 and the degrees of freedom for the pooled error is 14, according to the F-distribution table [20]. The F ratio values that are greater than 6.51 can be considered as having a significant effect on surface roughness. As a result, the burnishing force and the ball material played significant roles on the surface roughness. The burnishing force had the greatest contribution (59.05 %) among these factors. Table 5. ANOVA Table for S/N ratio of surface roughness. Source

SS

DF

MS

F

A

316.81

1

316.81

25.64

Contribution 23.94

B

775.09

2

387.54

31.37

59.05

C

2.44

2

1.22

0.09

-

D

37.44

2

18.7

1.51

-

E

1.42

2

0.71

0.06

-

F

47.4

2

23.7

1.91

-

Error

84.31

6

14.05

-

-

Total

1276.87

17

-

-

100

Pooled to error

172.96

14

12.35

-

17.01

4. Improvements on the surface hardness and residual stress and application 4.1. Improvements on the surface hardness and residual stress In addition to the surface roughness improvement, the surface hardness and the residual stress variation have also been investigated. The initial surface hardness of the test specimen was about HRc 51.0 on average. After finishing the L 18 experiments, the surface hardness on the eighteen areas have been measured and averaged by the Vickers micro-hardness test instrument, type MVK-H1, as shown in figure 8. The surface hardness was increased to HRc 52.5 (HV 510) on average, possibly, due to the squeezing of the grain boundary. The variation of the compressive residual stress on the burnished surface has been measured by an X-ray diffractometer, D8 DISCOVER µHR (Bruker AXS), as shown in figure 9. The residual stress on the fine turned surface was about 327 MPa,

215

216


whereas that on the burnished surface was increased up to 1047MPa, possibly, due to the modification of the microstructure.

Fig. 8. Measured surface hardness on the burnished surface.

Fig. 9. Variation of the compressive residual stress on the burnished surface.

4.2. Application to a tapered cylindrical surface The determined adequate cylindrical surface ball burnishing parameters using the rolling-contact type have been applied to the surface finish of a SUS420J2 specimen with different tapers. Concerning the inclination angle, the burnishing force F has been modified so that the required normal burnishing force F n could be obtained, based on the simple equation, F Fn / cos T , as shown in figure 10(a). A specimen configured with different taper angle, namely, 10, 20, and 30 degrees, respectively, has been designed, simulated, fine turned and burnished, as shown in figure 10 (b). The surface roughness R a of the tapered surfaces was improved from about 0.6 m to 0.05 m on average. The surface roughness improvement on the tapered surface of the test object using the burnishing process, was about 90%.

Fig. 10. (a) Modified burnishing force F on a tapered surface; (b) application of the adequate ball burnishing parameters to different tapered surfaces.


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5. Conclusion A new load-cell-embedded burnishing tool integrated with a CNC lathe has been developed, to improve the surface roughness, hardness, and residual stress of the SUS420J2 (equivalent to AISI420) stainless steel specimens economically and efficiently. The adequate cylindrical surface burnishing parameters for the rolling-contact type ball burnishing tool, have been determined after conducting the Taguchi’s experimental methods. The adequate surface rolling-contact-type burnishing parameters for the SUS420J2 stainless steel were the combination of the ball material of WC, the burnishing force of 650 N, the feed of 0.05 mm/rev., the velocity of 25 m/min., the cutting fluid as lubricant (emulsion concentration of 1:20), and the number of pass of 3. The burnishing force had the greatest contribution (59.05 %) to the surface roughness improvement among these factors based on the analysis of variance (ANOVA). The fine turned surface roughness of the test specimens could be improved from about R a 1.0 ȝP WR R a 0.025 ȝP(R max ȝm) on average, using the adequate burnishing parameters. The burnished surface hardness is increased from HRc 51.0 to HRc 52.5 on average. The residual stress on the burnished surface was also increased from about 327 MPa to 1047MPa. Applying the adequate ball burnishing parameters to a tapered surface, the surface roughness was improved from about 0.6 ȝm to 0.05 ȝm on average. Acknowledgements The authors are grateful to the National Science Council of the Republic of China for supporting this research under grant NSC 103-2221-E-011-MY2. References [1] Available on http://www.ecoroll.de/en/ecoroll.html. [2] L. N. López de Lacalle, A. Lamikiz, J. Muñoa, J. A. Sánchez, Quality improvement of ball-end milled sculptured surfaces by ball burnishing. International Journal of Machine Tools and Manufacture, 45 (2005) 1659-1668. [3] W. Grzesik, K. Zak, 2012, Modification of surface finish produced by hard turning using superfinishing and burnishing operations. Journal of Material Processing Technology, 212 (2012) 315-322. [4] F. J. Shiou, C. C. Hsu, Surface finishing of hardened and tempered stainless tool steel using sequential ball grinding, ball burnishing and ball polishing processes on a machining centre. Journal of Material Processing Technology, 205 (2008) 249-258. [5] P. Zhang, Z. Liu, Effect of sequential turning and burnishing on the surface integrity of Cr-Ni-based stainless steel formed by laser cladding process. Surface and Coating Technology, 276 (2015) 327-335. [6] V. Schulze, F. Bleicher, P. Groche, Y. B. Guo, Y. S. Pyun, Surface modification by machine hammer peening and burnishing. CIRP AnnalsManufacturing Technology, 65 (2016) 809-832. [7] I. S. Jawahir, H. Attia, D. Biermann, J. Duflou, F. Klocke, D. Meyer, et al., Cryogenic manufacturing processes. CIRP AnnalsManufacturing Technology, 65 (2016) 713-736. [8] W. Grzesik, K. Zak, Producing high quality hardened parts using sequential hard turning and ball burnishing operations. Precision Engineering, 37 (2013) 849-855. [9] B. H. Yan, C. C. Wang, H. M. Chow, Y. C. Lin, Feasibility study of rotary electrical discharge machining with ball burnishing for Al2O3/6061Al composite. International Journal of Machine Tools and Manufacture, 40 (2000) 1403-1421. [10]C. H. Fu, M. P. Sealy, Y. B. Guo, X. T. Wei, Austenite–martensite phase transformation of biomedical Nitinol by ball burnishing. Journal of Material Processing Technology, 214 (2014) 3122-3130. [11]F. J. Shiou, C. H. Chen, Determination of optimal ball burnishing parameters for plastic injection molding steel. International Journal of Advanced Manufacturing, 3 (2003) 177-185. [12]L. Luca, S. Neagu-Ventzel, I. Marinescu, Effects of working parameters on surface finish in ball-burnishing of hardened steels. Precision Engineering, 29 (2005) 253-256. [13]N. H. Loh, S. C. Tam, Effects of ball burnishing parameters on surface finish-a literature survey and discussion, Precision Engineering, 10 (1998) 215-220. [14] F. J. Shiou, C. H. Chuang, Precision surface finish of the mold steel PDS5 using an innovative ball burnishing tool embedded with a load cell. Precision Engineering, 34 (2010) 76-84. [15] Available on http://www.tokkin.com/materials/stainless_steel/martensite. [16] S. J. Huang, Research on the surface finish of a plastic mold steel using a new ball-burnishing tool embedded with a load cell on a CNC lathe, Master thesis (in Chinese), National Taiwan University of Science and Technology, Taiwan, 2016. [17] Available on http://sensing.honeywell.com/honeywell-testest-and-measurementeasurement-load-cell-range-guide-008886-2-en.pdf. [18] M. S. Phadke, Quality engineering using robust design. Prentice-Hall, Englewood Cliffs, New Jersey, 1989. [19] Yang Iron Works, Technical handbook of ML-15A CNC Lathe, Taiwan, 1996. [20] D. C. Montgomery, Design and analysis of experiments, fourth ed., Wiley, New York, 1997.