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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
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Precision surface finish of the mold steel PDS5 using an innovative ball burnishing tool embedded with a load cell Fang-Jung Shiou ∗ , Chuing-Hsiung Chuang Department of Mechanical Engineering, National Taiwan University of Science and Technology No. 43, Sec. 4, Keelung Road, 106 Taipei, Taiwan
a r t i c l e
i n f o
Article history: Received 26 May 2008 Received in revised form 17 October 2008 Accepted 13 March 2009 Available online 1 April 2009 Keywords: Load-cell-embedded burnishing tool Burnishing force Surface roughness Force compensation strategy
a b s t r a c t A load-cell-embedded burnishing tool has been newly developed and integrated with a machining center, to improve the surface roughness of the PDS5 plastic injection mold steel. Either the rolling-contact type or the sliding-contact type was possible for the developed ball burnishing tool. The characteristic curves of burnishing force vs. surface roughness for the PDS5 plastic injection mold steel using the developed burnishing tool for both the rolling-contact type and the sliding-contact type, have been investigated and constructed, based on the test results. The optimal plane surface burnishing force for the PDS5 plastic injection mold steel was about 420 N for the rolling-contact type and about 470 N for the sliding-contact type, based on the results of experiments. A force compensation strategy that results in the constant optimal normal force for burnishing an inclined surface or a curved surface, has also been proposed to improve the surface roughness of the test objects in this study. The surface roughness of a fine milled inclined surface of 60 degrees can be improved from Ra 3.0 m on average to Ra 0.08 m (Rmax 0.79 m) on average using force compensation, whereas the surface roughness was Ra 0.35 m (Rmax 4.56 m) on average with no force compensation. © 2009 Elsevier Inc. All rights reserved.
1. Introduction Design and manufacture of the molds play an important role in the forming and shaping processes. Some properties, such as hardness, toughness, corrosion resistance, and wear resistance, are basic requirements for mold materials. The PDS5 tool steel (equivalent to AISI P20) developed by Daido Co. in Japan, is popular for the molds of large plastic injection products in the field of automobile and domestic appliances. The hardness of this material is about HRC33 (HS46) [1]. One specific advantage of this material is that after machining, the mold can be directly used for further finishing processes without heat treatment due to its special pre-treatment. The surface finish quality of the molds is an essential requirement due to its direct effects on the appearance of the plastic product. Finishing processes such as hard cutting, grinding, burnishing, polishing, lapping, and coating, are all commonly used to improve the surface finish of the molds for plastics. The burnishing process, which is one of the surface finishing processes that results in a plastic deformation on the workpiece surface by using a tungsten carbide ball or a roller [2–8], as shown in Fig. 1, has been applied to improve the surface roughness, surface hardness, and fatigue resistance, in recent years. An advantage of the ball burnishing process is that the finish of a freeform surface was pos-
∗ Corresponding author. Tel.: +886 2 2737 6543; fax: +886 2 2737 6460. E-mail address: [email protected] (F.-J. Shiou). 0141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2009.03.003
sible [6]. Some dominant flat surface ball burnishing parameters for the mold steel were the lubricant, the ball material, the burnishing speed, the burnishing force, and the feed. Among these parameters, the burnishing force played an important role on the surface roughness improvement, based on the analysis of variance (ANOVA) [8]. Most of the burnishing force of the proposed burnishing tools was generated by either a spring or a hydraulic pressure device, and monitored by 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 [2]. As the burnishing force increases, the roughness parameter decreases to a point, and then starts to increase. The force at that point is termed as the optimal force. The model for the process of smoothing burnishing has been presented to determine an optimal burnishing force [9]. The improvement of the surface roughness due to the burnishing process generally ranged between 40% and 90% [2–9]. The aim of this study was mainly to develop an innovative ball burnishing tool used to decrease the surface roughness, to monitor the burnishing force economically and efficiently by integrating a load cell with the burnishing tool, without using an expensive dynamometer system, and to introduce a strategy of burnishing force compensation based on the determined optimal burnishing force. The design, manufacture, and calibration of the innovative burnishing tool embedded with a load cell are first introduced in Section 2. The optimal burnishing parameters and the interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact
F.-J. Shiou, C.-H. Chuang / Precision Engineering 34 (2010) 76–84
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factors have been considered in designing the novel ball burnishing tool: • The burnishing force can be measured precisely by a load cell embedded within the burnishing tool. • Ball burnishing of a smoothed surface or a surface with relatively large slope is possible. • Safety device is required to protect the load cell. • Pre-loading adjustment is possible for the burnishing of an inclined surface or a curved surface. • Integration of an optical edge detector with the burnishing tool is possible. • The burnishing ball and the helical spring are changeable to meet the need for burnishing different materials, such as steel, copper, aluminum, etc. • Improvement of the burnishing speed is possible. Fig. 1. Schematic diagram of the ball burnishing process.
ball burnishing of the plastic injection mold steel PDS5, were then described in Section 3. The strategies for the burnishing of an inclined surface and a curved surface based on the optimal burnishing force and the experimental results, were revealed in Section 4. 2. Design and fabrication of an innovative burnishing tool embedded with a load cell To simplify the monitoring of the burnishing force without using an expensive dynamometer system, an innovative ball burnishing tool has been designed and manufactured. The following
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 Fig. 2. This tool mainly consists of a clamping shank, a load cell, a mold helical spring, two spring guides, a safety device, a cover, a pre-load adjustment screw, a collect chuck, a ball holder, and a burnishing ball. The burnishing tool can be mounted in the spindle of the machining center via the clamping shank. A mold helical spring with the spring constant of 16.07 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-loading
Fig. 2. Design of a burnishing tool embedded with a load cell.
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of the spring can be adjusted by an adjustment screw, so that the burnishing of an inclined surface or a curved 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 used polished tungsten carbide burnishing ball is 0.375 in. To determine the origin of a workpiece to be burnished, an optical edge detector can be clamped by the collect chuck of the burnishing tool. Fig. 3 shows the photos of the fabricated ball burnishing tools of rolling-contact type and slidingcontact type, respectively. The rolling-contact type burnishing tool is appropriate for applying to the smoothed surface with the inclination angle within about ±30 degrees. For surfaces with large slopes, the sliding-contact type burnishing tool is available. The load cell embedded in the burnishing tool is a product of the Honeywell Sensotec Company, model 53. The induced burnishing force signal is amplified by an amplifier, type UV-10. The amplified signal is then connected to a 14-bit A/D interface card, type PCI703-16A, inserted in a PC. The maximum loading for the adopted load cell is 1110 N. A set of interface software programmed with the Visual Basic language has been developed to measure the burnishing force of the burnishing tool embedded with a load cell, as shown in Fig. 4. The calibration result of the measured load vs. the deformation of the mold spring for the developed burnishing tool is shown in Fig. 5. The linearity was good and the maximum error was about 1.3% of the maximum loading. 3. Optimal burnishing force for the plastic injection mold steel PDS5 using the innovative burnishing tool The effects of several parameters could be determined efficiently by conducting matrix experiments using Taguchi’s orthogonal array
Fig. 3. Photo of the developed ball burnishing tool (a) rolling-contact type and (b) sliding-contact type.
[10]. The optimal flat surface burnishing parameters using the rolling-contact-type of the innovative burnishing tool, with respect to the surface finish of the hardened and tempered STAVAX plastic mold stainless steel with HRC 50, were the combination of the
Fig. 4. Developed interface software of the burnishing force measurement system for the innovative burnishing tool embedded with a load cell.
F.-J. Shiou, C.-H. Chuang / Precision Engineering 34 (2010) 76–84
Fig. 5. Calibration result of the measured load vs. the deformation of the spring for the innovative burnishing tool embedded with a load cell.
lubricant of water-soluble oils (1:50), the ball material of WC (Co 6%), the burnishing force of 850 N, the feed of 800 mm/min, the stepover of 60 m, and the burnishing path orthogonal to the ball milling direction, after conducting the Taguchi’s L18 matrix experiments, ANOVA analysis, and full factorial experiments in [11]. Among these parameters, the burnishing force had the most important contribution to the surface roughness improvement based on the ANOVA analysis. These parameters instead of the burnishing force were applied to the test specimens of the mold steel PDS5 with the hardness of HRC 33, to develop a model of surface formation by ball burnishing, from which the ball burnishing force, resulted in the smallest surface roughness for the developed burnishing tool, could be determined. The experimental setup used to develop a model of surface formation by ball burnishing and to determine the optimal flat surface ball burnishing force of the innovative burnishing tool for the surface roughness improvement, is shown in Fig. 6. The burnishing force ranging from 150 N to 900 N with an interval of 50 N has been configured for both the rolling-contact type and the slidingcontact type ball burnishing experiments. The PDS5 specimens
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have been designed and fabricated so that they could be mounted on a dynamometer, type 9273 of Kistler Co., to compare and verify the burnishing force measured by the load cell. The surface to be burnished was divided into sixteen zones for different burnishing forces. The pre-machined surface roughness of the test specimens using face milling process, measured by Hommelwerke T4000 surface roughness measuring equipment, was about Ra 2.5 m (Rmax 10.0 m) on average. The ball burnishing experiments of the milled specimens had been carried out on the 3-axis machining center made by Yang-Iron Co. type MV-3A equipped with the NC-controller of FANUC Co. type 0 M [12]. An MP700 touchtrigger probe made by Renishaw Co. was also integrated with the machining center tool magazine to measure and determine the coordinated origin of the specimens to be machined. The NC codes needed for ball burnishing were simulated and generated by the CATIA CAD/CAM software. These codes were transmitted to the CNC controller of the machining center via RS232 serial interface. Three specimens for both the rolling-contact type and the sliding-contact type ball burnishing experiments have been carried out so that the average burnished surface roughness value was calculated from that of the sixteen zones. The burnishing forces measured by the load cell were about 1.5% more accurate than those measured by the dynamometer [11]. The plots of the measured surface roughness (Ra & Rmax ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing, are shown in Figs. 7 and 8, respectively. The optimal burnishing force resulted in the best surface roughness was about 420 N using the rolling-contact type burnishing tool, and about 470 N using the sliding-contact type burnishing tool, considering the corresponding values of the minimal surface roughness values of the fitted polynomials. When the burnishing force was smaller than 350 N, the burnished surface roughness increased as the burnishing force decreased, and the rolling-contact type burnishing tool had obviously better performance on the surface roughness improvement
Fig. 6. Experimental setup to determine the optimal ball burnishing parameters.
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Fig. 7. Plot of the measured surface roughness (Ra ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing.
Fig. 8. Plot of the measured surface roughness (Rmax ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing.
than the sliding-contact type burnishing tool. When the burnishing force was greater than the optimal burnishing force, the performance on the surface roughness improvement showed no great difference between the rolling-contact type burnishing tool and the sliding-contact type burnishing tool, but the burnished surface roughness increased gradually. Based on the experimental results, the recommended flat surface burnishing parameters for the developed burnishing tools have been summarized in Table 1. The surface roughness of the test specimens could be improved Table 1 Combination of the ball burnishing parameters for the PDS5 tool steel using the innovative ball burnishing tool. Factor
Level
A. Lubricant B. Ball material C. Burnishing force (N) D. Feed (mm/min) E. Stepover (m) F. Path direction
Water-soluble oil (mass concentration 1:50) WC (Co 6%) Rolling contact; 420 Sliding contact; 470 800 60 Orthogonal
from about Ra 2.5 m (Rmax 10.0 m) on average to Ra 0.07 m (Rmax 0.7 m) on average using the optimal flat surface burnishing parameters. The interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing (almost the same for the rollingcontact ball burnishing), is shown in Fig. 9. When the burnishing force was smaller than 400 N, the peaks (asperities) of the milled surface were burnished and resulted in almost no permanent penetration. The permanent depth of penetration, the distance between the valley of the starting burnishing position and the mean line of the burnished profile, was about 2 m under the optimal burnishing force. With the increase of the burnishing force greater than the optimal burnishing force, the depth of penetration increased obviously. The material was slightly extruded nearby the starting position of burnishing when the burnishing force was greater than 600 N, and resulted in the increase of the surface roughness as shown in Fig. 7. According to these results, the burnishing force, in practical application, is suggested to be increased gradually to the required optimal force to keep away from the permanent
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Fig. 9. Plot of the interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing.
penetration at the starting position, with the help of an appropriate burnishing path configuration (ex. use of the tangential inlet). 4. Force compensation for the burnishing of an inclined or a curved surface based on the optimal burnishing force 4.1. Strategies for the burnishing of an inclined surface and a curved surface based on the optimal burnishing force Burnishing an inclined or a curved surface using a 3-axis machine tool, the normal force component will decrease and result
in the increase of the surface roughness, as presented in Fig. 7 or Fig. 8. Consequently, a strategy of burnishing force compensation has been investigated for burnishing an inclined or a curved surface, to improve the burnished surface roughness. The basic force compensation strategy is to increase the required burnishing force, FRB , along the z-axis direction, so that the optimal normal burnishing force, FON , can be obtained, as shown in Fig. 10. The strategy for burnishing an inclined surface with a constant slope is that a preload will be set for the helical spring in order to avoid the possible lateral deflection of the burnishing rod, based on the required burnishing force. Nevertheless, the strategy for burnishing a curved
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compensated by the NC controller as the tool length compensation. (FPL )i = (FRB )i − [(a × 0.5) + b] y = a · x + b,
a = 160.908,
(2) b = −14.221
(3)
where (FPL )i : preload applied to the helical spring at position i; (FRB )i : required burnishing force along z-axis at position i; a,b: coefficients of the calibration result of the load cell. For burnishing a curved surface, the Z-values of the NC codes, (ZCNC ), will be modified based on the difference between the required burnishing force and the optimal burnishing force ((FRB )i − (FON )i ) at different angles ( i ). The amount of the compensation of Z-values for NC codes can be calculated by Eq. (4). (ZCNC )i = −
Fig. 10. Schematic diagram of the optimal normal burnishing force and the required burnishing force along z-axis.
surface with distinct slopes at different positions is that the Z-values of the generated NC codes will be compensated and modified based on the required burnishing force. The inclination angle i at position i having been determined, the required burnishing force along z-axis FRB can then be calculated by Eq. (1) based on the determined optimal normal burnishing force. (FRB )i = (FON )i × (seci )
(1)
where FRB : required burnishing force along z-axis; FON : optimal normal burnishing force; i : inclination angle at position i. In order to avoid the prompt lateral deflection of the burnishing rod induced by the burnishing force compensation for burnishing an inclined surface, an appropriate preload (FPL ), was applied to the helical spring by adjusting the preload screw in the burnishing tool in actual implementation in this work. With the intention that the contact between the burnishing tool and the workpiece can be ensured due to the possible positioning error of the machine tool, the preload for the helical spring has been first set at about 66 N (deflection of the helical spring: 0.5 mm), smaller than the required burnishing force, FRB . The preload can be calculated by Eq. (2), based on the calibration result of the load cell expressed by Eq. (3) presented in Fig. 5. Before executing the burnishing process, the amount of 0.5 mm for the deflection of the spring was further
((F ) − (F ) ) − b RB i ON i a
(4)
4.2. Experimental results on the burnishing of an inclined surface and a curved surface of PDS5 based on the optimal burnishing force Due to the limitation of the inclination angle within ±30 degrees for the rolling-contact type burnishing tool, the sliding-contact type burnishing tool was used for the experiments of force compensation about the burnishing of an inclined or a curved surface of PDS5, based on the combination of the optimal burnishing parameters listed in Table 1. Fig. 11 shows the configuration of the ball burnishing regions on the different inclined surfaces, including 30◦ , 45◦ , and 60◦ , respectively. The configuration of the ball burnishing regions at different positions on a curved surface with the radius of 185 mm, is shown in Fig. 12. Regions A and B are configured for burnishing with no force compensation, whereas regions C and D are configured for burnishing with force compensation, respectively. Region E on the inclined surface of 30◦ is configured for burnishing without applying the preload. The pre-machined fine milled surface roughness of the test objects measured by Hommelwerke T4000 surface roughness measuring equipment was about 3.0 m on average. The required burnishing forces and preloads, for different inclined surfaces with force compensation and with no force compensation calculated based on Eq. (1)–(3), are shown in Table 2. The NC codes needed for the ball burnishing have been generated by the CATIA CAD/CAM software after confirming the machining path simulation. With the increase of the slopes for different positions on the curved surface, the amount of the compensation of the Z-values increased, as shown in Fig. 13. The burnished test objects of the inclined surfaces and the curved surface are shown in Fig. 14. The preload has been set for
Fig. 11. Configuration of the ball burnishing regions on the different inclined surfaces.
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Fig. 12. Configuration of the ball burnishing regions on the curved surface.
Table 2 Calculated required burnishing forces and preloads for different inclined surfaces.
Table 3 Measured surface roughness values (Ra ) for different areas of the inclined surfaces.
Inclination angles of the workpieces
Inclination angles of the workpieces
30 deg. 45 deg. 60 deg.
With no compensation
With compensation
FRB (N)
FPL (N)
FRB (N)
FPL (N)
470 470 470
404 404 404
542 665 940
476 599 874
Fig. 13. Modified paths for different positions on the curved surface with the magnification ratio of five.
burnishing the A to D regions of the inclined surfaces. Applying the optimal burnishing force, by setting the required movement of the z-axis instead of setting a preload for the helical spring, to the region “E” on the inclined surface of 30◦ , an undesired scratched lay resulted by the lateral deflection of the burnishing rod, is shown in Fig. 14(a). As a result, the setting of the preload for the helical spring based on the required burnishing force is suggested for burnishing an inclined surface with a constant slope.
30 deg 45 deg 60 deg
Measured Ra value (m) With no compensation
With compensation
A
B
Mean
C
D
Mean
0.12 0.09 0.36
0.08 0.08 0.35
0.10 0.09 0.35
0.06 0.07 0.08
0.12 0.08 0.07
0.09 0.08 0.08
Table 3 summarizes the measured burnished surface roughness values Ra ,with no force compensation and with force compensation, on the different inclined surfaces including 30◦ , 45◦ , and 60◦ , respectively. The performance on the surface roughness improvement on the inclined surfaces of 30◦ and 45◦ showed no great difference between the burnishing with no force compensation and that with force compensation. However, with the force compensation, the surface roughness on the inclined surface of 60◦ has been greatly improved from 0.35 m on average to 0.08 m on average. The measured burnished surface roughness values Ra , with the original NC codes without force compensation and with the modified NC codes for force compensation, at different positions of the curved surface, respectively, was compared in Table 4. Similar to the inclined surface, the greater the slope (ex. Position 1), the greater the surface roughness improvement with the modified NC codes for force compensation. The performance on the surface roughness improvement at different positions on the curved surface, in general, was about 10% using the burnishing with force compensation.
Fig. 14. Photos of the burnished workpieces: (a) inclined surfaces and (b) curved surface.
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Table 4 Measured surface roughness values (Ra ) for different positions of the curved surface. Position
Position 1 Position 2 Position 3
Measured Ra value (m) With no compensation
With compensation
A
B
Mean
C
D
Mean
0.13 0.11 0.09
0.13 0.12 0.09
0.13 0.12 0.09
0.10 0.10 0.08
0.11 0.10 0.08
0.11 0.10 0.08
5. Conclusion The development of an innovative ball burnishing embedded with a load cell integrated with a machining center to improve the surface roughness of the PDS5 plastic injection mold steel, has been proposed in this study. The recommended flat surface burnishing parameters using the sliding-contact-type of the innovative burnishing tool for the PDS5 mold steel with HRC 33, were the combination of the lubricant of water-soluble oils (1:50), the ball material of WC (Co 6%), the burnishing force of 470 N, the feed of 800 mm/min, the stepover of 60 m, and the burnishing path orthogonal to the ball milling direction, based on the experimental results. The surface roughness of the test specimens could be improved from about Ra 2.5 m (Rmax 10.0 m) on average to Ra 0.07 m (Rmax 0.70 m) on average, using the optimal flat surface burnishing parameters. The interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing, has been investigated. The permanent depth of penetration at the starting position was about 2 m under the optimal burnishing force. A strategy of burnishing force compensation based on the optimal burnishing force has also been investigated for burnishing an inclined or a curved surface using the sliding-contact type burnishing tool, to increase the burnished surface roughness. The performance on the surface roughness improvement on either the inclined surfaces or the curved surface, was effective using the burnishing with
force compensation, based on the experimental results. An adaptive control of the required burnishing force along z-axis to obtain a constant optimal normal burnishing force for burnishing a freeform surface using the new burnishing tool could be further studied in the future. Acknowledgements The authors are appreciative to the National Science Council of the Republic of China for supporting this research under grant NSC 96-2221-E-011-107. References [1] http://www.daido.co.jp/english/products/tool/plasticmold.html. [2] Loh NH, Tam SC. Effects of ball burnishing parameters on surface finish-a literature survey and discussion. Precis Eng 1988;10(4):215–20. [3] Yu X, Wang L. Effect of various parameters on the surface roughness of an aluminum alloy burnished with a spherical surfaced polycrystalline diamond tool. Int J Mach Tools Manuf 1999;39:459–69. [4] Klocke F, Liermann J. Roller burnishing of hard turned surfaces. Int J Mach Tools Manuf 1996;38(5):419–23. [5] Mamalis AG, Grabchenko AI, Horváth M, Mészáros I, Paulmier D. Ultraprecision metal removal processing of mirror-surfaces. J Mater Process Technol 2001;108:269–77. ˜ [6] López de Lacalle LN, Lamikiz A, Munoa J, Sánchez JA. Quality improvement of ball-end milled sculptured surfaces by ball burnishing. Int J Mach Tools Manuf 2005;45:1659–68. [7] Luca L, Neagu-Ventzel S, Marinescu I. Effects of working parameters on surface finish in ball-burnishing of hardened steels. Precis Eng 2005;29:253–6. [8] Shiou FJ, Chen CH. Determination of optimal ball burnishing parameters for plastic injection molding steel. Int J Adv Manuf T 2003;3:177–85. [9] Korzynski M. Modeling and experimental validation of the force-surface roughness relation for smoothing burnishing with a spherical tool. Int J Mach Tools Manuf 2007;47:1956–64. [10] Phadke MS. Quality engineering using robust design. Englewood Cliffs, New Jersey: Prentice-Hall; 1989. [11] Chuang CH. Research on the automated surface finish of the tool steels using a load-cell embedded burnishing tool. Thesis of Master of Science. National Taiwan University of Science and Technology; 2007 [in Chinese]. [12] Yang Iron Works, Technical handbook of MV-3A vertical machining center. Taiwan; 1996.
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Precision surface finish of the mold steel PDS5 using an innovative ball burnishing tool embedded with a load cell Fang-Jung Shiou ∗ , Chuing-Hsiung Chuang Department of Mechanical Engineering, National Taiwan University of Science and Technology No. 43, Sec. 4, Keelung Road, 106 Taipei, Taiwan
a r t i c l e
i n f o
Article history: Received 26 May 2008 Received in revised form 17 October 2008 Accepted 13 March 2009 Available online 1 April 2009 Keywords: Load-cell-embedded burnishing tool Burnishing force Surface roughness Force compensation strategy
a b s t r a c t A load-cell-embedded burnishing tool has been newly developed and integrated with a machining center, to improve the surface roughness of the PDS5 plastic injection mold steel. Either the rolling-contact type or the sliding-contact type was possible for the developed ball burnishing tool. The characteristic curves of burnishing force vs. surface roughness for the PDS5 plastic injection mold steel using the developed burnishing tool for both the rolling-contact type and the sliding-contact type, have been investigated and constructed, based on the test results. The optimal plane surface burnishing force for the PDS5 plastic injection mold steel was about 420 N for the rolling-contact type and about 470 N for the sliding-contact type, based on the results of experiments. A force compensation strategy that results in the constant optimal normal force for burnishing an inclined surface or a curved surface, has also been proposed to improve the surface roughness of the test objects in this study. The surface roughness of a fine milled inclined surface of 60 degrees can be improved from Ra 3.0 m on average to Ra 0.08 m (Rmax 0.79 m) on average using force compensation, whereas the surface roughness was Ra 0.35 m (Rmax 4.56 m) on average with no force compensation. © 2009 Elsevier Inc. All rights reserved.
1. Introduction Design and manufacture of the molds play an important role in the forming and shaping processes. Some properties, such as hardness, toughness, corrosion resistance, and wear resistance, are basic requirements for mold materials. The PDS5 tool steel (equivalent to AISI P20) developed by Daido Co. in Japan, is popular for the molds of large plastic injection products in the field of automobile and domestic appliances. The hardness of this material is about HRC33 (HS46) [1]. One specific advantage of this material is that after machining, the mold can be directly used for further finishing processes without heat treatment due to its special pre-treatment. The surface finish quality of the molds is an essential requirement due to its direct effects on the appearance of the plastic product. Finishing processes such as hard cutting, grinding, burnishing, polishing, lapping, and coating, are all commonly used to improve the surface finish of the molds for plastics. The burnishing process, which is one of the surface finishing processes that results in a plastic deformation on the workpiece surface by using a tungsten carbide ball or a roller [2–8], as shown in Fig. 1, has been applied to improve the surface roughness, surface hardness, and fatigue resistance, in recent years. An advantage of the ball burnishing process is that the finish of a freeform surface was pos-
∗ Corresponding author. Tel.: +886 2 2737 6543; fax: +886 2 2737 6460. E-mail address: [email protected] (F.-J. Shiou). 0141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2009.03.003
sible [6]. Some dominant flat surface ball burnishing parameters for the mold steel were the lubricant, the ball material, the burnishing speed, the burnishing force, and the feed. Among these parameters, the burnishing force played an important role on the surface roughness improvement, based on the analysis of variance (ANOVA) [8]. Most of the burnishing force of the proposed burnishing tools was generated by either a spring or a hydraulic pressure device, and monitored by 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 [2]. As the burnishing force increases, the roughness parameter decreases to a point, and then starts to increase. The force at that point is termed as the optimal force. The model for the process of smoothing burnishing has been presented to determine an optimal burnishing force [9]. The improvement of the surface roughness due to the burnishing process generally ranged between 40% and 90% [2–9]. The aim of this study was mainly to develop an innovative ball burnishing tool used to decrease the surface roughness, to monitor the burnishing force economically and efficiently by integrating a load cell with the burnishing tool, without using an expensive dynamometer system, and to introduce a strategy of burnishing force compensation based on the determined optimal burnishing force. The design, manufacture, and calibration of the innovative burnishing tool embedded with a load cell are first introduced in Section 2. The optimal burnishing parameters and the interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact
F.-J. Shiou, C.-H. Chuang / Precision Engineering 34 (2010) 76–84
77
factors have been considered in designing the novel ball burnishing tool: • The burnishing force can be measured precisely by a load cell embedded within the burnishing tool. • Ball burnishing of a smoothed surface or a surface with relatively large slope is possible. • Safety device is required to protect the load cell. • Pre-loading adjustment is possible for the burnishing of an inclined surface or a curved surface. • Integration of an optical edge detector with the burnishing tool is possible. • The burnishing ball and the helical spring are changeable to meet the need for burnishing different materials, such as steel, copper, aluminum, etc. • Improvement of the burnishing speed is possible. Fig. 1. Schematic diagram of the ball burnishing process.
ball burnishing of the plastic injection mold steel PDS5, were then described in Section 3. The strategies for the burnishing of an inclined surface and a curved surface based on the optimal burnishing force and the experimental results, were revealed in Section 4. 2. Design and fabrication of an innovative burnishing tool embedded with a load cell To simplify the monitoring of the burnishing force without using an expensive dynamometer system, an innovative ball burnishing tool has been designed and manufactured. The following
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 Fig. 2. This tool mainly consists of a clamping shank, a load cell, a mold helical spring, two spring guides, a safety device, a cover, a pre-load adjustment screw, a collect chuck, a ball holder, and a burnishing ball. The burnishing tool can be mounted in the spindle of the machining center via the clamping shank. A mold helical spring with the spring constant of 16.07 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-loading
Fig. 2. Design of a burnishing tool embedded with a load cell.
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of the spring can be adjusted by an adjustment screw, so that the burnishing of an inclined surface or a curved 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 used polished tungsten carbide burnishing ball is 0.375 in. To determine the origin of a workpiece to be burnished, an optical edge detector can be clamped by the collect chuck of the burnishing tool. Fig. 3 shows the photos of the fabricated ball burnishing tools of rolling-contact type and slidingcontact type, respectively. The rolling-contact type burnishing tool is appropriate for applying to the smoothed surface with the inclination angle within about ±30 degrees. For surfaces with large slopes, the sliding-contact type burnishing tool is available. The load cell embedded in the burnishing tool is a product of the Honeywell Sensotec Company, model 53. The induced burnishing force signal is amplified by an amplifier, type UV-10. The amplified signal is then connected to a 14-bit A/D interface card, type PCI703-16A, inserted in a PC. The maximum loading for the adopted load cell is 1110 N. A set of interface software programmed with the Visual Basic language has been developed to measure the burnishing force of the burnishing tool embedded with a load cell, as shown in Fig. 4. The calibration result of the measured load vs. the deformation of the mold spring for the developed burnishing tool is shown in Fig. 5. The linearity was good and the maximum error was about 1.3% of the maximum loading. 3. Optimal burnishing force for the plastic injection mold steel PDS5 using the innovative burnishing tool The effects of several parameters could be determined efficiently by conducting matrix experiments using Taguchi’s orthogonal array
Fig. 3. Photo of the developed ball burnishing tool (a) rolling-contact type and (b) sliding-contact type.
[10]. The optimal flat surface burnishing parameters using the rolling-contact-type of the innovative burnishing tool, with respect to the surface finish of the hardened and tempered STAVAX plastic mold stainless steel with HRC 50, were the combination of the
Fig. 4. Developed interface software of the burnishing force measurement system for the innovative burnishing tool embedded with a load cell.
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Fig. 5. Calibration result of the measured load vs. the deformation of the spring for the innovative burnishing tool embedded with a load cell.
lubricant of water-soluble oils (1:50), the ball material of WC (Co 6%), the burnishing force of 850 N, the feed of 800 mm/min, the stepover of 60 m, and the burnishing path orthogonal to the ball milling direction, after conducting the Taguchi’s L18 matrix experiments, ANOVA analysis, and full factorial experiments in [11]. Among these parameters, the burnishing force had the most important contribution to the surface roughness improvement based on the ANOVA analysis. These parameters instead of the burnishing force were applied to the test specimens of the mold steel PDS5 with the hardness of HRC 33, to develop a model of surface formation by ball burnishing, from which the ball burnishing force, resulted in the smallest surface roughness for the developed burnishing tool, could be determined. The experimental setup used to develop a model of surface formation by ball burnishing and to determine the optimal flat surface ball burnishing force of the innovative burnishing tool for the surface roughness improvement, is shown in Fig. 6. The burnishing force ranging from 150 N to 900 N with an interval of 50 N has been configured for both the rolling-contact type and the slidingcontact type ball burnishing experiments. The PDS5 specimens
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have been designed and fabricated so that they could be mounted on a dynamometer, type 9273 of Kistler Co., to compare and verify the burnishing force measured by the load cell. The surface to be burnished was divided into sixteen zones for different burnishing forces. The pre-machined surface roughness of the test specimens using face milling process, measured by Hommelwerke T4000 surface roughness measuring equipment, was about Ra 2.5 m (Rmax 10.0 m) on average. The ball burnishing experiments of the milled specimens had been carried out on the 3-axis machining center made by Yang-Iron Co. type MV-3A equipped with the NC-controller of FANUC Co. type 0 M [12]. An MP700 touchtrigger probe made by Renishaw Co. was also integrated with the machining center tool magazine to measure and determine the coordinated origin of the specimens to be machined. The NC codes needed for ball burnishing were simulated and generated by the CATIA CAD/CAM software. These codes were transmitted to the CNC controller of the machining center via RS232 serial interface. Three specimens for both the rolling-contact type and the sliding-contact type ball burnishing experiments have been carried out so that the average burnished surface roughness value was calculated from that of the sixteen zones. The burnishing forces measured by the load cell were about 1.5% more accurate than those measured by the dynamometer [11]. The plots of the measured surface roughness (Ra & Rmax ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing, are shown in Figs. 7 and 8, respectively. The optimal burnishing force resulted in the best surface roughness was about 420 N using the rolling-contact type burnishing tool, and about 470 N using the sliding-contact type burnishing tool, considering the corresponding values of the minimal surface roughness values of the fitted polynomials. When the burnishing force was smaller than 350 N, the burnished surface roughness increased as the burnishing force decreased, and the rolling-contact type burnishing tool had obviously better performance on the surface roughness improvement
Fig. 6. Experimental setup to determine the optimal ball burnishing parameters.
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Fig. 7. Plot of the measured surface roughness (Ra ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing.
Fig. 8. Plot of the measured surface roughness (Rmax ) vs. normal burnishing force for the rolling-contact and sliding-contact ball burnishing.
than the sliding-contact type burnishing tool. When the burnishing force was greater than the optimal burnishing force, the performance on the surface roughness improvement showed no great difference between the rolling-contact type burnishing tool and the sliding-contact type burnishing tool, but the burnished surface roughness increased gradually. Based on the experimental results, the recommended flat surface burnishing parameters for the developed burnishing tools have been summarized in Table 1. The surface roughness of the test specimens could be improved Table 1 Combination of the ball burnishing parameters for the PDS5 tool steel using the innovative ball burnishing tool. Factor
Level
A. Lubricant B. Ball material C. Burnishing force (N) D. Feed (mm/min) E. Stepover (m) F. Path direction
Water-soluble oil (mass concentration 1:50) WC (Co 6%) Rolling contact; 420 Sliding contact; 470 800 60 Orthogonal
from about Ra 2.5 m (Rmax 10.0 m) on average to Ra 0.07 m (Rmax 0.7 m) on average using the optimal flat surface burnishing parameters. The interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing (almost the same for the rollingcontact ball burnishing), is shown in Fig. 9. When the burnishing force was smaller than 400 N, the peaks (asperities) of the milled surface were burnished and resulted in almost no permanent penetration. The permanent depth of penetration, the distance between the valley of the starting burnishing position and the mean line of the burnished profile, was about 2 m under the optimal burnishing force. With the increase of the burnishing force greater than the optimal burnishing force, the depth of penetration increased obviously. The material was slightly extruded nearby the starting position of burnishing when the burnishing force was greater than 600 N, and resulted in the increase of the surface roughness as shown in Fig. 7. According to these results, the burnishing force, in practical application, is suggested to be increased gradually to the required optimal force to keep away from the permanent
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Fig. 9. Plot of the interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing.
penetration at the starting position, with the help of an appropriate burnishing path configuration (ex. use of the tangential inlet). 4. Force compensation for the burnishing of an inclined or a curved surface based on the optimal burnishing force 4.1. Strategies for the burnishing of an inclined surface and a curved surface based on the optimal burnishing force Burnishing an inclined or a curved surface using a 3-axis machine tool, the normal force component will decrease and result
in the increase of the surface roughness, as presented in Fig. 7 or Fig. 8. Consequently, a strategy of burnishing force compensation has been investigated for burnishing an inclined or a curved surface, to improve the burnished surface roughness. The basic force compensation strategy is to increase the required burnishing force, FRB , along the z-axis direction, so that the optimal normal burnishing force, FON , can be obtained, as shown in Fig. 10. The strategy for burnishing an inclined surface with a constant slope is that a preload will be set for the helical spring in order to avoid the possible lateral deflection of the burnishing rod, based on the required burnishing force. Nevertheless, the strategy for burnishing a curved
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compensated by the NC controller as the tool length compensation. (FPL )i = (FRB )i − [(a × 0.5) + b] y = a · x + b,
a = 160.908,
(2) b = −14.221
(3)
where (FPL )i : preload applied to the helical spring at position i; (FRB )i : required burnishing force along z-axis at position i; a,b: coefficients of the calibration result of the load cell. For burnishing a curved surface, the Z-values of the NC codes, (ZCNC ), will be modified based on the difference between the required burnishing force and the optimal burnishing force ((FRB )i − (FON )i ) at different angles ( i ). The amount of the compensation of Z-values for NC codes can be calculated by Eq. (4). (ZCNC )i = −
Fig. 10. Schematic diagram of the optimal normal burnishing force and the required burnishing force along z-axis.
surface with distinct slopes at different positions is that the Z-values of the generated NC codes will be compensated and modified based on the required burnishing force. The inclination angle i at position i having been determined, the required burnishing force along z-axis FRB can then be calculated by Eq. (1) based on the determined optimal normal burnishing force. (FRB )i = (FON )i × (seci )
(1)
where FRB : required burnishing force along z-axis; FON : optimal normal burnishing force; i : inclination angle at position i. In order to avoid the prompt lateral deflection of the burnishing rod induced by the burnishing force compensation for burnishing an inclined surface, an appropriate preload (FPL ), was applied to the helical spring by adjusting the preload screw in the burnishing tool in actual implementation in this work. With the intention that the contact between the burnishing tool and the workpiece can be ensured due to the possible positioning error of the machine tool, the preload for the helical spring has been first set at about 66 N (deflection of the helical spring: 0.5 mm), smaller than the required burnishing force, FRB . The preload can be calculated by Eq. (2), based on the calibration result of the load cell expressed by Eq. (3) presented in Fig. 5. Before executing the burnishing process, the amount of 0.5 mm for the deflection of the spring was further
((F ) − (F ) ) − b RB i ON i a
(4)
4.2. Experimental results on the burnishing of an inclined surface and a curved surface of PDS5 based on the optimal burnishing force Due to the limitation of the inclination angle within ±30 degrees for the rolling-contact type burnishing tool, the sliding-contact type burnishing tool was used for the experiments of force compensation about the burnishing of an inclined or a curved surface of PDS5, based on the combination of the optimal burnishing parameters listed in Table 1. Fig. 11 shows the configuration of the ball burnishing regions on the different inclined surfaces, including 30◦ , 45◦ , and 60◦ , respectively. The configuration of the ball burnishing regions at different positions on a curved surface with the radius of 185 mm, is shown in Fig. 12. Regions A and B are configured for burnishing with no force compensation, whereas regions C and D are configured for burnishing with force compensation, respectively. Region E on the inclined surface of 30◦ is configured for burnishing without applying the preload. The pre-machined fine milled surface roughness of the test objects measured by Hommelwerke T4000 surface roughness measuring equipment was about 3.0 m on average. The required burnishing forces and preloads, for different inclined surfaces with force compensation and with no force compensation calculated based on Eq. (1)–(3), are shown in Table 2. The NC codes needed for the ball burnishing have been generated by the CATIA CAD/CAM software after confirming the machining path simulation. With the increase of the slopes for different positions on the curved surface, the amount of the compensation of the Z-values increased, as shown in Fig. 13. The burnished test objects of the inclined surfaces and the curved surface are shown in Fig. 14. The preload has been set for
Fig. 11. Configuration of the ball burnishing regions on the different inclined surfaces.
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Fig. 12. Configuration of the ball burnishing regions on the curved surface.
Table 2 Calculated required burnishing forces and preloads for different inclined surfaces.
Table 3 Measured surface roughness values (Ra ) for different areas of the inclined surfaces.
Inclination angles of the workpieces
Inclination angles of the workpieces
30 deg. 45 deg. 60 deg.
With no compensation
With compensation
FRB (N)
FPL (N)
FRB (N)
FPL (N)
470 470 470
404 404 404
542 665 940
476 599 874
Fig. 13. Modified paths for different positions on the curved surface with the magnification ratio of five.
burnishing the A to D regions of the inclined surfaces. Applying the optimal burnishing force, by setting the required movement of the z-axis instead of setting a preload for the helical spring, to the region “E” on the inclined surface of 30◦ , an undesired scratched lay resulted by the lateral deflection of the burnishing rod, is shown in Fig. 14(a). As a result, the setting of the preload for the helical spring based on the required burnishing force is suggested for burnishing an inclined surface with a constant slope.
30 deg 45 deg 60 deg
Measured Ra value (m) With no compensation
With compensation
A
B
Mean
C
D
Mean
0.12 0.09 0.36
0.08 0.08 0.35
0.10 0.09 0.35
0.06 0.07 0.08
0.12 0.08 0.07
0.09 0.08 0.08
Table 3 summarizes the measured burnished surface roughness values Ra ,with no force compensation and with force compensation, on the different inclined surfaces including 30◦ , 45◦ , and 60◦ , respectively. The performance on the surface roughness improvement on the inclined surfaces of 30◦ and 45◦ showed no great difference between the burnishing with no force compensation and that with force compensation. However, with the force compensation, the surface roughness on the inclined surface of 60◦ has been greatly improved from 0.35 m on average to 0.08 m on average. The measured burnished surface roughness values Ra , with the original NC codes without force compensation and with the modified NC codes for force compensation, at different positions of the curved surface, respectively, was compared in Table 4. Similar to the inclined surface, the greater the slope (ex. Position 1), the greater the surface roughness improvement with the modified NC codes for force compensation. The performance on the surface roughness improvement at different positions on the curved surface, in general, was about 10% using the burnishing with force compensation.
Fig. 14. Photos of the burnished workpieces: (a) inclined surfaces and (b) curved surface.
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Table 4 Measured surface roughness values (Ra ) for different positions of the curved surface. Position
Position 1 Position 2 Position 3
Measured Ra value (m) With no compensation
With compensation
A
B
Mean
C
D
Mean
0.13 0.11 0.09
0.13 0.12 0.09
0.13 0.12 0.09
0.10 0.10 0.08
0.11 0.10 0.08
0.11 0.10 0.08
5. Conclusion The development of an innovative ball burnishing embedded with a load cell integrated with a machining center to improve the surface roughness of the PDS5 plastic injection mold steel, has been proposed in this study. The recommended flat surface burnishing parameters using the sliding-contact-type of the innovative burnishing tool for the PDS5 mold steel with HRC 33, were the combination of the lubricant of water-soluble oils (1:50), the ball material of WC (Co 6%), the burnishing force of 470 N, the feed of 800 mm/min, the stepover of 60 m, and the burnishing path orthogonal to the ball milling direction, based on the experimental results. The surface roughness of the test specimens could be improved from about Ra 2.5 m (Rmax 10.0 m) on average to Ra 0.07 m (Rmax 0.70 m) on average, using the optimal flat surface burnishing parameters. The interface between the milled profiles and the burnished profiles under different normal burnishing forces (150–900 N) for the sliding-contact ball burnishing, has been investigated. The permanent depth of penetration at the starting position was about 2 m under the optimal burnishing force. A strategy of burnishing force compensation based on the optimal burnishing force has also been investigated for burnishing an inclined or a curved surface using the sliding-contact type burnishing tool, to increase the burnished surface roughness. The performance on the surface roughness improvement on either the inclined surfaces or the curved surface, was effective using the burnishing with
force compensation, based on the experimental results. An adaptive control of the required burnishing force along z-axis to obtain a constant optimal normal burnishing force for burnishing a freeform surface using the new burnishing tool could be further studied in the future. Acknowledgements The authors are appreciative to the National Science Council of the Republic of China for supporting this research under grant NSC 96-2221-E-011-107. References [1] http://www.daido.co.jp/english/products/tool/plasticmold.html. [2] Loh NH, Tam SC. Effects of ball burnishing parameters on surface finish-a literature survey and discussion. Precis Eng 1988;10(4):215–20. [3] Yu X, Wang L. Effect of various parameters on the surface roughness of an aluminum alloy burnished with a spherical surfaced polycrystalline diamond tool. Int J Mach Tools Manuf 1999;39:459–69. [4] Klocke F, Liermann J. Roller burnishing of hard turned surfaces. Int J Mach Tools Manuf 1996;38(5):419–23. [5] Mamalis AG, Grabchenko AI, Horváth M, Mészáros I, Paulmier D. Ultraprecision metal removal processing of mirror-surfaces. J Mater Process Technol 2001;108:269–77. ˜ [6] López de Lacalle LN, Lamikiz A, Munoa J, Sánchez JA. Quality improvement of ball-end milled sculptured surfaces by ball burnishing. Int J Mach Tools Manuf 2005;45:1659–68. [7] Luca L, Neagu-Ventzel S, Marinescu I. Effects of working parameters on surface finish in ball-burnishing of hardened steels. Precis Eng 2005;29:253–6. [8] Shiou FJ, Chen CH. Determination of optimal ball burnishing parameters for plastic injection molding steel. Int J Adv Manuf T 2003;3:177–85. [9] Korzynski M. Modeling and experimental validation of the force-surface roughness relation for smoothing burnishing with a spherical tool. Int J Mach Tools Manuf 2007;47:1956–64. [10] Phadke MS. Quality engineering using robust design. Englewood Cliffs, New Jersey: Prentice-Hall; 1989. [11] Chuang CH. Research on the automated surface finish of the tool steels using a load-cell embedded burnishing tool. Thesis of Master of Science. National Taiwan University of Science and Technology; 2007 [in Chinese]. [12] Yang Iron Works, Technical handbook of MV-3A vertical machining center. Taiwan; 1996.