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Development of a new precision internal machining process using an alternating magnetic field
Precision Engineering 27 (2003) 51–58
Development of a new precision internal machining process using an alternating magnetic field Hitomi Yamaguchi a,∗ , Takeo Shinmura b , Maki Takenaga b a
Faculty of Engineering, Utsunomiya University, Utsunomiya, Tochigi 321-8585, Japan b Graduate School of Engineering, Utsunomiya University, Tochigi 321-8585, Japan
Received 15 May 2002; received in revised form 29 July 2002; accepted 29 July 2002
Abstract Imparting compressive residual stress to a surface improves the fatigue structural integrity of components, which is particularly important for components used in such critical applications as high-pressure gas or liquid piping systems. This paper proposes a new precision internal machining process that controls the surface integrity of internal surface of these components. This process utilizes an alternating magnetic field to control the force and dynamic motion of the tools needed for machining. An experimental set-up was developed to test the processing principle. This study characterizes the in-process tool behavior—that is, the relationships between the magnetic field, the tool properties, and the tool behavior—and reveals the properties of the tools required to achieve the desired results: magnetic anisotropy and a specific geometric restriction. The surface roughness, hardness, and residual stress measurements following machining experiments demonstrate the effects of the tool behavior on the machining characteristics. This paper also proposes methods to obtain desired surface characteristics. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Precision machining; Internal machining; Magnetic field assisted machining; Surface characteristics; Surface modification
1. Introduction Imparting compressive residual stress to a surface improves the fatigue structural integrity of components. Shot peening is a widely-used process for controlling fatigue structural integrity [1,2]. Peening with shot, discharged either from a rotating wheel or from a nozzle with an air blast, causes deformation of the surface. This results in work hardening of the surface and imparts compressive residual stress to the surface. A shot-peened surface is a satin-finished surface and has a micro-pool and dimple effect. This increase in surface roughness is thought to adversely affect the fatigue structural integrity [3]. The technique of fine particle bombarding has recently been developed to take advantages of some of the benefits of shot peening mentioned above whilst reducing the inherent problems [4]. The process of fine particle bombarding imparts higher compressive residual stress to the surface than does conventional shot peening while having little effect on the initial surface roughness. A similar method for introducing compressive residual stress by use of a cavitating jet from a nozzle has also been proposed [5]. This process does ∗ Corresponding
author. Tel.: +81-28-689-6077; fax: +81-28-689-6077. E-mail address: [email protected] (H. Yamaguchi).
not require shot as in the case of shot peening. In addition, the running cost is lower, and there are no health hazards. These processes, however, are difficult to apply to internal surfaces of components, (e.g. high-pressure gas cylinders and pressure fuel pipes) because of the process delivery mechanisms. Moreover, there is no process that is equivalent to a peening process in fine control of surface modification. The development of a new process is therefore needed to solve the above-mentioned problems. Consequently, this study proposes a new precision internal machining process that controls the surface integrity of the component. The aims of the present research are to describe the processing principle, develop an experimental set-up to realize the principle, characterize the in-process tool behavior and its effects on the machining characteristics, and to explain the finishing mechanism. This paper also proposes methods to obtain desired surface characteristics.
2. Processing principle and experimental set-up This new process uses an alternating magnetic field to control magnetic tools inside the workpiece. The general principle of the process is shown in Fig. 1. Coils facing each other in a parallel circuit generate an alternating magnetic field
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Fig. 2. External photograph of experimental set-up.
Due to the effects of the alternating magnetic field acting on the pin, the pin exhibits random, active, three-dimensional motion by colliding with the inner surface of the workpiece, thereby achieving precision internal machining. The kinetic energy, K, of the pin exhibiting three-dimensional motion is calculated by the following equation: K = 21 (Mv 2 + I ω2 ),
Fig. 1. Schematic of internal machining process using alternating magnetic field.
around two magnetic poles. The two poles are positioned directly above the workpiece with an adjustable interpole separation. The vertical distance between the two poles and the workpiece is also variable. Magnetic tools, such as pins, inserted into the workpiece are influenced by the alternating magnetic field. When the magnetic axis of the pin is inclined at an angle θ to the magnetic field, the pin rotates to align the axis to the magnetic field. The pin having +m and −m poles in a magnetic field H, shown in Fig. 1(b), experiences a couple F at each pole. The couple F rotates the pin and generates a moment of the couple L: F = mH,
(1)
L = mlH sin θ,
(2)
where l is the distance between +m and −m poles, and ml is the magnetic moment. When the magnetic field H changes in the direction of x, as shown in Fig. 1(c), a magnetic force Fx acts on the pin, driving it: Fx = ml
∂H ∂H = V χH . ∂x ∂x
(3)
The magnetic force acting on the pin Fx is calculated as the product of volume V, susceptibility χ of the pin, and the intensity and gradient of the magnetic field H (∂H/∂x) [6].
(4)
where M is mass of the pin, v the velocity, I the moment of inertia on the pin, and ω is angular velocity. An experimental set-up developed to test this processing principle is shown in Fig. 2. A coil of copper wire (Ø1 mm, 3700 turns) was wound around a core of rolled steel (Ø30 × 140 mm), and a replaceable pole of rolled steel was attached to the core. Constant voltage was supplied to the coils through an ac power amplifier. The alternating magnetic field around the poles was adjustable by a function generator, and the distance between the opposed coils was adjustable. The workpiece was set under the poles, but the position of the workpiece was flexible in the vertical and horizontal directions. The smaller the distances between the two opposed magnetic poles and between the workpiece and magnetic poles are, the stronger is the magnetic field, inducing dynamic behavior of the pins in the alternating magnetic field. In the experiments, the clearances between the pole tips and between the workpiece and the poles were set at 3 and 2 mm, respectively.
3. Tools behavior in process Before machining experiments were performed, the dynamic behavior of the magnetic tools was investigated through observation of the pin motion using the experimental set-up. Table 1 shows the experimental conditions. A previous research study has shown that a required property for induction of effective motion of the tools in an alternating magnetic field is the magnetic anisotropy of the tools [7]. Since cold-worked SUS304 stainless steel pins exhibit ferromagnetism with magnetic anisotropy due to transformation of the crystal construction [7], they were used in
H. Yamaguchi et al. / Precision Engineering 27 (2003) 51–58 Table 1 Experimental conditions Workpiece
Magnetic tools
Coil
Acrylic resin tube (Ø20 × Ø17 × 100 mm) SUS304 stainless steel bright annealed tube (Ø19 × Ø16 × 83 mm) SUS304 stainless steel cold-worked pins (Ø0.5 × 5 mm, Ø0.5 × 2.5 mm, Ø0.5 × 5 mm, Ø1 × 5 mm) 3 g supplied Core: SS400 rolled steel (Ø30 × 140 mm) copper wire (Ø1 mm): 3700 turns
Pole
AC power
Sinusoidal current, 2.0 A (115 V, 15 Hz)
Measured parts
these studies. The magnetic axis of the pin is the same as the geometric axis of the pin. Eq. (2) shows that a moment acts on a magnetic particle regardless of the geometry but that the distance l between +m and −m poles has an influence on the motion of the pins. According to Eq. (3), the driving force of the pin is proportional to the distance l of the pin. This indicates that the distance l must have an influence on the motion as well as the machining force of the pin, controlling the machining characteristics. The distance between +m and −m poles, l, of the pin was adjusted by changing the length of the pin in the experiments. Pins with three different lengths, Ø0.5 × 0.5 mm, Ø0.5 × 2.5 mm, and Ø0.5 × 5 mm, were used in the experiments. An transparent acrylic resin tube, Ø20 × Ø17 × 100 mm, was used to observe the motion of the pins inside the tube. Fig. 3 shows a schematic of the pins in the alternating magnetic field. Only the Ø0.5 × 5 mm pins exhibited dynamic motion and mainly collided with top and bottom regions in
Fig. 3. Schematic of motion of the pins in cases of Ø0.5 × 5 mm and Ø0.5 × 0.5 mm.
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response to the alternating magnetic field, as shown in Fig. 3(a). It is known that Ø0.5 × 5 mm pins have higher susceptibility and lead to greater magnetic force than do Ø0.5 × 0.5 mm pins [8]. The greater magnetic force and moment of the pins resulted in dynamic motion of the Ø0.5 × 5 mm pins. The Ø0.5 × 2.5 mm and Ø0.5 × 0.5 mm pins were linked together by magnetic force, and these linkages of the pins trembled in response to the alternation of the magnetic field, as shown in Fig. 3(b). The degree of the tremor of the Ø0.5×2.5 mm pins was larger than that of the Ø0.5×0.5 mm pins because of the greater magnetic force acting on the pins. The results of these experiments revealed another required property for induction of dynamic motion of the pins: the distance l between the +m and −m poles of the pin.
4. Conditions for machining experiments In case of a magnetic field barrel finishing process, Ø1 × 5 mm pins in an alternating magnetic field collide against the target surface with greater force than Ø0.5 × 5 mm pins and succeed in deburring efficiently [7]. In the experimental set-up shown in Fig. 2, the Ø1 × 5 mm pins also exhibited dynamic motion similar to that of the Ø0.5 × 5 mm pins and collided with great force against the inner surface. As long as the pin has the distance l needed for dynamic motion, increasing its volume results in an increase in its driving force and collision force against the target surface. This enhances the machining performance. Machining experiments were, thus, conducted using Ø1 × 5 mm cold-worked SUS304 stainless steel pins to determine the fundamental machining characteristics of the process using non-ferromagnetic bright annealed SUS304 stainless steel tubes, which are commonly used for piping systems, as workpieces. Other conditions are shown in Table 1. The machining experiments resulted in changes in hardness and residual stress, in addition to the surface roughness and material removal, with elapse of machining time. A micro-hardness tester was used to measure the hardness of the machined surface. The size of the workpiece that could be used for the hardness test was restricted by the configuration of the tester. The experiments were therefore performed using a tube divided into four pieces in the axial direction, which included the top, lateral, and bottom regions, as shown in Table 1. During the machining experiments, the pieces were glued in place on the tube. Fig. 4 shows the magnetic field measured in the machining area. The highest magnetic flux density was obtained in the machining area corresponding to the center of the pole, X = 0 mm, and decreased with increase in the distance from the pole. The top region, which is the closest to the poles, showed the highest magnetic flux density and the sharpest decline in magnetic flux density with the distance X of the three regions because of the trapezoidal shape of the pole. The lateral and bottom regions showed gentle declines of the magnetic flux density with the distance X because of greater
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Fig. 4. Changes in magnetic flux density with distance X.
distance between the region and the pole. The unevenness of the magnetic field must affect not only dynamic motion of pins but also machining characteristics in each region.
5. Fundamental machining characteristics The material removal was hardly detected by an electronic force balance with 0.1 mg resolution in any experimental condition. This shows that the process generates a machined surface by an accumulation of plastic deformations as a result of collisions of the pins with the surface. Fig. 5 shows changes in surface roughness with machining time corresponding to the center of the pole, X = 0 mm. As shown in Fig. 4, the shorter the distance between machining area and pole is, the greater is the magnetic field, which is the predominant component of the collision force of the pins. This caused the greatest plastic deformation of the surface in the top region, making the roughest surface. In the lateral region, the pins caused less indentation on the target surface because of the geometric arrangement. This resulted in the
Fig. 5. Changes in surface roughness with machining time (X = 0 mm).
least change in surface roughness in the lateral region despite the fact that the magnetic field was stronger in the lateral region than in the bottom region. Fig. 6 shows photographs of microscopic changes in the top surface shapes with increasing machining time. Microscopic observation showed that the machined surface was generated by the accumulation of indentations made by the pins, that is, plastic deformation of the surface material. The surface shape was changed with the increase in the indentations made by the pins until a homogenous indentation pattern was achieved after 25 min. After 25 min, no significant changes in the surface shape were observed despite the continuous machining action of the pins. This agreed with the results in Fig. 5 that show the surface roughness increased at the start of machining then remained at a certain value despite further machining. The surface roughness value must be a characteristic of the conditions. Fig. 7 shows microscopic photos of the inner surface after 65 min of machining. At X = 0 mm, the sizes of indentations made by the pins were seen to be largest in the top region and smallest in the lateral region. This confirmed that the greatest
Fig. 6. Microscopic images of top surface with increasing machining time.
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Fig. 7. Microscopy of surface after machining for 65 min.
surface roughness Ry was in the top region and lowest surface roughness Ry was in the lateral region in Fig. 5. The top and lateral surfaces, however, were not fully covered by pin indentations in the machining area away from the pole after machining for 65 min. In contrast to these regions, the bottom surface was fully covered by pin indentations. The bottom surface is probably also altered by the accumulation of collisions of the pins assisted by gravity. This resulted in the evenly machined surface generated by the largest number of the indentations in the bottom region despite the unevenness of the magnetic field. Fig. 8 shows the changes in hardness HV with machining time. Regardless of the stagnancy of the changes in the surface roughness value after 25 min as shown in Fig. 5, the hardness of the area with which the pins collided continued to increase with elapse of machining time. The hardness depends on the transformation of the crystal construction of the surface caused by the collision of the pins with the surface. As long as the pins collided with the target surface, the pins continued to transform the crystal construction of the surface, causing work-hardening of the material. In the top and lateral regions, while the center of the machined area showed a large increase in hardness with elapse of machining time, the area away from the pole showed only a slight increase in hardness. According to Fig. 8(a), the hardness increased only slightly in the area beyond X = 14 mm in the top region, which is the outside the area covered by the pole. This made the variation of the hardness value range,
between 191 and 378 HV. The lateral region showed less variance in hardness values, between 238 and 316 HV. This resulted from the fact that the surface was not fully covered by pin indentations in the machining area away from the pole in the top and lateral regions, as shown in Fig. 7. The hardness of the bottom region increased most with machining time throughout the machining area, between 331 and 355 HV. In the bottom region, the collision force, consisting of the magnetic force and gravitation, acting on the pins can be smaller than that in the top region. However, the assistance of the gravitation on the pins increased the number of indentations made by the pins. This must be insufficient for the deformation of the surface shape but enough to transform the crystal construction of the surface, causing work-hardening. Another index for evaluating the machining characteristics is the residual stress of the machined surface. Tubes cut at an angle of 45◦ , as shown in Fig. 9, were used for measurements of residual stress of the machined surface by use of an X-ray stress measuring apparatus. Fig. 9 shows typical results of measurements of residual stress of the bottom surface at the area corresponding to the center of the pole, X = 0 mm, before machining and after machining for 80 min. The surface initially had compressive residual stress in both circumferential and axial directions. This stress must have been imparted by the previous manufacturing process, which involves rolling and forging, typical cold-working processes. The residual compressive stress at the bottom surface changed from −106.7 to −360.7 MPa in the axial
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not only surface shape but also surface integrity, such as by work-hardening and imparting of compressive residual stress to the machined surface. The experiments revealed that the non-uniformity of the alternating magnetic field in the machining area, which affects the motion of the pins, causes the unevenness of the surface characteristics of the machined surface. As mentioned previously, the geometry of the pins is another property required to change the motion of the pins. Thus, appropriate control of the magnetic field and the geometry of pin will enable the machining characteristics to be controlled, as will be discussed in the next section.
6. Effects of magnetic field and pin geometry on machining characteristics 6.1. Magnetic field in the machining area
direction and from −78.6 to −244.9 MPa in the tangential direction of the tube. The residual stress increased to a level about three times higher than the initial value. The results demonstrated that the proposed process enables control of
In the experiments, the alternating magnetic field in the machining area was adjusted by changing the shape of pole. As shown in Fig. 10, a pair of rectangular poles (30 mm × 20 mm × 10 mm) was used instead of the trapezoidal poles shown in Table 1, in the experimental set-up. Fig. 10 shows changes in magnetic flux density with distance X for the case of the rectangular poles. While the top region exhibits higher magnetic flux density over the entire area of the pole, the lateral and bottom regions exhibit values similar to the case with the rectangular poles, as shown in Fig. 4. According to the observations of the Ø1×5 mm SUS304 cold-worked pins using the rectangular poles under the same experimental conditions as those shown in Table 1, the pins showed dynamic motion over a wider area and covered the area corresponding to the entire pole width. Machining experiments using SUS304 tubes investigated the effects of the differences in dynamic motion of the pins in the cases of the trapezoidal and rectangular poles on the machining characteristics. Fig. 11 shows the microscopic photos of the surface in the top region after machining for 80 min. It is seen that the surface was evenly machined over the area corresponding to the pole width. Although not presented in this paper, microscopic photos of the surfaces in the lateral
Fig. 9. Changes in residual stress before and after 80 min of machining time.
Fig. 10. Changes in magnetic flux density with distance X.
Fig. 8. Changes in hardness with machining time.
H. Yamaguchi et al. / Precision Engineering 27 (2003) 51–58
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Fig. 11. Microscopy of top surface after machining for 80 min in case of rectangular pole.
and bottom regions showed that the surface in those regions was also almost evenly machined as done in the top region. Fig. 12 shows changes in hardness with increase in distance X for both trapezoidal and rectangular poles after machining for 80 min. Except for the top region, both conditions resulted in similar degree of hardness. In the top region, the hardness was consistent over the area covered by the pole in the case of the rectangular poles, and it was almost the same as that of the bottom region. This indicated that the motion of the pins can be controlled by adjusting the strength of the alternating magnetic field, thus achieving the desired machining. Here, the change in the alternating magnetic field reduced the non-uniform machining characteristics. Further, rotation of the tube during processing mechanically eliminates any non-uniformity caused by the geometric arrangement and variation in the magnetic field. 6.2. Geometry of pins Ø0.5 × 5 mm pins, which also showed three-dimensional dynamic motion, were used for machining experiments with the rectangular pole. The smaller the pin is, the smaller is the magnetic force required to drive the pin. According to Eqs. (1)–(3), the smaller pin can respond to an alternation of a magnetic field with smaller H. When an alternating current I is supplied into the coil providing inductance L, the alternating current I is inversely proportional to ωL, where ω is angular
Fig. 12. Changes in hardness with distance X of machined surface for 80 min.
Fig. 13. SEM photos of top surface and changes in hardness with distance X after machining for 80 min.
velocity (ω = 2πf , f is the frequency). A decrease in the alternating current I, that is, a decrease in the magnetic field H in Eqs. (1) and (3), leads to an increase in frequency f. This makes the smaller pins respond to the changes in the magnetic field at a range of higher frequencies. In the experimental set-up, the conditions which the Ø0.5 × 5 mm pins exhibited the most active motion responding to the alternation of the magnetic field were shifted from I = 2.0 A with and f = 15 Hz in case of the Ø1 × 5 mm pins to I = 1.85 A with and f = 20 Hz. These conditions were used for the machining experiments. Fig. 13 shows a photo of the top surface as seen by a scanning electron microscope (SEM) and changes in hardness with increase in distance X after machining for 8 min in the case of Ø0.5 × 5 mm pins. An SEM photo of the top surface after machining for 80 min in the case of Ø1 × 5 mm pins is also shown for comparison. The indentations made by the Ø0.5 × 5 mm pins are smaller than those made by the Ø1 × 5 mm pins, and the surface roughness after machining in the case of Ø0.5×5 mm pins (3.3 m Ry) was smaller than that in the case of Ø1 × 5 mm pins (5.6 m Ry). This was because the machining force acting on the Ø0.5 × 5 mm pins was smaller than that acting on the Ø1 × 5 mm pins, which is explained by Eqs. (2) and (3). However, as shown Figs. 12(b) and 13(b), the degrees of hardness were almost the same. In the experiments, Ø0.5 × 5 mm pins responded most to an alternating magnetic field with a frequency of which the 20 Hz, and the Ø1 × 5 mm pins to 15 Hz. This increased the number of indentations in the surface made by the Ø0.5 × 5 mm pins compared to those made by the Ø1 × 5 mm pins. The net result is that the hardening effect of the large number of smaller indentations of the Ø0.5 × 5 mm pins was similar to the smaller number of larger impacts of the larger Ø1 × 5 mm pins.
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7. Conclusions The results of this study can be summarized as follows: (1) A new precision internal machining process for controlling the surface integrity of internal component surfaces was proposed, and its processing principle was described. (2) Observation of the motion of tools using the developed experimental set-up revealed that the following properties of tools are required to achieve the desired result: magnetic anisotropy and geometric property, which is distance between +m and −m, l. (3) Machining experiments using SUS304 stainless steel tubes demonstrated that the process generates a machined surface by an accumulation of plastic deformations as a result of collisions of the tools with the surface and controls the surface integrity, including increases in hardness and compressive residual stress. Once the indentations covered the surface fully, the surface roughness became stagnant even if the process continues. By contrast, the hardness of the machined surface continues to change as long as the tools collide with the surface. (4) The machining behavior of the tools and machining force acting on the tools, which drives the machining characteristics, can be controlled by the alternating magnetic field and the geometry of the tools. In the study, reducing the tool diameter restricted the changes in surface roughness with machining time while still achieving the desired hardness improvement.
Acknowledgments The authors thank Mr. Takuya Shimizu of Rigaku International Corporation for his support in measurements of residual stress in the workpieces used in this study.
References [1] Kalpakjian S. Manufacturing engineering and technology. 2nd ed. Boston: Addison-Wesley; 1992. [2] Hirose M. Shot peening. Tokyo: Seibundo Shinkosha Publishing Company; 1955 [in Japanese]. [3] Nakazawa H, Honma H. Fatigue strength of metal. Tokyo: Yokendo Ltd.; 1982 [in Japanese]. [4] Kagaya C, Egami N. Trends of surface origination using high velocity impact phenomena of fine particles. Denki-Seiko (Electric Furnace) 2000;71:51–8 [in Japanese]. [5] Soyama H, Park JD, Saka M. Use of cavitating jet for introducing compressive residual stress. Trans ASME J Manuf Sci Eng 2000;122: 83–9. [6] Kawanishi K, et al. Magnetic engineering handbook. Tokyo: Asakura Shoten; 1998 [in Japanese]. [7] Yamaguchi H, Shinmura T, Horiuchi N. Study of the magnetic tool behavior and its relationship to the processing characteristics in a magnetic field assisted barrel finishing process. Trans NAMRI/SME 2001;24:197– 204. [8] Yamaguchi H, Shinmura T, Shinji W. Study of a magnetic field assisted internal finishing process using abrasive slurry circulation system. Effects of geometrical and magnetic properties of magnetic particles on the finishing characteristics. J Jpn Soc Precision Eng 2001;67(3):444–9 [in Japanese].
Development of a new precision internal machining process using an alternating magnetic field Hitomi Yamaguchi a,∗ , Takeo Shinmura b , Maki Takenaga b a
Faculty of Engineering, Utsunomiya University, Utsunomiya, Tochigi 321-8585, Japan b Graduate School of Engineering, Utsunomiya University, Tochigi 321-8585, Japan
Received 15 May 2002; received in revised form 29 July 2002; accepted 29 July 2002
Abstract Imparting compressive residual stress to a surface improves the fatigue structural integrity of components, which is particularly important for components used in such critical applications as high-pressure gas or liquid piping systems. This paper proposes a new precision internal machining process that controls the surface integrity of internal surface of these components. This process utilizes an alternating magnetic field to control the force and dynamic motion of the tools needed for machining. An experimental set-up was developed to test the processing principle. This study characterizes the in-process tool behavior—that is, the relationships between the magnetic field, the tool properties, and the tool behavior—and reveals the properties of the tools required to achieve the desired results: magnetic anisotropy and a specific geometric restriction. The surface roughness, hardness, and residual stress measurements following machining experiments demonstrate the effects of the tool behavior on the machining characteristics. This paper also proposes methods to obtain desired surface characteristics. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Precision machining; Internal machining; Magnetic field assisted machining; Surface characteristics; Surface modification
1. Introduction Imparting compressive residual stress to a surface improves the fatigue structural integrity of components. Shot peening is a widely-used process for controlling fatigue structural integrity [1,2]. Peening with shot, discharged either from a rotating wheel or from a nozzle with an air blast, causes deformation of the surface. This results in work hardening of the surface and imparts compressive residual stress to the surface. A shot-peened surface is a satin-finished surface and has a micro-pool and dimple effect. This increase in surface roughness is thought to adversely affect the fatigue structural integrity [3]. The technique of fine particle bombarding has recently been developed to take advantages of some of the benefits of shot peening mentioned above whilst reducing the inherent problems [4]. The process of fine particle bombarding imparts higher compressive residual stress to the surface than does conventional shot peening while having little effect on the initial surface roughness. A similar method for introducing compressive residual stress by use of a cavitating jet from a nozzle has also been proposed [5]. This process does ∗ Corresponding
author. Tel.: +81-28-689-6077; fax: +81-28-689-6077. E-mail address: [email protected] (H. Yamaguchi).
not require shot as in the case of shot peening. In addition, the running cost is lower, and there are no health hazards. These processes, however, are difficult to apply to internal surfaces of components, (e.g. high-pressure gas cylinders and pressure fuel pipes) because of the process delivery mechanisms. Moreover, there is no process that is equivalent to a peening process in fine control of surface modification. The development of a new process is therefore needed to solve the above-mentioned problems. Consequently, this study proposes a new precision internal machining process that controls the surface integrity of the component. The aims of the present research are to describe the processing principle, develop an experimental set-up to realize the principle, characterize the in-process tool behavior and its effects on the machining characteristics, and to explain the finishing mechanism. This paper also proposes methods to obtain desired surface characteristics.
2. Processing principle and experimental set-up This new process uses an alternating magnetic field to control magnetic tools inside the workpiece. The general principle of the process is shown in Fig. 1. Coils facing each other in a parallel circuit generate an alternating magnetic field
0141-6359/02/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 0 2 ) 0 0 1 7 7 - 0
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Fig. 2. External photograph of experimental set-up.
Due to the effects of the alternating magnetic field acting on the pin, the pin exhibits random, active, three-dimensional motion by colliding with the inner surface of the workpiece, thereby achieving precision internal machining. The kinetic energy, K, of the pin exhibiting three-dimensional motion is calculated by the following equation: K = 21 (Mv 2 + I ω2 ),
Fig. 1. Schematic of internal machining process using alternating magnetic field.
around two magnetic poles. The two poles are positioned directly above the workpiece with an adjustable interpole separation. The vertical distance between the two poles and the workpiece is also variable. Magnetic tools, such as pins, inserted into the workpiece are influenced by the alternating magnetic field. When the magnetic axis of the pin is inclined at an angle θ to the magnetic field, the pin rotates to align the axis to the magnetic field. The pin having +m and −m poles in a magnetic field H, shown in Fig. 1(b), experiences a couple F at each pole. The couple F rotates the pin and generates a moment of the couple L: F = mH,
(1)
L = mlH sin θ,
(2)
where l is the distance between +m and −m poles, and ml is the magnetic moment. When the magnetic field H changes in the direction of x, as shown in Fig. 1(c), a magnetic force Fx acts on the pin, driving it: Fx = ml
∂H ∂H = V χH . ∂x ∂x
(3)
The magnetic force acting on the pin Fx is calculated as the product of volume V, susceptibility χ of the pin, and the intensity and gradient of the magnetic field H (∂H/∂x) [6].
(4)
where M is mass of the pin, v the velocity, I the moment of inertia on the pin, and ω is angular velocity. An experimental set-up developed to test this processing principle is shown in Fig. 2. A coil of copper wire (Ø1 mm, 3700 turns) was wound around a core of rolled steel (Ø30 × 140 mm), and a replaceable pole of rolled steel was attached to the core. Constant voltage was supplied to the coils through an ac power amplifier. The alternating magnetic field around the poles was adjustable by a function generator, and the distance between the opposed coils was adjustable. The workpiece was set under the poles, but the position of the workpiece was flexible in the vertical and horizontal directions. The smaller the distances between the two opposed magnetic poles and between the workpiece and magnetic poles are, the stronger is the magnetic field, inducing dynamic behavior of the pins in the alternating magnetic field. In the experiments, the clearances between the pole tips and between the workpiece and the poles were set at 3 and 2 mm, respectively.
3. Tools behavior in process Before machining experiments were performed, the dynamic behavior of the magnetic tools was investigated through observation of the pin motion using the experimental set-up. Table 1 shows the experimental conditions. A previous research study has shown that a required property for induction of effective motion of the tools in an alternating magnetic field is the magnetic anisotropy of the tools [7]. Since cold-worked SUS304 stainless steel pins exhibit ferromagnetism with magnetic anisotropy due to transformation of the crystal construction [7], they were used in
H. Yamaguchi et al. / Precision Engineering 27 (2003) 51–58 Table 1 Experimental conditions Workpiece
Magnetic tools
Coil
Acrylic resin tube (Ø20 × Ø17 × 100 mm) SUS304 stainless steel bright annealed tube (Ø19 × Ø16 × 83 mm) SUS304 stainless steel cold-worked pins (Ø0.5 × 5 mm, Ø0.5 × 2.5 mm, Ø0.5 × 5 mm, Ø1 × 5 mm) 3 g supplied Core: SS400 rolled steel (Ø30 × 140 mm) copper wire (Ø1 mm): 3700 turns
Pole
AC power
Sinusoidal current, 2.0 A (115 V, 15 Hz)
Measured parts
these studies. The magnetic axis of the pin is the same as the geometric axis of the pin. Eq. (2) shows that a moment acts on a magnetic particle regardless of the geometry but that the distance l between +m and −m poles has an influence on the motion of the pins. According to Eq. (3), the driving force of the pin is proportional to the distance l of the pin. This indicates that the distance l must have an influence on the motion as well as the machining force of the pin, controlling the machining characteristics. The distance between +m and −m poles, l, of the pin was adjusted by changing the length of the pin in the experiments. Pins with three different lengths, Ø0.5 × 0.5 mm, Ø0.5 × 2.5 mm, and Ø0.5 × 5 mm, were used in the experiments. An transparent acrylic resin tube, Ø20 × Ø17 × 100 mm, was used to observe the motion of the pins inside the tube. Fig. 3 shows a schematic of the pins in the alternating magnetic field. Only the Ø0.5 × 5 mm pins exhibited dynamic motion and mainly collided with top and bottom regions in
Fig. 3. Schematic of motion of the pins in cases of Ø0.5 × 5 mm and Ø0.5 × 0.5 mm.
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response to the alternating magnetic field, as shown in Fig. 3(a). It is known that Ø0.5 × 5 mm pins have higher susceptibility and lead to greater magnetic force than do Ø0.5 × 0.5 mm pins [8]. The greater magnetic force and moment of the pins resulted in dynamic motion of the Ø0.5 × 5 mm pins. The Ø0.5 × 2.5 mm and Ø0.5 × 0.5 mm pins were linked together by magnetic force, and these linkages of the pins trembled in response to the alternation of the magnetic field, as shown in Fig. 3(b). The degree of the tremor of the Ø0.5×2.5 mm pins was larger than that of the Ø0.5×0.5 mm pins because of the greater magnetic force acting on the pins. The results of these experiments revealed another required property for induction of dynamic motion of the pins: the distance l between the +m and −m poles of the pin.
4. Conditions for machining experiments In case of a magnetic field barrel finishing process, Ø1 × 5 mm pins in an alternating magnetic field collide against the target surface with greater force than Ø0.5 × 5 mm pins and succeed in deburring efficiently [7]. In the experimental set-up shown in Fig. 2, the Ø1 × 5 mm pins also exhibited dynamic motion similar to that of the Ø0.5 × 5 mm pins and collided with great force against the inner surface. As long as the pin has the distance l needed for dynamic motion, increasing its volume results in an increase in its driving force and collision force against the target surface. This enhances the machining performance. Machining experiments were, thus, conducted using Ø1 × 5 mm cold-worked SUS304 stainless steel pins to determine the fundamental machining characteristics of the process using non-ferromagnetic bright annealed SUS304 stainless steel tubes, which are commonly used for piping systems, as workpieces. Other conditions are shown in Table 1. The machining experiments resulted in changes in hardness and residual stress, in addition to the surface roughness and material removal, with elapse of machining time. A micro-hardness tester was used to measure the hardness of the machined surface. The size of the workpiece that could be used for the hardness test was restricted by the configuration of the tester. The experiments were therefore performed using a tube divided into four pieces in the axial direction, which included the top, lateral, and bottom regions, as shown in Table 1. During the machining experiments, the pieces were glued in place on the tube. Fig. 4 shows the magnetic field measured in the machining area. The highest magnetic flux density was obtained in the machining area corresponding to the center of the pole, X = 0 mm, and decreased with increase in the distance from the pole. The top region, which is the closest to the poles, showed the highest magnetic flux density and the sharpest decline in magnetic flux density with the distance X of the three regions because of the trapezoidal shape of the pole. The lateral and bottom regions showed gentle declines of the magnetic flux density with the distance X because of greater
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Fig. 4. Changes in magnetic flux density with distance X.
distance between the region and the pole. The unevenness of the magnetic field must affect not only dynamic motion of pins but also machining characteristics in each region.
5. Fundamental machining characteristics The material removal was hardly detected by an electronic force balance with 0.1 mg resolution in any experimental condition. This shows that the process generates a machined surface by an accumulation of plastic deformations as a result of collisions of the pins with the surface. Fig. 5 shows changes in surface roughness with machining time corresponding to the center of the pole, X = 0 mm. As shown in Fig. 4, the shorter the distance between machining area and pole is, the greater is the magnetic field, which is the predominant component of the collision force of the pins. This caused the greatest plastic deformation of the surface in the top region, making the roughest surface. In the lateral region, the pins caused less indentation on the target surface because of the geometric arrangement. This resulted in the
Fig. 5. Changes in surface roughness with machining time (X = 0 mm).
least change in surface roughness in the lateral region despite the fact that the magnetic field was stronger in the lateral region than in the bottom region. Fig. 6 shows photographs of microscopic changes in the top surface shapes with increasing machining time. Microscopic observation showed that the machined surface was generated by the accumulation of indentations made by the pins, that is, plastic deformation of the surface material. The surface shape was changed with the increase in the indentations made by the pins until a homogenous indentation pattern was achieved after 25 min. After 25 min, no significant changes in the surface shape were observed despite the continuous machining action of the pins. This agreed with the results in Fig. 5 that show the surface roughness increased at the start of machining then remained at a certain value despite further machining. The surface roughness value must be a characteristic of the conditions. Fig. 7 shows microscopic photos of the inner surface after 65 min of machining. At X = 0 mm, the sizes of indentations made by the pins were seen to be largest in the top region and smallest in the lateral region. This confirmed that the greatest
Fig. 6. Microscopic images of top surface with increasing machining time.
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Fig. 7. Microscopy of surface after machining for 65 min.
surface roughness Ry was in the top region and lowest surface roughness Ry was in the lateral region in Fig. 5. The top and lateral surfaces, however, were not fully covered by pin indentations in the machining area away from the pole after machining for 65 min. In contrast to these regions, the bottom surface was fully covered by pin indentations. The bottom surface is probably also altered by the accumulation of collisions of the pins assisted by gravity. This resulted in the evenly machined surface generated by the largest number of the indentations in the bottom region despite the unevenness of the magnetic field. Fig. 8 shows the changes in hardness HV with machining time. Regardless of the stagnancy of the changes in the surface roughness value after 25 min as shown in Fig. 5, the hardness of the area with which the pins collided continued to increase with elapse of machining time. The hardness depends on the transformation of the crystal construction of the surface caused by the collision of the pins with the surface. As long as the pins collided with the target surface, the pins continued to transform the crystal construction of the surface, causing work-hardening of the material. In the top and lateral regions, while the center of the machined area showed a large increase in hardness with elapse of machining time, the area away from the pole showed only a slight increase in hardness. According to Fig. 8(a), the hardness increased only slightly in the area beyond X = 14 mm in the top region, which is the outside the area covered by the pole. This made the variation of the hardness value range,
between 191 and 378 HV. The lateral region showed less variance in hardness values, between 238 and 316 HV. This resulted from the fact that the surface was not fully covered by pin indentations in the machining area away from the pole in the top and lateral regions, as shown in Fig. 7. The hardness of the bottom region increased most with machining time throughout the machining area, between 331 and 355 HV. In the bottom region, the collision force, consisting of the magnetic force and gravitation, acting on the pins can be smaller than that in the top region. However, the assistance of the gravitation on the pins increased the number of indentations made by the pins. This must be insufficient for the deformation of the surface shape but enough to transform the crystal construction of the surface, causing work-hardening. Another index for evaluating the machining characteristics is the residual stress of the machined surface. Tubes cut at an angle of 45◦ , as shown in Fig. 9, were used for measurements of residual stress of the machined surface by use of an X-ray stress measuring apparatus. Fig. 9 shows typical results of measurements of residual stress of the bottom surface at the area corresponding to the center of the pole, X = 0 mm, before machining and after machining for 80 min. The surface initially had compressive residual stress in both circumferential and axial directions. This stress must have been imparted by the previous manufacturing process, which involves rolling and forging, typical cold-working processes. The residual compressive stress at the bottom surface changed from −106.7 to −360.7 MPa in the axial
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not only surface shape but also surface integrity, such as by work-hardening and imparting of compressive residual stress to the machined surface. The experiments revealed that the non-uniformity of the alternating magnetic field in the machining area, which affects the motion of the pins, causes the unevenness of the surface characteristics of the machined surface. As mentioned previously, the geometry of the pins is another property required to change the motion of the pins. Thus, appropriate control of the magnetic field and the geometry of pin will enable the machining characteristics to be controlled, as will be discussed in the next section.
6. Effects of magnetic field and pin geometry on machining characteristics 6.1. Magnetic field in the machining area
direction and from −78.6 to −244.9 MPa in the tangential direction of the tube. The residual stress increased to a level about three times higher than the initial value. The results demonstrated that the proposed process enables control of
In the experiments, the alternating magnetic field in the machining area was adjusted by changing the shape of pole. As shown in Fig. 10, a pair of rectangular poles (30 mm × 20 mm × 10 mm) was used instead of the trapezoidal poles shown in Table 1, in the experimental set-up. Fig. 10 shows changes in magnetic flux density with distance X for the case of the rectangular poles. While the top region exhibits higher magnetic flux density over the entire area of the pole, the lateral and bottom regions exhibit values similar to the case with the rectangular poles, as shown in Fig. 4. According to the observations of the Ø1×5 mm SUS304 cold-worked pins using the rectangular poles under the same experimental conditions as those shown in Table 1, the pins showed dynamic motion over a wider area and covered the area corresponding to the entire pole width. Machining experiments using SUS304 tubes investigated the effects of the differences in dynamic motion of the pins in the cases of the trapezoidal and rectangular poles on the machining characteristics. Fig. 11 shows the microscopic photos of the surface in the top region after machining for 80 min. It is seen that the surface was evenly machined over the area corresponding to the pole width. Although not presented in this paper, microscopic photos of the surfaces in the lateral
Fig. 9. Changes in residual stress before and after 80 min of machining time.
Fig. 10. Changes in magnetic flux density with distance X.
Fig. 8. Changes in hardness with machining time.
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Fig. 11. Microscopy of top surface after machining for 80 min in case of rectangular pole.
and bottom regions showed that the surface in those regions was also almost evenly machined as done in the top region. Fig. 12 shows changes in hardness with increase in distance X for both trapezoidal and rectangular poles after machining for 80 min. Except for the top region, both conditions resulted in similar degree of hardness. In the top region, the hardness was consistent over the area covered by the pole in the case of the rectangular poles, and it was almost the same as that of the bottom region. This indicated that the motion of the pins can be controlled by adjusting the strength of the alternating magnetic field, thus achieving the desired machining. Here, the change in the alternating magnetic field reduced the non-uniform machining characteristics. Further, rotation of the tube during processing mechanically eliminates any non-uniformity caused by the geometric arrangement and variation in the magnetic field. 6.2. Geometry of pins Ø0.5 × 5 mm pins, which also showed three-dimensional dynamic motion, were used for machining experiments with the rectangular pole. The smaller the pin is, the smaller is the magnetic force required to drive the pin. According to Eqs. (1)–(3), the smaller pin can respond to an alternation of a magnetic field with smaller H. When an alternating current I is supplied into the coil providing inductance L, the alternating current I is inversely proportional to ωL, where ω is angular
Fig. 12. Changes in hardness with distance X of machined surface for 80 min.
Fig. 13. SEM photos of top surface and changes in hardness with distance X after machining for 80 min.
velocity (ω = 2πf , f is the frequency). A decrease in the alternating current I, that is, a decrease in the magnetic field H in Eqs. (1) and (3), leads to an increase in frequency f. This makes the smaller pins respond to the changes in the magnetic field at a range of higher frequencies. In the experimental set-up, the conditions which the Ø0.5 × 5 mm pins exhibited the most active motion responding to the alternation of the magnetic field were shifted from I = 2.0 A with and f = 15 Hz in case of the Ø1 × 5 mm pins to I = 1.85 A with and f = 20 Hz. These conditions were used for the machining experiments. Fig. 13 shows a photo of the top surface as seen by a scanning electron microscope (SEM) and changes in hardness with increase in distance X after machining for 8 min in the case of Ø0.5 × 5 mm pins. An SEM photo of the top surface after machining for 80 min in the case of Ø1 × 5 mm pins is also shown for comparison. The indentations made by the Ø0.5 × 5 mm pins are smaller than those made by the Ø1 × 5 mm pins, and the surface roughness after machining in the case of Ø0.5×5 mm pins (3.3 m Ry) was smaller than that in the case of Ø1 × 5 mm pins (5.6 m Ry). This was because the machining force acting on the Ø0.5 × 5 mm pins was smaller than that acting on the Ø1 × 5 mm pins, which is explained by Eqs. (2) and (3). However, as shown Figs. 12(b) and 13(b), the degrees of hardness were almost the same. In the experiments, Ø0.5 × 5 mm pins responded most to an alternating magnetic field with a frequency of which the 20 Hz, and the Ø1 × 5 mm pins to 15 Hz. This increased the number of indentations in the surface made by the Ø0.5 × 5 mm pins compared to those made by the Ø1 × 5 mm pins. The net result is that the hardening effect of the large number of smaller indentations of the Ø0.5 × 5 mm pins was similar to the smaller number of larger impacts of the larger Ø1 × 5 mm pins.
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7. Conclusions The results of this study can be summarized as follows: (1) A new precision internal machining process for controlling the surface integrity of internal component surfaces was proposed, and its processing principle was described. (2) Observation of the motion of tools using the developed experimental set-up revealed that the following properties of tools are required to achieve the desired result: magnetic anisotropy and geometric property, which is distance between +m and −m, l. (3) Machining experiments using SUS304 stainless steel tubes demonstrated that the process generates a machined surface by an accumulation of plastic deformations as a result of collisions of the tools with the surface and controls the surface integrity, including increases in hardness and compressive residual stress. Once the indentations covered the surface fully, the surface roughness became stagnant even if the process continues. By contrast, the hardness of the machined surface continues to change as long as the tools collide with the surface. (4) The machining behavior of the tools and machining force acting on the tools, which drives the machining characteristics, can be controlled by the alternating magnetic field and the geometry of the tools. In the study, reducing the tool diameter restricted the changes in surface roughness with machining time while still achieving the desired hardness improvement.
Acknowledgments The authors thank Mr. Takuya Shimizu of Rigaku International Corporation for his support in measurements of residual stress in the workpieces used in this study.
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