International Journal of Machine Tools and Manufacture

Volume 45, Issue 1, January 2005, Pages 43–49

Study on the inner surface finishing of tubing by magnetic abrasive finishing

Yan Wang , , Dejin Hu

School of Mechanical and Power Engineering, Shanghai Jiaotong University 279, Shanghai 200030, People's Republic of China

Received 19 March 2004. Accepted 29 June 2004. Available online 11 September 2004.

http://dx.doi.org/10.1016/j.ijmachtools.2004.06.014 , How to Cite or Link Using DOI Cited by in Scopus (16)

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AbstractWith the development of industry manufacturing technology, fine surface finish is in high demand in a wide spectrum of industrial applications. Presently, it is required that the parts used in manufacturing semiconductors, atomic energy parts, medical instruments and aerospace components have a very precise surface roughness. Amongst them, vacuum tubes, wave guides and sanitary tubes are difficult to polish by conventional finishing methods such as lapping, because of their shapes. The surface roughness of these tubes affects the performance of the

entire system, but the finishing technology for these tubes is very scant in manufacturing fields. This project was proposed by a Shanghai Far East pharmaceutial and mechanical factory. They stated that the roughness of the inner surface must be less than 0.3 μm Ra after finishing. An internal magnetic abrasive finishing (MAF) process is proposed for producing highly finished inner surfaces of tubes used in this study. The process principle and the finishing characteristics of unbounded magnetic abrasive within internal tubing finishing are described first. MAF setup was designed for finishing three kinds of materials tubing, such as Ly12 aluminum alloy, 316L stainless steel and H62 brass. Experimental results indicated that finishing parameters such as polishing speed, magnetic abrasive supply, abrasive material, magnetic abrasive manufacturing process and grain size have critical effects on the material removal rate (MRR). How the inner surface micro shape changes course during finishing of an aluminous tube is demonstrated.

Keywords Magnetic abrasive finishing (MAF); Ly12; 316L; H62; Material removal rate (MRR)

1. IntroductionWith the development of industry manufacturing technology, fine surface finish is in high demand in a wide spectrum of industrial applications. Presently, it is required that the parts used in manufacturing semiconductors, atomic energy parts, medical instruments and aerospace components, have a very precise surface roughness. Amongst them, vacuum tubes, wave guides and sanitary tubes are difficult to polish by conventional finishing methods such as lapping, because of their shapes. The surface roughness of these tubes affects the performance of the entire system, but the finishing technology for these tubes is very scant in manufacturing fields [1] and [2]. A relatively new finishing method, magnetic abrasive finishing (MAF) is one such advanced machining process in which cutting force is primarily controlled by the magnetic field. This process can be used to produce efficiently good surface quality of the order of few nanometer finish on flat surfaces as well as internal and external surfaces of tube type workpieces. The method can not only machine ferromagnetic materials such as steel, but can also machine nonferromagnetic materials such as stainless steel and brass. In addition, MAF possesses many attractive advantages, such as self-sharpening, self-adaptability, controllability, and the finishing tool requires neither compensation nor dressing. The method was originally introduced in the Soviet Union, with further fundamental research in various countries including Japan [4]. Nowadays, the study of the magnetic field assisted finishing processes is being conducted at industrial levels around the world. Shinmura et al. [5] have studied basic principles of the MAF process and concluded that the stock removal and surface finish value (Ra) increase as the magnetic abrasive particle diameter “D” increases. Ra value of the final surface finish increases as the abrasive grain diameter “d” increases. In order to achieve smooth surface and

remove surface damage, the ferromagnetic particle diameter must be chosen as a compromise of material removal rate and resulting surface finish. Kim et al. [8] and [9] have modeled and simulated the MAF process and concluded that the magnetic flux density in the air-gap is affected greatly by the length of the air-gap; magnetic flux density increases as the air-gap length decreases. Yamaguchi et al. have studied the feasibility of using a magnetic field assisted finishing process with a mixture of conventional abrasive and ferrous particles for the internal finishing of alumina ceramic tubes and gained an understanding of the mechanism involved [10]. The authors have also studied inner surface finishing of a thin and long austenitic stainless steel tube and concluded that rotating speed of magnetic pole, magnetic flux density, magnetic abrasive mesh and finishing time are the parameters which significantly influence the change in surface finish value (Ra) [11].

This project was proposed by a Shanghai Far East pharmaceutial and mechanical factory. They stated that the roughness of the inner surface must be less than 0.2 μm Ra after finishing. This paper describes the process principle and finishing characteristics of unbounded magnetic abrasive within internal tubing finishing and also deals with the design and fabrication of MAF setup for finishing three kinds of materials tubing, such as Ly12 aluminum alloy, 316L stainless steel and H62 brass. The present work is aimed at studying the effects of finishing parameters such as polishing speed, magnetic abrasive supply, abrasive material, magnetic abrasive manufacturing processes and particle size on change in surface finish, percent improvement in surface finish and material removall rate (MRR) on the three kinds of materials tubing. The MAF setup has been designed and fabricated in the Research Institute of Manufacturing Technology and Automatic Equipment ( Shanghai Jiaotong University, China).

2. Principles of internal finishingFig. 1 shows a schematic of internal magnetic abrasive finishing processes using a stationary pole system. Magnetic abrasives introduced into the tube are conglomerated at the finishing zone by a magnetic field, generating the finishing force against the inner surface of the tube. The magnetic forces affect the abrasive at position “A” outside the working gap as follows :

(1)

where x is the direction of the line of magnetic force, y is the direction of the magnetic equipotential line, Fx is the x component of the magnetic force Fy is the y component of the magnetic force, χm is susceptibility of the abrasive particles, is permeability of vacuum, V is the volume of one magnetic particle, H is the magnetic field strength at point “A”, and ∂H/∂x and ∂H/∂y are gradients of magnetic field strength in the x and y directions, respectively.

Fig. 1. Schematic view of internal finishing by magnetic abrasives.

3. Characteristicabrasive behavior in processFig. 2 shows a two-dimensional (2-D) schematic of the abrasive behavior during the internal magnetic abrasive finishing process using a pole rotation system for a nonferromagnetic workpiece. In the process, magnetic abrasives introduced into the workpiece are attracted by the magnetic field and bear on the inner surface of the workpiece. The rotation of the poles, which consist of small permanent magnets, around the workpiece causes the rotation of the magnetic field at the finishing area [12].

Fig. 2. Two-dimensional schematic of abrasive behavior during the process.

The abrasive behavior is primarily determined by the relative strengths of two opposing forces: the magnetic force and the friction force acting on the abrasive against the inner surface of the tube. When the magnetic force is greater than the friction force, the abrasive follows the rotation of the magnetic field. This generates relative motion of the abrasive against the inner surface, removing material from the workpiece. The magnetic force on the abrasive is the predominant

component of finishing force, which is the result of the magnetic and centrifugal forces experienced by the rotating magnetic abrasive.

The abrasive behavior against the inner surface of the tube has a profound impact on the finishing characteristics and is characterized into four states, shown in Fig. 2 [13] . Fig. 2(a) shows a state where all abrasive is magnetically held at the finishing area and follows the movement of the magnetic field, showing smooth rotation along the inner surface. This facilitates the improvement of the surface roughness. Fig. 2(b)–(d) show the characteristic abrasive behavior under three different levels of magnetic force, wherein the magnetic force is successively weaker going from Fig. 2(b)–(d). Fig. 2(b) shows a condition in which the abrasive shows rotary motion against the inner surface, while also undergoing self-displacement within the abrasive mass. Self-displacement describes a condition where the abrasive exhibits motion within the abrasive mass. The self-displacement of the cutting edges of the abrasive may lead to increased material removal. In the states illustrated in Fig. 2(a) and (b), the surface modification is the accumulation of the unidirectional cutting marks of the smoothly rotating abrasive. A weaker magnetic force on the abrasive induces the behavior shown in Fig. 2(c). In this condition, part of the abrasive mass is barely held at the finishing area by the magnetic field. The abrasive is irregularly dispersed away from the finishing area, striking the surface and cutting into the material. This results in noticeable differences in the surface modification from the cases of Fig. 2(a) and (b) and a remarkable increase in the material removal caused by the aggressive contact of the abrasive against the surface. As the magnetic force is reduced further, the condition is ultimately reached in which the magnetic field cannot hold the abrasive at the finishing area at all. In this condition, the abrasive adheres to the surface of the rotating tube because of friction, regardless of the movement of the magnetic field, as shown in Fig. 2(d). This eliminates the relative motion of the abrasive against the inner surface needed for the finishing operation, and the initial surface is retained. Practically speaking, oversupply of the abrasive and high rotational speed of the magnetic field are factors that cause this irregular jumbling condition.

4. Magnetic abrasive finishing experimentAccording to the characteristic abrasive behavior in the process, a schematic diagram of the finishing experimental device which was designed and made is shown in Fig. 3. Four rare earth permanent magnets were fixed in the inner surface of magnetic yoke. The abrasive particles are divided into two parts in the magnetic field. The included angle between the two magnetic poles where magnetic abrasive conglomerated is 60 degrees. Fig. 4 shows the external view of experimental setup. The apparatus is mounted on the cross-slide of a lathe (HQ-350C). The yoke is driven by an electric motor through a gear drive. A series of experiments were made to finish different kinds of workpiece materials, such as Ly12, 316L and H62. The compositions of the test materials are shown in Table 1. The mechanical properties of the three kinds of workpiece materials are shown in Table 2 [14] .

Fig. 3. Schematic diagram of the rotating motion device.

Fig. 4. External view of experimental setup.

Table 1. Compositions of test materials

CompositionsLy12 Cu%=3.8~4.9%, Mn%=0.30~0.90%, Mg%=1.2~1.8%, Al%=92.4~94.7%

H62 Cu%=60.5%~63.5%, Pb%=0.08%,Fe%=0.15%, Sb%=0.005%, P%=0.01%,Zn%=36.255~39.255%

316L C%≤0.03%, Si≤1%, Mn≤2%,P≤0.035%, S≤0.03%, Ni=12%~15%, Cr=16~18%,Mo=2%~3%

Table 2. Mechanical properties of test materials

Ly12 H62 316LVickers hardness (HV) 129 139 178Tensile strength (Mpa) 431 760 620Yield strength (MPa) 274 590 310

The free-cutting machinability of the workpiece material is a critical factor which effects material removal rate in dry finishing process, whereas the chemical reaction is a critical factor which effects material removal rate in the finishing area with finishing liquid. Because the chemisorption film and physisorption film are formed between polar molecules in the finishing liquid and metal workpiece. The finishing liquid in the experiment consists of 4% weight percentage stearinic acid and 96% weight percentage transformer oil.

Fig. 5 shows that the material removal rate(MRR) of H62 workpiece is highest regardless of four kinds of magnetic abrasives, which are Al2O3/Fe(Al2O3 percentage by weight is 20%), TiC/Fe (TiC percentage by weight is 20%), TiC/Fe (TiC percentage by weight is 35%) and TiC/Fe (TiC percentage by weight is 7%). The four kinds of magnetic abrasives were made by different technology methods. Al2O3/Fe (20%) magnetic abrasive is palletized after sintered. TiC/Fe (20%) magnetic abrasive is sintered after palletizing. TiC/Fe (35%) magnetic abrasive is palletized after sintered. Whereas TiC/Fe (7%) magnetic abrasive is fabricated by an in situ reaction composite method. The MRR of Ly12 is higher than that of 316L, except for the case of TiC/Fe (7%) magnetic abrasive. By comparing TiC/Fe (35%) abrasive with Al2O3/Fe (20%) abrasive, it is evident that the MRR with TiC/Fe (35%) abrasive is higher than with Al2O3/Fe (20%) abrasive. This is because TiC abrasive weight percentage is greater than Al2O3 abrasive weight percentage. The amount of micro cutting edge of TiC/Fe (35%) abrasive contacting with the workpiece surface is much more than that of Al2O3/Fe (20%). The MRR of H62 workpiece with TiC/Fe (35%) abrasive is 75 mg/min, which is much higher than the MRR of Ly12 workpiece or 316L workpiece. So TiC/Fe (35%) abrasive is most suitable for finishing brass alloy.

Fig. 5. Relation between MRR and magnetic abrasive.

Fig. 6 shows that the rotational speed of magnetic pole has a significant influence on MRR. The MRR increases with the increasing of the rotational speed of the magnetic pole. They almost keep a linear relationship under given experimental condition. As long as the abrasive rotated smoothly, the material removal increased with increased pole revolution, generating a smooth surface. The abrasive exhibited irregular jumbling at higher pole rotational speeds. The faster the poles’ revolution, the higher the centrifugal force acting on each abrasive; this increases the frictional force. Therefore, under higher pole rotational speeds, a higher magnetic force is required to counter the frictional force and maintain smooth abrasive rotation. As the degree of the abrasive jumbling increased with pole revolution rate, the surface roughness improvement

decreased because of material over-removal caused by the aggressive strikes of the jumbling abrasive. The material removal decreased because of a lack of abrasive contact against the inner surface of the tube.

Fig. 6. Relation between MRR and speed.

As Fig. 7 shows, the MRR only increase initially in the magnetic finishing without liquid. However, after the surface roughness is saturated, the increase of MRR gradually slows. Finally the MRR reaches a stable value.

Fig. 7. Relation between MRR and finishing time.

Fig. 8 shows the effects of magnetic abrasive volume on material removal rate. The MRR increases with the increasing of the magnetic abrasive volume. There is an optimal magnetic abrasive volume that results in maximum material removal rate. This is because the magnetic pressure is maximum at this point. Fig. 9 shows the relation between MRR and magnetic

abrasive particle size. There is an optimal magnetic abrasive particle size 30–50%, which results in maximum material removal rate. The magnetic abrasive particle, which is smaller than the optimum, results in smaller MRR. The magnetic abrasive particle, which is larger than the optimum, also results in smaller MRR. This is because the magnetic force is proportional to abrasive particle size, a larger magnetic particle size produces greater magnetic force. But the number of larger magnetic abrasive particle effective micro cutting edges is fewer than it is for small particles , so the MRR decreases with increasing magnetic abrasive particle size.

Fig. 8. Relation between MRR and volume.

Fig. 9. Relation between MRR and grid.

Fig. 10 shows four different kinds of TiC/Fe magnetic abrasive manufacturing processes producing different MRRs. TiC/Fe (25%), TiC/Fe (20%) and TiC/Fe (15%) magnetic abrasives are pelletized then sintered, respectively. So their textures are all loose. However TiC/Fe (35%) magnetic abrasive is sintered then palletized, its texture is compact. A compact texture helps to

produce larger finishing pressure than loose texture produces. So the MRR of TiC/Fe(35%) is the largest among four kinds of magnetic abrasive particles.

Fig. 10. Relation between MRR and magnetic abrasive.

The Al2O3/Fe (20%) magnetic abrasive particle can polish most metals tube, such as Ly12, H62 and 316L with or without finishing liquid. A series of experiments were carried out to finish Ly12, H62 and 316L with transformer oil or finishing liquid (4% stearinic +96% oil). The magnetic abrasives of Al2O3/Fe were mixed together with transformer oil or finishing liquid before finishing workpiece. Fig. 11 shows that the MRR without liquid is relevant to free-cutting machinability of tube material. The MRR of H62 without finishing liquid is the largest among three kinds of tube material. The MRR with transformer oil or stearinic acid liquid is larger than that without liquid. This is because transformer oil helps to form physisorption film and stearinic acid helps to form chemisorption film. Both physisorption film and chemisorption film can obviously improve the MRR.

Fig. 11. Relation between MMR and tube material.

Fig. 12 shows the relation between roughness and finishing time. In this experiment, the workpiece is a 316L rolled tube, which is 40 mm long, Φ30 mm inner diameter. The initial value of surface roughness is between 0.8 and 1.5 μm. The actual values of surface roughness after 4 min finishing are all below 0.1 μm. If their finishing times are prolonged further, the values of surface roughness do not decrease but their dispersion degree decreases. These experimental results illustrate that the material removal mode changes from initial selective removal to later uniform removal. However, it is not economic to use uniform removal mode to decrease the value of surface roughness.

Fig. 12. Relation between roughness and finishing time.

The turned aluminium tube inner surface micro shape changes course during finishing process is shown in Fig. 13. Fig. 13(1)) show the initial surface topography. Fig. 13(2), (3) and (4) show the surface micro shape after 1 min magnetic finishing, 2 min magnetic finishing and 3 min magnetic finishing respectively. Fig. 13(5)) shows the surface micro shape after 6 min magnetic finishing, when the surface roughness decreases greatly. The final surface micro shape after 10 min magnetic finishing is shown in Fig. 13(6)), sags and crests distribute randomly. The value of surface roughness is the smallest among six surface micro shape change stages. The surface roughness value of the tube descends from original Ra9.6 μm to Ra0.24 μm.

Fig. 13. Surface micro shape change course.

5. ConclusionsThe results of this study can be summarized as follows:

1.

The magnetic field distribution defines the magnetic abrasive configuration and the magnetic force acting on the abrasive, and has a predominant effect on the abrasive behavior.

2.

The abrasive behavior determined by the finishing conditions against the inner surface of the tube, which has an impact on the finishing characteristics, can be characterized into four states.

3.

This study showed the feasibility of using a magnetic abrasive finishing with a mixture of conventional abrasives and ferrous particles for the internal finishing of three kinds of metal tubes, such as Ly12 aluminum alloy, 316L stainless steel and H62 brass and gained an understanding of the mechanism involved.

4.

The MRR increases with the increasing of the rotational speed of magnetic pole. They almost keep a linear relationship under given experimental conditions.

5.

There is a optimal magnetic abrasive particle size 30–50% for TiC/Fe (35%), which results in maximum material removal rate. Similarly, there is also an optimal magnetic abrasive volume that results in maximum material removal rate.

6.

The MRR only increase initially in the magnetic finishing without liquid. However , after the surface roughness is saturated, the increase of MRR gradually slows. Finally the MRR reaches a stable value.

7.

The internal magnetic abrasive finishing of three kinds of metals with or without liquid experiments were carried out. The results showed that the free-cutting machinability of the workpiece material is critical factor which effects material removal rate in dry finishing process. Whereas the chemical reaction is critical factor which effects material removal rate in the finishing area with finishing liquid. The material removal rate(MRR) of H62 workpiece is highest regardless of four kinds of magnetic abrasives, which are Al2O3/Fe(Al2O3 percentage by weight is 20%), TiC/Fe(TiC percentage by weight is 20%), TiC/Fe(TiC percentage by weight is 35%) and TiC/Fe(TiC percentage by weight is 7%).

8.

The inner surface micro shape change course of turned aluminium tube during finishing is demonstrated in the end. The surface roughness value of the tube descends from original Ra 9.6μm to Ra 0.24 μm.

AcknowledgementsThis project is sponsored by grants from the National Ministry of Education Doctor Foundation (97023834). The author acknowledges the help of Prof. Hu De-jin and Dr. Li Xue-quan in the research of this project.

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