Key Engineering Materials
ISSN: 1662-9795, Vols. 257-258, pp 505-510
doi:10.4028/www.scientific.net/KEM.257-258.505
© 2004 Trans Tech Publications, Switzerland
Online: 2004-02-15
A Study on the Magnetic Field Assisted Machining Process for Internal
Finishing using a Magnetic Machining Jig
Y. Zou and T. Shinmura
Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585,
Japan
Keywords: Magnetic Field-Assisted Machining, Magnetic Machining Jig, Internal Finishing,
Processing Principle, Magnetic Particles, Abrasive Slurry, Finishing Characteristics
Abstract. This paper describes a new efficient internal finishing process for a non-ferromagnetic
thick tube of 10-20 mm in thickness by the application of a magnetic field assisted machining process
using a magnetic machining jig, which consists of a rare earth permanent magnet, as a tool, instead of
the conventional magnetic abrasives. In contrast to the magnetic abrasives, it can generate a stronger
magnetic force, which is the finishing force, due to higher material susceptibility, and makes internal
finishing of a non-ferromagnetic thick tube possible. The principle and advantages of this process are
described. The finishing experiment was carried out and the finishing characteristics are described.
The results showed that this process enables precise internal finishing of thick non-ferromagnetic
tubes, such as the SUS304 stainless steel tube of 10 mm in thickness. The initial surface roughness of
4.5 µm in Ra was improved to 0.1 µm in Ra.
Introduction
An internal magnetic abrasive finishing process was developed for generating high-quality inner
surfaces of tubes and gas bombs used in critical applications such as in clean gas or liquid piping
systems. In the previous reports [1] [2], the finishing characteristics and mechanism of the thin
non-ferromagnetic tube (under about 4 mm in thickness) using magnetic abrasives have been clarified.
However, it is difficult to finish internal surfaces of thick tubes and clean gas bombs (10~20 mm in
thickness) by the conventional finishing process using the magnetic abrasives, because magnetic
force (finishing force) weakens when the thickness of tube becomes thick. A new efficient process for
internal finishing of a non-ferromagnetic thick tube has been proposed [3]. In this process, magnetic
field-assisted machining using a magnetic machining jig that consists of rare earth permanent
magnets wrapped with abrasive paper is employed instead of conventional magnetic abrasives.
Moreover, a model experiment in which a brass plate (10 mm in thickness) was used as a work was
carried out, and it was shown that finishing could be achieved by using this new process.
Moreover, this new process was developed using magnetic particles that are magnetically
attracted to the surface of a permanent magnet of a magnetic machining jig and using abrasive slurry.
While the magnetic machining jig follows the rotation of pole through the magnetic particles, the
relative motion against the inner surface of tube is generated, the abrasive behavior is indirectly given
from magnetic particles to abrasive grains, the precise finishing is achieved.
In this paper, the advantages of using a magnetic machining jig are clarified by comparing the
merits and demerits of the method using magnetic abrasives and the method using a magnetic
machining jig. Next, the principle and advantages of this new process for an internal magnetic
abrasive finishing using a magnetic machining jig are described. Finally, the magnetic machining jig
and the experimental setup applied to the internal finishing of thick tube were manufactured, and an
experiment on internal finishing of a thick tube using the magnetic machining jig was carried out for
the first time. A two-step process was used in this experiment to obtain a high-quality surface. In the
first step, an abrasive cloth was boned on the surface of the magnetic machining jig, and in the second
step, magnetic particles were magnetically attracted to the surface of magnet of the magnetic
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (#69811647, Pennsylvania State University, University Park, USA-18/09/16,09:50:28)
506
Advances in Abrasive Technology VI
machining jig instead of the abrasive cloth. The results of the experiment showed that precise internal
finishing of a thick non-ferromagnetic tube could be achieved by using this process.
Difference between Magnetic Machining Jig and Magnetic Abrasives
Reason for Proposal of a Magnetic Machining Jig. Figure 1 shows schematics of magnetic the
field assisted machining process using magnetic abrasives and using a magnetic machining jig. As
shown in Fig. 1(a), the advantage of using magnetic abrasives is that flexible processing behavior can
be obtained along the surface of workpiece. Therefore, the shape of workpiece was kept together with
the surface accuracy of the workpiece can be improved. However, the magnetic susceptibility of the
entire of magnetic particles group is extremely low, though the magnetic susceptibility of a single
magnetic particle is high, because there are many gaps inside the particles group. Therefore, the
magnetic force (finishing force) is considerably weak when the magnetic particles group was used as
a tool. So that, the processing will fall in impossibility when the thickness of workpiece becomes
thick, for example, the thick tube of 10~20 mm in thickness was used. To overcome this problem, a
new magnetic field-assisted machining process using a magnetic machining jig that consists of
permanent magnets was developed, as shown in Fig. 1(b). Greater machining force and movement
force (magnetic force) can be generated using the magnetic machining jig because of the greater
material susceptibility, and this makes internal finishing of a non-ferromagnetic thick tube possible.
Magnetic abrasives
Workpiece
(Non-ferromagnetic)
Magnet
Magnetic machining jig
N
Magnet
Table
(a) In the case of magnetic abrasives
N
Abrasive paper
Workpiece
(Non-ferromagnetic)
Table
(b) In the case of magnetic machining jig
Fig.1 Schematics of the magnetic field-assisted machining process using magnetic
abrasives and using a magnetic machining jig
Comparison of Magnetic Forces Acting on a Machining Jig and Magnetic Abrasives. Figure
2 shows a schematic of the measuring setup and various types of magnetic machining jig. A rare earth
permanent magnet (Fe-B-Nd, 18×12×10 mm) was used as a pole and was set up on an aluminum
alloy base. The magnetic force was detected with a strain gauge attached to a brass sheet (1 mm in
thickness), and the distance Z between the machining jig (tool) and the pole was changed in the range
of 2~10 mm, as shown in Fig. 2(a). Tool was used with electrolytic iron particles (510 µm in mean
diameter) filled enough in an acrylic container (inside size: 18×12×10 mm), and with a magnetic
machining jig including the same volume general rolling steel SS400, a single of magnet (Fe-B-Nd,
18×12×10 mm), and the N S type of machining jig shown in Fig. 2(b). In the case of an N-S type of
machining jig, a magnetic closed circuit can be formed. Therefore, the magnetic resistance in a
magnetic circuit is decreased and the magnetic force is increased. Moreover, because two points were
supported, the stability of processing can be obtained. It is thought that the high-efficiency machining
was realized, and the stability was obtained by using the N-S type of machining jig. Then, the
magnetic force was measured, and the results are shown in Fig. 3. The difference of the magnetic
force Fz using the magnetic particles and using various types of the magnetic machining jig as shown
in Fig. 3. It was clarified that the magnetic force Fz when SS400 steel was used was 1.8-times higher
than that when iron particles were used, that Fz when a single magnet was used was 12.7-times higher
Key Engineering Materials Vols. 257-258
507
than that when SS400 steel was used, and that Fz when an N-S type of machining jig was used was
3.8-times higher than that when a single magnet was used when the clearance between the tool and
pole was set at 10 mm.
Strain gage
Magnetic abrasives
Tool
FZ
Steel (SS400)
Abrasive paper
Various
types of tool
Z
S
Permanent magnet
Abrasive paper
N
Base
(Aluminum
alloy)
Workpiece
(Non-ferromagnetic)
18mm
S
N
N
S
Permanent magnet
N-S type of
machining jig
Abrasive paper
(b) Schematic of various types of tool
(a) Schematic of measuring setup
Fig.2 Schematic of measuring setup and various proposed types of tool
40
34.1
Magnetic force FmFz NN
35
30
25
Iron particles
Steel
SS400SS400
steel
Performent
mgnat
Permanent magnet
A pair
of of
magnets
N-S
type
machining jig
20
15
8.9
10
5
0.4
0.7
0
Kind of tool
of magnetic
jig
Fig.3 ChangesThe
in kind
magnetic
force Fmachining
z with various types of tool
Processing Principle
Figure 4 shows a schematic of the internal finishing process using a magnetic machining jig by the
use of a pole rotation system [1,2]. The magnetic machining jig placed in the tube is magnetically
attracted by the poles placed outside the tube, pushing the inner surface of the tube by the generation
of magnetic force. When the poles are rotated around the tube, the machining jig follow rotate along
the inner surface of the tube by magnetic force (finishing force) together with the rotating poles,
resulting in occurrence of relative movement between the magnetic machining jig and the tube.
If the rotating poles are driven in the direction of the tube axis, the magnetic machining jig is also
driven by the magnetic force in the direction of the tube axis while rotating along the inner surface of
the tube. As a result, finishing of the entire inner surface of the tube is achieved. An N-S type of
magnetic machining jig was used in this experiment as shown in Fig.4. When the abrasive paper was
wrapped on the magnetic machining jig, the finishing could be achieved easily like boned-abrasive
machining.
Moreover, a new process was proposed using magnetic particles, magnetically attracted to the
surface of the permanent magnet of magnetic machining jig, and using abrasive slurry in order to
508
Advances in Abrasive Technology VI
obtain a high-quality surface. While the magnetic machining jig follows the rotation of pole through
the magnetic particles, the relative motion against the inner surface of tube is generated. The abrasive
behavior is indirectly given from magnetic particles to abrasive grains. The precise finishing is
achieved. Because the problem of the abrasive paper longevity was solved and the size of magnetic
particle and abrasive grain can be selected freely by this technique, high-precision process could be
achieved.
Pole rotation
Pole
Yoke
(SS400 steel)
Magnetic particles
Rotation
Pole
Tube
Permanent magnet
Yoke (SS400 steel)
Magnetic machining jig
Fig.4 Schematic of internal machining process using a magnetic machining jig
Experimental Setup and Conditions
Figure 5 shows an external view of the experimental setup for the internal machining process using a
magnetic machining jig. A finishing unit, consisting of magnet poles attached inside a yoke, was set
up on the reciprocating table of a lathe machine, and a stainless steel tube as a workpiece was fixed
between the chuck and the tail stock center of the lathe machine. Ferrite magnets were used as poles
for safety of processing environment, and the poles are big (50×35×26mm) for the increase of the
finishing zone in this experiment, but sufficient magnetic force (finishing force) can be obtained
because that the magnetic machining jig was used. The poles were set symmetrically toward the tube
center in an N-S-S-N pole arrangement, and the pole arrangement is flexible for various tube
diameters. Even though the distance between the pole tip and magnetic machining jig is bigger, the
higher magnetic force can be achieved. In order to confirm that a tube of 10 mm in thickness can be
finished, a tube of 5 mm in thickness was used as a workpiece, and the distance between the pole tip
and the tube was set at 7 mm in this experiment. It is thought that the same result is obtained when a
tube of 10 mm in thickness was used, and the distance was set at 2 mm. Moreover, the finishing unit
can be driven in the direction of the tube axis with the table of lathe machine as the pole rotating. This
gives the magnetic machining jig motion in the direction of the tube axis in addition to rotation. The
experimental conditions are shown in Table 1. A two-step process was performed in this experiment.
An abrasive cloth and abrasive slurry were used in the first-step processing, and magnetic particles
and abrasive slurry were used in the second-step processing. The composition and size of the
magnetic machining jig in the two-step processing are shown in Table 1.
Experimental Results and Discussion
Figure 6 shows the results of finishing by the two-step processing, that is, changes in the surface
roughness and material removal with finishing time and photographs of the inner surface of the tube
before and after finishing. The results show that the initial surface roughness of 4.5 µmRa (31.4 µmRy)
Key Engineering Materials Vols. 257-258
509
was improved to 0.4 µmRa (4 µmRy) by the first-step processing and that the surface roughness was
improved rapidly from 0.4 µmRa (4 µmRy) to 0.1 µmRa (1.1 µmRy) by the second-step processing
when an SUS304 stainless-steel tube of 10 mm in thickness was used as the workpiece and the
finishing zone was set at 150 mm. The pattern drawn on the white sheet put in the tube is reflected in
the inner surface of the tube after finishing as shown in Fig. 6. This shows it is possible to obtain a
high-quality surface for thick tubes using a magnetic machining jig. This study is the first study in
which internal finishing of thick tube was achieved by using a magnetic machining jig.
Pole
Chuck
Tube Machining jig
Fixed cover
Motor Center
Carriage of
lathe
Tail stock
Fig.5 External view of the experimental setup using a lathe
Table 1 Experimental conditions
Workpiece
SUS304 stainless steel tube
Ø89.1×Ø79.1×500mm
Initial surface roughness: 31.4µmRy (4.5µmRa)
Permanent
Polymer
Revolution: 34min-1
magnet
Magnetic
Magnet: Nd-Fe-B rare earth
24
machining jig
permanent magnet
28
N
s
Yoke
Yoke: SS400 steel
12
Molding material: Polymer
50
50
Pole
Magnet: Ferrite magnet
50×35×26mm
Yoke: SS400 steel
Abrasive cloth
1st-step
Abrasive paper: #100 WA
Non-woven fabric
processing
resinoid-bonded abrasive cloth
#1000 WA slurry: 5wt%
Machining jig
(16µm in mean dia.)
2nd-step
Magnetic particle: Iron particles
Magnetic particles
processing
(510µm in mean dia.)
Diamond slurry: 7.5wt%
Non-woven fabric
(12µm in mean dia., 2.4g)
Machining jig
Pole revolution
415min-1
Pole feeding speed 1.2m/min
Clearance
7mm
510
Advances in Abrasive Technology VI
Finished
surface
Ra = 4.5µm
Ra = 0.1µm
10mm
After finishing (100 min)
Before finishing
2nd step
processing
1st step
processing
35
10000
10
Ry
30
Ra
M
8000
8
M
25
Material removal M g
Surface roughness Ry, Ra µm
10mm
20
6000
6
15
4000
4
10
2000
2
5
0
0
20
40
60
80
0
100
Finishing time min
Fig.6 Changes in surface roughness and material removal with finishing time
Conclusions
In this study, the difference between the finishing performance of a magnetic machining jig and that
of magnetic abrasives was examined. Moreover, a new efficient internal finishing process for a thick
tube was developed by the application of a magnetic field-assisted machining using a magnetic
machining jig. Finishing experiments were carried out to examine the finishing characteristics. The
results are summarized as follows:
The difference between the results obtained by using a magnetic machining jig and those obtained
by using magnetic abrasives were compared, and it was found that about an 80-times-higher magnetic
force (finishing pressure) can be achieved by using an N-S type of magnetic machining jig than by
using magnetic abrasives when the distance between the pole and the tool was set at 10 mm.
A new technique using magnetic particles that are magnetically attracted to the surface of the
magnetic machining jig and using abrasive slurry was developed to obtain a high-quality surface. The
principle and the advantages of this process were clarified.
It was confirmed that this process enables precise internal finishing of a thick tube (10 mm in
thickness) and that the initial surface roughness of 4.5 µm in Ra can be improved to 0.1 µm in Ra by
two-step processing.
References
[1] T. Shinmura and H. Yamaguchi: JSME Int. J., Part C, Vol. 38 (1995), p. 798.
[2] H. Yamaguchi, T. Shinmura and T. Kaneko: Int. J. Japan. Soc. Prec. Eng., Vol. 30 (1996), p.
317.
[3] Y. Zou and T. Shinmura: Trans. Japan. Soc. Mech. Eng., Part C, Vol. 68 (2002), p. 1575.