FIELD OF INVENTION
The present invention relates to a tool for producing variable magnetic field by
permanent magnets for nano finish surfaces particularly in blind holes, smaller
holes, deep grooves and vertical curved surfaces of components formed of
similar or dissimilar materials with different hardness.
BACKGROUND OF THE INVENTION
The global technology is moving into a new era in the field of machining and
finishing operation. In prior art, micro-finish was considered to be the end goal,
but now industries demand for nano-finish surface. This crates new challenges
to manufacturing industries to provide nano-finishing at low cost. To meet this
requirement of the industries new processes are being developed. The known
processes for finishing like lapping, honing employ high normal stresses and
temperature during finishing operation which causes sub-surface damage
including change in microstructure of the finished surface. Accordingly, a good
surface finish without causing sub-surface damage and micro-structural
change, is the objective of the present technology, which warrants an improved
control on polishing pressure during finishing operation. Magnetic Abrasive
Machining (MAM) is one of the prior art processes that has the ability of
producing high quality surface finish without changing the micro structure of
surface. In Magnetic Abrasive Machining the polishing pressure can be
precisely controlled by changing the magnetic field density in the working gap,
which affects the flexible magnetic abrasive brush (FMAB) formed in the gap.
In Magnetic Abrasive Machining process, the finishing medium used is a
Magnetic Abrasive Powder which is a mixture of an abrasive powder and iron
powder in different ratios. In some published journals, Aluminum oxide (AI203)
and Silicon carbide (SiC) have been used as the abrasive medium. Aluminum
oxide is recommended for soft materials and Silicon carbide is used for hard
material, as per requirement. Different types of abrasive is also used depending
on the hardness of the material to be finished for example, aluminum oxide is
used for softer material and for finishing hard material silicon carbide powder
is used. Different size of abrasive particle and iron particle are used for
achieving different results.
These particles of iron and abrasive material may be bonded or used directly as
the mixture. The bonded particle is found to have provided better results in
cylindrical magnetic abrasive finishing of internal surfaces of holes. However,
sintering of these particles requires both high pressure and temperature in an
inert gas atmosphere. Thus, unbonded particles are generally used.
The prior art teaches several processes under Magnetic Abrasive Machining,
which use magnetic field strength for brush formation to perform finishing
such as Magnetic Abrasive Finishing (MAF), Magnetic Float Polishing (MFP),
Magneto-Rheological Jet Finishing (MRJF), Magneto-Rheological Abrasive Flow
Finishing (MRAFF), Magneto-Rheological Finishing (MRF) and recently
developed Ball End Magneto-Rheological Process (BEMRP). These known
processes are applied for different kind of surfaces. For example, Magnetic
Float Polishing (MFP) is used for finishing the spherical balls. Ball End
Magneto-Rheological Process (BEMRP) is used for finishing the 3D surfaces,
Magneto-Rheological Abrasive Flow Finishing (MRAFF) process produces good
surface finish on the internal surface of pipes, bushes.
However, all these processes have limitations in finishing certain features like
blind holes, through holes, deep grooves. There are some known tools
developed for finishing holes like magnetic abrasive finishing tool [Georgy A.
Podoprigora in 1986] which can only be applied for larger holes because the
magnets are mounted on the periphery of a cylindrical magnet holder. Also due
to fixed magnetic field of the permanent magnets, it is not suitable for the
material of different hardness using same combination, because, for finishing
different kind of materials different combination of magnets are required.
A tool is also available specially developed for barrel finishing by magnetic
abrasive finishing operation. In this tool, the magnets are alternately arranged
on the bottom side of the tool holder so that magnetic field appears on the
peripheral edge of the tool for brush formation. But due to application of a
number of magnets in the tool it is restricted for finishing operation of bigger
holes.
Magneto Rheological abrasive flow finishing (MRAFF) is known to be suitable
for finishing pipes with small thickness. But, this process cannot be applied to
holes in the solid part. Because in this method the magnetic field is applied
from outside and the magnetic abrasive brush forms on the inner surface of
the hole, wherein the magnetic field should be applied from all peripheral
directions with equal strength. This magnetic field must be strong enough so
that stiff brush can form for finishing the surface and if the magnetic field is
weak then stiffness of the brush becomes less which cannot give desired
finishing effect. Magnetic field is known to decrease as it goes away from the
pole, therefore, for achieving higher magnetic field, the thickness of the pipe
cannot be very high. Thus, it becomes difficult to apply the magnetic field to
the holes in solid bodies. Due to this problem, MRAFF can only be applied for
polishing pipes with small thickness.
Magnetic barrel machining tool (Patent No. US5611725) and Device for
magneto abrasive machining of holes (US4599826A), use permanent magnet
for generating the magnetic field. But these tools can be only used for barrel
cleaning because too many magnets are arranged on the peripheral surface for
generating high magnetic field intensity. Due to this, the size of these tools
become larger, which cannot be used for cleaning of small holes. Also these
tools cannot produce variable magnetic field for producing different kind of
brush stiffness.
Magnetic Abrasive finishing operation as used in the prior art is a nanofinishing
operation which is mostly used for finishing flat and 3D surfaces, in
which Permanent magnet or electro magnet for generating field for brush
formation is used. For finishing operation this brush is rubbed on workpiece by
introducing relative motion between brush and workpiece. The strength of this
brush is responsible for finishing operation and the strength of this brush
depends upon strength of magnetic field. For finishing cylindrical surface,
cylindrical magnetic abrasive finishing is used. It is easy to finish cylindrical
surface externally by employing magnetic field from outside but it is very hard
to perform finishing operation on internal cylindrical surface which is generally
limited to workpiece with low wall thickness.
KR 100793112 teaches a cross hole deburring device to remove quickly burr
generated in the cross hole of a workpiece by using a cross hole inductor
through a magnetic grinding process. A cross hole deburring device includes a
CNC machine, and a cross hole inductor arranged in a spindle of the CNC
machine so as to remove burr from a cross hole in a workpiece.
The cross hole inductor includes a rod type permanent magnet having a
predetermined length, and magnetic powder at one end of the permanent
magnet. Due to use of permanent magnet the tool produces fixed magnetic
field, which cannot be used for different material with different hardness.
JP2007268689 discloses a magnetic abrasive finishing device, a method there
for, and machining tools for efficiently polishing an inner wall of a pipe or the
like made of a magnetic body, and polishing without applying an excessively
high pressure. In the magnetic machining device, a polishing object made of a
magnetic material is polished by using magnetic abrasive grains on the surface
of a machining tool having a permanent magnet. The magnetic machining
device has a workpiece holding means for holding at least the polishing object,
a spindle for transmitting a rotational movement and/or an axial vibration
movement to the machining tool by mounting the machining tool, and a
-6-
spindle rotation vibration drive means for applying the rotational movement
and/or the axial vibration movement to the spindle. The machining tool
wherein a bar-shape permanent magnet is embedded to be exposed or
projected from the surface can be used. Due to use of permanent magnet the
tool produces fixed magnetic field which cannot be used for different material
with different hardness.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a tool for producing variable
magnetic field by permanent magnets for nano finish surfaces in blind holes,
smaller holes, deep grooves and vertical curved surfaces of components formed
of similar or dissimilar materials with different hardness, which eliminates the
disadvantages of prior art.
Another object of the invention is to propose a tool for producing variable
magnetic field by permanent magnets for nano finish surfaces in blind holes,
smaller holes, deep grooves and vertical curved components formed of different
materials with different hardness, which is enabled to produce variable
magnetic field at the periphery allowing nano-finishing of irregular shaped and
substantially in-accessible surfaces.
A further object of the invention is to propose a tool for producing variable
magnetic field by permanent magnets for nano finish surfaces in blind holes,
smaller holes, deep grooves and vertical curved surfaces of components formed
of similar or dissimilar materials with different hardness, which produces
variable magnetic field even by using permanent magnets.
A still further object of the invention is to propose a tool for producing variable
magnetic field by permanent magnets for nano finish surfaces which is reliable
and simple in construction.
-7-
^UMMARY OF THE INVENTION
According to this invention, there is provided a tool for producing variable
magnetic field by permanent magnets for nano finish surfaces comprising of a
lower part fitted with an upper part, wherein the lower part comprising of a
magnet integrated with a Ferromagnetic piece and said upper part comprising
a magnet with a means to vary formation of magnetic field at the periphery of
the piece.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further objects and advantages of this invention will be more apparent from
the ensuing description when read in conjunction with the accompanying
drawings and wherein:
Figure 1 - shows tool according to the invention indicating orthographic view
and isometric view.
Figure 2 - shows a schematic view of the tool of Figure 1 with the abrasive
brush formed on the peripheral surface of the tool.
Figure 3- shows schematic of flow of magnetic field lines in the absence of
top magnet when the variation can't be achieved.
Figure 4- shows schematic of flow of magnetic field lines of magnetic field
formed on the periphery of Ferromagnetic piece.
Figure 5 - shows magnetic field strength and field vector when magnets of the
tool are in contact with the Ferromagnetic piece.
Figure 6 - shows the parameters of figure 5 when the upper magnet placed
away from the Ferromagnetic piece.
Figure 7 - shows a photographic view of the tool of the invention when
mounted on a machine for operation.
-8-
•Figure 8- shows finished and unfinished surface (F and U) of workpiece
using present invention.
DETAIL DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE
TO THE ACCOMPANYING DRAWINGS
The present invention is directed to a tool for producing variable magnetic field
by permanent magnets for nano finish surfaces particularly in blind holes,
smaller holes, deep grooves and vertical curved surfaces of components formed
of similar or dissimilar materials with different hardness. Now, reference may
be made to figure 1, wherein the tool comprising of atleast two parts i.e. upper
part and lower part.
The lower part comprising of a magnet (1) which is integrated with a
Ferromagnetic piece (2) on its top surface. The magnet with said piece is fixedly
disposed inside a casing of diamagnetic material (3).
The upper part comprising a magnet (4) integrally provided with a tool head (5),
top of which is having a nut arrangement (6), so that the top part can be fitted
inside the lower part with the help of screw (a plurality of threads) on the inner
surface of top portion of the lower part constituting tool of instant invention,
wherein similar poles of the magnets face each other and the piece (2) is
sandwiched between both magnets.
According to another embodiment of present invention, tool head with nut
arrangement is detachably attached to the upper magnet (4), thereby rendering
the tool into a device of three components.
The Ferromagnetic piece and the casing is made of for example mild steel and
copper respectively without restricting scope of the invention to the same.
Thus, other materials readily apparent to a person skilled in the art are
understood to be within scope of the invention.
The shape of both the magnets. Ferromagnetic piece, tool head and casing is
required to be similar so that the upper magnet can be telescopically provided
inside the casing accommodating lower magnet with the piece on the top. In
the exemplary embodiment of the invention, the shape of the magnets, piece,
tool head and casing is cylindrical.
The tool can be configured with small magnets for finishing small holes.
The copper casing is used to hold the magnets. The inner diameter of the
copper casing is slightly higher than the magnet diameter and the outer
diameter of the casing is equal to diameter of the Ferromagnetic piece. The
casing is integral to said ferromagnetic piece so that both the magnets can be
in contact with this ferromagnetic piece.
By means of screw and nut arrangement, the gap between the upper magnet
and Ferromagnetic piece is variable giving rise to variable magnetic field on the
periphery of said piece for creating different kind of brush stiffness on the piece
and lower magnet including its bottom i.e. formation of brush on the tool to
enable conducting nano-finish operation on blind holes, smaller holes, deep
grooves or curved surfaces of work pieces with different hardness as shown in
figure 2.
The magnets and the piece are arranged as per design to concentrate the
magnetic field of both the magnets. As can be seen from figure 4 when the
South Pole/ North Pole of both the magnets are in contact with the
Ferromagnetic piece, the magnetic field lines of both the magnets flow through
the piece and exit from its peripheral area.
In the absence of second magnet, the magnetic field appearing on the periphery
of the Ferromagnetic piece can't be varied as shown in figure 3. When the
south/north pole of the upper magnet approaches the top surface of the iron
piece, the total magnetic field gets concentrated on the periphery of the piece.
By varying the gap between the piece and upper magnet, the magnetic
-10-
concentration at peripheral surface of said piece can be changed as in figure.4.
This magnetic field can be used for brush formation of magnetic abrasive
particle (figure 2). By reducing the gap completely the combined effect of
magnetic field arising from both magnets is obtained on the periphery of
ferromagnetic piece (figure 4).
The copper is used for the casing, because the copper is diamagnetic and has
very less negative susceptibility with sufficient hardness. When magnetic field
lines of both the magnets come out of a very small area, these field lines repel
each other and try to spread. When any ferromagnetic material is used, these
field lines concentrate on the periphery of Ferromagnetic piece. Due to
diamagnetic property of the casing material, there is no leakage of magnetic
field from the casing.
The developed tool can be used for nano-finishing of through and blind holes
by using Magnetic abrasive finishing process. This tool can also be used for
finishing grooves and vertical surfaces. In prior art hole finishing operation
using MR/MA finishing, the magnetic field was introduced from outside and
the abrasive laden magnetic field used to be introduced from inside due to
which the thickness of the workpiece that can be machined from this known
process is limited. But the developed tool generates sufficient amount of
magnetic field to form the brush stiffness enough to perform finishing
operation. Due to introduction of the magnetic field from inside, holes of any
size can be machined. The tool can be mounted on a vertical milling machine
and used as side milling tool for finishing grooves and vertical surfaces.
Magnetic field of the tool can be varied through the gap, so for finishing
different material different magnetic field can be used depending upon the
hardness of the workpiece. This tool can be used for finishing both
ferromagnetic and non-ferromagnetic materials.
-11-
'As already discussed for obtaining a variable magnetic field a provision in the
form of a tool head is integrated in the tool. By rotating the tool head, a gap
between the upper magnet and the MS piece can be varied which results in
change of magnetic flux density at the periphery of the MS piece. Thus a
magnetic field density from 0.81 Tesla to 0.349 Tesla can be achieved using the
assembly of exemplary embodiment of invention as in figure. 1.
Thus, the brush on the peripheral surface of the tool can also be used for
finishing flat and curved vertical surfaces like a side milling tool. The lower
surface of the tool shows the bare surface of the lower magnet. Accordingly, the
brush is also formed at the lower surface of the tool which can be used to
finish flat vertical surfaces. The brush formed simultaneously at the periphery
of Ferromagnetic piece of the tool and lower surface of the tool and in case the
height of the lower magnet is low then these two brush get connected (figure 2),
because the magnetic field lines coming out of the peripheral surface of the
iron piece get converged into the lower surface of the lower magnet, so it can
also be used to finish blind holes.
The strength of the abrasive brush on tool lower position can be varied by
changing the gap between tool lower portion and the work piece surface. The
strength of the brush at the periphery of the iron piece can be varied by
changing the magnetic field strength of the tool by tightening and loosening the
upper part i.e. by changing the gap.
Further for the validation of the construction as stated above, some simulation
were performed, the results are shown in figure 5 and figure 6.
It can be seen from figure 5, that when the upper magnet is in contact with the
MS piece, the magnet field is intensified to 0.844 Tesla at the periphery of the
MS piece and the magnetic field at other surfaces of the tool is around .096
Tesla. So this proves that magnetic field is concentrated by the interplay of the
ferromagnetic and diamagnetic material.
-12-
»
When the upper magnet is moved 10mm away from the MS piece the magnet
field reduces to 0.438 Tesla at the periphery. So this result confirms that
validation of the concept is employed in construction of the tool.
Experimentation
From the generated results of simulation, it is clear that magnetic field lines
are trying to repel from each other and due to this, magnetic field lines of both
the magnets pass outside through iron piece. Due to this combined effect very
high magnetic field is generated at the periphery of iron piece.
When the gap between upper magnet and iron piece is increased the magnetic
field strength decreases as compared to magnetic field strength in case of no
gap. But due to very high permeability of iron and diamagnetic property of
copper the maximum field lines still crosses from iron piece.
This simulation results validate theory of present invention which got verified
by experimental value.
Some experiments were performed by the tool to check its finishing capability.
For the experiment. Stainless Steel (SS304) pipes were taken as work specimen
having a hardness of 180 Vickers. The internal diameter of all the pipes was
26mm so that when the tool was inserted inside the pipe to perform finishing
operation, a constant radial clearance of 1mm could be maintained. The tool
was mounted in the tool holder of a vertical milling machine and a vice fixed
with the bed of the milling machine held the pipe tightly. Some pilot
experiments were performed and based on the results the effective time for
finishing was 15 minutes, after which rate of improvement in the result
decreased.
a. Process parameters
For statistical analysis, four parameters are chosen as the process
parameters which are (a) magnetic field (b) RPM of the tool (c) abrasive
-13-
concentration (d) abrasive size. From prior research (R.S. Mulik, Magnetic
abrasive finishing of hardened AISI 52100 steel, 2010) it has been found
that some of this parameter affects the results nonlinearly, so a Central
Composite Design method was used for analyzing the effect of the process
parameters on the finishing capability of the tool. Pilot experiments were
performed by varying these process parameters and based on the result of
these experiments effective range for variables are decided (table 1).
Table 1 Different level of the parameters chosen for the experiment
Factors
Description
RPM
Magnetic Field (T)
Abrasive weight
percentage
Abrasive mesh
no.
Levels
-2
200
.4
20%
400
-1
300
.5
25%
600
0
400
.6
30%
800
1
500
.7
35%
1000
2
600
.8
40%
1200
Coded
un-coded
Table 2 DOE table
Exp.n
o.
1
2
3
4
5
Time
15
15
15
15
15
RPM
400
500
500
500
500
Magnetic
Field
0.6
0.5
0.7
0.7
0.7
Abrasive
Cone.
20
35
25
35
25
Abrasive
mesh no.
800
600
1000
600
600
Initial
(Ra)
0.5101
0.4917
0.4702
0.4451
0.4183
Final
(Ra)
0.187
0.2046
0.1261
0.1338
0.1459
Percentage
change in
(Ra)
63 34
58.39
73.18
69.94
65.12
^H-
6
. 7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
600
400
400
400
400
400
400
500
500
400
400
300
200
300
500
400
400
400
300
400
300
300
500
300
300
300
0.6
0.6
0.4
0.6
0.6
0.6
0.79
0.5
0.5
0.6
0.6
0.7
0.6
0.5
0.7
0.6
0.6
0.6
0.7
0.6
0.5
0.5
0.5
0.7
0.7
0.5
30
40
30
30
30
30
30
25
25
30
30
25
30
25
35
30
30
30
25
30
35
35
35
35
35
25
800
800
800
800
800
800
800
1000
600
800
400
600
800
1000
1000
800
1200
800
1000
800
1000
600
1000
1000
600
600
0.4985
0.5036
0.4825
0.4477
0.4631
0.5046
0.4102
0.4508
0.494
0.4981
0.432
0.5062
0.4655
0.4583
0.4501
0.5257
0.4984
0.4364
0.468
0.4956
0.4863
0.5473
0.4896
0.5122
0.4405
0.5086
0.2294
0.1622
0.1957
0.1725
0.1737
0.1531
0.0931
0.1754
0.2261
0.1677
0.1581
0.196
0.1949
0.1753
0.1485
0.1718
0.1563
0.1572
0.1368
0.167
0.1811
0.1932
0.2185
0.1497
0.1273
0.2258
53.98
67.79
59.44
61.47
62.52
69.66
77.30
61.09
54.23
66.33
63.40
61.28
58.13
61.75
67.00
67.32
68.64
63.98
70.77
66.30
62.76
64.70
55.37
70.77
71.10
55.60
Using the above experimental results (table 2) a statistical model was
developed for determining the best capability of the developed tool. The
developed statistical model is used to obtain the optimal value of the
parameters at which the statistical model gives best result. For checking the
-15-
precision of the tool few experiments were performed. The results of these
confirmation experiments (table 3) show that tool performs within statistical
permissible range.
Table 3 Experimental table of confirmation experiment
Ex
PNo.
1
2
3
Tim
e
(mi
n)
15
15
15
RP
M
500
500
500
Mag.
field
(Tesl
a)
.8
.8
.8
Abras
ive
weigh
t
perce
ntage
20
20
20
Mes
h
no.
120
0
120
0
120
0
Expecte
d
percent
age
change
in Ra
89.4
89.4
89.4
Confide
nee
interval
at 95%
confiden
ce level
89.413.3
3
89.4±3.3
3
89.4±3.3
3
Experimen
tal Results
of Exp.
Initia
1
Ra
(|um)
.533
.546
.579
Fin
al
Ra
(lam
)
.06
3
.05
6
.07
0
Exp.
percent
age
change
in Ra
88.05
89.70
87.88
It is to be noted that the present invention is susceptible to modifications,
adaptations and changes by those skilled in the art. Such variant embodiments
employing the concepts and features of this invention are intended to be within
the scope of the present invention, which is further set forth under the
following claims:-

WE CLAIM
1. A tool for producing variable magnetic field by permanent magnets for
nano finish surfaces comprising of a lower part fitted with an upper
part, wherein the lower part comprising of a magnet integrated with a
Ferromagnetic piece and said upper part comprising a magnet with a
means to vary formation of magnetic field at the periphery of the
piece.
2. A tool as claimed in claim 1, wherein the Ferromagnetic piece is
integrated on the top surface of the lower magnet.
3. A tool as claimed in claim 1 or 2, wherein the lower magnet with the
Ferromagnetic piece is integrally disposed inside a casing of
diamagnetic material constituting said lower part.
4. A tool as claimed in claim 3, wherein the casing is preferably made of
diamagnetic material to prevent leakage of magnetic field from the
casing.
5. A tool as claimed in any of the preceding claims, wherein the upper
part comprising a magnet attached to a tool head, top of which is
provided with a nut arrangement for fitting said upper magnet inside
the lower part with the help of screw on the inner surface of top
portion of said lower part.
6. A tool as claimed in claim 5, wherein the magnet may be detachably
attached to the tool head.
fB^•17-
7. A tool as claimed in claim 5, wherein the magnet may be integral to
the tool head.
8. A tool as claimed in any of the preceding claims, wherein the magnets,
similar poles of which face each other with the Ferromagnetic piece
there between.
9. A tool as claimed in any of the preceding claims, wherein the gap
between the upper magnet and Ferromagnetic piece is variable by
means of the nut and screw arrangement giving rise variation in the
magnetic field at the periphery of the ferromagnetic piece for creating
different kind of brush stiffness on the ferromagnetic piece.
10. A tool as claimed in any of the preceding claims, which can be
employed to produce nano-fmish surface area in blind holes, smaller
holes, deep grooves and vertical curved surfaces of components of
similar/dissimilar materials with different hardness and of irregular
shaped and substantially in-accessible surfaces.