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Machining of hardened steel using advanced tool materials, such as CBN, has certain advantages over
the traditional cutting-hardening-grinding practice in terms of improved fatigue strength of the
machined parts, increased productivity and reduced energy consumption [1, 2, 3]. Although CBN tools
offer excellent performance on fully hardened steels, the results on steels of medium hardness have
been challenged by other members of the tooling family, e.g. ceramic tools or even some new carbides.
In this paper the performance of CBN tools is investigated when machining steel hardened to 45 - 55
HRC. Since CBN tools are normally used in finishing operation and the cutting regime employed is
likely to generate large radial thrust force which may cause chatter and deteriorate machining quality,
understanding the changing patterns of cutting forces and surface finish is therefore important.
2 EXPERIMENTAL WORK
2.1 Materials
The cutting tools used were high concentration CBN compacts, referred to in this paper as CBN 1 and
CBN 2. The materials and geometric parameters of the tool inserts are detailed in Table 1. The
workpiece material used in the tests was hardened GB699-88 55 steel hardened to 45~55 HRC. The
compositions of the workpiece material are shown in Table 2.
2.2 Experimental procedure
Multivariate tests were performed to measure cutting forces and machined surface roughness. The
operating parameters were v = 56 ~ 182 m/min, f = 0.08 ~ 0.31 mm/rev and ap = 0.025 ~ 0.1 mm. In
addition, the tools with chamfered/unchamfered cutting edge and with different tool nose geometry
were used in certain tests. All tests were conducted dry under continuous turning conditions.
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3 RESULTS AND DISCUSSIONS
3.1 Cutting forces
3.1.1 Cutting force components
Cutting forces can be resolved into three components: feed force (Fx), radial thrust force (Fy) and
tangential cutting force (Fz). Usually the tangential cutting force is the largest of the three components,
though in finishing the radial thrust force is often larger, see Figures 1 ~ 3, while the feed force is
minimal. This arrangement in finishing can be explained by studying the particular cutting regime and
tool geometry used in the tests. From the tool geometry and the cutting conditions outlined in section
2.2, it is clear that the depths of cut (0.025~0.10 mm) are far smaller than the nose radii of the tools
(0.3~1.2 mm). Under such conditions the tool nose, i.e. the curved part of the cutting edge, performs
the whole cutting job, thus the acting cutting edge angle varies along the tool-work contact arc of the
tool nose. The largest value of the angle appears at the position where the cutting edge meets the
original work part surface as in Figure 4. The maximum cutting edge angle can be obtained from:-
Kr a
rr
p=
arccos
(1)
Where Kris the cutting edge angle,
r is the tool nose radius,
ap is the depth of cut.
If r = 1 mm, ap = 0.025 mm, then Kr= 128'. Such a small cutting edge angle is seldom used in metal
cutting, moreover, if considering the average value along the tool-work contact arc the angle is even
smaller. As the cutting edge angle decreases the horizontal component of the cutting force F xy will
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alter direction clockwise, see Figure 5. As a result, Fy will increase whereas Fx will decrease. Fz will
also increase but to a much less extent [4]. Eventually Fy will surpass Fz, and Fx will reduce to a
negligible quantity.
The increase in Fy can lead to instability through vibration. From this point of view the tool nose radius
should be kept as small as possible. This is not ideal however in respect of good surface finish. In
addition, this may also cause temperature concentration at the tool nose and increase the likelihood of
spalling, resulting in a short tool life.
3.1.2 Cutting regime vs cutting forces
With an increase of cutting speed both the radial thrust force and the tangential cutting force showed a
decrease, Figure 1. This is a standard effect when cutting most metals with carbide tools. Trent [5]
attributed the phenomenon in part to the softening of the workpiece material at high temperature and in
part to a decrease in tool-chip contact area owing to a thinner chip.
When the feed rate was increased the forces also increased, but the radial thrust forces generated by
CBN 1 tools, Figure 1, appeared not as sensitive to the change as those produced by CBN 2 tools,
Figure 2.
Depth of cut seemed to influence cutting forces more significantly than cutting speed and feed rate. In
fact, the feed force (Fx) showed visible changes only when increasing DOC. Substituting the tool nose
radius used, r = 1.2 mm, in the cutting force tests into Eqn.1, it can be seen that when the depth of
cut increases from 0.025 mm to 0.1 mm, as in Figure 1, the maximum cutting edge angle increases
from 113' to 233'. This is a major reason for Fx to increase, as illustrated in Figure 5.
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3.1.3 Edge geometry and cutting forces
Chamfered and unchamfered CBN inserts were used in the cutting force tests. It can be seen from
Figures 1 to 3 that all three force components generated by the chamfered tools were greater than those
recorded when using the unchamfered ones. The radial thrust force was affected the most. On the
chamfered tools, Fy was doubled or even tripled yet the increase in Fz was only about 10% - 50%.
There are other observations that may also be related to geometric parameters. In the cutting force tests,
CBN 2 inserts were ground to different nose radii yet were tested under otherwise identical cutting
conditions. It can be seen from Figure 3 that as the nose radius increases from 0.3 mm to 1 mm, Fy
increases by about 30%, whereas the changes in Fx and Fz are negligible. This phenomena can be
explained using Eqn.1. When the nose radius changes from 0.3 mm to 1 mm, for a depth of cut of 0.1
mm, the maximum cutting edge angle decreases from 48 to 26. Such change may turn the horizontal
component of the cutting force (Fxy in Figure 5) clockwise, then the radial component of the cutting
force increases.
3.1.4 Influences of tool wear on the cutting forces
It can be seen in Figure 4 that tool wear had a negligible influence on feed force and tangential cutting
force, however the radial thrust force showed a 90% - 150% increase when the wear land VBB had an
increment of about 0.18 mm.
Because the tool nose radius is much larger than the depth of cut, the flank wear land may almost be
parallel to the feed direction, thus the force normal to the flank wear land will be approximately in the
direction of the y axis. Meanwhile the friction force on the flank wear land is always in the direction of
the z axis. The fact that Fz changes only slightly while Fy increases dramatically seems to indicate that
either the increase of the normal force on the flank wear land does not lead to the increase of friction
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force, or the friction force on the flank face is too small to have significant influence on the total force
in the z direction.
3.2 Surface roughness
3.2.1 Hardness vs roughness
The majority of Ra data collected during the tests were summarised by using histograms, see Figures
6. The horizontal axis of the graphs represents the observed roughness readings and the vertical axis
gives the frequency of the readings. The graphs are able to show the variations of surface roughness
with the changing workpiece hardness.
From Figures 6, it is evident that the harder the workpiece material, the lower is the surface roughness
obtained for a given set of operating parameters. This phenomenon may be explained by a finding
presented by Usui [6].
In orthogonal cutting, the material flow is mainly two dimensional, on a plane normal to the cutting
edge. The deformation in the third direction, i.e. the direction parallel to the cutting edge, is usually
disregarded. However this deformation does exist and causes slight lateral plastic flow of the
workpiece material in the region adjacent to the two free surfaces, e.g. the internal and external
surfaces if a thin wall tube is used as the workpiece when conducting orthogonal cutting on a lathe.
When there is only one free surface as in turning a solid bar, the lateral flow on the constrained side
may increase the peak-to- valley height of the machined surface profile as in Figure 7. By increasing
the workpiece hardness, the plasticity of the workpiece material is reduced and so is the level of the
lateral plastic flow. As a result the surface roughness becomes lower.
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3.2.2 Influence of cutting regime
Surface finish was shown to be improved by increasing cutting speed, Figure 8, though the
improvement was very limited. Producing a better surface finish at higher cutting speed is not
something unusual in metal cutting, but the conventional explanations are usually related to BUE [4].
That is, the formation of built-up-edge is favoured in a certain range of cutting speed. By increasing
cutting speed beyond this region, BUE will be eliminated and as a result the surface finish will improve.
When hardened steel was machined under present cutting conditions. the cutting speeds adopted were
higher than that favouring BUE formation [5]. Indeed BUE was not apparently observed even at the
lowest speed of 56.5 m/min. Therefore the phenomenon needs further explanation. Two possible
reasons are given below:-
According to Liu [7], the properties of metals are influenced by the deformation velocity. The higher
the velocity, the less significant the plastic behaviour will be. Based on the reasoning in section 3.2.1,
the lateral plastic flow of the workpiece material along the cutting edge direction may increase the
peak-to-valley height of the surface irregularity. If the material presents less plasticity by increasing
cutting speed and hence deformation velocity, the surface finish can be improved as a result of less
significant lateral plastic flow and thus less additional increase of the peak-to-valley height of the
machined surface roughness.
The second possible reason is based on SEM observations. At low cutting speed grooves developed on
the flank wear land, Figure 9. When such cutting edge is engaged with a workpiece, the defects will in
part be copied on to the newly generated surface. In any event it is likely that the surface will be rough.
With an increase in cutting speed the grooves will gradually be reduced, thus the cutting edge and wear
land will become smoother, see Figure 10, as will the workpiece surface. The influence of wear land
grooves on surface roughness was also observed by Solaja [8], Ansell and Taylor [9]. They
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demonstrated that with the development of the grooves the surface finish deteriorated.
The roughness increases with increases in feed rate, see Figure 11, but the trend is less significant for
the tools with large nose radius. A recommendation is therefore made to the tool users that if the inserts
of 1 mm nose radii are used, feed rates as large as 0.3mm/rev may be used in order to promote
productivity when finishing without significant deterioration of surface roughness. However low
DOC should be used in order to reduce the tendency to chatter.
The DOC has little direct influence on the surface roughness, however with increases in DOC, chatter
may result causing degradation of the workpiece surface. Therefore if the tool-work system is not very
rigid, such as in cutting slender parts, very fine DOC should be employed to avoid chatter. In this way
very good surface finishes can be obtained. For example, when a DOC of 0.025 mm and feed rate of
0.2 mm/rev were employed, a roughness of Ra = 0.22 m was achieved using CBN 1 inserts, which is
compatible with grinding.
3.2.3 Influences of tool wear
As mentioned in the last section, large surface roughness values produced at low cutting speed
probably resulted in part from the grooves on the wear scars of the tools. It can be seen from Figure 12
that the roughness is also associated with the width of the flank wear land. The relationship may be
explained as follows:-
When a new insert starts to work, the machined surface is determined, for a given feed rate, by the
geometry of the fresh tool edge. If the DOC is far smaller than the nose radius, then the principal
geometric parameter is only the nose radius. As the tool wears however, the round corner becomes
flatter, in other words the nose radius increases substantially. As a result the machined surface finish
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improves. With the development of excessive flank wear however, increased cutting force and
temperature may destabilize the machining process and the surface quality is degraded.
CONCLUSIONS
1 When finish cutting of hardened steel radial thrust force (Fy) became the largest among the
three cutting force components and was most sensitive to the changes of cutting edge chamfer,
tool nose radius and flank wear. Although an unchamfered tool with small nose radius generated
low Fy and hence reduced the tendency to chatter, such geometry decreased tool life.
2 Lateral plastic flow of the workpiece material in front of a cutting edge increased roughness of
machined surfaces. Therefore the harder, and hence less plastic, the workpiece material, the
better the surface finish.
3 Surface roughness could be improved by increasing cutting speed. Two possible reasons are: (i)
workpiece material presents less plastic behaviour at higher deformation velocity and (ii) flank
wear scar becomes smoother at higher cutting speed.
4 Better surface finish could be produced using the tool with certain degree of tool wear, which
increased the tool nose radius. Excessive tool wear however resulted rough surface..
ACKNOWLEDGEMENTS
The authors would like to thank General Electric Co, USA, Shanxi Natural Science Foundation PRC
and Taiyuan Heavy Machinery Plant, PRC for funding the work.
REFERNCES
1. Y.Matsumoto, et al; Effect of machining process on the fatigue strength of hardened AISI3040
steel, Trans. ASME. Journal of Engineering for Industry, May 1991, Vol 113, P154-159.
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2. A.A.Panov; Intensifying components machining by means of tools provided with synthetic
superhard materials and ceramics, Soviet Engineering Research, V9 (1989) n11 P45-49.
3. N.G.Boim and I.N.Sokolov; The use of super-hard material and ceramic cutting tools in
machine tool construction, Soviet Engineering Research, V4 (1984) n7 P55-56.
4. South China Institute of Technology (editor); Principles of Metal Cutting and Design of
Cutting Tools (in Chinese), V1, Shanghai Science and Technology Press, 1979.
5. E.M.Trent; Metal Cutting (3rd edition), Butterworth- Heinemann Ltd, 1991.
6. E.Usui; The Principles of Cutting and Grinding (Chinese version translated from Japanese by
X.Gao and D.Liu), Machinery Industry Press, 1982.
7. H.Liu; Mechanics of Materials (in Chinese), The People's Education Press, 1979.
8. V.Solaja; Wear of carbide tools and surface finish generated in finish turning of steel, Wear,
V2 (1958/59) P40-58.
9. C.T.Ansell and J.Taylor; the surface finishing properties of carbide and ceramic cutting tools,
Proc. 3rd Int. MTDR Conf. 1962,
Figure 1 Cutting forces vs cutting regime and edge chamfer using CBN1.
(a) f = 0.15 mm/rev. ap = 0.05 mm (b) v = 95 m/min, ap = 0.05 mm
(c) v = 95 m/min, f = 0.15 mm/rev.
Figure 2 Cutting forces vs feed rate and tool nose radius using CBN2.
v = 85 m/min, ap = 0.1 mm
Figure 3 Cutting forces vs tool wear and edge chamfer with CBN1.
v = 95 m/min, f = 0.15 mm/rev. ap = 0.05 mm
Figure 4 The maximum cutting edge angle with a large tool nose radius and small depth of cut
Figure 5 The influence of cutting edge angle on the direction of Fxy
Figure 6 Surface roughness vs workpiece hardness
(a) By CBN1 tools (b) By CBN2 tools
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Figure 7 Additional increase of surface roughness caused by lateral plastic flow
Figure 8 Surface roughness vs cutting speed and tool geometry with CBN1
f = 0.1 mm/rev. ap = 0.1 mm
Figure 9 Wear scar of CBN 1 tool
v= 82.5 m/min, f = 0.2 mm/rev, ap = 0.025 mm
Figure 10 Wear scar of CBN1 tool
v = 145 m/min, f = 0.2 mm/rev, ap
= 0.025 mm.
Figure 11 Surface roughness vs feed rate and tool nose radius
v = 85 m/min, ap = 0.1 mm
Figure 12 Surface roughness vs tool wear
1. v= 82.5 m/min, f = 0.2 mm/rev, ap = 0.025 mm
2. v= 121 m/min, f = 0.1 mm/rev, ap = 0.1 mm
(a)
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(b)
(c)
Table 1 The tool inserts used in the tests
Tools CBN 1 CBN 2
Material CBN>90vol%
metallic binder
grain size 3m
CBN>90vol%
+Co,Fe,W
grain size 50 -
100 m
Major cutting edge angle() 7590(1) 75
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Orthogonal rake angle() -7 -7
Clearance angle() 7 7
Inclination angle () -5 -5
Nose radius (mm) 1.2 0.3
Chamfer 0/0.5mm 10 0
(1) Because the depth of cut was far smaller than the nose radius, the major cutting edge angle
was not practically functional.
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p
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Figure 7
Figure 8
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X60 X1200
Figure 9
X60 X1200
Figure 10
Figure 11
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1
2
Figure 12
International Journal of Machine Tools & Manufacture, 2000, 40 (3), 455-466, EI
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