Development of novel tool for cutting of carbon-fiber-reinforced plastics (Positive use of abrasive wear at tool edge for reduction in cutting force)

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This paper describes a novel design concept for a tool for cutting carbon-fiber-reinforced plastic (CFRP) composites. The cutting tool, which is termed a two-layer tool, was fabricated from two materials with a spatial distribution of hardness around the tool edge. In the two-layer tool, the rake face is made of a material with a relatively high wear resistance, i.e., polycrystalline diamond (PCD), whereas the flank face is made of a material with a relatively low wear resistance, i.e., tungsten carbide (WC-Co). The results of milling tests conducted with a unidirectional CFRP laminate and the two-layer tool showed that the existence of a hardness distribution works to reduce cutting forces, because the wear process of the two-layer tool develops with a constant roundness at the tool edge over relatively long cutting distances. A simplified model was developed to describe how friction force is reduced. Increasing the difference between the wear resistances of the PCD layer and the WC-Co substrate and increasing the clearance angle of the tool edge were found to be effective in reducing the cutting forces. The finding of this study will be helpful in the development of novel design concepts for extending the life of tools for cutting CFRP composites.

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# _Mechanical Engineering Journal_

## Development of novel tool for cutting of carbon-fiber-reinforced plastics (Positive use of abrasive wear at tool edge for reduction in cutting force)

Satoru MAEGAWA*, Shinya HAYAKAWA*, Fumihiro ITOIGAWA* and Takashi NAKAMURA*

*Nagoya Institute of Technology
Gokiso-cho Showa-ku Nagoya, Aichi 466-8555, Japan

[E-mail: maegawa.satoru@nitech.ac.jp](mailto:maegawa.satou@jnitech.ac.jp)

Received 1 June 2015

Abstract
This paper describes a novel design concept for a tool for cutting carbon-fiber-reinforced plastic (CFRP)
composites. The cutting tool, which is termed a two-layer tool, was fabricated from two materials with a spatial
distribution of hardness around the tool edge. In the two-layer tool, the rake face is made of a material with a
relatively high wear resistance, i.e., polycrystalline diamond (PCD), whereas the flank face is made of a
material with a relatively low wear resistance, i.e., tungsten carbide (WC-Co). The results of milling tests
conducted with a unidirectional CFRP laminate and the two-layer tool showed that the existence of a hardness
distribution works to reduce cutting forces, because the wear process of the two-layer tool develops with a
constant roundness at the tool edge over relatively long cutting distances. A simplified model was developed to
describe how friction force is reduced. Increasing the difference between the wear resistances of the PCD layer
and the WC-Co substrate and increasing the clearance angle of the tool edge were found to be effective in
reducing the cutting forces. The finding of this study will be helpful in the development of novel design
concepts for extending the life of tools for cutting CFRP composites.

_Key words:_ Carbon-fiber-reinforced plastics, Life of tool, Tool wear, Polycrystalline diamond, Abrasive wear,

Cutting forces

1. Introduction

Carbon-fiber-reinforced plastic (CFRP) composites are widely used because of their excellent mechanical
properties, such as their high-strength-to-weight ratios, high modulus-to-weight ratios, high damping capacities, and
good dimensional stability. However, severe tool wear of tools during machining of CFRP composites is a serious
problem (Sheikh-Ahmad, 2009; Dandekar and Shin, 2012; Che, et al., 2014) that increases the cost of machining CFRP
products.

The choice of material is the most important factor in the life of the tools used in cutting CFRP composites. In the
machining of CFRP composites, the abrasive wear on the flank face characterizes tool wear processes. Therefore,
high-hardness materials, such as polycrystalline diamond (PCD) and chemical vapor deposition (CVD) diamond, are
commonly used in CFRP cutting tools (Sakuma, et al., 1985; Ferreira, et al., 1999; Ferreira, et al., 2001; Lantrip, 2008;
Chatelain and Zaghbani, 2012).

In an effort for extending the life of cutting tool that is used in CFRP machining, our group has been focusing on
the positive use of the wear at the tool edge (Maegawa, et al., 2015). According to this concept, a two-layer tool with a
hardness distribution around the tool edge can be used to maintain a constant roundness at the tool edge over a long
cutting distance. The whole of the rake face is made of relatively high-hardness materials, i.e., PCD or CVD diamond,
and a large part of the flank face is made of WC-Co. The results of some experiments have shown that the rate of
increase in the cutting forces due to the progression of tool wear in a two-layer tool is smaller than that in a
conventional single-material tool.

Paper No.15-00295
J-STAGE Advance Publication date: 24 August, 2015

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

In this paper, we describe a method to enhance the cutting force reduction effect of the proposed two-layer tool.
Using three types of two-layer tools, each made of a PCD layer and a WC-Co substrate, milling tests were conducted
using a unidirectional CFRP laminate. Measurements of changes in the cutting forces and the edge profile during the
milling tests showed that the wear resistance ratio (which is defined as the ratio of the wear resistance of the PCD layer
to that of the WC-Co substrate) and the clearance angle of the tool edge are very important parameters influencing the
cutting forces. Increasing the wear resistance ratio and the clearance angle can enhance the cutting force reduction
effect. Based on the experimental results, a simplified model for the wear process of the two-layer tool that clarifies the
method for enhancing the cutting force reduction effect was developed.

2. Cutting force reduction effect of two-layer tool in cutting of CFRP composites

Previous research results demonstrating the cutting force reduction effect (Maegawa, et al., 2015), in which time
changes in cutting forces and shape of tool edge during a two dimensional milling were monitored, are shown in Fig. 1.
The left side of the figure shows changes in the average value of the cutting force parallel to the feed direction, _F_ X, and
the right side shows changes in the average value of the cutting force perpendicular to the feed direction, _F_ Y. It should
be noted that “ _F_ Y” in Fig. 1 corresponds to “ _F_ Z” in this current paper, because in this paper the Z axis is defined as the
perpendicular direction (see Fig. 5). In these experiments, three types of tools were used, as illustrated in the figure;
Tool I and Tool II were made of single materials, WC-Co and PCD, respectively. Tool III was made of two materials
with different hardnesses, i.e., a PCD layer on the rake face and a WC-Co substrate. Tool III corresponds to the
two-layer tool discussed in this paper.

As Fig. 1 shows, for all three of these cutting tools, the cutting forces _F_ X and _F_ Y gradually increase with increasing
cutting distance. This results from the loss of initial sharpness of the tool edge due to the progression of tool wear. The
wear resistance of Tool II and Tool III (made of PCD and PCD/WC-Co, respectively) is higher than that of Tool I
(made of WC-Co), and the cutting forces for Tool I increase much more rapidly than those of Tool II and Tool III.
Interestingly, although most of the flank face and the bulk regions are made of a relatively low-hardness material, i.e.,
WC-Co, the rate of increase in the cutting forces is lower for Tool III than for Tool II. This implies that the life of Tool
III is higher than that of Tool II. These results indicate that the use of a relatively low-hardness material can reduce the
rate of increase of the cutting forces. In this study, this is termed as “cutting force reduction effect”.

The mechanism of the cutting force reduction effect is shown schematically in Fig. 2, which illustrates the
difference between the wear processes around the tool edge of a conventional tool made of a single material and a
two-layer tool made of two materials with different hardnesses. In the figure, four profiles of worn edges at cutting
distances _L_ = _L_ 1, _L_ 2, _L_ 3, and _L_ 4, are illustrated, where _L_ 1 < _L_ 2 < _L_ 3 < _L_ 4. In both tools, the initial tool edge is gradually
dulled with cutting distance due to the progression of abrasive wear on the flank face. In the single-material tool,
illustrated in Fig. 2(a), a large curvature (roundness) of the flank face, which is denoted as _R_ in the figure, is formed.
The shape of the curvature is determined by the spatial distribution of the normal pressure acting on the flank face. In
contrast, in the case of the two-layer tool, illustrated in Fig. 2(b), severe abrasive wear occurs locally in the region of
the lower-wear-resistance material and reduces the curvature of the flank face. Consequently, the thrust cutting force

Fig. 1 Change in cutting force with cutting distance for three different cutting tools (Maegawa, et al., 2015).

[DOI 10 1299/ j 15 00295] © 2015 Th J S i t f M h i l E i

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

Fig. 2 Schematic of the difference in wear processes of (a) a conventional single-material tool and (b) the proposed

two-layer tool made of two materials with different hardnesses.

can be reduced, because the thrust force corresponds to the force needed to press the material under the tool, i.e., the
burnishing force. Small roundness decreases the pressing region, and it reduces the thrust cutting force. This is the
“cutting force reduction effect” suggested in this study. The figure also denotes the difference between the position of
the bottom faces at the high wear resistance layer and the low wear resistance substrate, i.e., Δ _P_ L2, Δ _P_ L3, and Δ _P_ L4.
These values are important factor to determine the cutting force reduction effect, which will be discussed at later.

It should be noted that the total wear volume of the conventional single-material tool shown in Fig. 2(a) is smaller
than that of the two-layer tool shown in Fig. 2(b). The reason for the difference in the total wear volumes is that the
single-material tool is made entirely of a relatively high-wear-resistance material, whereas the substrate of the
two-layer tool is made of a relatively low-wear-resistance material. Therefore, the wear volume at the tool edge is not
the only parameter that determines the life of the tool. It should be noted that, of course, the recession of the cutting
edge which rises with increasing tool wear results in a dimension error. However, the dimension error can be reduced
by changing the depth of cut with changes in cutting distance using a CNC control. In this case, it is needed to estimate
the relationship between the recession distance of the cutting edge and cutting distance in advance.

3. Experiment

3.1 Experimental setup

Figure 3 shows a schematic illustration of the experimental setup used in this study. A vertical machining center
was used for the milling tests performed. As Fig. 3 shows, a unidirectional CFRP laminate with a thickness of 3 mm
was fixed on a steel vise. The steel vise was fixed on a machining pallet via a three-directional dynamometer that was
used to monitor the cutting forces during the milling tests. Each tested cutting tool was fixed on a main spindle via a
tool holder. By changing the rotation and position of the main spindle of the machining center, the tool could be rotated
at a constant rotation speed and moved at a constant feed rate. For all the experiments performed in this study, the
cutting speed _V_ and feed rate _F_ were set to 30 m/min and 0.05 mm/revolution, which are in typical cutting conditions of
CFRP composites. However, in many cases, the cutting is performed in higher cutting speeds. In this study, in order to
avoid thermal effects on the cutting tool and the CFRP materials (Caprino, et al., 1996), experiments were performed at
a comparatively low cutting speed.

3.2 Specimens

Multidirectional CFRP laminates have been used in many studies of cutting tools to ensure uniform mechanical
properties in all directions. The use of unidirectional CFRP laminates simplifies the analysis; it is easy to understand
the mechanical interaction between a unidirectional CFRP laminate and a cutting tool. The mechanical response and

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

cutting mechanism are highly dependent on the relative angle between the orientation of the carbon fibers and the
traveling direction of the tool edge (Sakuma and Seto, 1983; Kaneeda, 1991; Wang, et al., 1995; Sreejith, et al., 2000)
However, this study was focused on the effects of the type of material and the clearance angle of a two-layer tool on the
reduction in cutting forces. In most of the tests conducted in this study, a unidirectional CFRP laminate with a thickness
of 3 mm was used. The orientation direction of the carbon fibers in the CFRP laminates was set to be perpendicular to
the feed direction of the tool edge. In the laminates, PAN-based carbon fibers are embedded in epoxy resin; tensile
strength, tensile elasticity, and a diameter of the carbon fibers are 4,900 MPa, 240 GPa, and 7.0 μm, respectively. In
order to avoid the effect of dramatic deteriorations in the base material of the CFRP laminates, the CFRP laminate was
sandwiched between two plastic plates or aluminum plates with a thickness of 1 mm. The results of preliminary
experiments showed that the forces required to cut these plates were much smaller than those observed in the cutting of
the CFRP laminates. Therefore, the cutting forces for the plastic plates can be neglected in the analyses.

3.3 Prepared cutting tools

Figure 4 lists the geometries of the cutting tools used in this study. Tools A, C, and E were made of PCD. The rake
face of Tools B, D, and F were made of PCD, and most of their flank faces and substrates were made of WC-Co (K10
grade). These three tools represent examples of two-layer tools. The size _d_ of the synthetic diamond grains of the PCD
regions of Tools A through D was 10 µm, and that of Tools E and F was 1 µm. The grain size is typically an important
factor in the resistance to abrasive wear during cutting of CFRP composites. Therefore, by comparing the differences in
the wear processes of these tools, the effect of the difference in the wear resistances of the PCD layer and the WC-Co
substrate on the life of the tools could be investigated. The other mechanical properties of PCD (10 µm), PCD (1 µm)
and WC-Co (K10) were as follows. The densities of them are 4.08, 4.37, and 14.7 g/cm 3, respectively. The fracture
toughness of them are 8.96, 13.4 and 10.50 MPa m 1/2, respectively. In this study, the thicknesses of the PCD layers
were determined to avoid accidental chipping of the tool edge. From preliminary experiments, it was found that when a
PCD layer with a smaller thickness is used, a large scale chipping of the PCD layer was observed. The PCD layer was
integrally bonded to the WC-Co substrate (PCD was sintered on the WC-Co substrate under extreme pressures and
temperatures), so that it did not peel off from the substrate during the milling tests. The clearance angle of Tools A and
B was 10°, while that of the other tools was 20°. It should be noted that the performance of Tools A and B was
examined in a previous study (Maegawa, et al., 2015). For Tools C through F, the radius of the tool edge was set to be
approximately 2–3 μm using a pulse laser grinding technique, which creates a sharp edge without mechanical defects or
cracks (Mabuchi, et al., 2015).

3.4 Experimental procedure

In the previous research by our group (Morikawa, et al., 2013), it was found that the cutting forces under a
down-milling process at _θ_ = 0° is larger than other conditions, i.e., down-milling at _θ_ = 90°, up-milling at _θ_ = 0°, and
up-milling at _θ_ = 90°. Thus, the reduction of cutting forces is most needed and expected under a down-milling process

Fig. 3 Schematic of the experimental setup: (a) top view and (b) side view.

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

Fig. 4 Geometries of prepared cutting tools; _H_ : thickness of PCD layer, _d_ : grain size of diamond in PCD.

at _θ_ = 0°. From this reason, in the most of experiments in this study, the plane down- milling process was investigated.

Figure 5 shows the configuration of the plane down-milling process used in this study. The edge of the tool rotates
at a constant speed _V,_ and it travels forward in the direction shown in the left of the figure at a constant feed rate of _F_ .
The undeformed chip thickness _h_ varies continuously with the change in the engagement angle _φ_ . The tilted thick black
lines in the figure indicate the orientation direction of the carbon fibers in the CFRP laminates. As described in the
previous section, most of our experiments were performed with _θ_ = 0°.

During the milling tests, the cutting forces in the _X_ and _Z_ directions, _F_ X and _F_ Z, were measured at a sampling rate
of 10 kHz using the dynamometer. At predetermined values of the cutting distance _L_, the edge profile of the tool edge
was measured with a surface roughness meter. In addition, the width of flank wear was measured from microscope
images.

To better understand the relationship between the cutting forces and the position of the tool edge, _F_ X and _F_ Z were
transformed into principle and thrust forces, _F_ P and _F_ T, as shown in Fig. 5, using the following equations:

?P = ?Xcos?+ ?Zsin?
?T = −?Xsin?+ ?Zcos? (1)

where _φ_ is the engagement angle.
The relative angle _θ_ rel between the orientation of the carbon fibers and the tool traveling direction during a single
milling pass is described using the engagement angle _φ_ as follows:

?rel = ?−? (2)

The engagement angle _φ_ changes during a single pass from _φ_ = _φ_ max = 24.62° (at the start position of the single pass)
to _φ_ = _φ_ min = 0° (at the end position of the single pass). Similarly, the undeformed chip thickness _h_ varies continuously
with the changing _φ_ as follows (Shaw, 1984);

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

Fig. 5 Configuration of down-milling process for CFRP composites.

ℎ= ?sin? (3)

where _f_ is the feed per tooth, which was set to 50 μm.

4. Experimental results

4.1 Changes in cutting forces during a single pass

Figure 6 shows typical results for the changes in the cutting forces _F_ P and _F_ T during single milling passes at cutting
distances _L_ = 10, 50, 100, 150, and 200 m for Tool A. At the onset of cutting, i.e., at _φ_ = _φ_ max = 24.62°, the cutting
forces _F_ P and _F_ T suddenly increase, and then they gradually decrease over time. This tendency is consistent with the
changes in the undeformed chip thickness _h_ during the down-milling process. As described by Eq. (3), _h_ gradually
decreases with decreasing _φ_ . The increases in the cutting forces _F_ P and _F_ T indicate that the initial sharpness of the tool
edge is gradually dulled due to the progression of tool wear.

At all cutting distances, the values of _F_ P were found to be much smaller than those of _F_ T. As noted by Koplev
(1983), the principal force corresponds to the sum of the forces needed to make a cutting chip and the friction force
acting on the flank face of the tool. The former, i.e., the chip formation force, typically does not depend on the
progression of tool wear. Therefore, small changes in _F_ P result from changes in the friction force, which can be
expressed as the product of the normal load and the friction coefficient. In contrast, the thrust force represents the
deformation resistance of the CFRP substrate. The value of the thrust force corresponds to the force needed to press the
material under the tool, i.e., the burnishing force. Therefore, as described in Fig. 2, _F_ T increases with increasing
penetration volume, which is determined by the roundness of the flank face of the cutting tool.

4.2 Changes in average cutting forces

To describe the dependence of the cutting forces on the cutting distance, we calculated the averaged cutting forces
over single passes, _F_ Pave and _F_ Tave, as follows:

?Pave = ∫? ? P??

? P?? ?Tave = ∫? ? T??

? T?? _,_ (4)

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

where _T_ is the time period of a single pass.

Figure 7 summarizes the changes in the average cutting forces, _F_ Pave and _F_ Tave over the cutting distance. The open
circles indicate the changes in the cutting forces for Tools A, C, and E (single-material tools), while the solid circles

Fig. 6 Typical results for changes in cutting forces during single passes at _L_ = 10, 50, 100, 150, and 200 m with Tool A.

Fig. 7 Changes in averaged cutting forces with increasing cutting distance; (a) open circles: Tool A, solid circles: Tool

B, (b) open circles: Tool C, solid circles: Tool D, and (c) open circles: Tool E, solid circles: Tool F.

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

indicate the changes in the cutting forces for Tools B, D, and F (two-layer tools). It should be noted that only for Tool A
was the cutting test terminated at a cutting distance _L_ of 200 m.

In general, the cutting forces of the single-material tools, i.e., Tools A, C, and E, were larger than these of the
two-layer tools, i.e., Tools B, D, and F. Thus, the reduction in the cutting forces of the two-layer tools, i.e., cuttign force
reduction effect, occurred for all two-layer tools, despite the differences in the composition of the PCD layer and the
clearance angle. In addition, a comparison of Figs. 7(a) and (b) or Figs. 7(b) and (c) shows that an increase in the
clearance angle and a decrease in the diamond grain size of the PCD are both effective in enhancing the reduction
effect of cutting forces. In addition, it was found that differences in the cutting forces between single-material tools and
two-layer tools occur after a certain cutting distance, as indicated by the down-pointing arrows in Fig. 7.

4.3 Measurement of changes in edge profile

Figure 8 shows changes in the profile of the tool edge. It should be noted that in Fig. 8(a), i.e., for Tool A, the edge
profiles were drawn only at cutting distances of _L_ = 50, 100, 200 m, because the cutting test was stopped befoer _L_ =
300 m. In contrast, in Figs. 8(b) to (f), i.e., for Tools B to F, the profiles on the flank face were drawn at _L_ = 50, 100,
200, 300, and 400 m.
The edge profiles shown in Fig. 8 conform to the shape shown in Fig. 2. In single-material tools, as shown in Figs.
8(a), (c), and (e), the tool edge has a large curvature, which corresponds to _R_ in Fig. 2. In contrast, in two-layer tools, as
shown in Figs. 8(b), (d), and (f), the position of the bottom face of the WC-Co substrate is located at a relatively high
position compared to the bottom faces of the PCD layer. This is the result of the difference between the abrasive wear
resistance of the PCD and that of the WC-Co region. For two-layer tools, abrasive wear occurs locally in the WC-Co
region. These results are consistent with those shown in Fig. 7: for the two-layer tools the thrust forces are smaller than

Fig. 8 Changes in profile of tool edge; (a) Tool A, (b) Tool B, (c) Tool C, (d) Tool D, (e) Tool E, and (f) Tool F.

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

for the single-material tools. Thus, as Fig. 2 shows, for two-layer tools, a small roundness at the tool edge is maintained
for a relatively long cutting distance, leading to slow the increase in cutting forces.

Figures 8(c) and (e) show that the wear volume of Tool E, which was fabricated of PCD with a grain size _d_ of 1
µm, was smaller than that of Tool C, which was fabricated of PCD with a grain size _d_ of 10 µm. These results show
that the abrasive wear resistance of PCD at a grain size of _d_ = 1 µm is higher than that of PCD at a grain size of _d_ = 10
µm. Thus, the wear ratio of the wear resistances of the PCD and WC-Co regions in Tool F is larger than that in Tool D.

5. Discussion

Figure 9 shows the changes in the wear width at the flank face of Tools A and B. There is a large difference
between the wear widths of Tools A and B beyond a cutting distance _L_ of 50 m. This means that at _L_ = 50 m, the wear
width due to abrasive wear on the flank face extends to the WC-Co region in Tool B. Beyond that point, in Tool B,
severe abrasive wear occurs in the WC-Co region, and the rate of increase of the wear width of Tool B is larger than
that of Tool A. Thus, the local abrasive wear occurs beyond _L_ = 50 m. These findings are consistent with the results
shown in Fig. 7, i.e., the differences in the cutting forces for Tools A and B are evident beyond _L_ = 50 m. The reduction
in the cutting forces was found to be attributable to the changes in the edge profile due to severe local abrasive wear on
the flank face of Tool B.

In order to investigate the effect of the grain size of PCD on the cutting force reduction effect, the thrust force ratio
_λ_, which is defined as the ratio of _F_ Tave for a two-layer tool (Tool D or F) to _F_ Tave for a corresponding single-material
tool (Tool C or E), was derived as shown in Fig. 10. The open circles indicate the changes in _λ_ = _F_ Tave for Tool D / _F_ Tave
for Tool C over the cutting distance _L_, and the solid circles indicate the changes in _λ_ = _F_ Tave for Tool F / _F_ Tave for Tool E.
Before _L_ = 100 m, _λ_ has a value of one for both conditions. This means that the effect of reducing the cutting forces of
the two-layer tools does not occur in the early stage of cutting. After _L_ = 100 m, the value of _λ_ gradually decreases with
cutting distance. It means that the reduction effect of cutting forces appears after _L_ = 100 m; it is consistent with the
results in Figs. 7(b) and (c). A visible large difference in cutting forces between single materials and two-layer tools
were observed after _L_ = 150 m, which are denoted as black arrows in the figure. Focusing on values under large _L_, it is
found that they approach to different values. Thus, we can clearly see the difference, denoted by Δ _λ_, at large _L_ values.
The reducing effect between Tools E and F was found to be larger than that between Tools C and D. Thus, a decrease in
the diamond grain size enhances the cutting force reduction effect. Furthermore, it was found that the difference
between the two layer tools with different grain size were observed after a certain cutting distance, i.e., approximately
250 m. The reason is not clear. However, after _L_ = 300 m, the value of Δ _λ_, keeps a constant value. It implies that the
difference of the cutting forces is observed under a certain equilibrium condition.

Figure 11 illustrates a simplified model of the wear process for the two-layer tools, i.e., Tools B, D, and F. The edge

Fig. 9 Changes in flank wear width for Tool A and Tool B.

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

Fig. 10 Changes in thrust force ratio _λ_ with cutting distance; open circles: ratio of _F_ Tave for Tool D to _F_ Tave for Tool C,

and solid circles: ratio of _F_ Tave for Tool F to _F_ Tave for Tool E.

Fig. 11 Schematic of simplified model of wear process for proposed two-layer tool.

of the tool consists of two different regions: region I, which is a high-hardness layer with high wear resistance, and
region II, which is a lower-hardness substrate with low wear resistance. The contact lengths of regions I and II are _l_ 1
and _l_ 2, respectively. In regions I and II, the normal loads _W_ 1 and _W_ 2 act on the contact interface between the rake face of
the tool and the CFRP substrate. These forces correspond to the restriction forces of the CFRP substrate. The gray areas
indicate the wear volumes Δ _V_ over the sliding distance Δ _S_ . It should be noted that the model is simplified; the wear
widths of the two regions in the vertical direction are treated as equal. It is difficult to base a quantitative analysis on a
simplified model because the simplified model cannot be used to simulate the wear process realistically. However, the
use of a simplified model is sufficient for the purposes of qualitative determination of the factors that influence the
cutting force reduction effect.

Assuming that the wear volume can be characterized by Archard’s wear law (Archard, 1953; Rabinowicz, 1965),
the wear widths in the regions Δ _z_ 1 and Δ _z_ 2 at a sliding distance Δ _S_ can be expressed as follows:

∆?1 =

?1∆S

?1?1 (5)

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

∆?2 =

?2∆S

?2?2 (6)

where _K_ 1 and _K_ 2 are wear constants determined by the wear resistance, depending on the types of materials, and _l_ 1 and
_l_ 2 are the contact lengths of regions I and II, respectively. The model assumes that the wear widths are directly
proportional to the normal loads _W_ 1 and _W_ 2 and that the sliding distance Δ _S_ and inversely proportional to the constants
_K_ 1 and _K_ 2.

When _L_ is a small value, the tool wear develops under a transient condition; here, Δ _Z_ 1 and Δ _Z_ 2 does not have a
same value. The transient wear process was already illustrated in Fig. 2, where the difference between two positions,
i.e., Δ _P_ Li, changes with varying cutting distance, i.e., Δ _P_ L2 ≠ Δ _P_ L3. This is also shown in Fig. 8. In order to develop the
cutting force reduction effect, therefore, it is required to increase Δ _P_ ; Δ _Z_ 2 > Δ _Z_ 1. The result is consistent with the
discussion in Fig. 2 that _K_ 1 > _K_ 2 is effective to avoid for decreasing the roundness of tool edge. After that, the wear
process reaches to an equilibrium condition, because the valance of the normal loads changes with developing tool
wear. When the position of the bottom face at region I is much higher than that at region II, _W_ 1 will be much larger than
_W_ 2. During the equilibrium condition (i.e., large _L_ condition), the values of Δz1 and Δz2 should be identical, i.e., Δz1 =
Δz2. Thus, using Eqs. (5) and (6), we can obtain following relationship:

?1
?1?1 =

?2
?2?2 (7)

The total thrust force _F_ T can be expressed as follows:

?T = ?1 + ?2 (8)

Using Eqs. (5) and (6), Eq. (8) can be written as follows:

?2
?1 ~~)~~ (9)

?T = ?1(1 +

?2
?1

As Eq. (9) shows, the dimensionless ratio _K_ 2/ _K_ 1, which corresponds to the ratio of the wear resistance in region I to
the wear resistance in region II, is an important factor in characterizing the thrust force _F_ T under the equilibrium
condition. In general, the used of a high-wear-resistance material reduces the volume of tool wear and slows the rate of
increase in the cutting forces. However, in the case of two-layer tools, the use of a low-wear-resistance material
decreases the thrust force. For example, it is expected that the combination of diamond layer and the high-speed steel is
more effective for enhancing the cutting force reduction effect. In addition, an increase in the clearance angle
corresponds to a decrease in the contact length _l_ 2. Thus, the simplified model developed in this study can explain the
results shown in Fig. 7.

In a typical case of CFRP cutting, the orientation direction of the carbon fibers is an important factor in
determining the wear process during the cutting process (Sakuma and Seto, 1983; Kaneeda, 1991; Wang, et al., 1995;
Sreejith, et al., 2000). In addition, the cutting configuration (i.e., up or down cut) is also an important parameter. Figure
12 summarizes the tool edge profiles after a cutting distance of 8 m (Morikawa, et al., 2013). The orientation direction
of the carbon fibers ( _θ_ ) and the cutting configuration were varied. In the images, the edge profiles are expressed using
the shape of the mold. After the cutting tests, the worn tools were molded by a thermosetting plastic. After the mold
was hardened, the tool was removed, and the cross section of the mold without the tool was examined using a
microscope. It should be noted that the entire tool was made of high-speed steel for the purpose of performing
acceleration tests.

Of cause, the wear processes of the high-speed steel material and other high hardness materials, e.g., PCD or
WC-Co, are not same. However, it is easily expected that the wear process of these materials will also strongly depend
on the cutting configuration (up cut or down cut) and orientation direction, if not exactly the same as the wear process
in high-speed steel tools. Thus, this implies that the magnitude of the cutting force reduction effect also depends on the
orientation direction and cutting configuration. Therefore, additional experiments and analyses are required to develop
a more realistic design guide for two-layer cutting tools. However, these experiments and analyses were beyond the
scope of this study, which was focused on the basic design of a two-layer tool.

Maegawa, Hayakawa, Itoigawa and Nakamura, Mechanical Engineering Journal, Vol.2, No.6 (2015)

Fig. 12 Profiles of tool edge depending on cutting configuration (up or down cut) and orientation direction of carbon

fibers _θ_ ; material of tool: high-speed steel, cutting distance _L_ : 8 m.

6. Conclusion

This study examined the cutting force reduction effect in two-layer tools used to cut carbon-fiber-reinforced plastic
(CFRP) composites. The rake face and a portion of each of the two-layer tools used in this study were made of a
material with a relatively high wear resistance, i.e., polycrystalline diamond (PCD). In contrast, the other region of the
flank face of each tool was made of a material with relatively low wear resistance, i.e., tungsten carbide (WC-Co). The
results of milling tests performed using unidirectional CFRP laminates and two-layer tools showed that the use of a
two-layer tool can reduce the thrust cutting force by taking advantage of abrasive wear on the flank face of the tool.
The wear resistance ratio, defined as the ratio of the wear resistance of the PCD layer to the wear resistance of the
WC-Co substrate, and the clearance angle of the tool were found to be important factors in the reduction of cutting
forces. A simplified model for the wear process of a two-layer tool was developed to describe how to enhance the
cutting force reduction effect of a two-layer tool. The findings of this study indicate that it is possible to take advantage
of the abrasive wear on the flank face of a cutting tool to reduce the cutting force without improvement of the tool
material.

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