Invited paper

Experimental Study on Inclined Planetary Mechanism Drillingfor Carbon Fiber Reinforced Plastic
Hidetake Tanaka1, Kazuki Ota1, Ryo Takizawa2, and Kazuhisa Yanagi1
1

Nagaoka University of Technology, Nagaoka, Japan
2
N.S.S Co., Ojiya, Japan

Abstract – The study deals with a proposal of a new drilling technique for CFRP. The authors have modified cutting mechanism
principle of the orbital drilling, named inclined planetary drilling. Its axis of tool rotation spindle is not parallel to the axis of planetary
revolution. The inclined angle is adjustable from 0 to 2 degrees. Because of tool rotation axis is inclined, penetration is not caused byoutermost cutting edges but inner cutting edges. If penetration could be occurred, the inner cutting edges penetrate firstly, then theoutermost edges enlarge drilled hole and these sequence can avoid generation of delaminations and burrs. The inclined planetarydrilling has been demonstrated by the fundamental drilling experiment by use of a high-speed spindle unit and a lathe. And practical
inclined planetary mechanism drilling system has developed. From the experimental result, capability of inclined planetary drillinghas been realized and no burrs and delaminations are observed on the workpiece.
Key Words: CFRP, Planetary drilling, Inclined milling, Spindle

1. Introduction
Recently, CFRP (Carbon Fiber Reinforced Plastic) are widely usedas structural material in aviation industries because of their

inclined planetary motion. Furthermore, a novel spindle unit ofinclined planetary mechanism drilling has been developed on thebasis of the fundamental research results.

advantages such as lightweight and high strength. However, those

From the experimental result, capability of inclined planetary

materials are mostly difficult to machine in conventional drilling

drilling has been realized and no burrs and delaminations are observed

techniques. The orbital drilling [1][2] has been realized as one ofon the workpiece.

these solutions. The orbital drilling is similar to helical milling bymachining center. In the case of the orbital drilling a cutting tool is

2. Cutting edge motion of planetary drilling

settled on a tool rotation spindle eccentrically from a planetary

We focused on a bottom edge motion on a cutting tool of the orbital

revolution spindle and it rotates independently. The holes machined

drilling which may inferentially affect machinability of CFRP drilling.

by the orbital drilling have higher roundness than holes machined by

A cutting edge on the bottom face is expressed as 2 position vectors

helical milling or conventional drilling.

summation as show in figure 1.

The authors had established cutting model of the orbital drilling

The velocity of a cutting edge on the bottom face depends onand clarified its machinability and proposed the cutting tools having

planetary revolution speed and tool rotation speed. Distribution of theparticular cutting edge for CFRP drilling according to the pastvelocity is not constant as shown in figure 2.

studies.[3-5] The orbital drilling has some drawbacks, e.g.,

An angle between the line to tool center to planetary revolution

delaminations of workpiece by its cutting principle and mechanicalcenter and the line to tool center to the point is named angularvibration.

difference and it can be also a significant factor. Direction of tool

In order to bring solutions to the orbital drilling technique, theauthors have modified cutting mechanism principle of the orbital

rotation around planetary revolution center is CCW and direction oftool rotation speed on its center axis is CW.

drilling which named as the inclined planetary drilling. Its axis of tool

Cutting edge

rotation is not parallel to the axis of planetary revolution. The inclined
angle is adjustable from 0 to 2 degrees. In the case of the orbital
drilling, the outermost cutting edges penetrate workpiece and thebottom layer is delaminated. On the other hand, in the case of thertinclined planetary drilling, penetration is caused by the inner cuttingedges not the outermost cutting edges because of inclined tool rotation

Tool center

axis. When penetration occurred, the inner cutting edges penetratefirstly then the outermost edges enlarge the drilled hole and thesequence can avoid generation of delaminations and burrs. In addition,

Planetaryrorevolution

inclined tool axis can reduce cutting force and mechanical vibration
by its principle. Above-mentioned properties of the inclined planetarydrilling have been demonstrated by the practical fundamental drilling
experiment by use of a high-speed spindle unit and a lathe as imitating

Fig. 1 Geometrical model of orbital drilling cutting edge

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

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Invited paper

Velocity of the cutting edge by tool rotation (vs) is defined by thePlanetary revolution motioneq. (1) where rt stands for tool radius and Tss stands for tool rotation
speed.
vs = 2 Tss rt

Planetary
revolution
center(1)

Angular difference
Tool center
Spindlerotation
Spindle rotation
Tool center

Velocity of the point by planetary revolution speed (vo) is definedby the eq. (2) where ro stands for eccentricity and Os stands forplanetary revolution speed.

(a) Cutting edge on the bottom facevo = 2 Os ro(2)

From eq. (1) and (2), the velocity of cutting edge is calculated andthe result is shown in figure 2. In the case of these parameters, Os andro are larger than actual orbital drilling parameter in order to ensure
the effect of orbital motion. Figure 2(c) shows the result in 0°. vs andvo cancel them each other when it is no angular difference. The zero
velocity point on bottom face can appear continuously and movingaround on the bottom cutting edge. Therefore it makes thrust loading(b) AngularCutting edge
angle = 45q
difference

larger.
In the case of conventional drilling, drilling resistance becomeslarger at the centre of chisel point where the cutting edge speed

Zero velocity point

theoretically being zero. Beside, the planetary revolution madedirection of the cutting edge velocity reversed at a singular point of
the bottom edge. The radius range where the singular point existing isdefined as counter velocity radius (rn). The counter velocity radius
might cause cutting resistance larger and negative effect to themachinability.
Counter velocity radius (rt) in planetary drilling is defined byplanetary revolution speed (Os), eccentricity (ro) and tool rotation

Counter velocity radius(c) Angular difference = 0q
(㨏) Cutting edge angle = 0q

speed (Tss).
Tool cutting edge speed from planetary motion speed is defined by

1m/secVelocity by tool rotation

the eq. (3).
Velocity by planetary revolution rotation
vo = 2Os( rt +ro )Composition velocity by 2 rotations(3)

Fig. 2 Cutting edge velocity components

If vo becomes equal to vs in zero velocity point,θ

vo = vs(4)

Tool rotation

Tool rotation

Cutting tool2ǸOs( rt +ro )= 2Tss rt

Feed

(5)

Feed
Tool length
Workpiece

Workpiece2ǸOs rn ( 1+ro / rn )= 2Tss rn

Cutting tool

Revolution(6)

Consequently, the counter velocity radius of bottom edge in orbital

Eccentricity

Revolution

Eccentricity

drilling is defined by eq. (7).
rn = ro /(Tss /Os -1 )(7)

The cutting edge can not machine effectively where the inner areaof tool radius = rn on the bottom surface of cutting tool. Thus
machining with the zero cutting velocity point and the inner areaof counter velocity radius should be avoided as far as possible inorder to improve the machinability of the orbital drilling.

(a) Orbital drilling(b) Inclined planetary drillingFig. 3 Schematic view of orbital and inclined planetary drilling
improvements of ball end-mill. Machining at the zero velocity
point(tip of ball end-mill) is avoided by tiled tool rotational axis. We
have adopted the same concept to the planetary drilling motion.
Figure 3 shows differences between the orbital drilling model and ourinnovated inclined planetary drilling. In the case of the inclined
planetary drilling, the tool rotational axis is inclined, in another words,

3. Inclined planetary drilling
The oblique cutting is generally known as one of machinability

24

the tool rotational axis is not parallel to the planetary revolution axis.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

Invited paper

Fig.4 Oblique cutting with ball end-mill
Fig.7 Whole view of experimental apparatusesx

Fig.5 Generation of burrs and delaminations

Fig.8 Geometry of drilling experimental instruments
The diameter of a drilled hole by inclined planetary drilling isdefined by geometrical parameters of the tiled planetary drilling
instrument configuration as shown in figure 6. Significant parameters
are rt; tool radius, TL; tool length and ; inclined angle. The diameterFig.6 Geometrical model of inclined planetary drilling
The counter velocity radius is calculated by eq. (7) with drilling
conditions which are ro, Tss and Os. For example; assuming practicalof a drilled  hole (D) is calculated by eq. (8). Especially thisequation can be used restricted utilization for ball end-mills. Utilizing
a ball end-mill does have control the diameter of the drilled holesimple.

drilling, counter velocity radius rn = 0.02mm with the conditions of

D = 2(rt + TL tan )fundamental experimental result where ro = 2, Tss = 30,000, Os = 300.

(8)Then, figure 4 shows model of oblique cutting with ball end-mill.
Here, tilt angle is  and rnt is radius of first contact position to aworkpiece surface of a ball end-mill. Supposing practical inclined
planetary drilling condition,  = 3 and rt = 3. rn = 0.157mm and it isenough to avoid the counter velocity radius. The inclined planetarydrilling can accordingly improve machinability in stead of thecommercial orbital drilling technique.
The other inevitable problem of the orbital drilling is delaminations
and burrs on the exit edge of drilled holes. By using of the orbital
drilling, delaminating and burrs are generated in principle as shown infigure 5 left. In the case of the orbital drilling, the outermost cuttingedge is the first penetration point of tool on backside of a workpiece.
And thin layer of workpiece can be peeled off when penetrating and

4. Experiment
In order to evaluate capability of the inclined planetary drilling,fundamental drilling experiments by using of a lathe and high-speed
spindle unit have been carried out. Layout of the experimental
apparatuses is shown in figure 7. A jig with a workpiece (CFRP) is
fixed on the main spindle of a lathe and a high-speed spindle unit is
attached to the tool post instead of turning tools. The tool post canadjust tilting angle against feed direction. In this case, the main
spindle rotation of the lathe performs as the planetary revolution.
Eccentricity x is calculated by eq. (9) as shown in figure 8.
x = L2 sin  – L1(1-cos )(9)that causes delamination and burr on the edge. On the other hand in
the case of inclined planetary drilling, the first penetration is notcaused by the outermost cutting edge but inner cutting edge. If
delaminating and burrs could be occurred when penetrating, theoutermost cutting edge would make the drilled hole deburred asshown in figure 5 right.

CFRP (PAN-Based Carbon Fiber; Specific gravity: 1.5, Tensile
strength: 1.6kN, Young̉s modulus: 120GPa, Glass transition point:
120 ͠ ) plates (50mm square, thickness: 5mm) are prepared as
experimental workpieces. In this case, inclined angle is 2 degree assame as the eccentricity: 2mm. The experiment has been carried out

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

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Invited paper

Table 2 Surface roughness of drilled holes
Ra [m]Rz [m]Table 1 Experimental conditions
Cutting tool (Cemented carbide)
R=4mm, ball end-mill
Eccentricity (Inclined angle)

2 mm (2q)

Inclined planetary drilling

1.17

8.41

Planetary revolution speed

560 min-1

Planetary drilling

1.20

12.05

Tool rotation speed

28000 min-1

Feed rate

30 mm/minNumber of trial

3

Table 3 Measured results of roundness
Roundness [m]Inclined planetary drilling

14.60

measuring instrument. Roundness strongly depend on the surface

Planetary drilling

17.89

roughness, in other words, the smoother surface roughness it have, thebetter roundness is obtained by using of same experimental
instruments. Table 3 shows the measurement result of roundness.
From the fundamental experimental result, the inclined planetarydrilling having availability of smoother drilling than the orbital
drilling is clarified.
6. Inclined planetary mechanism drilling system
The advantages of the inclined planetary mechanism drilling havebeen demonstrated through the experiment by use of a lathe. From thepoint of view of practical CFRP drilling factory e.g. aviation andaero-space industry, the size of workpieces to be drilled is much larger
than the test specimens. Practical CFRP drilling equipments require
following features;
1. The size is enough small and light to handle by human operators.
2. The drilling machine can drill the workpiece without anotherFig.9 Apperance of drilled holes
under the conditions as shown in table 1. Especially, drilling feeding
moved forward only.

machine tool.
3. The drilling machine can attach to an industrial robot as an
end-effecter.
We have designed a new small size spindle unit for the inclined

A ball end-mill is logically decided as optimum cutting tools basedon the geometrical cutting model of inclined planetary drilling.

planetary drilling, which consists of a novel inclination mechanism.
Figure 10 shows schematic views of the eccentric mechanism by

In order to distinguish usability of the inclined planetary drilling

inclination. The eccentric mechanism of the inclined planetary drilling

technique from the orbital drilling, experiment for comparing inclined

system is multi-cylinder mechanism. Figure 10(a) and 2(b) shows theconcentric state and eccentric inclined state, respectively. Frame a (Xa,planetary drilling with orbital drilling is carried out. Drilling conditionis as same as shown in table 1. The ball type cutting tool is selected
for the experiment by use of the orbital technique.

Ya, Za) is Cartesian coordinate system with origin at the center of theouter cylinder. Frame b (Xb, Yb, Zb) is Cartesian coordinate systemwith origin at the center of the inner cylinder. Frame b has inclined 
as the Ya-axis from frame a.

5. Experimental results

At this point, the tool tip position vector after eccentric is definedFirst of all, drilled holes are inspected visually to find burrs andby the Eq. (10) where, l stands for length of the inner cylinder and ti

delaminations on the edge. Figure 9 shows the difference of drilled

stands for tool length and  stands for distance that the axis of rotation

holes appearance by the orbital drilling and inclined planetary drilling.

was mapped to the top end surface from the center of the inner

Smooth finishing is observed and there are no burrs and delaminations

cylinder and  stands for the rotation angle around the Zb of frame b.

on the edge by the inclined planetary drilling. On the other hand, inthe case of the orbital drilling, burrs are occurred around the hole on
the both sides.
Surface roughness of drilled workpiece is measured by using of asurface roughness measurement instrument. Measured direction isparallel to the axis of the hole and surface roughness parameters arecalculated by average of four measured parts of three times trial
results. The measuring results indicate that the inclined planetarydrilling can drill with smoothest surface from the point of view ofsurface roughness as shown in table 2. At least it is found that the
inclined planetary drilling can drill with smoother surface roughness
than by using of the orbital drilling.

Fig. 10 Eccentric mechanism model

Roundness of drilled hole is measured by using of roundness

26

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

Invited paper

a

Pt 

cos 
0

0 sin 
1
0

0

0
1
sin  0 cos  0 
0
0
0
1 

Table 4 Specification of developed system
Max tool rotation frequency
60,000 min-1

 cos  

2t l   sin  

l 

l
2

1

(10)





a

Therefore, the center position of the tool tip( Pt) and theeccentricity(e) are as follows:

2t l 
 sin 2 l 2t l  sin  e  1 cos 
2 



2

600 min-1

Max eccentricity (Inclined angle)

4 mm (4q)

Size

175mm x 125mm x 188mm

Weight7,5 kgTable 5 Experimental conditions
Cutting tool (Cemented carbide)
R=3mm, ball end-mill
1 cos  


l
t
2

a
l

 tan  sin 
Pt 
2 

l 2t l  cos  sin  tan  cos  
2  
l

Max revolution frequency

(11)

2

 l 2t l  tan  sin   (12)

2 



Eccentricity (Inclined angle)

1, 2 mm (1q, 2q)

Planetary revolution speed

250 min-1

Tool rotation speed

28000 min-1

Feed rate

60 mm/minNumber of trial

3

Workpiece material

UDT800, UDT700,
UD Pitch,
3k Cross, 12k Corss

Figure 11 shows the 3D model on CAD, and figure 12 shows thecalculation result of the eccentricity where, l is 150 mm and tl is 50
mm. In addition, the acceleration was simulated by use of the 3Dmodel (figure 11) where, revolution speed is 200 /min and e is 3 mm.
The eccentricity which  is 3 degrees confirmed to increase linear
in the range of 0-35 degrees and, the acceleration is 3.8 mm/s2. When
 is 2 degrees, the necessary range of  is increase and linearity is lost.
In other words, the eccentricity can be obtained while keeping small dmore when  is 3 degrees. Also, the acceleration is 3.1 mm/s2 which 
is 2 degrees. There was not a significant difference. Therefore, we areconsidered as desirable that  is 3 degrees.
Fig. 13 Configuration of the inclined planetary mechanism unit

Revolution motor
Cutting toolFig. 11. 3D model of the eccentric mechanism

3.5

Eccentricity e [mm]Feeding stage

0.45

e ( = 3 deg )e ( = 2 deg )d ( = 3 deg )d ( = 2 deg )

4

3

0.4
0.35
0.3

2.5

0.25

2

0.2

1.5

0.15

1

0.1

0.5

0.05

0

0

-0.5

-0.05
0

10

20

30

40

50

Phase angle  [deg]Fig. 12. Calculation result of the eccentricity

Difference between the linearity d [mm]4.5

Fig. 14 Whole view of the developed system
On the basis of vibration simulation result, the configuration of theinclined planetary mechanism drilling unit is designed on 3D CAD as
shown in figure 13 and the developed apparatus is shown in figure 14.
Table 4 shows the specification of the developed inclined planetarymechanism drilling system. Maximum drilling ability of the system is
16mm in diameter and 15mm in depth. Material of the workpiece islimited for CFRP due to the rotation motor power limit.
In order to evaluate the performance of the developed system,
drilling experiment has been carried out. 5 kinds of CFRP plates

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

27

Invited paper
Table 6 Measurement results of surface roughness
Material
Ra [m]Rz [m]
UDT800Eccentricity :1 mmEccentricity :
2 mm

1.76

eccentricity made surface roughness worse as shown in table 6.

31.18

Table 7 shows the measurement result of roundness. Roundness

UDT7000.70

6.56

strongly depends on the trend of surface roughness except for

UD Pitch

0.93

19.08

UDT800. And especially, roundness of UDT800 is worst because of

3k Cross

0.67

6.91

12k Cross

0.70

8.17

UDT800

2.33

41.93

UDT7000.84

7.96

UDT800 and 12k Cross, a cutting tool coated by diamond CVD or

UD Pitch

1.20

22.22

PCD tool as well as suitable drilling conditions for them is required.

3k Cross

0.91

8.50

12k Cross

0.86

8.76

the same reason of the fiber filler.
From the experimental result, the developed inclined planetarymechanism drilling system having availability of smooth drilling isdemonstrated. In order to improve the drilled holes quality of

7. Summaryplanetary drilling has been established according to the analytical

Table 7 Measurement results of roundness
Material
Roundness [m]Eccentricity :1 mmEccentricity :2 mmIn order to improve machinability of orbital drilling, inclined
result of the motion of bottom cutting edge of orbital drilling.



Fundamental drilling experiments that imitate the motion of

UDT8004.85

UDT7003.85

UD Pitch

4.52

3k Cross

5.03

12k Cross

5.94

UDT8005.11

mechanism drilling system having availability of smoother

UDT7004.48

drilling than the orbital drilling is demonstrated.

UD Pitch

6.15

3k Cross

7.32

12k Cross

9.06

inclined planetary drilling with a lathe have been carried out.


Experiment for comparing inclined planetary drilling with orbital
drilling is carried out. From the experimental result, thesuperiority of the inclined planetary drilling is clarified.



From the experimental result, the developed inclined planetary

References[1] http://www.novator.nu/[2] E.Brinksmeier, Sascha Fangmann, I.Meyer. 2010.

Production(50mm square, thickness: 5mm) are prepared as experimental

Engineering Research and Development. 2:277-283

workpieces as shown in Table 5. In this case, eccentricity is 1 or 2 mm.

[3] NEBUKA Teppei, TANAKA Hidetake, and YANAGI Kazuhisa.,

The experimental conditions are shown in table 5.

2007, Proceedings of Asia symposium for Precision Engineering andDrilled holes are inspected visuall. Figure 15 shows the appearance

Nanotechnology, 309-312.

of drilled holes. Smooth finishing is observed generates no burrs and[4] ISHIBASHI Tatsuya, TANAKA Hidetake, and YANAGI

delaminations on the edge except for UDT800 and 12k Cross. On the

Kazuhisa., 2009, An analytical model for orbital drilling and its

other hand, in the case of UDT800 and 12k Cross, burrs are occurred

applicability to, Proceedings of the Twenty-Fourth Annual Meeting ofaround the hole on both sides. It seems that the fiber filler having highthe American society for Precision Engineering.

tensile strength is hard to cut off with the normal ball end-mill.

[5] Hidetake TANAKA, Shinji Obata and Kazuhisa YANAGI,

Surface roughness of drilled workpiece is measured. Measured

Optimization of cutting edge configuration and machining conditions

direction is parallel to the axis of the hole and surface roughness

in orbital drilling, Proceedings of 4th CIRP International Conference

parameters are calculated by average of four measured parts of three

on High Performance Cutting, 2010, p. 201-206

times trial results. The measuring results indicate that larger

Fig. 15 Appearances of drilled hole by developed inclined planetary mechanism drilling unit
28

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.