METALLOGRAPHIC ORIENTATION EFFECTS ON BURR FORMATION
IN MACHINING OXYGEN-FREE-COPPER USING SINGLE
CRYSTALLINE DIAMOND MICRO-TOOLS
X Ding1,2 *, D L Butler1,2, G C Lim1, Y C Liu1, K C Shaw1, H Y Zheng1
1
Singapore Institute of Manufacturing Technology (SIMTech),
71 Nanyang Drive, Singapore 638075
2
School of Mechanical and Aerospace Engineering,
Nanyang Technological University, Singapore
ABSTRACT
A study was carried out to investigate the
mechanism of burr formation in micro-scalemechanical machining (henceforth referred to as
‘micro-machining’)
of
Oxygen-Free-Copper
(OFC) using a 5-axis ultra-precision machine.
The single crystalline diamond (SCD) microtools with a cutting contact length of around 30
µm on the primary clearance face were
employed in this study. Burrs could be observed
when the cutting depth is deeper than 0.9 µm.
The crystallographic orientation exerts a great
influence on the formation of the burrs.
Modifying the machining parameters, such as
reducing the cutting depth or the cross-feed rate
could minimize the burr size. This study
contributes to the understanding of the physics
of mechanical machining with micro-tools.
cutting properties, leading to a distinct difference
between the traditional macro-mechanical
machining and the micro-machining. A number
of studies have been carried out to arrive at a
better understanding of the effects of
metallographic structure on machining single
crystal materials [3-9]. Polycrystalline OFC is
widely used for optical components since optical
grade surfaces can be produced with diamond
turning. However, so far, there is very little
reported information on the introduction of burr
formation as well as the minimization of burrs in
micro-machining OFC. There fore, there is a
needed to carry out a study to investigate the
fundamental
mechanisms
in
micro-burr
formation for the purpose of predicting and
controlling the burr formation during micromachining OFC
Key words: Micro-size diamond tool, micromachining, FIB, grain orientation, burr formation
MACHINING EXPERIMENTS
All experiments were conducted using a Moore
Nanotech
350
ultra-precision
Freeform
Generator. A FEI dual beam focused ion beam
(FIB) system (Nova Navolab) which integrates
ion and electron beams for FIB and SEM
functionality in one machine was used to
fabricate the micro-size SCD tools and observe
the machined work pieces. A Wyko NT3300
optical profiler was used to measure the
machined surface after experiments. Kistler
dynamometer (9256C1 Minidyn Multicomponent
Dynamometer) was used to detect the force
signals during machining. A Nano-indentor
(NanoTest 550) was used to test the micro
hardness on the top surface of the work
material.
INTRODUCTION
Micro-burrs have been observed in the micromilling of stainless steel, brass, aluminium and
cast iron [1, 2]. The suppression of burr
development in machining with a micro-tool is
very important because, unlike in macromachining, post-processing cannot always be
applied to remove burrs on miniature fabricated
parts. De-burring may introduce dimensional
errors and residual stresses in the component.
Therefore, the best solution is to prevent or
minimize burr formation during micro-machining.
For the implementation of this approach it is
critical to understand the basic mechanisms
involved in the burr formation in micro-machining
using micro-tools. However, since
the
metallographic phase size is often of the same
order of magnitude as the cutter size, the
metallurgical structures will affect the overall
OFC is a high purity copper with less than
0.05% residual deoxidants. Table 1 show the
material’s properties. Prior to micro-machining
and nano-indentation, an SCD tool with nose
radius of 1.0 mm was used to shape the top
surface of work materials. The shaped top
surface of the work material can achieve an
optical quality finish with R a as low as 7nm and
flat waviness of 0.1 m Peak-Valley value.
The SCD micro-tool was fabricated using a FIB
[10] to obtain the following profile: cutting edge
radius ranging from 15 µm to 18 µm, rake angle
of -1.5 º, and primary and side clearance angles
of 7 º. The cutting contact length at the toolworkpiece interface is around 30µm. Since
machining is carried out at small depth-of-cuts
using a micro-size tool, it was necessary to align
the tool carefully in order to achieve high
alignment accuracy.
RESULTS AND ANALYSIS
Grooves were formed on the top surface of the
work material with the groove width equivalent to
the cutting contact length at a tool-workpiece
interface of around 30 µm and a cutting speed of
1.0mm/min. Deformation or pile-up could be
observed along the groove edge when the
cutting depth was deeper than 0.9 m with the
existence of burrs between A and B as shown in
Fig. 1 (a). The burr was generated at the
crystallographic grain right to the grain boundary
and can be observed in the close-up view in Fig.
1 (b). Previous research undertaken by the
authors concluded that, as the micro-size tool
traverses within a grain with a higher hardness,
the work material in front of the machining tool
may deform severely, leading to thicker chip,
striation at the chip back, higher machining
force, reduced shear angle and a degraded
machined surface [11]. A similar phenomenon
was displayed in Fig. 1 and Fig. 2, such as the
build-up at the chip, degraded machined surface
and a significant increased cutting force at the
instance when the burr was formed during
micro-machining. The micro-hardness test
indicated that the metallographic phase between
A and B is harder (2.6 GPa) than that obtained
in the grain to the left of the boundary (1.7 GPa).
Thus when the micro-tool traverses within a
grain of relative higher hardness the likelihood of
burr formation is increased.
The formation of the burrs attributed to the
crystallographic orientation could be avoided by
modifying the machining parameters. Reducing
the cutting depth helps to minimize the burr size.
Fig. 3 shows that there were no burrs observed
with a variation in grain orientation at a cutting
depth of less than 0.35 m at the same cutting
speed without coolant, although the machined
surface changed with the crystallographic
orientation.
Three grooves were formed through the same
grain (upper dashed curve) by a micro-tool as
shown in Fig. 4 (a). The depths of grooves from
the 1st to 3rd groove were 0.9 m, 1.4 m and
1.7 m respectively. The 1 st and 2nd grooves
rd
along with Zone Z1 at the 3 groove were
formed using a micro-tool with the top layer
being removed in one pass and the layer width
equalling that of the cutting contact length at the
primary clearance face using a machining speed
of 1.0mm/min. The Zone Z2 in the 3 rd groove
was generated by a series of passes with the
same machining speed and direction; however a
cross-feed of 1m/pass was used resulting in
material on the right side of the tool being
repeatedly removed. No coolant was used in
the micro-machining experiments. Burrs could
be found at all groove edges except the right
edge of the 3rd groove which was generated with
a cross-feed of 1m/pass. Fig. 5 show the forces
obtained from forming the right edge of the 3 rd
groove with a cross-feed of 1 m/pass.
Comparing to the significant change in cutting
forces displayed in Fig. 2, the invariant cutting
forces were observed though the micro-size tool
cut as it crossed various phases with the full
cutting distance of 2.4 mm at a reduced crossfeed. The reduced cutting force in micromachining plays an important role in minimizing
the burr size.
CONCLUSIONS
Burrs could be observed when the cutting depth
is deeper than 0.9 µm. The burrs may form if the
pile-ups or cracks/fractures occurred at the
groove edges once the localized stresses builds
up to a threshold when the cutting depth is
deeper
than
a
specific
value.
The
crystallographic orientation exerts an influence
on the formation of the burrs. Modifying the
machining parameters, such as reducing the
cutting depth or the cross-feed could minimize
the burr size.
TABLE 1. Mechanical properties of OFC
Physical properties
Mechanical properties 20°C
Density Young’s Modulus Ultimate Tensile Yield Tensile Elongation Hardness
3
(g/cm )
[GPa]
Strength (MPa) Strength (MPa)
(%)
(HB)
8.94
117
331
303
16
93
20 m
Chip
Ra:
<30 nm
Ra:
<30 nm
Phase
boundaries
Ra:60140 nm
Burr
B
Build-up
Burr
A
5 µm
B
Ra:
60-140 nm
A
(a)
(b)
FIGURE 1. Grain orientation effects on the burrs formation in micro-machining. (a) Overview; (b) Close-up
view.
Cut t I ng f or ce ( N)
0.085
Cutting direction
0.055
0.025
A
B
-0.005
0
2000
4000
6000
8000
10000
FIGURE 2. Cutting force obtained during micro-machining.
5 m
Ra: 40-55 nm
Grain
boundary
Ra<25 nm
FIGURE 3. Micro-grooves machined at a cutting speed of 1 mm/min and at a cutting depth of 0.35 m
without coolant.
50 m
Z1
100 m
Z2
Z 1 Z2
Burr
Right
edge
Right
edge
Grain
boundary
1st
groove
nd
nd
rd
2
groove
rd
st
2
3
1
groove groove groove
3
groove
(a)
(b)
FIGURE 4. Three micro-grooves formed within one metallographic phase by a commercial micro-size
SCD tool. (a) Scanning Electron Microscope image; (b) Scanning Ion Microscope image
Cu t t i n g f o r c e ( N)
0.085
Cutting direction
0.055
0.025
-0.005
1600
3600
5600
7600
9600
11600
13600
15600
17600
rd
FIGURE 5. The cutting force obtained from forming the right edge of the 3 groove.
REFERENCES:
[1]Damazo Bn, Davies MA, Development of a drilling
burr control chart for stainless steel. Transactions
of the North American Manufacturing Research
Institution of SME. 2000; 28: 317-322
[2]Schaller T, Bohn L, Mayer J, Schubert K.
Microstructure Grooves with a Width of Less Than
50m cut With Ground Hard Metal Micro End Mills.
Precision Engineering. 1999; 23: 229-235
[3]Ding X, Butler D L, Lim G C, Cheng C K, Shaw K C,
Liu K, Fong W S and Zheng H Y. Machining with
micro-size single crystalline diamond tools
fabricated by a focused ion beam, Journal of
Micromechanics and Microengineering. 2009;19/2:
025005-15
[4]Lee W B, Cheung C F and To S. A mesoplasticity
analysis of cutting friction in ultra-precision
machining, Journal of Materials Processing
Technology. 2003;140: 292-7
[5]To S, Lee W B and Chan C Y. Ultraprecision
diamond turning of aluminium single crystals,
Journal of Materials Processing Technology. 1997;
63: 157-62
[6]Moriwaki T and Okuda K. Machinability of copper in
ultra-precision micro diamond cutting CIRP. 1989;
38/1: 115-8
[7]Kim J D and Kim D S. Theoretical analysis of
micro-cutting characteristics in ultra-precision
machining. Journal of Materials Processing
Technology. 1995; 49: 387-98
[8]Weule H, Huntrup V and Tritschler H. Micro-cutting
of steel to meet new requirements in
miniaturization CIRP. 2001; 50/1 61-4
[9]Lee W B, Cheung C F and To S. A microplasticity
analysis of micro-cutting force variation in ultraprecision
diamond
turning.
Journal
of
Manufacturing Secience and Engineering. 2002;
124: 170-7
[10]Ding X, Lim G C, Cheng C K, Butler D L, Shaw K,
C, Liu K and Fong W S, Fabrication of a micro-size
Diamond Tool Using a Focused Ion Beam, Journal
of Micromechanics and Microengineering. 2008;
18/7:075017-26
[11]Ding X, Butler D L, Lee K C, Crystallographic
Orientation Effects on Machined Surface Integrity
with Polycrystalline Oxygen Free Copper by Single
Crystalline Diamond Micro-Tools, Submitted to
Journal of Micromechanics and Microengineering.