Ultrasonic-assisted cutting of wood

Abstract

Ultrasonic-assisted cutting experiments have been performed on two wood species, spruce and beech in dry and wet state. Cutting forces and normal forces are measured and analysed with respect to uncut chip thickness (i.e. infeed depth) and linear correlation is found. Compared to conventional cutting reduction of cutting forces in the order of 50% is achieved at relatively small vibration amplitudes of 8μm. The results are interpreted in terms of reduced friction forces caused by ultrasonic vibration of the cutting knife.

Full text

Journal of Materials Processing Technology xxx (2005) xxx–xxx
Ultrasonic-assisted cutting of wood
G. Sinn, B. Zettl, H. Mayer∗, S. Stanzl-Tschegg
Institute of Physics and Materials Science, Department of Materials Sciences and Process Engineering,
BOKU, Peter-Jordan Str. 82, A-1190 Vienna, Austria
Received 3 November 2003; received in revised form 27 October 2004; accepted 8 April 2005
Abstract
Ultrasonic-assisted cutting experiments have been performed on two wood species, spruce and beech in dry and wet state. Cutting forces
and normal forces are measured and analysed with respect to uncut chip thickness (i.e. infeed depth) and linear correlation is found. Compared
to conventional cutting reduction of cutting forces in the order of 50% is achieved at relatively small vibration amplitudes of 8 ␮m. The results
are interpreted in terms of reduced friction forces caused by ultrasonic vibration of the cutting knife.
© 2005 Published by Elsevier B.V.
Keywords: Wood; Spruce; Beech; Ultrasonic cutting; Ultrasonic processing; Cutting force
1. Introduction
Cutting is the most important process in machining wood
and wood composites. Many machining processes, like saw-
ing, planing or milling, are based on cutting processes. The
surfacequalityobtained,thequantityofwaste,theservicelife
of the used tools and the energy consumption for the separa-
tion process are measures for the applicability and efficiency
of certain cutting processes. Due to the importance of cutting
for wood machining, improvements are of great technical as
well as economic interest.
A promising technology to enhance cutting processes is
ultrasonic-assisted cutting (UC). With this technique, the
cutting tool is stimulated to vibrations at frequencies of typi-
cally20kHz.Thecutting knife, which machines the material,
performs periodic oscillations at amplitudes in the order of
several micrometers, with beneficial influences on the cut-
ting process. One effect of UC is to reduce cutting forces
comparedtoconventionalcutting(CC).Thiscould be demon-
strated by turning different materials including aluminium,
bronze copper, carbon and stainless steel [1] as well as metal
matrix composite (SiC particle reinforced aluminium) [2].
Newprocesses arepossibleusing UC, like the turning of ultra
∗Corresponding author. Tel.: +43 1 47654 5161; fax: +43 1 47654 5159.
E-mail address: herwig.mayer@boku.ac.at (H. Mayer).
thin wall cylinders [3], in which the surface quality could be
improved and the surface roughness reduced. In machining
of optical plastics to mirror surface, which is a multi-step
processof grinding and polishing, UC couldbe used as a sub-
stitute for both processes [4]. The roundness of the working
piece turning hardened steel could be significantly improved
by UC [5]. Improved surface finish by UC is also described
for ultraprecision machining of glasses [6].
Reduction of cutting forces and improved surface quality
areimportant goals for wood cutting processes; however,few
studies exist on this subject. Kato et al. [7] studied the appli-
cation of ultrasonic vibrations on cutting tools in two basic
cases: longitudinal vibration (i.e. vibration in cutting direc-
tion) and lateral vibration (i.e. vibration normal to the cutting
direction). UC led to lower cutting forces and improved qual-
ity of the machined surfaces compared to CC. Chips formed
during cutting were affected by ultrasonic vibrations, i.e.
break type chips dominate in CC whereas flow type chips
prevail in UC.
The present study serves to investigate ultrasonic-assisted
cutting for machining of spruce (Picea abies L.) and beech
(Fagus sylvatica L.), which are two of the most important
commercial wood species in Europe. Knife setting parame-
ters are chosen similar to conventional cutting, and commer-
cially available cutting blades are used. This allows direct
comparison of UC and CC for commonly used geometries.
0924-0136/$ – see front matter © 2005 Published by Elsevier B.V.
doi:10.1016/j.jmatprotec.2005.04.076

2G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx
Cutting forces were measured with respect to the uncut chip
thickness (i.e. the infeed depth) and amplitude of the ultra-
sonic vibration. To include variations of mechanical prop-
erties with moisture content, both wood species have been
investigated in dry and wet condition.
2. Materials and methods
2.1. Wood species
Two different wood species were used in this study:
spruce (P. abies L.) as representative for European softwood,
and beech (F. sylvatica L.) as representative of European
hardwood. Blocks of wet and dry wood were prepared. The
sample size was: length 100mm, width 10mm and height
25mm. Moisture content of wet wood was above fibre
saturation point. The dry wood samples were stored in a
climate chamber of 65% relative humidity (RH) and 20◦C
until equilibrium moisture content was reached. Mean raw
density determined for dry spruce was 415±23kg/m3and
the raw density of dry beech was 723±39 kg/m3.
2.2. Ultrasonic cutting device
The mechanical components of the ultrasonic cutting
system used in the present investigation are shown in Fig. 1.
Vibrations of approximately 20kHz are generated by a
commercially available piezoelectric transducer (ultrasonic
converter), which transforms electric power into mechanical
movements. Direction of movement is in longitudinal (axial)
direction,i.e.themovementis back and forth and is causedby
theelongation and contraction of the ultrasonic transducer.In
order to achieve maximum vibration amplitudes at minimum
power consumption, the system vibrates in resonance.
Vibration nodes, i.e. places where the vibration amplitude is
zero, form approximately in the centre of each mechanical
part, and the node in the mounting part serves to clamp the
Fig. 1. Mechanical components of the ultrasonic cutting system. The trans-
ducer generates ultrasonic longitudinal (back and forth) resonance vibra-
tions. The (longitudinal) vibration amplitude varies along the length of the
mechanical parts, as shown in the lower part of the figure. The vibration
node in the centre of the mounting device is used to clamp the whole sys-
tem. Maximum amplitude of back and forth motion is at the place of the
knife’s edge.
system.Thisallowssuperimposingstaticcutting forces to the
ultrasonic system without damping the vibrations. The cross-
section area of the ultrasonic tool holder decreases along its
length, which serves to magnify the vibration amplitude at
the free end where the cutting knife is mounted by soldering.
A commercially available steel knife is used in the exper-
iments. Conventional cutting experiments are performed
using the same equipment and turning off the ultrasonic
vibration.
The cutting geometry was as follows: the whole mechan-
ical equipment was mounted at an angle of 40◦with respect
to the surface of the wood blocks. Due to the decreasing
diameter of the ultrasonic tool holder, clearance angle of
20◦of the knife resulted. The wedge angle of the steel
blade was 45◦. The ultrasonic cutting device was mounted
on an instrumented microtome in order to cut slices with
well-defined chip thickness.
The amplitude of the cutting knife was measured in two
ways: ultrasonic vibration gauges (induction coils used in
ultrasonicmaterial testing equipment[8])servedto determine
the vibration amplitudes along the length of the tool holder
and of the cutting knife. However, these vibration gauges
may be damaged during cutting. Strain gauges mounted on
the tool holder serve to additionally determine the vibration
amplitudes in an indirect way. The strain at the node in the
centre of the tool holder and the vibration amplitude at the
place of the knife are proportional [9]. Thus, the strain gauge
signal served to keep the vibration amplitude constant at a
pre-selected value (accuracy approximately ±5%). Numeri-
calmodellingof resonance vibration of the tool holder (based
on the equilibrium of an infinitesimal element under elastic
stresses [9]) served to optimise the design and to calculate
strain amplitudes and vibration amplitudes along the length
of the tool holder.
To study ultrasonic-assisted cutting, prototype equipment
with 1000 W electric power has been developed. This system
is in principal similar to commercially available equipment
used for ultrasonic welding, for example, and is based on
the principle of auto-resonant systems [10,11]. However, for
the purpose of wood processing, it was adapted to obtain
good energy efficiency. Controls of resonance frequency and
vibration amplitudes are performed in closed loop circuits.
2.3. Cutting experiments
Orthogonal cutting tests (λ=0◦) in longitudinal direction
[12] with a sharp tool were performed. Cutting speed, vC
was held constant at vC=170mm/s in all experiments. Con-
ventional cutting and ultrasonic-assisted cutting experiments
were accomplished.
Forces were measured by means of two three-dimensional
piezoelectric force sensors fixed below the specimens. Two
forces were evaluated: cutting force and normal force. Posi-
tive normal forces indicate compression forces on specimen
fixture whereas negative normal forces correlate to tension
forces. Cutting force and normal force have been sampled

G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx 3
with a frequency of 28kHz and filtered with sixth order
Butterworth low-pass filter (cut-off frequency 10kHz). Mea-
surements were recorded by computer and mean forces have
been calculated numerically.
Two series of experiments were carried out:
- To investigate the influence of chip thickness on UC,
the vibration amplitude was kept constant at 8␮m and
forces were measured as a function of uncut chip thick-
ness. Uncut chip thickness is the thickness of the removed
material before cutting, i.e. the infeed of material. CC
experiments served for comparison. All experiments have
been repeated five times to include statistical scatter in the
evaluations.
- To investigate the influence of ultrasonic vibrations on
cutting forces, experiments at different vibration ampli-
tudes of the cutting knife have been performed. In these
experiments, the uncut chip thickness was kept constant at
200␮m.
3. Results
In CC as well as UC experiments, continuous chips were
formed. No fracture of chips within the measurement length
of 100mm was observed and chip deformation appeared
similar in CC and UC tests. Cutting forces (forces acting
in cutting direction) and normal forces (normal to cutting
direction) are measured. Both forces are normalized with the
specimen thickness of 10mm (cutting width) in the follow-
ing, and will be presented in unit of N/mm.
3.1. Influence of chip thickness on normalized forces in
CC and UC
In Fig. 2, the measured normalized cutting forces in CC
and UC experiments are shown for dry spruce (a), wet spruce
(b), dry beech (c) and wet beech (d). Error bars indicate stan-
dard deviation of normalized forces measured in five consec-
utive experiments using the same experimental parameters.
Fig. 2. Normalized cutting forces (forces per unit thickness) depending on chip thickness measured in conventional cutting experiments (CC, 䊉) and ultrasonic-
assisted cutting experiments (UC, ) for dry spruce (a), wet spruce (b), dry beech (c) and wet beech (d).

4G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx
Table 1
Linear regression analysis of normalized cutting forces fCas a function of chip thickness, h(Eq. (1)) in conventional cutting (CC) and ultrasonic-assisted cutting
(UC) experiments
Wood Condition Methodology fC0(N/mm) kC(N/mm2)r
Spruce Dry CC 2.92 ±0.59 30.62 ±4.68 0.96
UC 1.82 ±0.47 17.14 ±3.74 0.92
Wet CC 2.05 ±0.33 15.25 ±2.15 0.95
UC 0.95 ±0.15 7.85 ±0.98 0.96
Beech Dry CC 4.87 ±0.58 25.74 ±4.61 0.94
UC 3.08 ±0.45 10.84 ±3.59 0.80
Wet CC 1.83 ±0.13 12.59 ±0.66 0.99
UC 0.78 ±0.07 7.93 ±0.36 1.00
fC0is the hypothetical normalized cutting force for zero chip thickness; kCis slope of regression line; ris the correlation coefficient.
The normalized cutting force, fCincreases with increasing
uncut chip thickness, hfor both materials in dry and wet
condition. Data may be reasonably well approximated by
linear regression using:
fC=fC0+hkC(1)
where fC0is the hypothetical normalized cutting force for
zero chip thickness and kCis the increase of normalized
cutting force with increasing chip thickness. The minimum
correlation coefficient, rwas 0.80 in UC tests of dry beech
(Table 1). Cutting forces for zero chip thickness and the slope
of the linear regression function are smaller in UC than in CC
Fig. 3. Normalized normal forces depending on chip thickness measured in CC (䊉) and UC experiments () for dry spruce (a), wet spruce (b), dry beech (c)
and wet beech (d).

G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx 5
Table 2
Linear regression analysis of normalized normal forces, fNas a function of chip thickness, h(Eq. (2)) in conventional cutting (CC) and ultrasonic-assisted
cutting (UC) experiments
Wood Condition Methodology fN0(N/mm) kN(N/mm2)r
Spruce Dry CC 3.28 ±0.41 −6.87 ±3.27 −0.72
UC 2.16 ±0.25 −6.69 ±2.00 −0.86
Wet CC 3.77 ±0.18 −6.12 ±0.80 −0.95
UC 2.10 ±0.33 −5.03 ±2.05 −0.71
Beech Dry CC 3.72 ±0.50 −12.70 ±3.95 −0.85
UC 1.94 ±0.35 −9.86 ±2.79 −0.84
Wet CC 1.45 ±0.19 −5.99 ±0.96 −0.95
UC 1.25 ±0.16 −4.47 ±0.82 −0.94
fN0is the hypothetical normalized normal force for zero chip thickness; kNis slope of regression line and ris the correlation coefficient.
experiments with the same material (Fig. 2). UC and CC tests
ofwetspruce and beech generally deliverlower cutting forces
than the respective tests in dry wood.
In Fig. 3, the normalized normal forces, fNare shown
for both species and both moisture contents with respect to
uncut chip thickness, h. Similar to above, linear correlation
may be used to approximate data.
fN=fN0+hkN(2)
where fN0is the hypothetical normalized normal force for
zero chip thickness and kNconsiders the variation of normal
force with cutting thickness. In Table 2, the parameters
as well as the correlation coefficients are shown. Scatter
of normal forces is larger than scatter of cutting forces,
and the absolute values of the correlation coefficients are
smaller. Minimum correlation coefficient was −0.71 for UC
of wet spruce. Parameters kNand correlation coefficients
are negative, which means that normal forces decrease with
increasing chip thickness. These forces may even become
negative for large chip thickness, as observed in wet beech
(Fig. 3d). Additionally, the parameter kNis similar whether
the materials were cut in CC or UC experiments. Normal
forces in UC are smaller than in CC tests of spruce and dry
beech, whereas approximately similar values were found in
wet beech. The water content of wood samples has different
influences on normal forces. Normal forces for CC of wet
spruce are slightly higher than for dry spruce, whereas no
difference is found in UC experiments. CC of dry beech
leads to greater normal forces than CC of wet beech, whereas
similar normal forces were found in UC experiments for
both moisture contents.
3.2. Influence of ultrasonic vibration amplitudes on
normalized cutting forces
Cutting forces at different ultrasonic vibration amplitudes
of the steel knife are presented in Fig. 4. Results are shown
relative to the mean cutting force measured in CC experi-
ments (vibration amplitude zero) with the same equipment.
In dry spruce (Fig. 4a) as well as dry beech (Fig. 4b), ultra-
sonic vibrations significantly reduce the cutting forces, and
vibration amplitudes of 10␮m lead to cutting forces which
are in the order of 40% of the respective values found in CC
experiments. At vibration amplitudes of 30␮m, the cutting
forces are approximately 20% (spruce) or 30% (beech) of the
respective values found in CC tests.
Fig. 4. Influence of ultrasonic vibration amplitudes of the steel knife on cutting forces for chip thickness 200 mm in dry spruce (a) and dry beech (b). Cutting
forces at vibration amplitude zero (=100%) determined with CC experiments serve for comparison.

6G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx
4. Discussion
The results presented above show that ultrasonic
vibrations of the cutting knife significantly affect forces
cutting wood. Several influences, including wood species,
moisture content, chip thickness and ultrasonic vibration
amplitude have been found and will be discussed in the
following.
The influence of wood species on the measured cutting
forces may be mainly attributed to the different mass density
[12]. Comparing normalized cutting forces of dry spruce and
dry beech, larger values are found for wood with the larger
density, whereas normal forces are almost similar. Besides
mass density, the moisture content has great effects on the
mechanical properties of wood, and thus on cutting forces.
Cutting forces are reported to decrease with increasing
moisture content from 10% RH to fibre saturation [12,13].
This general behaviour was reproduced in this study, and
lower cutting forces were found in wet instead of dry spruce
andbeech.One reason is the decreasing strength and stiffness
of wood with increasing moisture content below fibre satura-
tion, whereas at higher moisture contents these mechanical
properties remain almost constant [14–18]. This implies that
continuous chips formed during cutting are bent and inelasti-
callydeformed at lower forces at the higher moisturecontent,
which additionally reduces the necessary cutting force. From
a fracture mechanics point of view, the fracture toughness
of wood decreases with increasing moisture content [19,20].
This may be similarly used to account for lower cutting
forces found in the wet samples. Moisture affected cutting
forces in beech in a more pronounced way than in spruce.
While greater cutting forces were measured for dry beech
than dry spruce, forces were approximately similar in the wet
condition.
For both species and both moisture contents, the cutting
forces show linear dependence on uncut chip thickness. The
regressionline has a finite slope, which intersects the ordinate
at positive cutting force for hypothetical zero chip thickness.
According to Kivimaa [12], the non-zero axis intersection
indicates that the cutting force consists of two different con-
tributions: one is the force needed to actually cut the material
and the second accounts for the deformation of the separated
chip.More recent studies regardingfracturemechanical mod-
elling of cutting [21] assign the axis intersection at zero chip
thickness to the work of fracture (energy associated with sep-
aration of the material) and the slope of the regression line to
plastic work consumed by deforming unit volume. Refined
cutting models include friction on the rake face of the tool
and predict deviations from linear behaviour towards lower
cutting forces at small chip thickness [22,23]. Additionally,
friction on the rake face may influence the slope of the lin-
ear regression line of the normalized cutting force and chip
thickness [23].
Ultrasonic vibrations of the knife significantly reduce
cutting forces. The intersection with the ordinate at zero chip
thickness is at lower cutting forces in both species and both
Fig. 5. Movement of the knife at vibration amplitude, u=8␮m, cutting
speed,vC= 170mm/s and mounting the equipment 40◦inclined to the cutting
direction.
moisture contents, and the slope of the linear approximation
lines are smaller. Kato et al. [7] reports similar findings in
ultrasonic cutting of wood using different tool geometry
and at a factor of 65 lower cutting speed. Friction forces
are prominent contributions to cutting forces, which are
significantly reduced by superimposed ultrasonic vibrations.
Friction of the cutting tool is caused by the contact of the
clearance face with the cut surface and of the rake face with
the chip.
The experiments show that extrapolation of normal force
to zero chip thickness does not intercept at zero force, i.e. at
zero chip thickness the cutting tool is pushed on the cutting
surface and friction on the clearance face results. Analysis
of knife movement with respect to the specimen for the
investigated cutting speed shows (Fig. 5) that the cutting
tool performs an inclined sinusoidal trajectory, since the
ultrasonic cutting system is inclined by 40◦. Following the
trajectory from zero position, the tool penetrates into the
material until maximum depth. Then, the tool moves back,
compression forces on the wood surface are released and the
tool starts moving forward again. The effect of this forward
and backward moving of the tool reduces the sliding friction
forces. Studies of ultrasonically vibrating tools on wood
surfaces give experimental evidence for reduced friction
forces [24] and can be accounted for in Coulomb friction
models [25,26]. Reduced normal forces in UC experiments
were found in spruce and dry beech, whereas wet beech
showed similar forces in UC and CC experiments. Besides
positive compression forces on the cut surface from the
clearance face, bending of the chip causes negative contri-
butions to the measured normal force, and the respective
contributions are obviously different, depending on the
material.
The effect of ultrasonic vibrations to reduce cutting forces
is the more pronounced the larger the vibration amplitudes
(Fig.4).Normalized cutting forces in ultrasonic experiments,
fC(UC) depend on the vibration amplitude, u and may be
approximated using the following equation:
fC(UC) =fC(CC) −fvibr 1−exp −u
u0 (3)
fC(CC) is the normalized cutting force found in conven-
tional cutting experiments (vibration amplitude, u=0) and
fvibr and u0are experimental constants. The parameters

G. Sinn et al. / Journal of Materials Processing Technology xxx (2005) xxx–xxx 7
Table 3
Parameter of regression analysis using Eq. (3)
fvibr/fC(CC) (%) u0(␮m) r2
Dry spruce 88.0 ±4.6 9.9 ±1.2 0.93
Dry beech 74.8 ±3.2 8.7 ±0.9 0.92
fvibr/fC(CC) is the principally possible reduction of cutting force at high
vibration amplitudes with respect to the conventional cutting force; u0is the
ultrasonic vibration amplitude, where 37% of this reduction is reached and
r2is the coefficient of determination.
obtained by fitting Eq. (3) to the experimental data are
shown in Table 3. The vibration force, fvibr is understood as
the principally possible reduction of cutting force by ultra-
sonic vibrations. u0is the vibration amplitude, where 63%
of the possible reduction is actually found. This vibration
amplitude, u0is in the range of 10␮m for dry spruce and
dry beech.
Considering possible applications of ultrasonic-assisted
cutting in wood processing, it is most interesting that
reduction of cutting force is considerable already at vibration
amplitudes of 10␮m. It should be noted, however, that the
cutting velocity investigated was far below the cutting speed
in technical applications. Theoretical considerations predict
that the ultrasonic amplitude must be increased to obtain sig-
nificant effects as the cutting speed increases [1]. Increase of
vibrationamplitude is possible by several methods, including
varying the mechanical design of the components and using
different types of ultrasonic transducers. Cyclic stresses
at large amplitudes, however, may damage the mechanical
components due to material fatigue and additionally the
power requirement typically increases by an exponent
of two or more with increasing vibration amplitude. The
present investigation shows that cutting forces are reduced
already at relatively small amplitudes, which makes ultra-
sonic cutting a promising technology in the field of wood
processing.
5. Conclusions
Ultrasonic vibrations of the cutting knife reduce cutting
forces in machining dry and wet softwood and hardwood.
Reduced friction forces acting at the cutting knife mainly
cause this effect.
Linear correlation between cutting and normal force on
one hand, and uncut chip thickness on the other hand is
observed in ultrasonic-assisted cutting and conventional
cutting experiments. Ultrasonic vibrations reduce the neces-
sary cutting forces for hypothetical zero chip thickness and
additionally, the increment of cutting forces with increasing
chip thickness is less in ultrasonic-assisted cutting than in
conventional cutting.
Cutting forces decrease as the ultrasonic vibration
amplitudes increase. At vibration amplitudes of 30␮m, the
necessary cutting forces are approximately 20–30% of the
respective forces measured in conventional cutting exper-
iments. The influence of vibration amplitudes on cutting
forces may be approximated by Eq. (3) (see Section 4).
Acknowledgements
This work was financed by the Austrian Federal Ministry
of Transport, Innovation and Technology (BMVIT) subpro-
gram “Factory of Tomorrow”, which is gratefully acknowl-
edged. The authors wish to thank P. Beer for his valuable
help.
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