2006 cao spindle model
TRANSCRIPT
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International Journal of Machine Tools & Manufacture 47 (2007) 1342–1350
Modeling of spindle-bearing and machine tool systems for virtual
simulation of milling operations
Yuzhong Cao, Y. AltintasÃ
Department of Mechanical Engineering, University of British Columbia, 2054-6250, Applied Science Lane, Vancouver, BC, Canada V6T 1Z4
Received 27 June 2006; received in revised form 26 July 2006; accepted 3 August 2006
Available online 9 October 2006
Abstract
This paper presents a general, integrated model of the spindle bearing and machine tool system, consisting of a rotating shaft, tool
holder, angular contact ball bearings, housing, and the machine tool mounting. The model allows virtual cutting of a work material with
the numerical model of the spindle during the design stage. The proposed model predicts bearing stiffness, mode shapes, frequency
response function (FRF), static and dynamic deflections along the cutter and spindle shaft, as well as contact forces on the bearings with
simulated cutting forces before physically building and testing the spindles. The proposed models are verified experimentally by
conducting comprehensive tests on an instrumented-industrial spindle. The study shows that the accuracy of predicting the performance
of the spindles require integrated modeling of all spindle elements and mounting on the machine tool. The operating conditions of the
spindle, such as bearing preload, spindle speeds, cutting conditions and work material properties affect the frequency and amplitude of
vibrations during machining.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Spindle; Chatter vibration; Finite element method; Milling
1. Introduction
The successful application of high-speed machining
technology is highly dependent on spindles operating free
of chatter vibration without overloading the angular
contact ball bearings. Unless avoided, vibration instability
in the metal-cutting process leads to premature failure of
the spindle bearings [1]. The spindle, tool-holder, and tool
are the main sources of chatter vibrations on high-speed
machines. The objective of the design engineer is to predict
the cutting performance of the spindle during the designstage by relying on engineering model of the process and
system dynamics.
Early spindle research focused mainly on static and
quasi-static analysis, whereas current research is extended
to optimal design by using dynamic analysis. Ruhl et al. [2]
is one of the earliest researchers to use the finite element
(FE) method for modeling of rotor systems. His model
includes translational inertia and bending stiffness but
neglects rotational inertia, gyroscopic moments, shear
deformation, and axial load. Nelson [3] used the
Timoshenko beam theory to establish shape functions
and formulate system matrices, including the effects of
rotary inertia, gyroscopic moments, shear deformation,
and axial load.
In the past, little research has been conducted to model
the coupling of bearings and spindles. The effects of
preload and spindle speeds on bearing stiffness and thedynamics of the spindle system are seldom studied. Wardle
et al. [4] presented a very simplified model for describing
the dynamics of a spindle-bearing system with a constant
preload. The theoretical maximum operating speed of the
spindle system is increased by maintaining a constant
preload, but Wardle neglected the softening of bearing
stiffness due to rotational speeds. Chen et al. [5] built a
model for determining the response of a spindle-bearing
system at high speeds with an analytical method. His model
considers the spindle as a uniform Euler–Bernoulli beam
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0890-6955/$- see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2006.08.006
ÃCorresponding author. Tel.: +1 604822 5622; fax: +1 604822 2403.
E-mail addresses: [email protected]
(Y. Cao), [email protected] (Y. Altintas).
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supported by a pair of angular contact bearings. Using
Jones’ [6] bearing model, Chen analyzed the dynamic
behavior of the spindle around the trivial equilibrium
configuration with zero end loads. Only the axial preload is
considered in this model. First, the Newton–Raphson
iteration method is used to calculate the bearing stiffness
matrix at a given spindle speed, then the dynamic behavioris computed at this speed using the obtained bearing
stiffness. Li and Shin [7] presented a coupled spindle-
bearing model that includes thermal effects to predict the
bearing stiffness and natural frequencies of the spindle
system, using DeMul’s bearing model. The bearing
configuration, however, is limited to several cases and the
gyroscopic effect is not included.
All of the above models predict the natural vibration and
frequency response for a specific spindle design, and
consider only the spindle shaft and bearings. The effects
of the machine tool on the spindle dynamics are neglected.
Neither centrifugal force nor gyroscopic effect is included
in modeling the spindle shaft. The contact forces on
bearing balls and the time response of the spindle-bearing
system under dynamic cutting forces have not been
reported in the literature.
In this paper, a general method is presented for modeling
the spindle machine tool system, which consists of the cutter,
tool-holder, spindle shaft, bearings, housing, and the machine
tool. A simplified model, representing the dynamics of the
whole machine tool without the spindle, is developed by
means of experimental modal analysis which needs to be done
only once for every machine tool. The model of the whole
machine tool system is then created by coupling the spindle
model developed by the authors [8,9] with the simplified
model of the machine tool without the spindle. The assembly
of the spindle unit and spindle head is modeled through
contact springs. The proposed method is validated by
performing frequency response and cutting tests.
2. FE model of spindle-bearing and machine tool system
An instrumented, experimental spindle is mounted on a
vertical machining center. The spindle moves vertically
with the spindle head, which travels on the guideway
attached to the machine column. The spindle head acts like
a cantilever beam elastically supported on the column due
to the contact with the guideway, therefore, the flexibility
of the spindle mounting has to be reflected in the model of
the spindle-machine system.
The spindle has five bearings in overall back-to-back
configuration as shown in Fig. 1. Three rear bearings are
placed in a floating housing, which can be moved by a
hydraulic preload unit. Through a hydraulic pump, the
preload is applied to the rear bearings by the floating
housing. At the same time, the force is transmitted to the
spindle shaft by the nut, and moves towards the rear. As a
result, the force is applied to the front bearings through
step A of the spindle shaft. The spindle housing prevents
the spindle shaft from moving further to the rear by step B
of the spindle housing. The whole spindle is self-balanced
in the axial direction under the preload.
A general FE model of the spindle-bearing and machine
tool system is presented (Fig. 1). The Timoshenko beam is
used to model the spindle shaft and housing. In the FE
model, the black dots represent nodes, and each node has
three translational displacements in the X -, Y-, and Z -axes,
ARTICLE IN PRESS
Nomenclature
A cross sectional area of the beam
E Young’s modulus of material
G shear modulus of material
I diametral moment of inertia for the beamJ polar moments of inertia for the beam
N number of bearing balls for each bearing
P axial force on the beam
k index of bearing balls
m mass of the bearing ball
O rotational speed of the shaft
r density of material
Dm pitch diameter of the bearing measured from the
ball center
J b mass moment of inertia for the bearing ball
K i; K o contact constants between bearing balls and
inner rings, outer rings, respectively, depending
on the geometry and the material of the bearingballs and bearing rings
k s transverse shear form factor, k s is 0.9 for the
circular cross-section beam
m y; mz distributed moments per unit length about axes
y and z, respectively
OE orbital speed of the bearing ball
OB angular speed of the bearing ball about its own
center
ak angle between the vector of the bearing ball’sangular velocity about its own center and shaft
axis
di; do normal contact deformations between the cen-
ters of the ball and inner ring, and the ball and
outer ring, respectively
dð1Þ; dð4Þ displacement vectors of the shaft and housing,
respectively
dð2Þ; dð3Þ displacement vectors of inner and outer ring,
respectively
dix; d
i y; d
iz; g
i y; g
iz displacements of the inner ring
dox; d
o y ; d
oz ; g
o y ; g
oz displacements of the outer ring
yi; yo inner and outer ring contact angles of the
bearingy y; yz rotations about axes y and z, respectively
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and two rotations about the Y - and Z -axes. The pulley is
modeled as a rigid disk, the bearing spacer as a bar
element, and the nut and sleeve as a lumped mass. Thespindle has two front bearings (nos.1 and 2) in tandem and
three bearings (nos. 3–5) in tandem at the rear. The preload
is applied on the outer ring defined as node A3, which can
move along the spindle housing with nodes A4 and A5.
The forces are transmitted to inner rings B3–B5 through
bearing balls, then to the spindle shaft through inner ring
B5, which is fixed to the spindle shaft. Finally, the forces
are transferred to the front bearings by inner ring B1,
which is also fixed to the spindle shaft, then to the housing
by outer ring A2, which is fixed to the housing. An initial
preload is applied during the assembly, and can be adjusted
later through the hydraulic unit. The inner ring and outer
ring of the bearing are related by nonlinear bearing
equations, from which bearing stiffness is obtained by
solving equations of the spindle machine tool system.
The tool is assumed to be rigidly connected to the tool
holder which is fixed to the spindle shaft rigidly or through
translational and rotational springs. An equivalent cylinder is
used to represent the spindle head. First, the modal
parameters are identified for the spindle head before the
spindle is installed. Then, two dominant modes from
experimental modal analysis are used to configure a simplified
model for the spindle head by using springs and the mass of
the spindle head. The spring constants are estimated through
the mass and natural frequencies of the spindle head. Springs
are also used between the spindle housing and spindle head,
whose stiffness is obtained experimentally. The modeling of
each part is described as follows.
2.1. Equations of motion for the spindle shaft with rotating
effects
The equations of motion for the spindle shaft with
centrifugal force and gyroscpic moment due to the rotating
are as follows [8]:
rAd2u
dt2À EA
q2u
qx2À qx ¼ 0,
rAd2v
dt2À
q
qxk sAG
qv
qxÀ yz À P
qv
qx ! À q y
ÀO2rAv ¼ 0,
rAd 2w
dt2À
q
qxk sAG
qw
qxþ y y
À P
qw
qx
!À qz
ÀO2rAw ¼ 0
rI d2y y
dt2þ OrJ
dyz
dtÀ EI
q2y y
qx2þ k sAG
qw
qxþ y y
Àm y ¼ 0,
rI d2yz
dt2À OrJ
dy y
dtÀ EI
q2yz
qx2À k sAG
qv
qxÀ yz
Àmz ¼ 0. ð1Þ
ARTICLE IN PRESS
ShaftHousing BearingHydraulic
fluid
ToolholderTool
Spindle nose
Pulley
Preload
Clamping
Displacement sensors
unit
Step A Step B NutBearing housing
Preload
Housing
Shaft
Node Bearing Rigidly connected Movable
Pulley
Spacer
Tool and tool-holder
Linear spring
Spindle head
Rotation spring
Inner ring
Outer ring A1A2
A3 A4A5
B1B2
B3 B4B5
Fig. 1. An experimental spindle and its Finite Element Model.
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Eq. (1) is also suitable for the spindle housing by setting
spindle speed to zero.
The following equations of the spindle shaft and housing
in matrix forms can be obtained by using the FE method:
bM bcf €qg À ObG bcf _qg þ bK bc þ bK bcPÀÀO
2
bM b
cCÁ
fqg ¼ fF b
g, ð2Þ
where bM bc is the mass matrix, bM bcC is the mass matrix
used for computing the centrifugal forces, bG bc is the
gyroscopic matrix which is skew-symmetric, bK bc is the
stiffness matrix, bK bcP is the stiffness matrix due to the
axial force, and {F b} is the force vector, including
distributed and concentrated forces. The superscript b
represents the spindle shaft and housing. The details of the
matrices are shown in our earlier publication [8]. The
damping matrix is not included here, and is estimated from
experimentally identified modal damping, which is mostly
constant for each spindle-bearing family developed by the
manufacturers.The bar element takes only the axial stiffness of the beam
element, and the rigid disk is treated as a short beam with a
large diameter by setting the Young’s modulus to zero.
2.2. Nonlinear bearing model
Jones’ bearing model, which considers the bearing balls
and rings as elastic parts, is used in this paper, see Fig. 2.
The Hertzian contact theory is used to calculate the contact
force and displacement. Contact forces between the bearing
ball and bearing rings:
Qi ¼ K id3=2i ; Qo ¼ K od
3=2o . (3)
Centrifugal force (F c) and gyroscopic moment (M g) are [6]:
F c ¼1
2mDmO
2 OE
O
2
, (4)
M g ¼ J bO2 OB
O
OE
O
sin a. (5)
The force acting on the bearing ring is
F ¼ XN
k ¼1
f di; do; dð2Þ; dð3Þ; yi; yo; Qi; Qo; F c; M gÀ Á. (6)
The derivative of force with respect to the displacement is
the bearing stiffness matrix as follows [8]:
KB½ � ¼KI ÀKI
ÀKo Ko
" #, (7)
where KI and Ko are 5 Â 5 matrices. The bearing stiffness
matrix depends on the displacements which are in turn
affected by the stiffness of the bearing.
2.3. Modeling of machine tool without the spindle
In order to avoid complex modeling of the wholemachine tool, a simplified model is used to simulate the
dominant vibration of the machine tool without the spindle
system. The spindle head is a casting which connects the
spindle to the machine tool (Fig. 3); therefore, it is used to
represent the dynamics of the whole machine tool for the
purpose of structural assembly of the spindle. The
dynamics is different in the X - and Y -directions because
of the asymmetry of the spindle head and machine column.
The experimental modal analysis in the X -direction is
presented here; however, the same method is applied in the
Y -direction.
An equivalent cylinder is used to represent the spindlehead. Both translational and rotational stiffness of the
springs supporting the spindle head is estimated by using
two dominant modes from the modal analysis. The same
equivalent cylinder is used for both X - and Y -directions,
but the stiffness of the springs is different. The simulated
and measured FRF at node 1 are illustrated in Fig. 3,
ARTICLE IN PRESS
inner ring
ball
δ = (δx, δy, δz, γ y, γ z)
δ(3)
δ(1)
δ(2)
δ(4)
outer ring
housing
shaft
θo
θi
Fc
Mg
Qi
Qo
outer ring
inner ring
δy
δz
δx
γ y
γ z
Fig. 2. Elastic model of the bearing.
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In order to predict stability lobes in the frequency
domain, the FRF at the tool tip in both X - and Y -
directions needs to be evaluated. A CAT40 (i.e.,
CV40TT20M400) shrink-fit tool-holder and a four-fluted
carbide end mill with a diameter of 20 mm and a stick-out
of 50 mm were used in all experiments and simulations
presented in the paper. Both tool and holder are assumed
to be connected rigidly to the spindle shaft.
The magnitudes of FRF at the tool tip in both X - and
Y-directions for the rigid tool-holder–spindle connection
are shown in Fig. 6, where the bearing preload is 1200N.
The simulation matches measurements very well at lower
frequencies, but the errors increase at higher frequencies.
3.2. Prediction of stability lobes and chatter test
The predicted stability of the system was experimentally
evaluated by milling Aluminium 7050. The chatter stability
theory of Budak et al. [10] is used to predict the stability
lobes by considering both simulated and measured FRF at
the tool tip. The results are shown and verified by cutting
tests conducted under the cutting conditions marked with
boxes or circles in Fig. 7. It is shown that the simulated
FRF, which was obtained from the proposed FE model
of the spindle, can correctly predict the stability lobes.
It must be noted here that the inclusion of machine
tool–spindle connection dynamics is important to achieve
such accuracy.The measured cutting forces and the machined surfaces
for both stable and unstable depths of cut are shown
in Figs. 8 and 9. The process was stable in Fig. 8, where
the depth of cut was 2 mm. Although the depth of cut was
only doubled in Fig. 9, the process was unstable and the
cutting forces increased more than 500% due to chatter
vibrations. The dominant frequency of the cutting forces
at the depth of cut of 2 mm is the tooth passing frequency
of 400 Hz, while chatter occurs at the spindle mode of
1028Hz at 4mm.
The measured and simulated displacements at the tool-
holder for both depths of cut are shown in Figs. 10 and 11.
The simulated displacements are very close to the measured
displacements although the distribution is a little different.
However, the spectrum of displacements from both
simulation and measurement match very well.
The simulated radial stiffness of bearings nos. 1 and 5
for the two cutting tests is illustrated in Fig. 12.
The stiffness of the first bearing is affected more than
the fifth bearing. The bearing stiffness can even reach
zero during chatter. All forces are treated as dynamic
forces in simulation. The preload is applied to the
bearings first, and the cutting forces are applied to the
tool tip after the transient vibrations due to the preload
diminish.
ARTICLE IN PRESS
500 1000 1500 2000 2500 3000 35000
0.5
1
1.5
2
2.5
3
3.5
4x 10-8 FRF at spindle nose in the X direction
M a g n
i t u d e [
m / N ]
Frequency [Hz]
Measurement
Simulation (with the influence of the machine tool)
Simulation (without the influence of the machine tool)
Fig. 5. Comparison of the influence of the machine tool on spindle
dynamics.
500 1000 1500 2000 2500 3000 3500 4000 45000
2
4
6
x 10-7
FRF at tool tip in the X direction (preload =1200 N )
M a g n
i t u d e
[ m / N ]
500 1000 1500 2000 2500 3000 3500 4000 45000
2
4
6
x 10-7
FRF at tool tip in the Y direction (preload = 1200 N )
Frequency [Hz]
M a g n
i t u d e
[ m / N ]
Measurement
Simulation
Fig. 6. FRF at the tool tip for rigid connection of the tool-holder.
2000 4000 6000 8000 100000
2
4
6
8
10
Spindle speed [rpm]
D e p
t h o
f c u
t [ m m
]
From measured FRF
From simulated FRF
No chatter
Chatter
Fig. 7. Predicted stability lobes from measured and simulated FRF.
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Contact forces on bearings nos. 1 and 5 are shown in
Fig. 13. Similar to the case of bearing stiffness, the first
bearing experiences higher contact forces than the fifth
bearing. Bearing stiffness and contact forces cannot be
measured directly. The correct prediction of the FRF and
displacement response, however, indirectly proves the
validity of their simulation since bearing stiffness is closely
related to the contact forces.
3.3. Effects of preload and speed on the dynamics of spindle
machine tool systems
In general, the natural frequencies of all modes
increase with preload due to increased bearing stiffness,
but decrease with spindle speed due to centrifugal
forces. In order to compare the simulati on and
measurement more clearly, the influence of the preload
and spindle speed on the second dominant natural
frequency is plotted separately in Fig. 14, by fixing
either the spindle speed or preload. The frequency
i ncreases from 1068 to 1142H z w hen preload i s
increased from 600 to 1800 N. However, the frequency
drops from 1140 to 1090 Hz when the speed is increased
from stationary to 10,000 rpm. It is shown that the
proposed model can correctly predict the effects of the
ARTICLE IN PRESS
-400
-200
0
200
0
500
C u t t
i n g f o r c e [ N ]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
-400
-200
0
200
Time [s]
Z direction
Y direction
X direction
(a)
(b)
Fig. 9. The measured cutting forces and machined surface. Spindle speed:
6000rpm, depth of cut: 4 mm, feed rate: 0.1 mm/flute, chatter frequency
1028Hz.
0 0.01 0.02 0.03 0.04 0.05
-6
-4
-2
0
D i s p
l a c e m e n
t [ μ m
]
0 0.01 0.02 0.03 0.04 0.05-6
-4
-2
0
Time [s]
D i s p
l a c e m e n
t [ μ m
]
Measurement
Simulation
Fig. 10. The measured and simulated displacements in the X -direction at
the tool-holder. Spindle speed: 6000rpm, depth of cut: 2 mm, feed rate:
0.1 mm/flute, no chatter.
0 0.01 0.02 0.03 0.04 0.05-40
-20
0
20
40
D i s p
l a c e m e n
t [ μ m
]
0 0.01 0.02 0.03 0.04 0.05-40
-200
20
40
Time [s]
D i s p
l a c e m e n
t [ μ m
]
Measurement
Simulation
Fig. 11. The measured and simulated displacements in the X -direction at
the tool-holder. Spindle speed: 6000rpm, depth of cut: 4 mm, feed rate:
0.1 mm/flute, chatter.
-150
-100
-50
150
200
250
C u
t t i n g
f o r c e
[ N ]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
-100
-50
0
Time [s]
X direction
Y direction
Z direction
(a)
(b
Fig. 8. The measured cutting forces and machined surface during a stable
cut. Spindle speed: 6000 rpm, depth of cut: 2 mm, feed rate: 0.1mm/flute.
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preload and spindle speed on the dynamics of spindle
machine tool systems.
4. Conclusions
The numerical model of the spindle machine tool system
is developed to simulate the virtual cutting performance of
the machine–spindle system. It is shown that the reliability
of virtual cutting with the spindles require integrated
modeling of bearings, spindle shafts, tool and holders,
bearing preload, connection between the spindle and
machine tool housing, speed and machining process. The
study also demonstrates that the modeling of spindle alone
does not lead to correct prediction of its dynamics on the
machine tool, unless its mounting joints are included in the
mathematical models.
The experimentally verified mathematical model predicts
that the preload can increase the bearing stiffness, leading
to increased natural frequencies, which shifts the stability
lobes to the right towards higher speeds. However, higher
preload reduces the damping, which decreases the dynamic
stiffness at the tool tip hence reduces the chatter free, depth
of cuts. The preload cannot efficiently improve the stability
lobes, but it can enhance the static stiffness and reduce the
forced vibrations.
Acknowledgment
This research is jointly sponsored by NSERC, Pratt &
Whitney, Canada, Boeing Commercial Plane, and Weiss
Spindle Technology.
ARTICLE IN PRESS
0
2
4x 108
S t i f f n e s s
[ N / m ]
0
2
4x 108
S t i f f n e s s
[ N / m ]
Bearing No.1
Bearing No.5
Preload
period Cutting period
0
2
4x 108
S t i f f
n e s s
[ N / m ]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
0
2
4x 108
Time [s]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
Time [s]
S t i f f n e s s
[ N / m ]
Bearing No.1
Bearing No.5
Preloadperiod Cutting period
(a)
(b)
Fig. 12. Radial bearing stiffness under cutting forces (spindle speed:
6000 rpm, feed rate: 0.1 mm/flute).
0
100
200
C o n
t a c
t f o r c e
[ N ]
0
100
200
C o n
t a c
t f o r c e
[ N ]
Preloadperiod Cutting period
0
200
400
600
C o n
t a c
t f o r c e
[ N ]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
0
100
200
Time [s]
0 0.01 0.02 0.03 0.04 0.05
0 0.01 0.02 0.03 0.04 0.05
Time [s]
C o n
t a c
t f o r c e
[ N ]
Preloadperiod
Cutting period
Bearing No.5
Bearing No.1
Bearing No.5
Bearing No.1
(a)
(b)
Fig. 13. Simulated bearing contact forces under cutting (spindle speed:
6000 rpm, feed rate: 0.1 mm/flute).
600 800 1000 1200 1400 1600 18001050
1100
1150The second dominant frequency (spindle speed = 0 rpm)
Preload [N]
F r e q u e n c y
[ H z
]
Measurement
Simulation
0 2000 4000 6000 8000 100001050
1100
1150The second dominant frequency (preload = 1200N)
Spindle speed [rpm]
F r e q u e n
c y
[ H z
] MeasurementSimulation
Fig. 14. The influence of the preload and spindle speed on the second
natural frequency.
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ARTICLE IN PRESS
Y. Cao, Y. Altintas / International Journal of Machine Tools & Manufacture 47 (2007) 1342–13501350