error derivation and compensation for cnc machine tools

SISOM – 2004, BUCHAREST, 20-21 May
TOOLHOLDER/SPINDLE INTERFACES FOR HIGH SPEED MACHINE TOOLS
*)
Vlad Grigore LASCU *) , Cornel Mihai NICOLESCU **) , Nicoleta CARUTASU ***)
Institute of Solid Mechanics – Romanian Academy, Bucharest e-mail: lascuv@pcnet.ro
**)
Kungl Tekniska Hogskola, Department of Production Engineering, Stockholm, Sweeden
***)
Politehnica University Bucharest, Romania
Abstract. The high speed machining is now recognized as one of the key manufacturing technologies
for higher productivity. Availability of toolholders is of prime importance for uninterrupted
production in a highly automated manufacturing environment. Even in relatively small machining
shops a great variety of tools is required. Many industrial firms, suppliers of cutting tools, research
organisations, industrial consultants and software houses have contributed to an optimisation of
toolholder use. These efforts were aimed at simplifying tool management. To determine the goals and
areas for future research work in tool management, the authors of this contribution attempt to describe
and evaluate the present situation and to point out some future trends.
1. INTRODUCTION
Machine tools are now facing a turning point as a result of the interactions and changes of various
technical, economical and social factors surrounding them. When predicting the future trend of the evolution
of machine tools in connection with the development of the flexible manufacturing system and the
rationalization of the design procedure for machine tools /1,2/, it can be pointed out that the modular
construction system is likely to be one of the most important technological developments /7/.
The development within production engineering is accompanied by increasing quality requirements of
the produced workpieces. In addition to the product-related high-quality features such as the shape,
dimensional tolerances and surface qualities, the effectiveness and controllability of the manufacturing
process are relevant factors.
To make the increased capability of the cutting-tool materials available, strong machine tools and
cutting tools are required. The shape accuracy of the toolholders is determined by the kinematic machine tool
behaviour and the static, dynamic and thermal stiffness of the machine tool system. The surface quality that
can be achieved depends on the geometry of the tool edge, the machining parameters and the dynamic
behaviour of the system machine tool - toolholder - cutting tool - workpiece. The metal removing capacity
that can be achieved without chatter vibrations is clearly defined by the dynamic machine tool behaviour.
For machining complex shapes of dies and moulds, usually tools with a long overhang are used.
Equally the machining of the integral components of aircrafts and cars requires the use of tools with a large
length/diameter-ratio. Also for the machining of bore holes, for example in gear cases, and for the inside
machining of cylindrical workpieces long cutting tools, known as boring bars, are required.
With increasing overhang the tool becomes the dominating weak point in the system of machine tool -
toolholder - cutting tool - workpiece. As a result the stiffness at the tool center point is determined by the tool
initially.
High speed machining is a desirable technology for the following reasons:
◆The life of the cutting tool is longer than at conventional cutting speeds.
◆A wide, stable cutting area is predicted at the high spindle speeds, according to the stability lobe
diagram theory.

81
Toolholder/spindle interfaces for high speed machine tools
◆Most of the heat generated in cutting is transmitted to the chip.
Drilling is a widely used machining process; it is estimated that in 1991 approximately $1.62 billion
was spent in the production of drill bits in the U.S.A. /12/. Important applications can be found in the
automotive, aircraft and computer industries as well as fields such as medical science and equipment.
Despite its wide use, drilling remains a poorly understood manufacturing process where current difficulties
include poor hole quality, burr formation, chip clogging, excessive heat generation, and tool breakage. Drill
bit vibration /9/, in particular, has been recognized as an underlying cause of some of the problems
associated with the drilling process. Therefore, the design of the toolholders requires solving specific
questions.
2. INTERFACE TOOLHOLDER/ SPINDLE OF MACHINE TOOLS
The result of high speed cutting depends decisively on the interface toolholder/spindle and on the
clamping system which must operate under especially difficult conditions. The interface is situated directly
in the force flow between workpiece and machine. The optimum design must guarantee the rapid automatic
tool change and high performance functions as well as highest changing and repeating accuracy. In addition
to the general requirements on cutting (for example the transmission of torque and cutting forces), there are
additional demands on high speed cutting machining: small balance error, high concentricity, high runout
tolerance and position accuracy, reduced centrifugal force influenced by small radial dimensions and masses.
There are many design approaches to attaching tooling structures to the machine tool spindle /3,13/.
Historically, national and international standards for toolholders have not been comprehensive or precise
enough to meet the accuracy requirements of new tooling systems or machine tools designed to operate at
higher cutting speeds. Toolholders do not often fit well into the spindle shafts, since clearances due to
manufacturing tolerances are present, which is unacceptable in many cases; this allows for toolholder tilting
and out-of-roundness, which results in poor accuracy, repeatability and rigidity. Therefore, toolholder
quality, dimensional tolerances, and axial alignment vary broadly from manufacturer to manufacturer /14/.
The important structural and dynamic characteristics of a tooling structure interface are the
manufacturing tolerances, static and dynamic runout /8/, radial and axial positioning accuracy and
repeatability, connection rigidity (static and dynamic stiffness), force transmission capability, momentum
and torque characteristics, clamping forces, balance requirements, fatigue life and durability, retention force
requirements, safety, locking/unlocking forces, coolant capability, maintenance requirements, sensitivity to
contamination, and cost.
The total deflection of the tool is the sum of the deflections of the tool body, holder, bearings, and
spindle (including the machine structure). It is obviously desirable to reduce the total deflection.
Fig.1 Chuck deformations influenced by centrifugal forces
Because of the high speed there are centrifugal effects both on the spindle cone and the tool taper
(Fig.1). Here the spindle expands more than the tool, so that the tool axially displaces itself by the clamping

Vlad Grigore LASCU, Cornel Mihai NICOLESCU, Nicoleta CARUTASU 82
force. By these deformations the contact surfaces are diminished and therefore the frictionally engaged
transmission of torque occurs. Furthermore the centering of the tool is no longer guaranteed. After stopping
the spindle rotation the elastic deformation diminishes, a press fit develops which is difficult to release. The
axial offset can be prevented by implementing facing stops /3/. Strait shank tools can be clamped by a chuck
or a hydraulic expansion chuck. Their functional properties are no longer sufficient by modern standards.
3. OPTIMIZATION OF THE TOOL HOLDING FIXTURE
A further possible optimization of the dynamic behaviour of tools with a long overhang is offered by a
damped tool holding fixture. Here, a statically flexible element with a reasonably high damping is arranged
between the tool and tool holding fixture into the line of force. In spite of the static compliance the element
has to guarantee a high guidance accuracy of the cutting tool.
A straight shank holder can be mounted in a hydraulic expansion sleeve in the hydraulic chuck that is
activated manually or semi-automatically. This provides 360° uniform pressure which clamps the toolholder
concentrically, and uniform contact is achieved over the full length of engagement; the clearance between
the toolholder shank and the hydraulic sleeve should be smaller than 0.015 mm for proper gripping.
The simplified model for the calculation of the additional element corresponds to that of a two-degreeof-freedom
system. The tool is approximated as a system with one-degree-of-freedom. Its mass and stiffness
are determined by the first natural bending mode /4,5,6/. The damping of the tool shaft results from the
material internal damping only. The mass at the bottom of the two-degree-of-freedom system represents the
additional element for the optimization of the dynamic tool behaviour. The static stiffness at the tool center
point decreases as a result of the additional element of the damped tool holding fixture placed in series.
However, a raised static compliance is often approved in the several applications of machining if the
dynamic system stability is increased clearly by it. Providing a suitable calculation and design of the
damping element in the tool holding fixture, this can be achieved. For the generation of high damping the
visco-elastic material effect seems to be suitable.
Fig.2. Basic design of the damped tool holding fixture
The basic design of the implemented tool holding fixture with raised damping is shown (Fig.2). The
tool is tied to the tool holding fixture by a membrane that operates as a spring. The visco-elastic material
operates in the rear end of the system located between the extended tool shank and the tool holding fixture.
Circumferential grooves serve as oil reservoirs from which the replaced oil flow into the gap.
Through realizing a suitable calculation and design of the spring and of the damping element the tool
holding fixture's dynamic behaviour can be improved considerably. A comparison of the dynamic frequency

83
Toolholder/spindle interfaces for high speed machine tools
responses for a drilling cutter with diameter D = 20 mm and a length/diameter ratio of 8, measured at the
TCP was realized (Fig.3). The frequency response is represented for the fixture of the tool in a hydraulic
chuck, in the dampened tool holding fixture without and with visco-elastic materials.
Fig.3 Measured dynamic frequency response of the damped tool holding fixture.
Using the damped tool holding fixture the static compliance raises from 0,65 µm/N to 0,9 µm/N. Here,
the dynamic behaviour is optimized such that no relevant resonance ratio accumulates.
4. RECEPTANCE COUPLING SUBSTRUCTURE ANALYSIS MODEL OF
TOOLHOLDER/SPINDLE
We have also considered the application of Receptance Coupling Substructure Analysis (RCSA) to
prediction of the tool point frequency response function (FRF) in high speed machining /5,6,10, 11/. In this
method, the FRF of the tool (derived from an analytic or finite-element model) is coupled to an experimental
FRF measurement of the toolholder/spindle through appropriate connections to determine the assembly FRF.
Previous results have shown that the dynamic response after system change may be predicted, thereby
reducing the number of required measurements in some applications. For example, RCSA is particularly
relevant to tool tuning /11/, a technique that modifies the system dynamics by adjusting the tool overhang
length in an effort to improve material removal rates (MRR).
The RCSA model of the toolholder/spindle assembly (fig.4) includes two components, the tool, A, and
the toolholder/spindle, B, connected by linear and rotational springs and dampers.

Vlad Grigore LASCU, Cornel Mihai NICOLESCU, Nicoleta CARUTASU 84
Fig.4. RCSA model of the toolholder/spindle.
Using this model, prediction of the tool point FRF proceeds in three steps. First, the component
frequency responses are obtained. The end mill is treated as a free-free beam with an appropriate crosssectional
profile and an analytic or finite-element model developed for the dynamic response (direct and
cross FRFs are required at the two ends of the beam). In the mean time, the selected toolholder is placed in
the spindle and the free end direct FRF is measured (impact testing was used in this study). Second, the tool
is placed in the toolholder/spindle and the connection parameters (k x, c x , k θ , and c θ ) are identified usind a
single measurement and fit to the experimental data. Third, the complete system model is used to predict the
tool point FRF for other tool lengths. The assembly tool point FRF, G 11 (ω), is calculated from the
component receptance and mobility terms and experimentally determined the connection parameters:
where
E
E
E
E
1
2
3
3
= ( k xH
= ( k xH
= ( kθ
P
= ( k N
θ
G
33
33
33
−1
−1
11 = H11
− H12E1
E2
− L12E3
k N 21 + cθ
N'
21 )
21
−1
+ k xH
22 + cxH
' 33+
cxH
' 22 −1)
− E3
E4
( k xL
−1
+ cxH
' 21 ) − E3
( kθ
N 21 + cθ
N'
21 )( k xL33
+ k
+ kθ
P22
+ cθ
P'
33+
cθ
P'
22 + 1)
+ k N + c N'
+ c N'
)
θ
22
θ
33
θ
22
( θ − E E E
(1)
33
x
22
4
−1
1
+ k xL22
+ cxL'
33+
cxL'
L + c L'
+ c L'
)
The receptance terms relate displacement to force, H, displacement to moment, L, rotation to force, N,
and rotation to moment, P . The mobility terms, denoted H’, L’, N’, and P’, are defined similarly, but they
describe the time rate of the change of displacement and rotation to applied force and moment. Further
details of the development of this equation will be communicated.
x
33
2
x
22
22
)
(2)
5. CONCLUSION/ SUGGESTIONS FOR FUTURE DIRECTIONS
▲ Future development work in the field of tool management can no longer be approached as an
isolated area. In view of the central importance of the tool between machine tool and machining process,
various influencing factors must be tachen into consideration.
▲ Future impruvements in the materials employed for tools and new coating techniques will enable
more precise and subsequently more process-oriented classification of tools, including toolholders.
Toolholder design and the selection of tool materials will thus acquire even greater importance than
previously. The available performance potential can be fully utilized only when an optimal combination of
tool and machining task is achieved.
▲ The increase of the technologically optimized cutting speeds and feed rates is mainly a questions of
the velocity of machine tools control systems and the feed drive dynamics. In order to reduce the linear and
rotating masses with equal rigidity for all moving parts, the light-weight construction technology must be
consequently promoved.
▲ Machine security devices are still insufficient. Furthermore, there are no design and control
directives for fast rotating tools.

85
Toolholder/spindle interfaces for high speed machine tools
▲Material substitution as well as geometric shape optimization leads to increased dynamic stability of
drills, boring bars and milling cuters with a high length/diameter - ratio, more and more inclination to
dynamic instability appears. By the use of passive damping elements, integrated in the toolholding fixture,
the dynamic behaviour of the toolholder/spindle system can be optimized.
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