Vibrations in Metal Cutting
Measurement, Analysis and Reduction
Linus Pettersson
Ronneby, March 2002
Department of Telecommunications and Signal Processing
Blekinge Institute of Technology
372 25 Ronneby, Sweden
c Linus Pettersson
Licentiate Dissertation Series No. 01/02
ISSN 1650-2140
ISBN 91-7295-008-0
Published 2002
Printed by Kaserntryckeriet AB
Karlskrona 2002
Sweden
v
Abstract
Vibration and noise in metal cutting are ubiquitous problems in the workshop.
The turning operation is one kind of metal cutting that exhibits vibration related
problems. Today the industry aims at smaller tolerances in surface finish. Harder
regulations in terms of the noise levels in the operator environment are also central.
One step towards a solution to the noise and vibration problems is to investigate
what kind of vibrations that are present in a turning operation. The vibrations
in a boring operation have been put under scrutiny in the first part of this thesis.
Analytical models have been compared with experimental results and the vibration
pattern has been determined.
The second part of the thesis deals with active vibration control in external
turning operations. By embedding a piezo-ceramic actuator and an accelerometer
into a tool holder it was possible to obtain a solution that can be fitted in a standard
lathe. The control system consists of the active tool holder, a control system based
on the filtered-X LMS algorithm and an amplifier designed for capacitive loads.
The vibration level using this technique can be reduced by as much as 40 dB
during an external turning operation.
vii
Preface
The work presented in this licentiate thesis has been performed at the department
of Telecommunications and Signal Processing at Blekinge Institute of Technology.
This licentiate thesis summarizes my work within the field of vibration measurement, analysis and control. It consists of two parts, which are based on one research
report, two submitted articles and one accepted conference paper and the parts are
Part I Vibration Analysis of a Boring Bar.
Part II Active Control of Machine-Tool Vibration in a CNC Lathe Based on an
Active Tool Holder Shank with Embedded Piezo Ceramic Actuators.
ix
Acknowledgements
I am indebted to Lars Håkansson, my nearest coworker, for the help and support
in my research. Lars can be anything from a good friend to a nagging supervisor.
I would also like to pay my gratitude to professor Ingvar Claesson for inspiration
and guidance thruoghout my studies.
I would like to thank all my colleagues at the Department of Telecommunications and Signal Processing. They have helped me in several ways and without
them my time at the department would have been more (of) boring.
Many thanks to my family, for their support and encouragement. Finally I
would like to express all my love to my fiancée Cecilia, who helped me in many
ways, especially in relaxing from work during the weekends.
Linus Pettersson
Ronneby, March 2002
xi
Contents
Publication list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Part
I
Vibration Analysis of a Boring Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21-88
II
Active Control of Machine-Tool Vibration in a CNC Lathe Based
on an Active Tool Holder Shank with Embedded Piezo
Ceramic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89-101
13
Publication list
Part I is published as:
L. Pettersson, “Vibration Analysis of a Boring Bar” Research Report, ISSN 11031581, Feb. 2002.
Parts of this research report has been revised and submitted for publication as:
L. Pettersson, L. Håkansson A. Brandt and I. Claesson, “Identification of Dynamic Properties of Boring Bar Vibrations in a Continuous Boring Operation”,
submitted to Mechanical Systems & Signal Processing, Dec. 2001.
L. Pettersson, L. Håkansson A. Brandt and I. Claesson, “Analytical and Experimental Investigation of the Modal Properties of a Clamped Boring Bar”, submitted
to Mechanical Systems & Signal Processing, March 2002.
Part II is published as:
L. Pettersson, L. Håkansson, I. Claesson ans Sven Olsson, “Active Control of
Machine-Tool Vibration in a CNC Lathe Based on an Active Tool Holder Shank
with Embedded Piezo Ceramic Actuators”, The 8th International Congress on
Sound and Vibration, Hong Kong SAR, China, 2-6 July 2001.
15
Introduction
In turning operations the cutting tool is subjected to a dynamic excitation due
to the deformation of work material during the cutting operation. The relative
dynamic motion between the cutting tool and the workpiece will affect the result
of the machining, in particular the surface finish. Thus vibration related problems
are of great interest in turning operations.
Introduction to the Lathe
The lathe is a very useful and versatile machine in the workshop, and is capable of
performing a wide range of machining operations. The workpiece is held by a chuck
in one end and when possible also by a tailstock at the opposite end. The chuck is
mounted on a headstock, which incorporates the engine and gear mechanism. The
chuck is holding the workpiece with three or four jaws and a spindle engine causes
the chuck and workpiece to rotate. A tool-post is found between the headstock
and tailstock, which holds the cutting tool. The tool-post stands on a cross-slide
that enables it to move along the workpiece. An ordinary lathe can accommodate
only one cutting tool at the time, but a turret lathe is capable of holding several
cutting tools on a revolving turret.
Two common types of turning operations are external longitudinal turning operations and boring operations. Both turning operations are usually possible in
general purpose lathes. External longitudinal turning operations are performed on
the outside of a workpiece and the cutting tool is mounted on a tool holder shank.
Boring operations are performed in pre-drilled holes in the workpiece, i.e. inside
the workpiece, and the cutting tool is mounted on a boring bar.
PART I - Vibration Analysis of a Boring Bar
In internal turning or boring operations, vibration is a problem. The industries are
having problems performing specific boring operations. The vibrations involved
during the cutting operation influence the surface finish and the manufacturers are
having problems with small tolerances in boring operations. When cutting in predrilled holes the cross sectional area of the boring bar is limited. Since a general
boring bar is long and slender it is sensitive to external excitation and thereby
inclined to vibrate.
A thorough investigation of the vibrations involved in boring operations is therefore needed. Part I of this thesis scrutinizes the vibrations in boring operations.
A solid foundation was achieved from both theoretical and experimental methods
in order to analyze the vibrations involved. The theoretical methods derive from
16
knowledge of the dimensions of the system and its suspension or boundary conditions. The experimental methods are all derived from analysis of data acquired
from accelerometers and force transducers mounted on the boring bar.
Evolution in Active Vibration Control in Turning
Operations
A project in active vibration control in external turning operations was initiated
in 1997. It concerned external longitudinal turning operations and was inspired by
a project at Dept. of Mechanical Engineering, Lund Institute of Technology, LTH.
A working solution was developed where the vibrations were reduced by approximately 40 dB and resulted in a Ph D thesis for Lars Håkansson. The magnetostrictive design was however not suitable for industrial purposes thus further improvements were needed. A schematic picture of the first test design developed at
LTH is presented in Fig. 1.
Figure 1: The first working model developed at LTH.
The first magneto-strictive test design was not possible to incorporate into a
standard lathe used in the industry without severe modifications. The design also
needed improvements in its mean time between failure. A solution was found in
17
piezo-ceramic actuators. The first version of the new generation of active tool
holders was based on piezo plates glued on the surface of the tool holder as in
Fig 2. It had good potentials, but unfortunately the actuators were unsufficient for
the forces in a cutting operation.
Figure 2: The first attempt on an active tool holder solution using piezo ceramic
technique.
PCB piezotronics who was also involved in developing the active tool holder
with piezo plates was contacted to develop a more powerful active tool holder. The
unsatisfactory result is presented in Fig. 3. This tool holder was to weak, both in
structure and actuator and the mean time between failure was far from sufficient.
Figure 3: The PCB piezotronics active tool hoder solution using a piezo ceramic
actuator.
The demand of having embedded actuators was abandoned at this stage. Fig. 4
shows the first external solution based on piezo ceramic actuators that was developed at BTH. This solution was possible to incorporate into the lathe used in the
experiments without modifications. The experiments that were carried out showed
that the vibrations were reduced by approximately 40 dB. The surface finish was
also improved significantly.
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Cutting tool
Actuator
Tool holder shank
Figure 4: The first solution developed at BTH using a piezo ceramic actuator
mounted outside the tool holder.
Now, it was time to regain the demand of having the actuator embedded in
the design. The used actuator in the previous design was small enough to fit in
a modified standard tool holder. By embedding the actuator into a standard tool
holder enables the active vibration control technique to be used in a standard lathe
in the industry. An accelerometer was also embedded in this design. A CAD model
of the embedded design is presented in Fig. 5.
Accelerometer
Cutting
tool
Tool h
older
shank
Embedded and sealed
piezo ceramic actuator
Figure 5: A standard tool holder with an embedded piezo ceramic actuator.
The tool holder design has been improved by incorporating a socket into the
design in cooperation with Active Control Sweden AB, a company trying to develop
and market the active control technique in turning operations. The socket connects
the power to the actuator and the acceleration signal from the accelerometer with
19
the amplifier and control system. The result is called ActiCutTM and is presented
in Fig. 6.
Figure 6: The active tool holder called ActiCutTM with an embedded piezo ceramic
actuator and accelerometer.
Part II - Active Control of Machine-Tool Vibration in a CNC Lathe Based on an Active Tool
Holder Shank with Embedded Piezo Ceramic Actuators
In the turning operation chatter or vibration is a common problem affecting the
result of the machining, and, in particular, the surface finish. Tool life is also influenced by vibration. Severe acoustic noise in the working environment frequently
occurs as a result of dynamic motion between the cutting tool and the workpiece.
These problems can be reduced by active control of machine-tool vibration. However, machine-tool vibration control systems are usually not applicable to a general
lathe and turning operation. The physical features and properties of the mechanical constructions or solutions involved regarding the introduction of secondary
vibration usually limit their applicability. An adaptive active control solution for
a general lathe application has been developed. It is based on a standard industry tool holder shank with an embedded piezo ceramic actuator and an adaptive
feedback controller. The adaptive controller is based on the well known filtered-x
LMS-algorithm.
Part I
Vibration Analysis of a Boring
Bar
Part I is published as:
L. Pettersson, “Vibration Analysis of a Boring Bar” Research Report, ISSN 11031581, Feb. 2002.
Parts of this research report has been revised and submitted for publication as:
L. Pettersson, L. Håkansson A. Brandt and I. Claesson, “Identification of Dynamic Properties of Boring Bar Vibrations in a Continuous Boring Operation”,
submitted to Mechanical Systems & Signal Processing, Dec. 2001.
L. Pettersson, L. Håkansson A. Brandt and I. Claesson, “Analytical and Experimental Investigation of the Modal Properties of a Clamped Boring Bar”, submitted
to Mechanical Systems & Signal Processing, March 2002.
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