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 measure- ment, 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 H˚akansson, 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 Telecommunica- tions 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 fianc´ee 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 VibrationAnalysisofa BoringBar .................................. 21-88 II Active Control of Machine-Tool Vibration in a CNC Lathe Based on an Active Tool Holder Shank with Embedded Piezo CeramicActuators ................................................. 89-101 13 Publication list Part I is published as: L. Pettersson, “Vibration Analysis of a Boring Bar” ResearchReport, ISSN 1103- 1581, Feb. 2002. Parts of this research report has been revised and submitted for publication as: L. Pettersson, L. H˚akansson A. Brandt and I. Claesson, “Identification of Dy- namic Properties of Boring Bar Vibrations in a Continuous Boring Operation”, submitted to Mechanical Systems & Signal Processing, Dec. 2001. L. Pettersson, L. H˚akansson A. Brandt and I. Claesson, “Analytical and Experi- mental 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. H˚akansson, I. Claesson ans Sven Olsson, “Active Control of Machine-Tool Vibration in a CNC Lathe Based on an Active Tool Holder Shank withEmbedded Piezo Ceramic Actuators”, The8thInternational 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 op- erations and boring operations. Bothturning 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 pre- drilled 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 there- fore 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 con- ditions. 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 approx- imately 40 dB and resulted in a PhD thesisfor Lars H˚ akansson. The magneto- strictive design was however not suitable for industrial purposes thus further im- provements 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 devel- oped 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. 18 Cutting tool Tool holder shank Actuator 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 holder 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 withActive Control Sweden AB, a company trying to develop and market the active control technique in turning operations.