Basic Investigations of the Incremental Sheet Metal Forming Process on a CNC Milling Machine

by Sanjay Jadhav

Submitted to the department of Mechanical Engineering in partial fulfillment of the requirements for the award of the degree of “Doktor-Ingenieur” at the University of Dortmund Dortmund, Germany December 2004

Supervisor:

Prof. Dr.-Ing. Matthias Kleiner

Co-supervisor:

Prof. Dr.-Ing. Dr. h.c. Klaus Weinert

Date of the oral examination:

17th December 2004

Dortmunder Umformtechnik

Sanjay Jadhav

Basic Investigations of the Incremental Sheet Metal Forming Process on a CNC Milling Machine

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D 290 (Diss. Universität Dortmund)

Shaker Verlag Aachen 2005

Bibliografische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar. Zugl.: Dortmund, Univ., Diss., 2004

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Copyright Shaker Verlag 2005 Alle Rechte, auch das des auszugsweisen Nachdruckes, der auszugsweisen oder vollständigen Wiedergabe, der Speicherung in Datenverarbeitungsanlagen und der Übersetzung, vorbehalten. Printed in Germany

ISBN 3-8322-3732-1 ISSN 1619-6317 Shaker Verlag GmbH • Postfach 101818 • 52018 Aachen Telefon: 02407 / 95 96 - 0 • Telefax: 02407 / 95 96 - 9 Internet: www.shaker.de • eMail: [email protected]

ACKNOWLEDGEMENTS

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ACKNOWLEDGEMENTS The presented dissertation on the Basic Investigations of the Incremental Sheet Metal Forming Process on a CNC Milling Machine in its complete form is the outcome of the industrious efforts over three years at the Institute of Forming Technology and Lightweight Construction (IUL). The director of the IUL, Prof. Dr.-Ing. Matthias Kleiner has guided me by his exclusive methodology during the course of the research. He has been always a source of encouragement for which, I am highly indebted to him. The research was also guided by Chief engineers, Dr.-Ing. W. Homberg and Dr.-Ing. F. Kleiner. I express my sincere thanks for their research oriented guidelines. Further, I hereby also acknowledge the efforts of Mr. R. Göbel, Mr. R. Shanker, Mr. U. Dirksen, Dr.-Ing. A. Klaus, and Dr.-Ing. habil. S. Chatti for the fruitful discussions until the completion of this dissertation. The Graduate School of Production Engineering and Logistics has organized several scientific and non-scientific events for advancing the professional standard of the students. Being a student of the school, I express an immense pleasure to thank the committee as well as the program coordinator, Mrs. M. Syrou. I also acknowledge the financial support from the German Research Foundation [Deutsche Forschungsgemeinschaft (DFG)]. My appreciation to Mr. A. Brosius, Mr. M. Trompeter, Dr. rer. nat. S. Key, Dr.-Ing. S. Du, Mr. R. Govindarajan, Mr. N. Ridane, Mr. H. Ludwig, Mr. H. Klünder, Mrs. B. UlmBrandt, Mrs. J. Luxat, and other IUL as well as Graduate school colleagues for their direct or indirect cooperation. I am also indebted to Mr. D. Hoffmann, Mr. U. Wornalkiewicz, and Mr. W. Feurer for coordination during the experimental work. The dissertation preparation was assisted by committed efforts of N. Krishna and Raghunandan, many thanks to them. In the end, I express heartily gratitude towards my parents for that extraordinary level of care during all educational phases of my life. My special thanks also go to my brother Mr. R. J. Jadhav for his concern from the very first day of my study in Germany and my uncles, especially Mr. S. Bargal for cheering me throughout the painstaking period of compiling the dissertation. Further, I thank my uncle, Mr. J. K. Jadhav for his occasional help in academic life. Along with other well-wishers, I praise the cooperation from Sainath, Mahesh, Bhimraj, Rafik as well as the entire Jadhav and the Chavan family. My appreciations to the Prerna social group and V. P./ Z. P. schoolteachers for timely motivating conversations. Finally, my special appreciation to my wife, Adv. Kavita Jadhav for her care and kindness at all the time. Dortmund, December 2004

Sanjay Jadhav

ABSTRACT

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ABSTRACT The conventional sheet metal forming processes need part dependent tooling, which costs in terms of time and money. Due to these factors along with increasing variants variety in the sheet metal part fabrication, highly flexible forming processes are being developed. The Incremental Sheet Metal Forming (ISMF) is one of the emerging flexible forming technologies in the sheet metal engineering, which rather uses universal tooling that is mostly part independent. Hence, the process offers higher flexibility reducing the product development time greatly, and making it very suitable for low volume production. Fundamentally, a desired shape is formed on a fixed form defining support tool using an active small size-forming tool in the ISMF process. The aim of the presented dissertation is to realize the ISMF with different flexible supporting strategies compared to the form defining support strategies in order to acquire some basic information for building a machine for an advanced flexible forming process called as the Kinematical Incremental Sheet Metal Forming (KISMF). The tool path is one of the main process controlling variables and hence, different tool paths were studied to know the unique strengths of each tool path. Thereafter, several experiments were conducted using a CNC milling machine and a generalpurpose CAD/CAM system to avail the benefits, which are often confined to a special machine and software. The main objectives were the optimization of the process variables, including the tool path optimizations in order to form higher quality parts with minimum tooling and experiments. The experiments conducted from simple to complex shape categories revealed different suitable process controlling variables and tool paths. Further, different techniques to attain competitive part quality using simple flexible supports are reported. The competitive part quality is also governed by the thickness distribution that is dependent on the apex angle. Therefore, the optimum offset essential to overcome the limitations of forming with the sine law as per the apex angle and associated thinning are proved by forming a complex shape with almost vertical walls. Subsequently, an appropriate approach using preformed sheets was explored during the research to minimize thinning at lower apex angles. Based on the ISMF research results, some basic ideas for the realization of the KISMF process are presented. Finally, the conclusions of the contributed work have directed the research on the key potential areas, including the KISMF. The scope of this work summarizes the directions for production of parts with simple flexible tooling for expanding the process competence spectrum.

KURZZUSAMMENFASSUNG

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KURZZUSAMMENFASSUNG Bei konventionellen Blechumformverfahren kommen spezielle, werkstückspezifische Werkzeuge zum Einsatz, die unter hohen Zeit- und Kostenaufwand entwickelt und produziert werden. Die Inkrementelle Blechumformung (ISMF) ist eine neue Fertigungstechnologie mit in der Entwicklungsperspektive universal einsetzbaren, werkstückunabhängigen Werkzeugen. Daher bietet dieses Verfahren eine höhere Flexibilität und eine wesentliche Verkürzung der Produktentwicklungszeiten und ist besonders für die Kleinserienfertigung geeignet. Es arbeitet normalerweise mit einem fixierten, formgebenden Stützwerkzeug und nur einem kleinen, bewegten Werkzeug, das die Sollgeometrie auf das Blech abbildet. Ziel dieser Arbeit sind grundlegende Untersuchungen zur Inkrementellen Blechumformung sowie die Untersuchung verschiedener flexibler Abstützungsstrategien, um erste Kenntnisse für die Realisierung einer Umformmaschine für einen hochflexiblen Prozess, dem sog. Kinematischen Inkrementellen Blechumformen (KISMF), zu gewinnen. Ein wichtiger Schwerpunkt dieser Untersuchungen war die Analyse geeigneter Werkzeugbahnen innerhalb eines universellen CAD/CAM-Systems als wesentliche Vorraussetzung für die Entwicklung einer Prozesssteuerung bei der ISMF. Die berechneten Werkzeugbahnen wurden auf einer konventionellen CNC-Fräsmaschine realisiert und die speziellen Einsatzgrenzen des CAD/CAM-Systems sowie der Maschine bestimmt. Die experimentellen Ergebnisse bildeten die Voraussetzung für eine Optimierung von Prozessvariablen einschließlich der Werkzeugbahnen. So sollte die Bauteilqualität erhöht und der Werkzeugeinsatz vereinfacht werden. Die Versuche an zunehmend komplexen Versuchsbauteilen führten zu Aussagen über günstige Prozesssteuerungsgrößen, entsprechende Werkzeugbahnen und die Beeinflussung der Bauteilqualität. In diesem Zusammenhang wird über verschiedene Techniken einer flexiblen Werkstückunterstützung berichtet, insbesondere über die Blechdickenreduzierung in Abhängigkeit vom eingestellten Spaltmaß zwischen Stützwerkzeug und Formgebungsstempel. Der Einfluss optimierter Spaltmasse wurde an einem komplexen Bauteil mit fast senkrechten Seitenwänden nachgewiesen. Es wurde gezeigt, dass die die Anwendung des Sinus-Gesetzes zur Beschreibung der Blechdickenabnahme für diese extremen Scheitelwinkel nicht geeignet ist. Die Forschungsarbeiten schließen mit der Erarbeitung eines Ansatzes zur Umformung vorgeformter Bleche, durch deren Anwendung die Ausdünnung auch bei sehr kleinen Scheitelwinkeln reduziert werden konnte. Basierend auf den Untersuchungsergebnissen zum ISMF werden einige Ideen zum KISMF vorgestellt sowie zukünftige Potentiale und Entwicklungsfelder für eine Blechteilproduktion mit einfachen, flexiblen Werkzeugen sowie die Erweiterung des Einsatzspektrums für dieses Fertigungsverfahren werden benannt.

CONTENTS

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CONTENTS Acknowledgements...................................................................................................... i Abstract ....................................................................................................................... iii Nomenclature and Abbreviations ............................................................................. xi 1 Introduction............................................................................................................. 1 1.1 Background ........................................................................................................ 1 1.2 Focus of the Dissertation .................................................................................... 2 1.3 Organization of the Dissertation ......................................................................... 2 2 State of the Art........................................................................................................ 5 2.1 Flexible Forming Processes with Incremental Approach .................................... 5 2.2 The Incremental Sheet Metal Forming Process................................................ 16 2.2.1 Working principle ........................................................................................ 16 2.2.2 Process variants ......................................................................................... 18 2.2.3 Manufacturing cycles .................................................................................. 21 2.3 The ISMF Process Competence Overview....................................................... 22 2.4 Reported Scientific Work on the ISMF Technology .......................................... 23 3 Research Aim and Objectives ............................................................................. 29 3.1 Problem ............................................................................................................ 29 3.2 Aim ................................................................................................................... 31 3.3 Objectives......................................................................................................... 33 4 Experimental Setup and Feasibility Study ......................................................... 35 4.1 The CNC Machine and Experimental Setup ..................................................... 35 4.1.1 Tools and accessories for the ISMF ........................................................... 37 4. 2 Feasibility Experiments.................................................................................... 43 4.3 Implemented Manufacturing Cycle ................................................................... 48 4.4 Summary of the Chapter................................................................................... 50

CONTENTS

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5 Tool Path Generation and Development............................................................. 51 5.1 Main Aspects for Tool Path Generation ............................................................ 52 5.2 Adaptation of CAM Systems for the ISMF Tool Paths ...................................... 54 5.3 Tool Path Strategies ......................................................................................... 55 5.3.1 Tool path programming............................................................................... 59 5.3.2 Proposed tool path strategies ..................................................................... 63 5.4 Summary of the Chapter................................................................................... 65 6 Optimization of Process Variables and Enhancement of Geometrical Accuracy ............................................................................................................... 67 6.1 Analysis and Optimization of Basic Process Variables ..................................... 67 6.1.1 Analysis of effects and optimization............................................................ 73 6.2 Optimization of Tool Paths................................................................................ 76 6.2.1 Forming of rotational symmetrical shapes with part specific support and unspecific support....................................................................................... 76 6.2.1.1 Comparison of aspects with and without specific support..................... 81 6.2.1.2 Improvements after optimization........................................................... 82 6.2.2 Forming of non-rotational symmetrical shapes with specific support and unspecific support....................................................................................... 83 6.2.2.1 Analysis of the results........................................................................... 88 6.2.2.2 Optimization of the variables................................................................. 95 6.2.2.3 Correction strategies for enhancement of the geometrical accuracy .... 97 6.3 Forming of Complex Non-Rotational Shapes ................................................. 105 6.3.1 Offset modeling and tool path generation ................................................. 106 6.3.2 Optimization experiments and results....................................................... 107 6.4 Competitiveness and Benefits of the Simple Unspecific Support Forming ..... 116 6.5 Summary of the Chapter................................................................................. 120 7 Introduction to Forming of Non-Flat Sheets .................................................... 123 7.1 Forming of Curved Preformed Sheets ............................................................ 123 7.1.1 Experimental investigations ...................................................................... 124

CONTENTS

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7.2 Forming of Auxiliary Form Elements............................................................... 127 7.2.1 Experimental observations ....................................................................... 128 7.3 Forming Using Curved and Inclined sheets .................................................... 129 7.3.1 Demonstration of curved sheet forming .................................................... 130 7.3.2 Demonstration of inclined sheet forming................................................... 132 7.4 Summary of the Chapter................................................................................. 134 8 Some Ideas on the Kinematical Incremental Sheet Metal Forming Concept Realization........................................................................................................... 135 8.1 Kinematical Incremental Sheet Metal Forming Process ................................. 135 8.1.1 Working principle ...................................................................................... 135 8.1.2 Principal aspects for realization of the KISMF process............................. 136 8.1.3 The KISMF concept proposal on the existing CNC milling machine ......... 142 8.1.4 Summery of the proposal.......................................................................... 144 8.2 Summary of the Chapter................................................................................. 145 9 Conclusions and Future Developments ........................................................... 147 9.1 Conclusions .................................................................................................... 147 9.2 Future Developments ..................................................................................... 149

References ............................................................................................................... 151 List of Tables ........................................................................................................... 161 List of Figures.......................................................................................................... 163

NOMENCLATURE AND ABBREVIATIONS

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NOMENCLATURE Symbol

Unit 2

Description

A

mm

D

Degree

Contour half-apex angle

Main tool cross-sectional area

Di

Degree

Initial half-apex angle

Dc

Degree

Corrected half-apex angle

dft

mm

Main tool head diameter

Fr

mm/min

Forming speed or feed rate

Fx

N

Horizontal forming force along X-axis

Fy

N

Horizontal forming force along Y-axis

Fz

N

Vertical forming force along Z-axis

Ft

N

Total resultant forming force

Vb

MPa

Bending stress

Vc

MPa

Compressive stress

W

MPa

Shear stress

įavg

mm

Average geometrical deviation

įmax

mm

Maximum geometrical deviation

įmin

mm

Minimum geometrical deviation 4

I

mm

Ls

mm

Moment of inertia Sampling length for surface roughness

L

mm

Length for maximum bending moment

M

-

True strain

M

N·mm

Maximum bending moment

Ra

μm

Roughness average

Rmax

mm

Maximum concavity radius

Rmin

mm

Minimum concavity radius

S1

mm

Width of pyramid top

Tavg

mm

Average formed thickness

To

mm

Initial blank thickness

Tf

mm

Formed thickness

Wi

mm

Initial width of pyramid bottom

Wc

mm

Corrected width of pyramid bottom

y

mm

Distance from the neutral axis

Zinc

mm

Vertical increment of a tool path cycle

NOMENCLATURE AND ABBREVIATIONS

ABBREVIATIONS Abbreviation

Expansion

AFE

Auxiliary Form Element

Al99.5 W7 (Al99.5)

Pure aluminum, EN AW 1950A W7

Al99.5 H14

Pure aluminum, EN AW 1950A H14

AlMg3

Aluminum Magnesium alloy, EN AW 5754

Bi

Bidirectional tool path

CAD

Computer Aided Design

CAE

Computer Aided Engineering

CAM

Computer Aided Manufacturing

CMM

Coordinate Measuring Machine

CNC

Computer Numerical Control

DIN

Deutsches Institut für Normung

EN

European Norms

EP

European Patent

FEM

Finite Element Method

FLC

Forming Limit Curve

FLD

Forming Limit Diagram

G01

Linear interpolation

G02

Clockwise circular interpolation

G03

Counterclockwise circular interpolation

ISMF

Incremental Sheet Metal Forming

KISMF

Kinematical Incremental Sheet Metal Forming

IUL

Institut für Umformtechnik und Leichtbau

MS

Mild steel

NC

Numerical Control

PVD

Physical Vapor Deposition

RP

Rapid Prototyping

R&D

Research and Development

Ug

Unigraphics CAD/CAM/CAE system

Uni

Unidirectional tool path

2D

Two Dimensional

3D

Three Dimensional

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