The Automotive Body Manufacturing Systems and Processes E-Book

Mohammed A. Omar

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 485

Veröffentlichungsjahr: 2011

Beschreibung

The Automotive Body Manufacturing Systems and Processes

The Automotive Body Manufacturing Systems and Processes

A comprehensive and dedicated guide to automotive production lines, The Automotive Body Manufacturing Systems and Processes addresses automotive body processes from the stamping operations through to the final assembly activities. To begin, it discusses current metal forming practices, including stamping engineering, die development, and dimensional validation, and new innovations in metal forming, such as folding based forming, super-plastic, and hydro forming technologies. The first section also explains details of automotive spot welding (welding lobes), arc welding, and adhesive bonding, in addition to flexible fixturing systems and welding robotic cells. Guiding readers through each stage in the process of automotive painting, including the calculations needed to compute the number of applicators and paint consumption based on vehicle dimensions and demand, along with the final assembly and automotive mechanical fastening strategies, the book’s systematic coverage is unique.

The second module of the book focuses on the layout strategies of the automotive production line. A discussion of automotive aggregate planning and master production scheduling ensures that the reader is familiar with operational aspects. The book also reviews the energy emissions and expenditures of automotive production processes and proposes new technical solutions to reduce environmental impact.

• Provides extensive technical coverage of automotive production processes, discussing flexible stamping, welding, and painting lines
• Gives complete information on automotive production costing as well as the supplier selection process
• Covers systems from the operational perspective, describing the aggregate and master production planning
• Details technical aspects of flexible automotive manufacturing lines
• Methodically discusses the layout and location strategies of automotive manufacturing systems to encompass the structural elements
• Features topic-related questions with answers on a companion website

Leseprobe

Table of Contents

Cover

Table of Contents

Halftitle page

Title page

Copyright page

Dedication

Preface

Foreword

Acknowledgments

Abbreviations

1 Introduction

1.1 Anatomy of a Vehicle, Vehicle Functionality and Components

1.2 Vehicle Manufacturing: An Overview

1.3 Conclusion

2 Stamping and Metal Forming Processes

2.1 Formability Science of Automotive Sheet Panels: An Overview

2.2 Automotive Materials

2.3 Automotive Stamping Presses and Dies

2.4 Tailor Welded Blanks and their Stamping

2.5 Advances in Metal Forming

2.6 Stampings Dimensional Approval Process

2.7 Stamping Process Costing

3 Automotive Joining

3.1 Introduction

3.2 Fusion Welding Operations

3.3 Robotic Fusion-Welding Operations

3.4 Adhesive Bonding

3.5 Welding and Dimensional Conformance

3.6 Advances in Automotive Welding

3.7 The Automotive Joining Costing

4 Automotive Painting

4.1 Introduction

4.2 Immersion Coating Processes

4.3 Paint Curing Processes, and Balancing

4.4 Under-Body Sealant, PVC and Wax Applications

4.5 Painting Spray Booths Operations

4.6 Material Handling Systems Inside the Painting Area

4.7 Painting Robotics

4.8  Paint Quality Measurements

5 Final Assembly

5.1 Basics of Final Assembly Operations

5.2 Ergonomics of the Final Assembly Area

5.3 Mechanical Fastening and Bolting

6 Ecology of Automotive Manufacturing

6.1 Introduction of Automotive Manufacturing Ecology

6.2 Energy Consumption and Accounting

6.3 The Automotive Materials’ Ecological Impact

6.4 The Painting Process Ecology

6.5 Ecology of the Automobile

7 Static Aspects of the Automotive Manufacturing Processes

7.1 Introduction

7.2 Layout Strategies

7.3 Process-oriented Layout

7.4 Cell-based Layout Design

7.5 Product-based Layout

7.6 Lean Manufacturing Tools for Layout Design and Optimization

7.7 Locational Strategies

8 Operational Aspects of the Automotive Manufacturing Processes

8.1 Introduction

8.2 Aggregate Production Planning

8.3 Master Production Scheduling (MPS)

8.4 Material Requirement Planning (MRP)

8.5 Production Line Control and Management Style

8.6 Selection and Management of Suppliers

8.7 An Overview of the Automotive Quality Tools

References

Index

THE AUTOMOTIVE BODY MANUFACTURING SYSTEMS AND PROCESSES

This edition first published 2011

© 2011 John Wiley & Sons Ltd.

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Library of Congress Cataloging-in-Publication Data

Omar, Mohammed A.

The automotive body manufacturing systems and processes / Mohammed A Omar.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-97633-3 (hardback)

1. Automobiles–Bodies–Design and construction. I. Title.

TL255.O43 2011

629.2'34–dc22

2010045644

A catalogue record for this book is available from the British Library.

Print ISBN: 9780470976333 [HB]

ePDF ISBN: 9780470978474

oBook ISBN: 9781119990888

ePub ISBN: 9781119990871

To Rania and Yanal, my sources of inspiration.

Preface

This book addresses the automotive body manufacturing processes from three perspectives: (1) the transformational aspect, where all the actual material conversion processes and steps are discussed in detail; (2) the static aspect, which covers the plant layout design and strategies in addition to the locational strategies; and, finally, (3) the operational aspect. The transformational aspect is discussed in Chapters 2, 3, 4, 5, and 6; while the static aspect is given in Chapter 7 and the operational aspect with its two different levels—operational and strategic—is presented in Chapter 8.

The transformational perspective starts by covering the metal forming practices and its basic theoretical background in Chapter 2. It also addresses the potential technologies that might be used for shaping and forming the different body panels using lightweight materials with a lower formability window, such as aluminum and magnesium. The text discusses the automotive joining processes in Chapter 3, covering the fusion-based welding technologies, mainly the metal inert gas (MIG), the tungsten inert gas (TIG), and the resistance welding practices. These welding technologies are discussed to explain their applicability and limitations in joining the different body panels and components. The welding schedules for each of these technologies are explained and the spot-welding lobes and dynamic resistance behavior are also explained. Additionally, Chapter 3 describes the adhesive bonding practices and the different preparations and selection process needed to apply and decide on the correct adhesive bonds. The different strategies applied by automotive OEMs to enable their welding lines to accommodate different body styles using intelligent fixtures and control schemes are also discussed. Finally, the robotic welders and their advantages over manual applications, in addition to discussing potential joining practices such as friction stir welding, are addressed in this chapter.

Chapter 4 discusses the automotive painting processes and its different steps; starting from the conditioning and cleaning, then the conversion and E-coating, followed by the spray-based painting processes. Also, this chapter describes the automotive paint booths’ design and operation, while addressing the difference between the solvent-borne, and power-coat-based booth designs. Other miscellaneous steps that include the sealant, PVC and under-body wax application and curing steps are presented. In Chapter 5, the final assembly area and the different processing applied to install the different interior and exterior trim parts into the painted car shell, are presented, in addition to the marriage area where the power-train joins the painted shell. The mechanical joining and fastening practices are given in detail, explaining the different strategies that automotive OEMs use to ensure the right tension loads are achieved in their mechanical joints.

In Chapter 6, the automotive manufacturing ecological aspects, from the materials used and their utilization in addition to the energy expended in the manufacturing process, are discussed. The ecological chapter includes comprehensive analyses of the energy and resources footprint for each of the transformational aspects. Additionally the painting process is discussed in detail to explain its air conditioning requirements, water usage and treatment, and finally its air emissions. Also, the effect of reducing the current automobiles’ weight on their overall environmental footprint, especially in the usage phase, is presented.

Chapter 7 starts the discussion of the static aspect of the automotive manufacturing processes; it explains the different strategies used to plan the factory layout from the process-based, the product-based, and the cell-based layouts. Additionally, the different details in regard to the factors that affect the manufacturing plant location are presented; also the factor rating method, the center of gravity method, and the transportation table are described. Chapter 8 provides the operational and strategic management aspects of automotive manufacturing. This chapter explains the aggregate planning process and the master production scheduling, then it further discusses the material requisition planning (MRP) steps and its basic operation.

This book can be perceived to be composed of four basic modules: module one starts with Chapter 1 that provides a basic introduction to the automotive manufacturing processes from the assembly and the power-train manufacturing steps, in addition to explaining the basic vehicles’ functionalities and performance metrics and the industry basic drivers and changers. The second module is focused on the transformational aspect, which is found in Chapters 2, 3, 4, 5 and 6. The third module is concerned with the static aspect of automotive manufacturing, which is found in Chapter 7. The fourth and final module is in Chapter 8, where the operational and strategic tools are discussed. Dividing the automotive manufacturing processes into these four modules enables the reader to gain a comprehensive understanding of the automotive manufacturing processes, control schemes, and basic drivers, in addition to their environmental impact.

Foreword

The automobile is the most complex consumer product on the market today. It affects every aspect of our lives. It also requires significant intellectual, capital and human investment to produce. The market related to automotive production is second to none, and vehicle production drives multiple sectors of national and international interest including areas related to energy, emissions and safety. Manufacturing a vehicle requires a multi-billion dollar investment, and is one of the highest tech operations in the manufacturing sector. Certainly, automotive plants are one of the largest wealth generators in the industrial world. Furthermore, there is also no doubt that the architecture of the automobile will change rapidly over the next several product generations. Such a rapid enhancements will induce a significant strain on vehicle production in the future. Much of the technology and concepts employed in vehicle manufacture will, by necessity, change to meet the growing demand for rapidly changing technology, higher quality, improved safety, reduced emissions and improved energy efficiency in new vehicles.

Mohammed Omar has significant experience in a variety of automotive manufacturing environments. He has taken these experiences and developed a number of thorough and innovative courses at the Clemson University—International Center for Automotive Research. This text is the culmination of over a decade of these industry, research and teaching efforts.

This text is presented to the reader in four main modules that clearly and concisely present automotive technology and vehicle manufacture. The first module provides an introduction to automotive engineering and the key manufacturing processes necessary to successfully product the modern vehicle. Basic vehicle functions and performance metrics are presented in this module, as well as typical drivers and changers in the automotive industry. Within this model the base processes such as welding/joining, paint/coat and assembly are presented. Such processes are critical not only in final product quality and capability, but also define the resource needs of the overall production process. This leads directly into the second module, which targets the transformational aspects of automotive production as they relate to the environment and the economy. In the second section, issues from material utilization to energy and resource consumption are analyzed and discussed. The text highlights these factors and their overall impact on the resource footprint of both the product and its manufacturing process. The third module shifts from the production processes to the static aspect of vehicle manufacture. Issues such as the overall plant design, manufacturing cell integration, operation and optimization strategies are presented along with several examples of successful implementations from various corporate strategies. Finally, the fourth module addresses operational and strategic tools used in automotive manufacturing. Issues such as aggregate planning process, master production scheduling and Material Requisition Planning (MRP) are discussed.

The integration of these four modules provides a fresh and innovative perspective on automotive manufacture that enables the reader to have a comprehensive understanding of the automotive production processes, control schemes, basic drivers, in addition to environmental impact. The text is a must have for the modern manufacturing engineer, and will provide the reader with a state-of-the art foundation for modern manufacturing. I highly recommend Dr. Omar’s timely book. I believe it will benefit many readers and is an excellent reference.

Thomas Kurfess

Professor and BMW Chair of Manufacturing

Director, Automotive Engineering Manufacturing and Controls

Clemson University

Acknowledgments

This book reflects the work of thousands of mechanical and automotive engineers and researchers, whose dedication to their engineering profession has led to great advancements in science and mobility products that served and continue to serve us all.

Sincere and special dedication is due to Professor Kozo Saito (University of Kentucky) for his continuous academic and personal guidance. I would like also to thank the mechanical engineering professors at the University of Kentucky for their encouragement and mentoring during my PhD studies. Additionally, I would like to thank all my colleagues at the Clemson University automotive and mechanical engineering departments for their continuous support and enriching discussions. Special thanks are due to Professor Imtiaz Haque for his invaluable guidance and support. Also, I would like to recognize all my students for their dedication and hard work; especially Yi Zhou (my first PhD student) and Rohit Parvataneni (artwork) for their selfless work.

I would like to recognize; Jürgen Schwab, Brandon Hance, and Ali Al-Kilani for their technical contribution and discussions. Finally, I would like to thank my high school mathematics teacher, Mr. Mohammed Edrees.

Abbreviations

ACEEE

American Council for an Energy Efficient Economy

AHSS

advance high strength steel

AISI

American Iron and Steel Institute

AIV

Aluminum Intensive Vehicles

APQP

Advanced Product Quality Planning

ARB

accumulative roll bonding

BH

Bake harden-able

BiW

body in white

BoM

bill of material

BUT

Bending-Under-Tension

CAD/CAM

Computer Aided Design and Manufacturing

CAFE

Corporate Average Fuel Economy

CBS

Cartridge Bell System

CCD

charged coupled devices

CGA

circle grid analysis

CMM

Coordinate Measuring Machine

CNC

computer numerically controlled

CO

Change-Over

COPES

Conductive Paint Electrostatic Spray system

CT

cycle time

DBS

Draw Bead Simulator

DC

Deformation Capacity

DoC

Degree of Cure

DP

Dual-Phase

DQ

Draw Quality

DQSK

Drawing Quality Special Killed

DSC

Differential Scanning Calorimetry

EDDQ

Extra Deep Draw Quality

EGA

electro-galvanized Ze-Fe alloy

ELU

Environmental Load Unit

EPA

Environmental Protection Agency

EPI

Energy Performance Indicators

EPS

Environmental Priority Strategy

ERP

enterprise requisition planning

FE

Finite Element

FLC

forming limit curve

FLD

forming limit diagram

FMEA

Failure Mode and Effect Analysis

FSW

friction stir welding

FTIR

Fourier Transform Infrared Spectroscopy

GD&T

Geometric Dimensioning and Tolerancing

GIS

Geographical Information System

GMAW

Gas Metal Arc Welding

GQA

general quality agreement

HAP

Hazardous Air Pollutants

HAZ

Heat Affected Zone

HDGA

hot-dip galvanneal

HSLA

High Strength Low Alloy

HSS

high strength steel

HVLP

high volume of air supplied at low pressure

IF

Inter-terrestrial Free

IOI

Industrial Origami Incorporated

JIT

Just In Time

LDH

Limiting Dome Height

LDR

Limiting Draw Ratio

LED

Light Emitting Diode

LIEF

Long-Term Energy Forecasting

LM

Lean Manufacturing

MIG

metal inert gas

MMCs

Metal Matrix Composites

MPS

master production schedule

MRP

material requirement or requisition planning

MTBF

Mean Time Between Failures

NC

Numerically Controlled

NVH

noise, vibration and harshness

OCMM

Optical Coordinate Measuring Machines

OEMs

original equipment manufacturers

PLCs

programmable logic controllers

PPAP

production parts approval process

PUEL

post-uniform elongation

RDC

remaining deformation capacity

RH

relative humidity

RHT

Ring Hoop Tension

SHA

Systematic Handling Analysis

SLP

Systematic Layout Planning

SMCs

sheet molding compounds

SMED

Single Minute Exchange of Die

SoP

Start of Production

SPM

Strokes Per Minute

STS

Shape Tilt Strength

% TE

% total elongation

TFE

Tube Free-Expansion

TGA

Thermo-Gravimetric Analysis

TME

Temper Mill Extension

TPS

Toyota Production System

TRIP

TRansformation-Induced Plasticity

TSA

thickness strain analysis

TWB

Tailor Welded Blank

TWB/C/T

tailor-welded blanks, coils, and tubes

TWC

Tailor Welded Coil

UEL

uniform elongation

UTS

Ultimate Tensile Strength

VOC

Volatile Organic Compounds

VSM

Value Stream Mapping

WHP

Work Hardening Potential

WHR

Work Hardening Rate

WIP

Work In Process

YS

Yield Strength

1

Introduction

1.1 Anatomy of a Vehicle, Vehicle Functionality and Components

Customers today perceive the value of an automobile based on its structure, its mobility function, its appearance, and other miscellaneous options such as infotainment. This fact motivates the automotive engineers to develop engineering metrics to judge each of these perspectives in a quantitative manner to help them improve their design, benchmark their vehicles against their competitors and, more importantly, meet the legal regulations. For example, the performance of a vehicle structure is dependent on the following criteria: Crash-worthiness (or passive safety), service life (or durability), its noise, vibration and harshness (NVH) characteristics, in addition to new metrics that have recently been viewed as value-adding, such as structure recyclability and weight efficiency.

Crash-worthiness defines the vehicle structure ability or capacity to absorb dynamic energy without harming its occupants in an accident, while the durability is the probability that the structure will function without failure over a specified period of time or frequency of usage. The NVH describes the structure performance in absorbing the different vibration levels and providing a desired (designed) level of comfort. The noise is defined as vibration levels with low frequency (<25 Hz), while harshness is the term for vibrations at (∼25–100 Hz). All the above structural requirements are controlled by intrinsic (density, Young’s modulus) and extrinsic (thickness, geometry, and shapes) material properties and joining strategies. So the material, its shape selections, and the manufacturing process control the overall performance of vehicular structures.

The vehicle mobility function is controlled by its ride and handling dynamics in addition to the drive-line and power-train systems’ reliability. Again, the choice of material (weight and stiffness) and the design geometries (center of gravity location) affect the vehicle’s performance. The vehicle appearance can be described in its styling, which is controlled by the panels’ shape, its geometrical fit (gaps, flush setting, etc.), and its final paint finish. Human visual perception evaluates the vehicle’s finish in terms of specific visual qualities: Color properties, encompassing three color attributes, in addition to color matching between the different vehicle parts such as the steel body and the plastic trim. Also, surfaces’ spatial properties as well as the vehicle’s geometric attributes such as gloss, texture, and haze control the customer’s perception. For example, if the paint on a vehicle suffers from orange peel, i.e. the paint looks like the peel of an orange, the customer might mistakenly observe this as a defect (variation) in the sheet metal roughness.

The vehicle’s main components and sub-systems can be categorically listed as: Power-train, chassis, exterior and interior trims, and the body in white (BiW) or vehicle body-shell. The power-train is composed of the prime-mover (the internal combustion engine, or electric motor), the gear system, and the propulsion and drive shafts, while the chassis includes the suspension and steering components, in addition to the wheel, tires, and axles. The interior and exterior trims compose the front and rear ends, the door system, and the cockpit trim. Finally, the body in white is made up of the closures (doors, hood, tail-gate) and the frame, see Figure 1.1). The frame can be of a uni-body design (Figure 1.2 (a) uni-body), a body-on-frame (Figure 1.2 (b)), or a space-frame (Figure 1.2 (c)). The uni-body design features stamped panels, while the space-frame is made up of extrusions and cast parts. The BiW closures are selected based on the vehicle’s constituent material dent-resistance properties (i.e. yield strength) while the frame is designed to provide specific torsional and bending stiffness.

Figure 1.1 Left: the vehicle body structure without closures, right: the complete vehicle BiW

Figure 1.2 Top left: (a) a uni-body design, top, right: (b) truck platform; and bottom right: (c) space-frame design

1.2 Vehicle Manufacturing: An Overview

After reading Section 1.1, we can conclude that vehicle performance is judged based on design strength, stiffness, energy absorption, dent resistance, and surface roughness. However, before designers select a material or design a specific shape, they should consider manufacturability. The manufacturability from an automotive body structure’s point of view is described in terms of the design formability, the joining ability (weldability and hemming ability), the achieved surface finish and surface energy, and its overall cost. This fact motivates a deeper understanding of the automotive manufacturing processes and systems, because it will ultimately decide the design’s overall cost, final shape, and functionality, that is, the design validity.

The automotive manufacturing activities can be analyzed on two levels: the manufacturing system and process levels. The manufacturing system view is typically investigated from three different perspectives: the production line (the structural aspect) which covers the machinery, the material handling equipment, the labor resources, and its allocations to the different activities. The transformational aspect includes the functional part of the manufacturing system that is the conversion of the raw materials into finished or semi-finished products. The transformational activities include all the stamping, casting, welding, machining and painting efforts within the plant. The third aspect is the procedural aspect which describes the operating procedures and strategies, which is further viewed from two different levels; the strategic level which identifies the product type, and volume (product planning), given the operating environment conditions (customer demands and regulatory issues). Additionally, the strategic plan includes the resources’ allocation in the manufacturing enterprise. The second level is the operational level which is focused on production control, i.e. meeting the strategic plan objectives through planning, implementing, and control and monitoring activities. These operational activities are further categorized by [1]:

1. aggregate production planning which suggests product plans based on the required product volume, using a generic unit such as the vehicle platform not type, to increase the level of confidence from the forecast information;

2. production process planning which controls the production techniques to be used, in addition to process routes and sequence;

3. production scheduling to determine an implementation plan for the time schedule for every job in the process route;

4. production implementation which is the execution of the actual production plan according to the time schedule and allocated resources;

5. production control to measure and reduce any deviations from the actual plan and time schedules.

Another important view on the automotive manufacturing systems relates to the information, materials, and value-added (cost) flows within the plant. The raw materials and supplier parts flow from upstream to downstream through the material supply system, the material handling system and finally through the material distribution system. However, the information flows in the opposite direction, that is from downstream to upstream, to synchronize the rhythm of production and control its quality; this information flow is typically called the pull production system to indicate that the customer side controls the quantity and quality (product type) of the production. On the other hand, the old push system meant that the manufacturing plant outputs vehicles according to a mass production scheme without any feedback from the customer side.

The automobile manufacturing processes are divided into two plants: the assembly plants and the power-train plants. Both of these plants specialize in different transformational processes and convert different raw materials into final parts. However, both are synchronized in time to integrate their final outputs into complete vehicles.

1.2.1 Basics of the Assembly Processes

An automotive assembly plant is responsible for the fabrication of the complete vehicle BiW, starting from a steel and/or aluminum coil and ending with a complete painted car shell. Additionally, the power-train, chassis components, interior and exterior trims are all integrated into the BiW at the end of the assembly process in the final assembly area.

The assembly sequence starts with the receiving area for the coil (which is typically made out of steel and aluminum), which also includes a testing laboratory to check material thickness and surface characteristics. After passing the testing, the coil is either stored or staged for blanking. The blanks are then transferred to the stamping press lines to form the different vehicular panels. A typical BiW consists of about 300–400 stamped pieces, however, only a few main panels affect the overall geometry, fit and finish. These panels are the roof, the trunk (inner, outer, and pan), the hood (inner and outer), the under-body, the wheel-house, the body-side, A and B pillars, the floor pan, the front module (engine cradle, crush zones, shock towers), the quarter panels, and doors (inner, outer). Some of these panels are displayed in Figure 1.3.

Figure 1.3 The different panels of the vehicle structure

After the stamping process, some of the panels are joined to create sub-systems in specialized cells, as in the case for the doors where their inners and outers are adhesively bonded, hemmed and spot-welded. Additional cells exist in the stamping area for other components which are then fed to the body-weld or body-shop area. The stamping process utilizes mechanical and hydraulic presses with different tonnage, accessories and dies, so it can handle different panels ranging in shape and size, from 0.1–6.5 mm in thickness and with dimensions as small as 1 x panel thickness to as large as 500 x panel thickness.

In the body-shop area, the different panels are joined to form the car shell, starting with the under-body and then the body-side (left- and right-hand) outers. The joining of such panels is first done using tack welding to hold the pieces in place, followed by permanent spot welds. A typical vehicle shell has around 5000 spot welds, achieved through robotic welders working in designated cells and programmed offline. The completed body shells will also go through a dimensional check process using laser illumination with a charged coupled devices (CCD) camera system to monitor the shell gaps, flush setting and fit. The body-weld also features metal inert gas (MIG) welding for the under-body.

The robotic welding cells are controlled and monitored through separate programmable logic controllers (PLCs) which are then connected through a main controller to enable the complete line control through a master PLC.

The completed BiW is then transferred to the paint-line. The paint booth area cleans the car shells in immersion tanks and applies a conversion coating layer (iron phosphate or zinc phosphate) followed by an electro-coat or e-coat layer. The subsequent paint layers require drying or curing, through a combination of convection and radiation-based ovens. Spray paint booths follow the immersion stages, to apply the primer, top coat and clear coat layers. Also the paint booth area features other important steps such as applying the under-body wax and sealants followed by their curing process. Inspection for paint quality in terms of thickness, color match and contaminants is also important in the paint-line. In the paint-line, the vehicles might be taken out of the overall production sequence to create color batches, thus reducing the paint color change time. However, at the end of the line, all vehicles are arranged back in sequence.

After the paint-line, vehicles are transferred to the final assembly area, where the interior (cockpit, seats, etc.) and exterior trims are installed. The final assembly area consists mainly of manual labor using power-tools and fixtures for the ergonomics, in addition to autonomous carriers that transfer the power-train components (engine, transmission, etc.) for assembly work (installing the cables, fuel hoses, and controllers) and then to the marriage area. The marriage area is where the power-train is installed in the vehicle body. The final assembly area features a variety of mechanical fastening and riveting operations to install the different trim components in the vehicle shell. Additionally, a variety of sensory systems is used to check the dimensional fit of the different components, in addition to ensuring the proper torque for each joint.

The final step in the assembly process tests the vehicle operation and build, using a chassis dynamometer and a water-test chamber.

The assembly plants require a sophisticated control system that not only monitors the different areas’ performance (stamping, body-weld, paint and final assembly) but also synchronizes these activities with the reception of parts from the suppliers’ network and with the power-train facility.

The flow of parts and semi-finished vehicles within an assembly plant go through different layouts within each assembly area. In the stamping area, the parts are distributed between the different stamping presses depending on the press tonnage and the dies assigned to that press. Also the staging and storing of stamped pieces are done on racks and then transferred to the body-shop or to specialized cells separately, see Figure 1.4. In other words, the layout in the stamping area is similar to a product-based layout not a process-based one. A product-based layout is similar to the ones found in small workshops or a carpenter shop, where the flow of pieces (panels) and equipment allocation (dies) changes according to the product type (vehicle type).

Figure 1.4 A schematic of a typical stamping line layout

In the body-weld, there is a main assembly line where the sub-assemblies are fed to be joined to the main body frame. So the body-shop layout is a process-based layout, because the focus is on repeating the same process for all product (vehicle) types. The body-weld overall layout is similar to a spine, where the specialized cells that create the door sub-assemblies (joining inner and outer), the hood, the under-body, feed the main line that joins them to the body main shell, see Figure 1.5. The paint-line layout starts with a single straight line for the cleaning and the conditioning steps, the conversion coating (phosphate), and the e-coating immersion tanks. Then the vehicles are sent to a selectivity bank area (with a flexible conveyor system) so batches of vehicles of the same color are created for the spray booths. Some original equipment manufacturers (OEMs) like Toyota do not use the color-batching strategy but instead developed their paint-line booths to use a cartridge color system, where the robots can switch between different cartridges to change colors, thus eliminating the need to clean the paint supply line every time the color is changed. The overall flow within the paint-line is displayed in Figure 1.6, illustrating the different painting steps and layout. The final assembly area follows a process-based layout using a straight or a horseshoe-shaped assembly line.

Figure 1.5 The layout of a body-weld line

Figure 1.6 The basic processes in an automotive paint-line

1.2.2 Basics of the Power-train Processes

The power-train facilities are mainly responsible for building the vehicle power-train and drive-line components such as the engine and transmission. The power-train plants feature different transformational manufacturing processes from those found in the assembly plants. The power-train plants use a variety of forging, casting and machining operations to fabricate the engine components and the transmission. For example, the engine cylinder blocks are made of cast iron or are cast out of aluminum or in some cases from aluminum with a magnesium core to reduce the total weight of the engine. After the entire engine and the transmission components have been manufactured, they are assembled manually. For example, after casting and machining the engine cylinder block and the exhaust manifold, forging the pistons and the crankshaft, and finishing the valves, the crankshaft is installed manually in the cylinder block and secured by the bearing caps, which are torqued automatically. Then the pistons are lubricated and installed in the cylinder block carefully to prevent scratching the cylinder lining. Then the cylinder head is mounted and torqued to hold the valves assembly. The inlet and exhaust manifolds are installed next and fastened mechanically. The testing of the engine operation is done next, using an engine dynamometer.

The power-train plant relies mainly on in-house parts and components, in contrast to the assembly plants. However, new trends in the power-train manufacturing have reduced the number of parts manufactured in-house, so most OEMs now manufacture using basic engine components: the cylinder block and head, camshaft, crankshaft, and the connecting rods. The cylinder block goes through several machining processes that consist of rough and final millings, in addition to a variety of drilling, reaming and tapping processes. In general, a typical cylinder block will go through around 70 processes. The typical cycle time in an assembly process is around 60–80 seconds per station, however, the typical cycle time for a power-train operation is around 3 minutes, which highlights the need for advanced or improved material transfer technologies in addition to high speed machining centers and multi-spindle drilling. This fact has motivated the use of specialized tooling and fixture systems along with multi-spindle head-changer and multi-slide accessories. Additionally, the three-axis computer numerically controlled (CNC) machining centers have increased the power-train flexibility and agility. Figure 1.7 shows an estimated cost-based comparison between the high speed machining centers and the multi-spindle drilling at different vehicle production levels. Additional advances in power-train machining include the use of super-abrasive tooling for the boring, milling and honing operations. For example, the crank-boring operation used to rely on tungsten carbide tooling to achieve close tolerances in the range of 0.02 mm, however, the use of a poly-crystalline diamond tool has led to better quality at higher boring speeds.

Figure 1.7 An estimated cost comparison between the multi-spindle drilling and high speed machining centers

To manufacture the cylinder head, around 50 drilling operations are used to apply around 70 holes, so flexible transfer lines and cell-to-cell automation help to reduce the cycle time. Other specialized operations like the cam boring include the use of a long-line boring bar with custom fixture, to lower or raise the cam. On the other hand, to manufacture the crankshaft, the OEMs have to apply a series of different operations with tight tolerances, that include balancing the mass of the forged steel material, turning of both edges for clamping, and turning for the main and pin bearings, drilling the oil holes, finish grinding for the main and pin bearings, then superfinish the main and pin bearings. Finally, the crankshaft is washed, balanced and inspected. The balancing is done using an intelligent fixture that rotates the shaft and compensates for any imbalances by drilling holes.

The camshaft follows a similar processing sequence to that of the crankshaft, with changes in the tooling used and with the addition of a hardening process, where the shaft is heated using induction coils, then cooled rapidly.

The power-train manufacturing processes are also responsible for making the transmission components, mainly the gear system. The typical material for the different automotive gears is based on alloyed steel that provides the hard finish for the gear teeth while the core is soft and tough, so that it resists continuous use in terms of fatigue and wear resistances. These requirements motivate the use of different heat treatment steps to achieve the hard teeth and ductile core, which include a carburizing step to increase the carbon content within a controlled depth within the gear surface, a quenching process to increase the hardness, and a tempering step to improve the core toughness.

The basic operation used to form the gear is based on hot forging, followed by variety of hobbing and shaping cutting steps to generate the gear teeth. In the shaping process, a cutting gear with the designed profile is used to generate a similar tooth profile in the blank gear, however, in the hobbing process, a worm-like cutter cuts teeth on a cylindrical blank to generate the teeth, hence the hobbing process cannot be used to generate internal gears. Other subsequent operations include gear shaving, where a helical gear-like cutter, with closely spaced grooves, meshes with the gear so that a controlled material is removed from the gear teeth surfaces.

The standard processes within a power-train facility are displayed in Figure 1.8. So, the overall functional look of the vehicle manufacturing processes can be shown in Figure 1.9.

Figure 1.8 The basic processes within a power-train facility

Figure 1.9 The basic processes in an automotive assembly plan

1.3 Conclusion

The automotive manufacturing processes (assembly and power-train) play a major role in deciding on the vehicles’ design characteristics and the overall cost. Thus it is very important for designers and engineers to understand the current manufacturing infrastructure available in their company’s production lines. This will pinpoint the manufacturing capabilities and limitations. At the end of the day, the designer will specify the design tolerances but the machine will control the achieved tolerances. Additionally, designers should consider the materials’ compatibility from the joining process welding point of view, to avoid galvanic corrosion issues.

Additionally, designers should be aware that the vehicle design complexity in terms of number of parts and intricate shapes results in additional manufacturing steps (added cost and processing time). Also, the number of robotic welders and stamping presses should be taken into account due to their direct impact on the production rate. However, one should recognize that different OEMs make their decisions with regard to the vehicle type, volume and design based on their business models, which might be based on one of the following;

1. competing based on differentiation;

2. competing based on cost; or

3. competing based on time to market.

A company competing based on different and distinguished vehicle types might add complications to their manufacturing systems to achieve new added features or provide a wide range of options. However, an OEM competing on cost aims to reduce the manufacturing cost through less complicated designs and options. The OEMs who compete based on response time typically utilize common components and designs shared between different vehicle types and between old and new models of the same vehicle type. For example, the engine cradle design can be shared between old and new models without affecting the customer’s perception of the vehicle as a new model, at the same time it helps the OEM to cut the development time and cost, in addition to reducing the set-up changes in the manufacturing plants.

Additionally, recent changes in the automotive market have forced the automotive OEMs to increase their product portfolio to accommodate new demands from emerging markets, mainly in Brazil, Russia, India, and China or the BRIC countries. This increase in vehicle models has shifted the OEMs’ manufacturing models from the economy of scale to an economy of scope, which motivates further understanding of the manufacturing environment because such a shift adds complications in the following areas: the sequencing of the different models, the production capacity forecasting, and parts (suppliers) and sub-systems’ diversity. So new manufacturing and design strategies should be implemented and explored to alleviate some of these challenges such as the use of modular systems and subsystems between the different models, which reduce the parts’ diversity and the variations within the processes. At the same time, the modularity might have negative impact on the OEMs’ overall flexibility [2].

The impact of the above challenges on the automotive industry have led the automotive OEMs to revise their production and business strategies through mergers with other OEMs and by implementing efficient manufacturing procedures such as lean manufacturing and its different derivatives and versions created to suit each company style and product type. The number of automotive OEMs has dropped from 36 in 1970, to 21 in 1990, and to 14 in 2000. However, the number of automobiles produced is around 55 million vehicles [3] with the majority of production taking place in Asia (around 18 million vehicles), followed by Western Europe (17 million vehicles) and then the USA (around 11.5 million vehicles).

Exercises

Problem 1

In your own words, describe the current metrics used to judge the automobiles, by a typical customer and by an automotive engineer.

Problem 2

What is the difference between the noise and vibration within the context of the automotive NVH requirement?

Problem 3

Explain the three manufacturing system perspectives.

Problem 4

What is the difference between the operational and strategic operations, within the procedural aspect?

Problem 5

List the basic manufacturing processes involved in the making of the automobile BiW, and comment on each process layout and drivers.

Problem 6

List the main differences between the automotive power-train and assembly processes, from the following perspectives: the dependence on suppliers’ parts, and the nature of the processes utilized.

Problem 7

What is the difference between the gear hobbing and shaping processes?

Problem 8

What is the impact of economy of scale and the economy of scope on the automotive manufacturing process?

Problem 9

Automotive OEMs might compete based on different criteria, what are they?

Problem 10

List three challenges that have impacted the automotive industry in the past decade.

2

Stamping and Metal Forming Processes

The formability of sheet metals is one of the most important steps in the automobile manufacturing activities, because it is the decisive factor in the vehicle shell shape (styling), geometry (fit and finish), and performance (wind noise, water leakage). Additionally, the development and approval of the stamping dies are the most expensive and time-consuming efforts during a new vehicle design and launch. The die approval process can consume about 50 weeks before the Start of Production (SoP) to achieve the final dimensional validation.

The stamping of sheet metals can be defined as the process of changing the shape of the sheet metal blank into a useful shape in the plastic deformation state, using a die and a mechanical press; stamping is considered a net shaping process. However, the stamping engineering efforts are not limited to production engineering (i.e. the stamping process) but also include the development of the required tooling (i.e. stamping engineering). Such tooling includes the die making in addition to the fixtures and the automation tools such as the transfer mechanisms typically equipped with suction or electromagnetic cups. For the die making process, stamping engineering starts with the desired panel shape provided by the designer in a CAD file, in addition to the sought panel mechanical properties such as dent resistance (i.e. yield strength). Then, the engineers start with the material selection, i.e. selecting the steel grade, thickness and heat-treatment from what is typically provided by the steel mill. Feasibility analyses follow for each selected material, which lead to a process plan (process settings). After that, the die surface design starts with Finite Element (FE) simulation and numerical trials, followed by the actual (experimental) testing. Successful die designs will then be constructed and validated through a series of try-outs in the die-maker facility and then at the stamping line, using different number of parts (prototypes) and dimensional validation strategies (functional build, event-functional build). Finally, the automation and auxiliary tooling are constructed for each approved die for series production.

On the other hand, the stamping process starts with the steel and aluminum coils provided by the mills with specific thickness, surface topography, widths, and heat treatments. Additional inputs to the stamping press are: the die (toggle, progressive), the lubricants (water or oil), the tonnage conditions, and other process settings such as clearances. Generally, the stamping process constitute following main operations; blanking (or blank preparation), stamping (forming), and assembling activities.

A typical material flow in the stamping area is shown in Figure 2.1. The sequence in Figure 2.1 describes the following main operations:

1. Blank preparation: involves a cutting action about a closed shape that is the piece retained for future forming (i.e. the blank). The blank shape is composed of any number of straight and curved line segments. A more detailed look at blanking shows that it is further composed of slitting and shearing. Slitting is the process of cutting of lengths (usually coils) of sheet metal into narrower lengths by means of one or more pairs of circular knives. This operation often precedes shearing or blanking and is used to produce exact blank or nesting widths. Shearing is done by a blade acting along a straight line. The sheet metal is placed between a stationary lower blade and a movable upper blade and is sheared by bringing the blades in contact.

Other cutting operations exist in the automotive stamping for developed blanks, such processes include: (1) piercing which is the forming of a hole in sheet metal with a pointed punch without metal fallout; (2) lancing that creates an opening without completely separating the cut piece from the body of the sheet metal, such as the case for louvers; (3) trimming is the process of removing unwanted metal from the finished piece that was required for some previous stamping operation, such as binder areas, or was generated by a previous stamping operation, such as the earing zone on the top of a deep drawn cup; and (4) parting operations are used to separate two identical or mirror image stampings that were formed together (typically for the expediency of making two parts at one time or to balance the draw operation of a nonsymmetrical part). Parting also is an operation that involves two cut-off operations to produce contoured blanks from strip.

2. First forming operations: which aim at forming the blank into a semi-developed blank that has the initial shape. First forming operations include bending and flanging through either a shrink flanging, where the length of the flange shrinks as it is formed, or stretch flanging where the material is stretched as it is flanged.

3. Drawing operations: in the automotive press shop, most dies are called draw dies because the metal is drawn into the die cavity. However, most of the deformation modes are based on biaxial stretch over the punch or a bend-and-straighten from the flange. Drawing, sometimes known as cup drawing, radial drawing, or deep drawing, has a very specific set of conditions which differentiate it from other operations. The unique attribute of deep drawing is the deformation state of the flange. As the blank is pulled toward the die line, its circumference must be reduced. This reduction in the circumference generates a compressive stress in the circumferential direction, resulting in a radial elongation as the metal is extruded in the opposite direction.

4. Subsequent operations: most of the automotive panels require a sequence of forming steps because the degree of forming (flange angle, etc.) cannot be accomplished in a single step. Such operations include the re-strike step which comes after the metal has been stretched over a large radius punch (to avoid splitting), to spread the metal into the desired shape without any additional tension in the stamping line. Another typical subsequent forming operation is the redrawing. Limits are imposed on the blank diameter which can be drawn into a cup of a given diameter (this will be discussed in further detail in Section 2.1.3). Should a deeper cup be required, an intermediate diameter cup is drawn first; then the cup is redrawn in one or more subsequent stages to achieve the final diameter and height.

5. Assembling activities: these include variety of specialized cells for combining panels to form BiW components such as joining the door inners and outers. Additionally, other assembling might be done in die-joining strategies.

Figure 2.1 The sequence and basic steps in the stamping line

2.1 Formability Science of Automotive Sheet Panels: An Overview

The formability can be defined as the extent to which a sheet metal can be formed or worked into a specific shape without failure (cracking) and/or forming other undesirable features (e.g. Lueder bands). Formability is neither a material property nor a process property but it is a system property, dependent on the intrinsic and extrinsic sheet metal properties in addition to the process conditions.

In general, the variables that control formability of sheet metals within automotive production are:

1. materials variables: such as its thickness, width, n-value, r-value, m-value, surface topography, coating type, tensile and yield strengths, etc.;

2. blank variables: size, location, contour, flatness, edge conditions, pre-bend, etc.;

3. die variables: surface finish, rigidity, clearance, draw-beads, wear-plate tolerance, punch and die radii, etc.;

4. press variables: ram and bed flatness, shut height, inner ram load, press type and action, punch guidance, punch speed profile, etc.;

5. other variables: material temperature, die temperature, atmospheric conditions, etc.

The stamped pieces quality are typically judged based on the panel appearance, the resulted strains (patterns, directions), and its final dimensions. Such criteria can be further quantified through stamped panels’ final geometric characteristics, which include one or a combination of four main geometric shapes: plane, tunnel feature, dome element, and irregular features. Additionally, the resulted strains values and gradient describe the metal flow pattern. The most severe of stamped defects is the formation of a split or crack in shaped panels. Researchers have tried since the 1950s to develop formability metrics and theories with a focus on split or crack avoidance in sheet metal. According to Wang (in [4]), research teams focused on correlating the split occurrence with the material n-value and r-value through utilizing fracture mechanics. Further work by Keeler and Goodwin [5, 6] has established the splitting criterion in the plane stress states, i.e. in the bi-tension deformation state and the tension-compression deformation state. This led to the forming limit curve (FLC) or forming limit diagram (FLD), which describe the split tendency and the material deformation capacity in relation to two plane strains, called the major and minor strains. The application of the FLC will be discussed in more detail in Section 2.1.4.

Later work by Yoshida [7] focused on the stamping surface defects such as the splits, and the formed panel shape change through spring-back and/or distortion. Yoshida developed the first stamping indices, anti-fracturability and the shape-fixability. However, recent advances in the stamping process and engineering required further investigations due to the addition of new steel grades, mainly the high strength steel (HSS) and the advance high strength steel (AHSS), in addition to the use of more stamped aluminum in vehicle bodies. This chapter will focus on the Universal Formability Theory as proposed and developed by Xu [4] to describe the different formability indices; however, the text will not discuss the mathematical background or derivation in detail but will focus on the application of such theory to the evaluation of automotive stamped panels.

The main stamping defects analyzed by the Universal Formability Theory are as follows:

1. Splitting in the stamped panel: local necking rupturing in the stamped panel away from the edge.

2. Splitting at the edge of the panel: rupturing near the edge of the stamping due to the lower deformation capacity at the edge due to the shear zones (edge burrs and cracks).

3. Wrinkling: surface waviness resulting from compressive plastic instability.

4. Shape change: this is the elastic recovery within the panel caused by distortion and spring-back. The spring-back can be a first spring-back or a second spring-back, depending on its occurrence after the first or the second unloading of the panel from sequential stamping processes.

5. Low stretch: causing a lower work hardening performance of the formed panel, thus affecting its dent resistance.

6. Surface soft or low oil canning load ability: typically caused by the residual stresses from the different loadings in sequential stamping.

From the above, one can summarize the mechanisms that form defects as: defects due to extreme stresses and strains (as in the case of splits); the stress and strain gradients; the deformation history of the panel; and the residual stresses after unloading from the die cavity. The following discussion focuses on the formability indices developed to address each of the above stamping defects and their formation mechanisms.

The Universal Formability Theory suggests six formability indices to comprehensively address these stamping defects:

1. The anti-fracturability index: to address the splitting of the stamped components due to tensile stresses (away from the edge). The sheet metal passes through four stages of deformation before it splits. These stages are: (a) the elastic deformation when stressed within the yield-stress of the material; (b) the uniform elongation when the applied stress reaches the yield-stress; (c) then the diffuse necking stage; and finally, (d) the local necking which leads to fracture. Figure 2.2 illustrates these four stages on the stress-strain diagram. The deformation when the local necking occurs is the material maximum deformation capacity (DC) that can be safely utilized in automotive stamping. Knowing the material DC enables the engineers to predict the remaining deformation capacity (RDC) of the shaped sheet metal after the first forming operation. This can be done by measuring the strain difference between the local necking point (i.e. DC) and the strain after the first forming.

Knowing the panel’s RDC helps the stamping engineers to qualify their stamping practices as safe, marginal, or critical. Furthermore, the forming limit curve (FLC) helps the engineer visualize the stamping strain state relative to the material available DC. Figure 2.3 shows a typical forming limit diagram for a steel panel. The diagram includes the plane strains on the x and y axes, while the solid line represents the DC of the material. Thus one can measure the RDC at any point in the stamping by measuring the linear straining path or distance (measured from the origin) from that point to the solid line. A marginal region is typically established, around 10% for steel and 8% for aluminum samples to include a safety factor. This safety factor is typically measured as an increment in the major strain as shown in Figure 2.4 which shows the RDC and the ΔFLD or safety factor. However, one should note that the safety factor represents an actual value for the plane strain deformation; in other word, the case when the minor strain is zero, thus the RDC is a more accurate description of the strain state (safe, marginal, or critical). At the same time good stamping practice requires that the forming consumes at least 50% of the material DC. More details about the FLC and how to measure the major and minor strains using the Circle Grid Analysis is given in Section 2.1.4.

2. The anti-edge fracturability index: this index describes the weakened deformation capacity of the material near an edge. The shearing action along the material edge creates micro-cracks and burrs that tend to create a work-hardened region, that ranges in width from 50–70% of the material thickness. Additionally, the burrs and cracks work as fracture factors/initiators near the edge. The material RDC near the edge can then be evaluated in the same manner as in the case of the anti-fracturability of the stamping away from the edge. However, the difference in straining path should be taken into consideration for the case of simple tension and one should also take the burr height into consideration. Simple calculations can assume that the burr height is equal to the panel thickness; thus, the RDC is the difference between the DC (in simple tension) and the major strain.

3. The anti-wrinkle-ability index: wrinkles resulting from unbalanced compressive stresses are typically evaluated based on their geometric characteristics as three types: Types I, II and III. Type I is the most severe with height greater than or equal to the gap between the upper/lower die and the sheet metal; and Type III with height that can only be measured using specialized optical illumination.

4. Shape fixability and shape change index: upon lifting the die surface, the formed panel still possesses some elasticity in the form of spring-back and distortion that can lead to changing the formed shape. The spring-back is typically considered a shape change that causes the product dimensions to be out of tolerance toward one side of the reference surface, while the distortion is when the product dimensions are out of tolerance with both sides of the reference surface.

Spring-back is more severe for aluminum than steel, because spring-back decreases as the materials’ Young’s modulus increases. Steel has three times the Young’s modulus that of aluminum, however, the spring-back increases as the yield strength increases. This means that the spring-back in the case of aluminum is less than three times that of steel. This also explains why the dies made for aluminum are typically larger than those made for steel, for panels with the same shape and dimensions. Also the spring-back in the case of high strength steel (HSS) is higher than that of low mild steel. The spring-back effect can be visualized in Figures 2.5 (a) and 2.5 (b).

Typically, spring-back is more pronounced in channels and under-body structures and classified as an angular change, side-wall curl, and twist. The angular change happens when the bending edge line deviates from that of the forming tool and is typically caused by stress difference in the sheet thickness direction, when a sheet metal bends and unbends over a die radius. This stress difference in the sheet thickness direction creates a bending moment at the bending radius after dies are released, which results in the angular change. Sidewall curl is the curvature created in the side wall of a channel. This curvature occurs when a sheet of metal is drawn over a die/punch radius or through a draw bead. The main reason for this unevenness in the thickness direction is due to the stress generated during the bending and unbending process. The inside surface initially generates compressive stresses while the outer surface generates tensile stresses. During the bending and unbending sequence, the deformation histories for both sides of the sheet are unlikely to be identical. This usually manifests itself by flaring of the flanges, which is an important area for joining to other parts. The resulting sidewall curl can cause assembly difficulties for rail or channel sections that require close tolerances between the mating interfaces during joining. The twist spring-back is developed due to the torsional displacement developed in the panel cross-section as a consequence of the residual stresses acting in the part to create a force couple which tends to rotate one end of the part relative to another.

5. Stretchability index: this index is especially important in the case of flat, large exterior panels, because of the dent resistance and oil canning requirements. The oil canning tests for the maximum static, normal load cause surface elastic instability, while the denting for doors, as an example, can occur from stone impacts (dynamic denting). Denting can occur because the door surface is smooth and may not have sufficient curvature to resist “door slamming” (quasi-static denting) or along prominent feature lines where “creasing” can occur. The typically used standards, those defined by American Iron and Steel Institute (AISI), define a minimum dent resistance of 9.7 Joules. Based on testing using the practical techniques outlined, empirical formulae predicting the force and energy required to initiate a dent, can be shown in Equation 2.1:

(2.1)

where W is the denting energy, K is a constant, YS is the material yield strength, t is the panel thickness and S is the panel stiffness. Panel stiffness depends upon the elastic modulus, the panel thickness, shape and geometry.

For such panels, the stamping process should achieve a critical strain value so a certain amount of work hardening is achieved. Work hardening is required so that the final flow stress is able to satisfy the dent requirements because the initial blank has lower yield strength; the larger the metal deformation, the higher the final yield strength of the shaped panel. The stretchability defect is sometimes called “low stretch.”

6. The anti-buckling index: