CHAPTER 1
INTRODUCTION
1.1 AUTOMOTIVE BODY ENGINEERING
Automobile car body is a rigid shell which accommodates all the
functional and aesthetic parts of a vehicle. Body-on-chassis and Uni-body are
the two major body types used in automobile. Typically, sheet metal
components of various steel grades and thicknesses are used to build a rigid
body structure. Aluminum and composites are also used to make bodies with
limited quantity requirements and for special purposes.
The basic body structure of an automobile ranging from 200 to 400
parts, according to the model, is fabricated by spot welding method. The
range of spot weld in completing body structure varies from 3000 to 6000
spots. Figure 1.1 Shows the comparison of parts vs. spots distribution of
typical automobile bodies.
No. of Spots
No. of Parts
Figure 1.1 Number of parts vs. Number of spots comparison
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Body components and joineries are designed according to the
styling surface of the vehicle and the product designer assigns appropriate
number of spot welds to the welding joineries, according to the Computer
Aided Engineering (CAE) simulation outcome. A typical body structure of a
uni-body design is represented in figure 1.2.
Figure 1.2 Automotive body structure
Fabrication of automotive body is crucial because ultimately it
freezes the safety, convenience and comfort. Reduction of product
development lead time and control over product cost are the major
challenges in body engineering [1].
Body engineering department has six main areas of focus; sheet
metal, body metal, fenders, hood, radiators and general hardware [2]. Body
engineering is responsible for designing complete body structure with weld
spots, sealants and hardware details. Spot welds are generated in a spot
cloud module and populated in the body structure with spot weld annotations.
Process planners extract the body design and spot cloud data for
process planning from a design software platform (e.g. CATIA), and distribute
the spot welds in various weld stations in a body shop layout. This virtual
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plan is implemented in the physical body shop by the process engineers.
Simultaneous engineering, quick design change, and seamless processes
from the pre-styling idea development stage to the manufacturing stage are
very critical in product development [3]. Figure 1.3 represents the typical spot
weld cloud of an automotive body and figure 1.4 represents the body
structure with the spot weld cloud.
Figure 1.3 Automotive body spot weld cloud
Figure 1.4 Automotive body structure with spot weld cloud
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1.2 AUTOMOTIVE BODY MANUFACTURING
A typical body structure consists of three major assemblies,
figure 1.5
1) Under body (or) Floor complete
2) Upper body (or) Side complete
3) Closure (doors, hood and tail gate) assemblies
Basic structural knowledge of an automobile body is key for a body
design engineer to achieve the major product development targets of cost,
weight and performance [4].
The shop floor in which the bodies are built or assembled, is known
as a body shop or weld shop. In the body shops, around 200 components get
assembled in different welding stations or stages, classified as geometry
stations and re-spot stations. Body manufacturing shops are mainly classified
into three types; automated, semi-automated, and manual types. Automated
body shops require a larger space in the plant layout and the investment is
higher compared to the other two types. The reduction in investment and life
cycle operating costs can be achieved through implementation of modular
concepts in body manufacturing systems [5].
Figure 1.5 Body assembly family tree
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1.3 AUTOMOTIVE BODY QUALITY
1.3.1 Introduction
In the current automobile trend, quality has emerged as one of the
major priorities for all the manufacturers across the globe [6]. Focus on
quality has extended the need for optimization in processes and functions
that are both directly and indirectly related to the manufacturing processes
and design specifications [7]. In BIW, the quality of the car body is said to be
achieved, when the specification demanded by design is fully satisfied. The
purpose of quality control is to assure and maintain the characteristics of the
car body within the acceptable quality level. The success of automobile
companies is decided with the fulfillment of customer perceived quality [8].
Figure 1.6 Elements of BIW quality control
Figure 1.6 represents the four major elements that need to be
planned and controlled for achieving the specified BIW quality.
1. Dimensional Quality
2. Weld Quality
3. Fit & Finish
4. Sealing & Rust Prevention
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All four elements are defined in the product design phase and the
roles of the process design and manufacturing are vital in achieving the four
quality elements in continuous production.
Dimensional quality is directly linked with the other elements such
as fit & finish and weld integrity. Dimensional control of a part or sub
assembly begins with GD&T calculations (Geometric Dimensioning &
Tolerancing), which is one of the key activity in product design. The assembly
sequence and tolerancing for each part is derived from GD&T calculations.
The derived calculation will be incorporated in the product design and a
controlled process is required for achieving the same in manufacturing. A
dynamic and modern engineering approach will automatically reduce cost
and assures the highest possible on time product quality. Exact dimensional
management is the effective method of quality assurance [9].
Weld quality is another critical element to be planned and
controlled to achieve stiffness and reliability of the vehicle. Spot weld integrity
is highly significant as it is directly associated with the safety of the
passengers. Failure of critical spot welds in a vehicle may lead to major
failures of functions, which may lead to breakdown or severe damage of the
vehicle, causing bodily harm to the occupants.
1.3.2 Body Quality Assurance System
To manufacture a qualitative BIW, the quality function of a body
shop must have a defined and structured philosophy of work. It is necessary
to adopt an appropriate defect deduction system in the shop floor for filtering
defective assemblies. The level of periodic quality assurance of production
processes and verification of reliability in the operations are critical to
maintain quality [10]. A typical quality control system of a body shop employs
the following quality control measures.
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Body quality inspection plan is given in figure 1.7.
Figure 1.7 Body quality inspection plan
1.3.3 Panel Quality Inspection
Panel quality plays a vital role in automotive body manufacturing
processes [11]. Quality of sheet metal panels are confirmed through visual
and dimensional measurements. Checking Fixture (CF) is a quality
inspection gauge used to confirm individual panel dimensions. A checking
fixture has surface contours as per the component drawing specification to
check the dimensional quality of a component or assembly. Checking fixtures
are available for all press formed panels used in a body shop.
Checking fixture is used to inspect;
Panel trim line & break line conditions
Tooling & part mounting holes position
Panel profile to specification
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A typical checking fixture of a fender panel is represented in figure 1.8.
Figure 1.8 Checking fixture of a fender panel
1.3.4 Sub-Assembly Inspection Process
Inspection Fixture (IF) is a quality gauge used to confirm assembly
condition of welded (or) bolted panels. An inspection fixture replicates the
tooling pins of a welding fixture, thus part seating in inspection fixture
ensures (or) predicts the proper seating of parts in the welding fixture.
Inspection figure of a door assembly is given in figure 1.9.
Figure 1.9 Inspection fixture of a door assembly
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Inspection fixtures play a crucial role for consistent monitoring of
door hinge position, gap & flushness of door assembly. Sub-assembly
variation levels can be controlled through optimized datum-shifts between
assembly stations, measurement gauges and assembly process [12].
A BIW inspection fixture is used to replicate the main framer
condition, to confirm the matching of the upper body with the under body.
The main framer is the most crucial welding station in a body shop, which is
used to assemble all the major assemblies such as under body, side
assemblies, roof panel, roof headers and rails to form a body structure, figure
1.10.
Inspection fixture is used to inspect;
Dimensional quality of an assembly (Gap & Flush)
Mounting locations of closure parts
Parts seating condition in welding fixture
Figure 1.10 BIW main framing station
1.3.5 BIW structure dimensional inspection using CMM
Dimensional quality is a measure of conformance of the actual with
the specification. In the automotive body assembly process, dimensional
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control and its maintenance is critical to the product quality [13]. Dimensional
integrity of every part, every material, every tool and every process contribute
to the overall build quality [14].
Dimensional co-ordinates of a vehicle are represented by XYZ, figure 1.11.
X – Longitudinal direction
Y – Transverse direction
Z – Height direction
“X & Z” Coordinate references
“Y” Coordinate references
Figure 1.11 Vehicle co-ordinate system
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Three-dimensional (XYZ) measurements of a body structure are
confirmed using a Co-ordinate Measuring Machine (CMM). Position of major
mounting points and surface points with respect to vehicle co-ordinates are
confirmed through CMM Inspection. A Coordinate Measuring Machine
(CMM) is a device used for measuring the physical geometrical
characteristics of an object with respect to Computer Aided Design (CAD)
data of an object. The object can be a small part or a large BIW. CMM
machines are broadly classified in to two types;
1. Column mounted controlled movement CMM machine
2. Portable CMM machine
BIW CMM inspection is used to measure and analyze the below parameters
• Three-Dimensional accuracy of the BIW (XYZ positions)
• Interior / Exterior Fit & Finish
• Fitment of functional parts
Improving the range in measurement with latest technology is
critical to achieve high precision quality [15].
1.3.6 Body Building Process
1.3.6.1 Types of sheet metal joining
The complexity of new car body calls for the necessity of a
concurrent engineering approach. The best properties can only be achieved
by an integrated process of materials choice, body concept and joining
methods [16].
The familiar methods for sheet metal joining are,
1. Resistance spot welding
2. Metal Inert Gas (MIG) Welding
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3. Laser welding
4. Riveting
5. Adhesive bonding
However, resistance spot welding is the most widely used joinery
process across the globe for mass manufacturing of automotive bodies as it
holds significant benefits in the automobile context compared to the other
processes.
The salient merits of resistance spot welding (RSW),
1. Most suitable for mass production compared to any other
technique
2. Low capital investment and less operation cost
3. Moderate speed and environmentally friendly
4. Quality of the weld strength is high as the base metals are
fused after melting
5. Safe process and doesn’t require any special control system
6. Requires moderate skill for weld gun operation
7. Welding equipment is compatible with both manual and robotic
welding processes
8. Process time is very short compared to other techniques
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Body sheet metals can be joined in different methods, figure 1.12.
Figure 1.12 Sheet metal joining techniques
1.3.6.2 Resistance spot welding
In a car body, multiple types of high strength steels are used to
form a rigid structure. The sheet metal panels are typically joined by
resistance spot welding [17]. The strength of body assembly parts is mainly
dependent on the strength of the spot-welded joints. The weld quality of the
body is confirmed by destructive and non-destructive methods. Compared to
other welding processes, the resistance spot welding process has a low
process cost and requires a lower operator skill. The process is faster and
has a better weld accessibility compared to other welding methodologies.
This makes the resistance spot welding (RSW) process is ideal for robot
automatization and therefore makes it an idyllic choice for automotive
production [18].
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Figure 1.13 Spot welding illustration
Resistance spot welding is an application of Joule’s first law of
heating. The heat generated due to the resistance of a conductive workpiece
to the flow of current is a function of three parameters; weld current,
conductive workpiece resistance and the time duration of flow of current.
Spot welding illustration of a sheet metal combination is given in figure 1.13.
Joule’s first law of heating is represented as H=I2 RT.
Where,
H = Heat,
I = weld current,
R = conductive workpiece resistance
T=Time duration of flow of current.
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A resistance spot weld joint is typically designed such that a
welded joint has a shear force acting on it, when the sheets are subjected to
tension or compression.
Inspection of a spot weld is being done with two major methods,
figure 1.14.
1) Non-destructive testing
2) Destructive testing
Figure 1.14 Spot weld inspection methods
1.3.6.3 Non – destructive inspection
Non-destructive inspection of spot welds is carried out without any
deformation of the sheet metals to find ‘Pass’ or ‘Fail’ of the welding by
examining the surface of the welded spot. This way of inspection is done by
visual evaluation, dye penetrants and ultrasonic wave echo inspection.
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1.3.6.4 Destructive inspection
Chisel test
This is a method to confirm ‘Accepted’ or ‘Not Accepted’ spot
welds by using chisel and hammer, figure 1.15.
Chisel test is used to find the strength of a spot weld between the
two panels and the test is performed manually.
Figure 1.15 Spot weld chisel test
Teardown:
Teardown is one of the common methods employed to find the
spot weld strength in a BIW assembly.
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Figure 1.16 represents the process of tear down spreader tool is used to
open the welded joineries of the sheet metals with externally applied force.
Figure 1.16 Spot weld tear down spreader
1.4 INTEGRATED DESIGN AND MANUFACTURING - IDAM
In automotive industry, product and process design activities are
integrated and development proceeds simultaneously to deliver quality
products. Product and process designers work together to materialize the
virtual final design of the product, with quality and easy manufacturability.
Body design release activity happens with the concurrence of the
manufacturing engineering team to produce quality product with better
manufacturability.
BIW geometry co-ordinates of each part are defined by the product
designer in the design phase and it is the responsibility of process designer
to plan appropriate processes to set geometry between the parts in the
physical build. Spot distribution between the welding stations is a process
planning activity which is simultaneously engineered during the product
design stage.
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The given flow chart of the integrated product and process elaborates the
function of IDAM, figure 1.17.
Figure 1.17 Flow chart of integrated product and process design
1.5 NEED FOR QUALITY AND PRODUCTIVITY IMPROVEMENT
Quality and productivity are the two crucial elements of any
manufacturing facility, and continuous improvement of both the elements is
vital to the growth of any organization. Shanin has studied and highlighted
the role of quality in improving the productivity of operations. He determined
a strong correlation between quality and productivity [19]. Manufacturing
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systems and quality management practices are found to be critical in
maintaining competitive advantages [20].
Customer demands, globalization, shorten product development
time, cost cutting are the unprecedented challenges for the manufacturing
industry [21]. In the automotive industry, well organized and robust
manufacturing processes are required to achieve craftmanship targets [22].
Optimization of fixture design by reducing non-standard units in the weld
fixtures is critical to reduce the time for designing the fixture and reducing the
development cost [23]. Linking concept design with manufacturing
methodologies is essential for leaner design and leaner production [24].
Stability in product and process equipment is critical in reducing the quality
variations in a car body [25]. Optimization of fixture layout in a body shop has
a significant positive impact on the reduction of manufacturing costs [26].
Lean manufacturing concepts emphasize the reduction of process
times in manufacturing facilities to optimize the resources. Mothersell has
studied the brownfield conversion from mass manufacturing to lean
production in a large automotive company. He has explained the coherent
transformational model for converting the brownfield facility into a lean
production facility.
The following key technical components were considered in the
lean conversion study,
1) Changes in the assembly layout
2) Changes in TAKT time and jobs per hour (JPH), [27].
Improvements or innovations for improving quality and productivity
happen independently, without any mutual negative impact. In some cases,
both must rise together and this study is one among them.
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1.6 BODY SHOP WELDING FIXTURES
Geometry stations are designed and manufactured to locate the
parts and weld the key spots that set the geometry of the assembly.
Geometry fixture is a fixture that locates precisely, all the parts that are
added in that fixture and ensures that the geometry is set along with the key
spots welded in that fixture. Figure 1.18 represents the construction of a
typical geometry welding fixture. The assembled parts become a single piece
after the geometry station; which can be moved further in the system without
being concerned about the dimensional distortions. This means that all the
defined nominals of the two parts are fixed within the allowable defined
tolerance limits.
Figure 1.18 Geometry welding fixture
Re-spot stations are designed to complete the rest of the spots
defined by the Product designer. Re-spot welding fixture is just a work-
holding device or support device that holds the parts output from a geometry
fixture. No additional parts are added in this fixture and no additional units for
parts location are needed in these fixtures.
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Figure 1.19 represents the typical re-spot fixture of a body shop.
Figure 1.19 Re-spot welding fixture
Spot allocation among the stations is a critical process planning
activity; improper planning of spot allocation will impact quality and
productivity of the shop. Allocation of fewer spots in geometry station will lead
to quality deterioration and allocation of higher spots than the required spots
will lead to productivity loss.
1.7 GLOBAL TREND OF GEOMETRY AND RESPOT STATIONS
Global trend of geometry and re-spot distribution of various body
shops has been studied with the data collected from weld line builders
located in South Korea. Weld line builders deal with various automotive
manufacturers of the globe and are an appropriate source for getting
authentic data of various weld shop fixture distribution percentages.
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Geometry, and re-spot fixture quantity and JPH details are
obtained to understand the global trend. The weld line builders have shared
the requested details without disclosing the manufacturer details. However,
percentage of fixtures is considered as the focus of this study and not the
manufacturer. The models of United States, Europe and Asia are considered
to obtain generalized global trend in this study. The geometry and re-spot
fixture percentage is studied with the perspective of automation level used by
the various automakers.
Figure 1.20 Global trend of geometry & re-spot stations for automated
lines
Figure.1.20 shows the global trend in distribution of geometry and re-
spot stations for 100 % automated lines. The 100% automated lines fixture
details are denoted from X1 to X6. The global trend in distribution of welding
stations in 100% automated lines clearly indicates that the geometry stations
percentages are ranging between 31% and 45%. The re-spot percentage of
the models are ranging between 55% and 69%. The percentages of
geometry stations of the same capacity lines are designed with a 14%
difference. This trend also indicates that the geometry stations percentage is
low compared to re-spot stations percentages.
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Figure 1.21 Global trend of geometry & re-Spot Stations for semi-
automated lines
Figure 1.21 represents the global trend of geometry and re-spot
weld percentages of semi-automated and manual lines. The models are
denoted from Y1 to Y6 and plotted against the automation percentage. The
automation levels of these models ranging from 10% to 90%. The
observation from the trend chart clearly indicate the influence of automation
in selection of geometry and re-spot stations. The semi-automated/manual
lines are typically dedicated to specific models. Sharing of semi-
automated/manual lines by similar platform models is very rare. In both
cases, semi-automated and manual lines have dedicated geometry and re-
spot stations for each model. Automated lines are typically multi model lines
with changeover geometry fixtures and shared re-spot fixtures among the
models across a wider range. This is one of the major differences between
automated and semi-automated lines.
The trend of semi-automated and manual lines, given in figure
1.21, indicates the following key points.
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1) Geometry stations percentage is lower when the automation
level is above 40%
2) In the given weld lines with over 40% automation, geometry
stations percentage ranges from 33% to 45%. A difference of
12% observed on weld lines with same capacity models with
almost equal automation percentage.
3) The percentage of geometry stations is almost double in lines
with automation below 40%, compared to the lines with above
40% of automation. This drastic increase is the result of weld
lines being used predominantly for dedicated models rather
than multiple models.
4) The geometry stations percentages in weld lines with less than
40% automation lines are ranging between 53% and 82 %. A
difference of 29% observed on weld lines with models of same
capacity.
1.8 SUMMARY
The geometry stations percentage varies from 31% to 82% across
weld lines of different automation levels. The difference in geometry stations
percentage between semi-automated/manual lines could be related to the
automation levels of the weld lines. The difference in weld lines with similar
automation levels with same capacity shows that the difference is a result of
lack of optimization.
The variation percentage of same capacity weld lines are
summarized below.
1) Variation in 100% automated lines – 14%
2) Variation in 40% to 90% semi-automated lines – 12%
3) Variation in below 40% semi-auto / manual lines - 29%
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Higher the percentage of geometry stations in a body shop, lower
will be the productivity of the shop. It is observed that the cycle time of a
geometry station is considerably higher than a re-spot station and this is one
of the major focus area for productivity improvement. Addressing this issue
will lead to a significant improvement in the productivity of a body shop.
Based on studies, it is observed that the time taken to weld the
same spot is around 35% higher in a geometry station than when done in a
re-spot station.
The designation of minimum spot welds to set the geometry in the
geometry station must be done with a scientific approach. This ensures that
there is no negative impact such as issues in dimensional quality. Spot welds
required to set the geometry is one of the most critical factors which affect
dimensional integrity. Lower the number of spot welds welded than the
minimum required quantity, higher will be the probability of dimensional
variation in the assembled parts.
Productivity of a re-spot station is comparatively higher than that of
a geometry station because of the difference in process time, i.e., the time
taken to complete one spot weld in a geometry station is higher compared to
a re-spot station. In a manual geometry station as considered in this case
study, the average time taken to weld a spot is 7 seconds, whereas in the re-
spot station it takes only 4.5 seconds to complete a spot weld. This is
because of the complexity of the geometry station as there are more fixture
units; constraining the weld gun access in those stations. Comparatively, a
re-spot station is less complex due to fewer number of fixture units. It must
be noted that there is a mean difference of 2.5 seconds per spot weld; a 35%
reduction of the weld time is possible to achieve irrespective of manual or
automated processes.
In this study, this issue is addressed through a scientific approach
by distributing the minimum required spots in any geometry station. This
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enables assembly of more number of parts at the same geometry station.
This paves way for welding maximum number of spot welds at re-spot
stations. This will result in improved productivity, irrespective of the
automation influence on number of stations.
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