RECONFIGURABLE TOOLING FOR AIRFRAME ASSEMBLY

1. INTRODUCTION
The major forces influencing todays aerospace manufacturing environments are not different
from other manufacturing industries: global competition, shortened product life cycle, increasing
requirements for quality and reliability, faster paced advances in increasingly complex
technology, rapidly expanding options in materials and processes and increased unpredictable
surroundings. The ability of an aerospace enterprise to take advantage of these forces is the key
to any successful aerospace manufacturing strategy. The overriding goal of the manufacturing
enterprise is to achieve rapid flexible integrated design and manufacture of innovative products
in batch sizes that are getting smaller at a price the customer is prepared to pay [1]. To thrive in
the emerging aerospace market condition it is therefore important that the manufacturing
enterprise should be capable of rapidly responding to market trends by utilizing intelligent
technology within a rapid product development environment capable of very short times to
market. To meet such requirements, technological advances have been made during the past few
decades with respect to machine and cutting tools. Nevertheless, the fixturing technology in
machining is still lagging behind despite its criticality and importance.
Fixturing has been considered as one of the main problems to improve flexibility, productivity
and part quality, particularly in the aerospace industry. It is one of the most direct threats to cost
effectiveness and operational efficiency. The case for flexibility and automation is reinforced
further by crucial socio-economic issues such as the high cost of capital, the high cost of direct
labor, and the shrinking skilled-labor pool. As a result, a great deal attention has been directed
towards the development of Flexible Manufacturing Systems (FMS) in the past decades.
Fixtures are devices designed to repeatedly and accurately locate a workpiece in a desired
position and orientation relative to a global reference frame (e.g. machine tool), and securely
hold it in that location throughout the manufacturing process. Modern manufacturing
environments impose a continuous technological pull towards more flexible and efficient
equipment. The field of tooling and fixturing could not constitute an exemption of this trend. If
perfection could be achieved, fixturing could be produced at a high speed and have the ability to
hold any complex part. Generally, in manufacturing environments, fixture design is based on
past experience and trial-and-error approach. Therefore, fixturing design is expected to be a long
process with prohibitive cost. As FMS fills the gap between high-volume transfer lines and a
highly flexible manufacturing situation, it is adopted to respond quickly, smoothly and cheaply
to as yet unknown changes in product markets and production technology. It is economical in the
low-to-medium volume range, because of the short time and the low cost involved for the set-up
to accommodate a newly designed component.


2. AIRCRAFT ASSEMBLY TODAY

Flexibility in the production area and flexible production systems is being spoken of since the
early 1960s. One clear reason for this is that most manufacturing industries more or less have
experienced the rising number of product variants with simultaneously decreased product
lifespan. Nowadays, more things are done in smaller quantities, which increase the
demands on the production systems. The system will handle more product variants, with
differing customer demands and the ability to change product generation frequently. The
consequence of all this is that if a company uses product or variant specific tools, the cost
needs to be distributed among fewer products, which gives heavily increasing product costs as
a result. Ac c o r d i n g t o Bu l l e n ( 1 9 9 9 ) , big cost drivers are involved in aircraft
assembly and that assembly-related operation account for over 40% of total airframe
manufacturing costs.
Traditionally, the design of an aircraft is also used to produce the aircrafts assembly tools with
which to manufacture that specific aircraft. Initially, master tooling gauges are produced to
an extremely high standard and calibrated using national standards. These gauges are used
to calibrate the fixed jigs and tooling which in turn are used in the manufacturing process. The
gauges form a physical standard to which the aircraft is manufactured. However, there is a
significant lead-time associated with the manufacture and calibration of the tools. Jigs and
tooling constitute a significant proportion of the cost of manufacturing aircraft structures.
The commercial aerospace sector is intensively competitive manufacturers attempt to
drive down the acquisition and operating costs of their aircraft (Gooch, 1998).
Almost all the parts of an aircraft are assembled by riveting. However, currently the riveting
processes are done mostly by human operators, especially in small or middle scale
companies of aircraft manufacture. Most of the existing automatic riveting systems for airplane
fastening are very big and very expensive. In addition, the jigs for airplane assembly are high in
cost (Li et al, 1996). The cost of designing and fabricating the diversity of jigs to satisfy the jig
requirements of a manufacturing system can amount to 10-20% of the total system (Nee et al,
1995), and the storage of dedicated jigs occupies lots of space. Jigs are critical in the
development of new manufacturing techniques and largely dictate the level of flexibility a
manufacturing system can achieve.

3. EXISTING FLEXIBLE AND RECONFIGURABLE SOLUTIONS

Many fixturing techniques have been investigated during the past few decades and majority were
developed to meet changes in manufacturing industry. The following are some of the typical
questions that are considered in fixture design
1. Does the fixture accurately locate the part relative to the cutting tool?
2. Does the fixture ensure that the part is totally constrained?
3. Can the part be easily loaded into/unloaded from the fixture?
4. What is the role of type and number of contacts on fixturability of the part?
5. What is the minimum clamping force needed to restrain the part during machining?
The flexibility of a whole FMS is restricted by the flexibility of any of its components, including
fixture systems. The cost of designing and fabricating the fixtures in an FMS can amount to 10
20% of the total system cost. Traditionally, the function of a fixture is to hold a part in order to
keep that part in a desired position and orientation while the part is in manufacturing, assembly,
or verification processes.
Custom-oriented dedicated fixtures are not only time-consuming and costly to build, but they
also do not have the flexibility to deal with parts or assemblies of different shapes and sizes. To
reduce the cost of a manufacturing system, the fixture system should be designed to be
competent in fixturing as many workpieces as possible. In low-to-medium volume production,
FFSs that are competent in fixturing different kinds of workpieces become a prospective way of
reducing the unit cost of a product.
Fixturing systems can be divided into the following systems.
i. Flexible/Reconfigurable: can be used for more than one part and for multiple operations
e.g. modular fixtures, pin array, phase change, etc
ii. Permanent/Dedicated: specially designed to hold one part for a limited number of
operations; commonly used in high volume production.
iii. General purpose: mechanical vise, lathe chucks.

3.1 Modular Tooling
A modular fixturing system is a fixturing system that uses a collection of reusable standard
components to construct a complete variety of special-purpose work-holding devices. It reduces
fixture manufacturing time, makes fixture modification easier and eliminates dedicated fixture
storage space. Modular fixture design was mainly based on trial and error until the development
of computer systems to aiding the design process [5, 7].
Modular Tooling is a tooling technology using modular thinking to get flexibility. The
modularity is about building fixtures from a collection of standard parts. The details can be
attached with ordinary screws and have some kind of slots, which enables the parts to be
adjusted and therefore becomes flexible as shown in figure 1. Aluminum is the most
commonly used material in these collections. Besides the flexibility, it is often possible to
recycle these standard parts, which enables parts to be reused for the next generation of fixtures.

Figure 1: Extruded aluminum profiles connected with a common screw
As with all modular systems, the interfaces between different parts in the system are the
most crucial features. For a jig with the purpose to support, position and control an
assembly, the parts of the jig can be divided in three different categories the frame, the
adapters (positioning and holding the parts) and the links between the frame and the
adapters, who generally are called the pick-ups.
There is a lot of effort put in research projects, to develop effective modular assembly jigs. The
jig parts easiest to standardize is the frames, who need to have flexible interfaces towards
the pick-ups (this is one reason why extruded aluminum is interesting). To standardize the rest
of the jig system, there are two main tracks to follow:
A small number of standard adapters, which sets requirements on the details to have
standard interfaces
A small number of standardized pick-ups, who need to be a six degrees of freedom
system to overlap the distance and angular differences between the frames and the
adapters for all possible configurations
Advanced measuring technology together with computer support, simplifies building
methods like Modular Tooling. Using a metrology system for the calibration of fixtures
opens up for the use of digital master tools as well as for simplifying the build process itself.
However, it is also possible to use measurement systems with more mobility, as
photogrammetry or laser tracker systems. Important to understand is that the general idea with
Modular Tooling is not about the use of extruded aluminium profiles, but for the idea of
using modular thinking. The reason why aluminium often is used in Modular Tooling is
because the suppliers of aluminium parts often offer the standardized collections of profiles and
features to enable the building of fixtures with standardized parts, and in a flexible and modular
manner. Modular Tooling has proven to reduce the development costs of the fixtures, as well as
the time to develop them.
One factor, which is often forgotten in the manufacturing industry, is that there are
environmental advantages to reuse production equipment. In fact, the European Community is
about to set new demands on the industry in Europe, to force them to have control over what is
brought in to the country and how to handle material waste, in order to more effectively use our
natural recourses.

3.2 Reconfigurable Tooling
Reconfigurable toolings generally imply that they have been designed for a family of workpiece
geometries and/or manufacturing operations. These types are most suitable for batch and
production and job shop environments where they can be used by many different products. The
concept of fixture reconfigurability was developed in the 1960's for the machine tool
manufacturing industry, where such fixtures consisted of kits of standard modular components
such as, locators, V-blocks, and clamps assembled on a baseplate.
The conformable clamping system developed in [7] was designed primarily for fixturing turbine
blades. The system comprises two-section clamps, where the lower half of each clamp employs
pneumatic plungers. The plungers conform to the shape of the blades. A similar system was
developed in [8], where multi- point contact with the workpiece is achieved via lockable spring-
loaded plungers. This enables local conformability to the workpiece geometry.
There has also been research in the area of utilizing the phase-changing properties of certain
materials in order to achieve conformability to odd shaped workpieces [9]. The phase changes
can be temperature or electrically induced.
A way to achieve Reconfigurable Tooling is through the use of pogo sticks, see figure 2. One
of these applications is developed by Kostyrka Ltd. (Kostyrka, 2000). They use flexible sleeves
made from a compound of metal and plastic, which are axially held in housings. The sleeves
surround the part and clamp it by applying hydraulic pressure to the sleeve jacket. These pogo
sticks can be moved from one position to another either actively or passively. The active
pogo sticks are individually adjustable and programmable by their own controller and servos
to conform to part shape. The passive pogo sticks consist of actuators only capable of
extending, retracting and clamping. These pogo sticks are positioned through external means,
such as a robotic gantry or the machining centre itself. The gantry system then sets the pogo
sticks to the correct heights. The pogo sticks are placed in a fixture bed, where they are
positioned in a matrix. There is a vacuum cup on top of every pogo stick and by the extraction
of each pogo stick, they together can form a pattern and can hold plates and skins with varied
configuration.


Figure 2: Pogo sticks
Another company using reconfigurable tooling techniques with pogo sticks is TORRESTOOL
from mTorres in Spain. Their tool is a universal holding fixture, conceived to support aircraft
components in space. It may be arranged as horizontal, vertical or round configurations, either
active (servo driven) or passive (pneumatic) type of motions. Mostly the Reconfigurable
Tooling is used to hold plates and skins for trimming, drilling and milling.

3.3 Flexible fixture design

When designing a fixture, the general design requirements that need to be considered include
positive location, rigidity, ruggedness, repeatability, minimum distortion of workpieces,
tolerance to small variations in workpiece geometry, and reliability. The additional design
requirements specific to the development of the reconfigurable fixture, on the other hand, include:
1. Modularity: the fixture must be composed of standard modules, which can be
assembled on a baseplate.
2. Automatic reconfigurability: the fixture must be reconfigurable by a robot, (i.e., the
fixture components should be designed for robotic assembly).
3. Sensory feedback controllability: the fixture components must be integrated with
sensors for feedback controllability.
4. Programmability: the operation of the fixture must be programmable by a computer.
Design of a flexible fixture system refers to two level tasks: the high-level task is to determine
the overall flexible fixture system based on the features of part families. The low-level task is to
determine a concrete fixture configuration, including flexible variables or assemblies based on
the features of a special workpiece in the families. In most previous works, the whole flexible
fixture system is supposed to be given, and only the low-level task is involved.
Design process in determining a fixture configuration
This design process refers to selecting the candidate elements, and to determining their internal
variables and external assembly based on fixturing requirement supposing the overall FFS is
given. Fixture design is both a science and art. There are many manufacturing-related criteria and
considerations that help in the development of a procedure or methodology to design a fixture for
a given product and for a specific manufacturing operation. Four design phases are involved: the
description of the design problem, fixture analysis, fixture synthesis, and configuration
verification.
Description of design problem
A design problem can always be defined as an optimization problem. An optimization problem
has three elements: design variables, design constraints and design objectives. Appropriate
models should be established to perform the solving of an optimization problem, e.g. analysis
modeling between the design variables and the constraints, the evaluation modeling between the
design variables and the design objectives.
a. Design variables
Design variables are determined by the architecture of a given FFS. The concept of variables
represents a broad meaning. In this context, the selection of alternative elements, the selection of
the assembly between the elements, and adjustable parameters within a modular element may all
be defined as design variables. They can be a discrete or continuous. At the beginning of a
design process, all the changeable parameters or factors in an FFS are defined as design variables
in some way. It is a non-trivial issue to define the variables reflecting these various design
options.
b. Design constraints
The function of a fixture is to hold a workpiece in order to keep the workpiece in the desired
position and orientation when it is in its manufacturing, assembly, or verification processes. This
statement also provides the fixturing requirement and is further expressed as design constraints
in a design process.
(1) Form closure. The wrenches are used to hold the object are such that they can
balance, by a combination of their actions, any external tone acting on the object. This
requirement has been expressed as follows in the literature.
Resting stability: all supporting components must maintain contact with the
workpiece so that the workpiece rests fully on the supports. When a workpiece is
placed into a fixture, it should first assume equilibrium resting.
Clamping stability: when clamps are applied on the workpiece in a sequence, the
clamping forces should not upset the stable and accurate position previously
assumed by the workpiece. After clamps are applied, the fixture should
completely restrain the workpiece to counter any possible cutting forces and
couples in the machining stages.
Processing stability: In favorable processing cases, where major cutting forces are
absorbed by the supporting and locating components, only small forces need to be
absorbed by the clamping components.
(2) Accessibility/ detachability. The concept of fixturing accessibility/detachability
covers the aspects of interference free conditions, and spatial geometric constraint
satisfaction. Two types of accessibility/detachability should be considered. The first is the
reachability of an individual workpiece surface; the second one is the easiness of loading
and unloading the workpiece into a fixture.
Spyridi, Spitz and Requicha [10][11] developed both analytical and discretized
accessibility analysis methods. Chen and Woo [12] first developed the concept of
visibility map and provided geometric algorithm.
The accessibility/visibility methods were applied to minimize the number of workpiece
setup in CNC machining and CMM inspection [12] and compute the die opening
directions for removing fabricated workpieces [13].
(3) Deformation constraints. Workpiece deformation during fixture set-up and process
operation is the most important consideration in the fixture design process.
The design constraints may change with respect to special situations. For example, Brook et al.
(1998) thought the form closure was too restricted for robotic grasping.
Fixture calibration is of paramount importance for product quality since a significant number of
fixture-related failures are related to fixture installation and maintenance [3][4]. The application
of Reconfigurable Assembly Systems (RASs) brings new challenges to fixture calibration since
reconfigurable tooling elements need to be calibrated in multiple positions. However, currently
there are no methodologies to determine the best positions of the measurement system such as
laser tracker to fully calibrate a given fixture or minimize the number of setup positions of a
measurement system.
Fixture analysis
In fixture analysis, the relational models that map from the design variables to the design
constraints, and from the design variables to the design evaluations, have to be established.
Many algorithmic and heuristic methods are developed to synthesize and analyze setup plans and
fixture configurations for a single rigid part [5][6]. As for design and analysis of fixture for
single compliant/flexible part, Menassa and DeVries [7] proposed optimization techniques to
assist in the design and evaluation of 3-2-1 fixtures for prismatic parts. Cai et al. [8] proposed an
N- 2-1 locating layout for sheet metal parts fixturing.
In the area of assembly systems for a family of parts, Lee et al. [9] presented a workspace
synthesis analysis for fixturing of family of stamped parts using the genetic algorithm.
These models are used to verify whether a fixture configuration satisfies the design requirements.
i. Kinematic analysis: refers to the kinematic models from the design variables to
kinematic constraints. It is necessary that the proposed fixturing arrangement does not
interfere with the expected tool path, the fixture does not restrict access to features
being machined, and that the fixturing elements themselves can access desired faces
or the features for clamping. For correct location, the fixturing elements should
completely specify the position and orientation of the part with respect to desired
datum surfaces, but should not over-determine the location.
ii. Force analysis: refers to the static models from the design variables to the static
constraints. It is concerned with checking that the forces applied by the fixtures are
sufficient to maintain static equilibrium in the presence of cutting forces.
iii. Deformation analysis: refers to the tolerance models ranging from the design
variables to workpiece deformation. It is the most computationally intensive step. The
concern is that a part may deform elastically and/or plastically under the influence of
cutting and clamping forces so that the desired tolerances will not be achieved.
Deformation is particularly a concern with flexible parts and with parts in which a
great deal of material is removed. Hockenberger (1995) discussed the effect of
machining fixture design parameters on workpiece displacement.
Evaluation models: refers to how the fixturing performance is evaluated. The following indices
are often used to evaluate the performance of the configuration candidates:
number of wrenches
clamping forces
workpiece equilibrium
workpiece stability
workpiece deformation
fixture dexterity
fixture set-up time
The evaluation models are used to obtain these performance indices.
Fixture synthesis
Fixture synthesis determines a set of design variables for a fixture configuration that can satisfy
the design constraints while achieving the best performances. For an FFS with a small number of
design variables, the synthesis activity is relatively simple using the models obtained from the
fixture analysis. However, fixture synthesis may become very complex if there are many design
variables in an FFS.
Consider a modular fixture system as an example, to reduce the calculation and improve the
design efficiency, the synthesis activity is decomposed into several sub-activities: selection of
types of modules, determination of locate and support points, determination of clamping, the
assembly planning of fixture configuration, and so on.
Design verification
Fixture verification is an integrated part of the design process and must allow for the detection of
any interference that may occur during the fixture construction (Shirinzadeh and Tie 1995).
Verification of a design solution is necessary for the following reasons:
(1) There are too many factors involved in the design process; it is very difficult to
establish accurate analysis models.
(2) Design constraints are considered individually; some contradicting constraints may be
produced when they are considered together.
(3) Fixture design has a close relationship with other activities (such as Computer-Aided
Process Planning, and Computer-Aided Manufacturing) in a manufacturing system; the
design solution needs to be verified practicable for the whole manufacturing system.
Verification or monitoring is also needed in the use of a fixture system to justify whether the
system is in a good condition. Choudhuri and Meter (1999) analyzed the tolerance caused by
machining fixture locators, and Ceglarek and Shi (1996) used pattern recognition to perform
diagnosis of fixture failure in autobody assembly.
Selection, evaluation and design of a FFS
One of the most important topics is how to select, evaluate and design an FFS for one family of
workpieces. This is more difficult than the determination of a fixture configuration, because the
fixturing objects have uncertain requirements. Actually, this situation often happens. When a
new enterprise is built or some new products are introduced, a decision on whether to buy or
design an optimal FFS for the family of workpieces has to be made. When an enterprise changes
a large-scale product paradigm into a low-to-medium product paradigm, the owner has to
determine whether dedicated fixtures are replaced by FFSs, and which is better: to buy
commercial FFS or to develop a special FFS for the family of workpieces.
To select, evaluate and design an FFS, more considerations should be included in the evaluation
models, such as cost, efficiency, suitability, and lead-time. The analysis process becomes most
difficult because there is an uncertain relationship with the fixture requirements. Empirical
methodologies are, in practice, applicable to the overall process of selecting and evaluating.

3.4 Jigless Aerospace Manufacturing (JAM)

Another approach to reducing the cost and increasing the flexibility of tooling systems for
aircraft manufacture is Jigless Aerospace Manufacturing (JAM). According to Burley et al
(1999) JAM is not a single, mysterious, as yet undiscovered technology. Rather it is a growing
number of related and linked technologies. Many of these are already well established and
considered robust. This approach strives for the minimisation of product specific jigs,
fixtures and tooling. A new integrated methodology has been developed, which uses a number
of building blocks and tools, to enable design for jigless assemblies as a result of a logical,
step-by-step process (Naing, 2000). In the traditional way of building aircraft, previously
described as Dedicated Tooling, the parts are located on reference set jig location. These jigs are
dedicated to one assembly; therefore they have no influence on flexibility. The parts held in the
jig, are drilled and fastened manually and deburring is required. This is a very labor
intensive process. By using JAM instead, parts may be assembled as part-to-part, where
two mating parts are drilled in isolation from each other and deburred. The holes in the
parts are then used to locate one part to another. No jig locations are used. Jigs only function
as support cradles, therefore giving flexibility. No deburr operation is required and the
process will be less labor intensive. With this technique the final position of the parts in the
assembly is defined in the detail manufacturing process. This on the other hand is sets higher
demands on the manufacturing process, where the risk of mismatch from one part to another
increases.
Another way of using JAM is done by virtual reference. Here, a robotic arm holds the part
together. No jig location feature is necessary, therefore it is flexible and if enough pressure is
applied when drilled then no deburring operation is necessary. This technique sets high
demands on both the labor and the assembly system (Engstrm, 1998).
The Boeing Company has made an approach similar to JAM, called Determinant Assembly
(DA) (Williams, 1998), (Swanstrom & Hawke, 2000). With DA they can reduce the number of
tool-located components, by using coordinated fastener holes. All coordinated fastener holes are
drilled under size during part manufacture. The parts are then determinately located through the
use of coordinated fastener holes.
A dedicated fixture
A dedicated fixture is a single-purpose device which is designed to locate and constrain a
specific part or component. Once the manufacturing process is completed, the fixture is then
stored for later use. This traditional approach is costly and time consuming since it requires a
special fixture to be manufactured for every part or component. These drawbacks have motivated
researchers to develop modular fixturing system [5, 6].
Computer aided fixture design ranges from expert systems [8] to a kinematics approach [9] to
using genetic algorithms [10]. The modular programmable conformable clamping system
developed by M.R. Cutkosky, et al." was designed for fixturing a variety of turbine blade
forgings. This system consists of two-section clamps, where the lower half of each clamp
employs pneumatic plungers such that, when released, are free to conform to the profile of the
turbine blade. The modular fixturing system developed by J.L. Colbert, et al. ~ was primarily
designed for the machining of prismatic workpieces. This fixture includes a baseplate, tool points
units, and clamps. The baseplate has two sets of hole patterns-- one for the mounting of the tool
points and clamps, and the other for hydraulic fluid supply to the clamps. The tool point unit is
equipped with a micro switch activated by the motion of the tool point when a workpiece is in
contact.
Contact fixturing system utilizes magnetic fixturing, instant freeze chucks and phase change
fixturing system [11, 12]. The main problem in using magnetic work-holding methodology in
aerospace manufacturing is that the fixture should take the shape of the component which results
in the same problems of dedicated fixturing. Also residual magnetism could be a problem in
cleaning the swarf off the component. Instant freezing chucks [12] can hold any material by
placing the component on a plate covered by a thin film of water. When the chuck is switched on,
the water freezes holding the component. Phase change fixturing is based on the ability of certain
materials to change from a fluid to a solid and back to a fluid again [13]. When material is in
liquid phase, the part is immersed to the required depth. Then the materials solid phase is
introduced, providing rigid support and work-holding force for the component. Upon finishing
the manufacturing procedure, the component is removed by reversing the phase of the work-
holding material. The main advantages are holding complex shape components with uniformly
distributed forces. The main disadvantage of this technique is the possible contamination from
the phase change material and the difficulty of machining the immersed parts of the component.
The automatically reconfigurable fixture developed by H. Asada and A.B. By 4 was specifically
designed for assembly. This modular fixture includes locating pins (locators), guides, and clamps,
and can be configured using a robotic device. The main limitation of this fixture is that the
baseplate is a magnetic chuck, and therefore, only useful for nonmagnetic workpieces. The
automatic modular and adaptable fixture developed by J.H.Buitrago and K. Youcef-Toumi s
consists of multipin modules that can conform to the workpiece geometry. The modules, in turn,
consist of three main parts, i.e., array of pins, a shape memory alloy (SMA) actuator, and a
modular interface
4. METROLOGY SYSTEMS

To be able to move from the old tradition of using Dedicated Tooling to the flexible tooling
technique, both the tools as well as measuring system need to be changed. When
determining accuracy of robotic arms, accuracy is separated into two categories, repeatability
and positional. Today industrial robots have fairly good repeatability accuracy, about 0.1mm
sometimes even better (ABB, 2001). The positioning is much worse. Most robotics
manufacturers do not even mention positional accuracy when performance is specified. This is
not an attempt to cover up some weakness; it is simply not considered a very interesting factor
in most robot applications. This partly due to the way robotic arms are traditionally
programmed, namely by teach-in. The teach-in method has the advantage that the positioning
error is compensated for. If the arm is repeatable and the work-piece is placed in the same
location in front of the robot, the end-effector will be able to perform its task in the right place
every time, despite the fact that the location for this task is more or less unknown
(Whinnem, 2000). When the process uses offline programming and not teach-in operations, the
positioning accuracy of robots is not enough. In the aircraft industry in general the
fixturing devices that hold parts together when building aircrafts must have position accuracy
better than 0.2 mm. A drilled hole in an airframe must have even better accuracy. Some
external measuring device is necessary to get the accuracy needed to drill, rivet or assemble
any aircraft structure. Today different measuring technologies have been brought about to
handle this problem. One of them is Photogrammetry and another is Laser technology.

4.1 Photogrammetry
Photogrammetry is the science of obtaining reliable information about the properties of surfaces
and objects without physical contact with the objects, and of measuring and interpreting this
information. Richard Gooch made an interesting description of photogrammetry. He advocated
that the photogrammetry uses the known position of several camera stations together with the
projected angles of rays passing from the images of targets detected on the image plane of
each camera, to establish the 3D location of targets by determining the point in space at which
these rays intersect (Gooch, 1998). Richard explained the difference between Optical Metrology
and Machine Vision. The Optical Metrology uses high contrast optical targets that provide
the highest possible measurement accuracy, thus the need for sophisticated image
processing is eliminated in optical metrology. Machine Vision needs high performed computing
to extract and recognize natural features of an object. The goal of Optical Metrology is to reduce
costs and lead times while improving quality. Other important factors are the elimination of
Dedicated Tooling and increased level of automation. Richard advocates that optical
instruments containing advanced electro-optics, embedded processors and digital interfaces
are opening up new horizons for the application of optical measurement in manufacturing
automation. Metrology is an enabling technology and applications are being extended from
inspection to control of manufacturing process itself.
Another system that uses photogrammetry is 3D Image Metrology that has made large
improvements over the last years, particularly in three areas. The first area is quality control,
where production personnel and/or specialists operate the systems. In the second area, machine
control consisting of black boxes, which provide 3D positional feedback to CNC machines or
robots. The third area is in-process inspection, where the systems are integrated to CNC
machines, robots or production lines and perform measurements on the fly (Beyer, 1999). One
system that uses the 3D Image Metrology is the TI
2
technology at The Boeing Company.
This approach is based on the idea of controlling the location of a drill bit directly in relation to
the part using 3D Image Metrology instead of relying on expensive mechanical systems. The
TI
2
system consists of the Tricept robot from Neos Robotics, the Imetric 3D Image Metrology
system from Imetric and the IGRIP simulation software from Deneb. The TI
2
system uses a 3D
Image Metrology to inspect hole locations, trimming paths, and other machining operations on
the fly. But, the measured data is not directly fed back to update the machine path in this
system but to provide information for an inspection report and statistical process control
(Beyer, 1999)
4.2 Laser Measuring
The rear structure of the Airbus A340-600 has been assembled with two laser
interferometers, manufactured by Leica. According to the expertise the traditional construction
tools could be replaced by supports to hold the work pieces together in space. The laser trackers
use the angles derived from the virtual CAD model to measure the real object. Consequently the
work pieces can be positioned in real time to each other using reference targets on each part.
Thus all sections could be adjusted immediately; the tooling cost could be cut by half. The laser
tracker positions all work pieces to be assembled. Earlier, the measurements had to be taken
manually, now they are generated automatically (Leica, 1999).
As mentioned in section 3.3, The Boeing Company has done an approach to achieve flexible
tooling, by using Determinant Assembly (DA). This technology results in a flexible and
more accurate assembly system. DA eliminates the need for master tooling gauges by
building the jigs and tooling to CAD and calibrating these directly using optical
measurement systems such as the laser tracker. The changes to assemblies can be accomplished
by modifying the feature locations in a part NC program. This is contrasted to Dedicated
Tooling where a physical component of an assembly fixture must be relocated or a new index
fabricated and installed on the assembly fixture (Williams, 1998).

5. A NEW TOOLING CONCEPT

The relationship between the existing tooling concepts today and the Affordable Reconfigurable
Tooling concept is illustrated in figure 3. It is positioned in between two technologies. To the
left are tools for aircraft assembly and to the right fixtures working as holding devices for
aircraft part manufacturing. This section will briefly describe the five different tooling
concepts. They will be compared through their ability to reconfigure between different
configurations in a short period of time.


Figure 3: Five Tooling Concepts with different ability to reconfigure and different degrees of
flexibility.

5.1 Assembly Jigs

Concept 1 in figure 3 is called Dedicated Tooling. This concept is the most commonly used
tooling technique in aircraft assembly today. Because Dedicated Tools are tailor made, they
have the ability to assemble all kinds of airframe structures. Every tool is designed to assemble
one particular structure. On the other hand if changes are required, the jig has to be sawed apart;
new parts designed, manufactured and finally welded or perhaps screwed into the jig to the
right position. The consequence of this is that this kind of assembly jigs never is reconfigured.

Concept 2 in figure 3 moves us to Modular Tooling, which is built on standard
aluminum profiles. Those jigs might be changed, but in a limited range. By using pick-ups to
adjust to the datums (e.g. fixturing points), those jigs may handle minor changes from the
original configuration. They are flexible but not very reconfigurable. Reconfiguration between
different airframe structures is hardly an option. But suppose that the Modular Tools has to be
reconfigured, to assemble another structure type, the jig would first have to be disassembled
and then rebuilt again to apply the new configuration. Compared to Dedicated Tooling,
the Modular Tooling has advantages. Most of the jig structure is re-usable, the parts are
standardized and the jig is adjustable, but not very reconfigurable.

5.2 Manufacturing Fixtures

Concept 3 in figure 3 is the Pogo-fixturing concept, which is previously described as a
reconfigurable tooling. This solution is today mainly used to fix airframe parts for milling and
drilling. The fixture is most often used to suck plates with vacuum cups, so that a gantry robot
can mill the edges on the plate or drill holes. Although these fixtures are most commonly used
for fixturing details for manufacturing there do exist applications where assembly is done.
One example is the assembly of stiffeners to plates. Although this is assembly, it has a very
restricted change over rate. The ability to reconfigure between different airframe structures
is possible, but rather small.
Concept 4 in figure 3 shows a concept called Hyper-flexible concept. This solution is used
mostly for holding parts for manufacturing (e.g. flexible NC-fixtures). The change over rate is
rather small, but the time to reconfigure is fast. This technique is old and well known and very
much developed. Although the NC fixtures may reconfigure quickly, they have geometrical
limitations in the ability to change over between different types of structures. They are most
commonly used to fix smaller details for manufacturing. No assembly is involved.

5.3 Affordable Reconfigurable Tooling
By using the ideas from reconfigurable manufacturing fixtures combined with the ideas from the
modular thinking, a new concept has been developed. This concept is called Affordable
Reconfigurable Tooling. Basically this solution uses an industrial robot to do the
reconfiguration task. The jig will need some kind of pogo sticks, similar to the passive pogo
sticks from Kostyrka, although they will almost certainly need to be modified. The pogo sticks
have some kind of locking device, which is inactive in order to be reconfigured and active to be
fixed. The reason why they are locked when the system is deactivated is because there is
always a risk of leakage with a pressurized system if the tool is not reconfigured in longer
periods of time.
Although the assembly system goes from being dedicated to reconfigurable, there will be
limitations in the changeover rate that the system will manage. Perhaps the system has the ability
to reconfigure within one family of structure types (e.g. planar structures, wing structures or
aircraft bodies). This might be enough for some aircraft assemblers who have a specific niche
on the market, for example medium sized wing structures. But if there still are demands to
handle a reconfiguration between product families the need to reconstruct the tool, the pogo
sticks should be modular in order to making it possible to reconstruct the tool to a bigger
changeover than the pogo sticks will manage. Because the pogo sticks are modular they can be
dismounted and applied in some other configuration. This will probably be done manually and
therefore take more time, but perhaps that is acceptable for some assemblers, where longer
reconfiguration time is acceptable.
To clarify the distinction between Pogo-fixturing and Affordable Reconfigurable Tooling
one can say that in the latter case there is the advantage to use a robot in the reconfiguration
process. It probably will be cheaper to buy passive pogo sticks. Their only task is to be flexible
when unlocked and rigidly fixed when locked. No expensive equipment is needed in each pogo
stick to give the high accuracy in positioning. By using a robot to do the reconfiguration task,
no built-in servo is needed in every pogo stick to move it. Investing the money in a robot will
probably be the cheapest investment in this comparison, not only because of the cheaper pogos
required, but also because of possibility of using the robot to do drilling, riveting as well as
other material handling tasks.

SUBMISSIONS

A short introduction was given on how Affordable Reconfigurable Tooling may be a way to
fill the gap between todays assembly jigs and part manufacturing fixtures. The question is
probably how much time we can afford for the reconfiguring process. The University of
Linkping in cooperation with nine other Airplane manufacturers and suppliers, are now
under way to continue this work, which also is founded by the European Community
Automation for Drilling, Fastening, Assembly Systems Integration, and Tooling (ADFAST).
Still there is a lot more work required in the research of how the Affordable Reconfigurable
Tooling system shall be designed to accompany a wider range of reconfiguration, robustness
and accuracy.
Another interesting area, where a lot of progress has been made recently is in the metrology
area. Optical measurement sensors are increasingly available, often finding application in
measurement and inspection of manufactured products. For example, theodolites and laser
trackers are already used to calibrate jigs and tooling. Digital photogrammetry is used in
dimensional inspection of assemblies such as aircraft wings. Such tasks demand high
performance sensors with 2D and 3D capability, large working envelopes, high accuracy,
low measurement latency and increased flexibility. The availability of sensors, which meet and
exceed such criteria, is fuelling new possibilities in the manufacturing process itself. Dedicated
Tooling may be eliminated and replaced by Affordable Reconfigurable Tooling under the
control of embedded sensor systems. But a lot more research is required before the accuracy in
the machine controller is to perform with enough precision and speed.
As the interaction of 3D models and 3D metrology is making it possible to close the link
between designs and manufacturing, the vision to start using virtual manufacturing, which is
about going from CAD solids to accurate assembled aircraft structures is coming to be a
reachable strive. But considerable effort is needed in the reconfiguration programming process
in order to shorten the long lead times in Dedicated Tooling. The time and effort must not
be translated in complex offline programming procedures and end up in continuous long time
and thereby high costs. The offline programming system needs further development in order to
function as an operation planning system as well. This makes the programming and operation
planning of the system. Today there exists softwares for this kind of processes, for example
RobCad from Technomatix, or IGRIP from Deneb. If the kinematics of the tool is defined in
the offline system, simple drag and drop methods is one easy way of solving it. But still, a lot
of work and effort is needed before we get there.
A lot of work is still left to be done to achieve Affordable Reconfigurable Tools, but if they
become a reality, industrial robots will finally find their way into the aircraft industry, and
reduce the labor intensive assembly process as well as drilling and fastening, which would
make the aircraft manufacturers more Lean, Agile and Flexible.




















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