Fixture Design Using Configurators

Abstract

Design and manufacture of fixtures are among one of the major cost drivers in product industrialization. Modular or reconfigurable fixture solutions that may be adapted to encompass a large variety of parts or products have been researched and employed in applications ranging from machining to assembly. These solutions have not only the potential to reduce fixture manufacturing cost, but they also render it possible for different solutions to facilitate and speed up actual design work. The process of designing fixtures today is complicated, time consuming and require long experience by the tool designer. In this paper we present the Configurator approach -add on programs to the CAD-software which aids the designer in the design process. The Configurators are semi-automated and interactive, designed to use in compliance with the ART-concept, a reconfigurable fixture concept for assembly applications. The Configurator approach has been tested on industrial cases and parts of the results are presented in this paper.

Full text

Fixture design using Configurators
M. Jonsson1, H. Kihlman1
1Linköping University, Department of Management and Engineering, Linköping, Sweden
2Delfoi, Gothenburg, Sweden
Marie.Jonsson@liu.se
Henrik.Kihlman@delfoi.com
ABSTRACT
Design and manufacture of fixtures are among one of the major cost drivers in product
industrialization. Modular or reconfigurable fixture solutions that may be adapted to
encompass a large variety of parts or products have been researched and employed in
applications ranging from machining to assembly. These solutions have not only the
potential to reduce fixture manufacturing cost, but they also render it possible for different
solutions to facilitate and speed up actual design work. The process of designing fixtures
today is complicated, time consuming and require long experience by the tool designer. In
this paper we present the Configurator approach - add on programs to the CAD-software
which aids the designer in the design process. The Configurators are semi-automated and
interactive, designed to use in compliance with the ART-concept, a reconfigurable fixture
concept for assembly applications. The Configurator approach has been tested on
industrial cases and parts of the results are presented in this paper.
Keywords: Fixture design, Reconfigurable fixtures, Configurators.
1. INTRODUCTION
The increase in global competition and more rapid
changes in customer demands result in a trend of
higher product variety and innovation, shorter product
lifecycle, lower unit cost and thus shorter lead time. This
has spurred companies to adapt new flexible
approaches to manufacturing and automation, such as
FMS (Flexible Manufacturing Systems) and RMS
(Reconfigurable Manufacturing Systems) [3, 7]
During the course of manufacturing processes, such as
machining, assembly or inspection, it is necessary to
locate, support and immobilize the workpiece or
product. This is referred to as “workholding” or
“fixturing”. Traditionally, workholding devices are
designed and manufactured intended for a specific part
and operation, i.e. the fixtures are “dedicated”. This
single-purpose approach is costly due to the long lead
time and effort required for design and manufacture, but
also often due to manual set-up or modifications when a
manufacturing process is completed or parts and
operations are modified. Cost is also induced by the
need to store and retrieve dedicated fixtures.
In the manufacturing industry the design and
manufacture of fixtures and other dedicated tooling for
locating and positioning workpieces or products are
among the major cost drivers in product
industrialization. The design and manufacture of
dedicated fixtures typically amount to 10-20 % of total
manufacturing cost [4]. For the automotive industry the
cost of redesigning, manufacturing and installing
fixtures is on the order of $ 100 million/plant/year [2].
Hardware solutions and research has mostly focused
on NC machining applications resulting in commercially
available modular tooling kits. Other solutions for
flexible fixturing include sensory based assembly,
reconfigurable fixtures, programmable clamps,
adaptable clamps and phase-change fixtures of which
[11] presents a comprehensive review. Construction
and design of fixtures and other workholding devices
roughly up to 30-35 % of total fixture cost. Due to its
complexity the different aspects of fixture design has
been extensively researched starting back in the
1940’s. The focus has been on understanding the
fixture design process and capturing the designer’s
knowledge and skill through the use of computers and
AI [5]. The ultimate purpose has been to fully automate
the fixture design process and focus has been directed
towards different design aspects for modular tooling kits
for machining rather than assembly processes. Also
issues that arises when a flexible fixturing system are
put to industrial use has been neglected [1].
1.1 Fixture design
The fixture design process is, as stated earlier, a
science as much as an art. [9] presents the following
generic flowchart for the fixture design process (figure
1). The process requires information on the product to
be constrained (commonly a 3D-model of the product
along with description of tolerances) the manufacturing
operation and the equipment to be utilized. After
considering all these aspects in the planning phase the
Fig 1: Generic fixture design process [9].

tool designer (the person who designs the fixture) works
out the number, type and location of the workholding
elements (i.e. the datum points, the points of contact
between the fixture and the product). He or she then
designs the fixture body connecting these elements,
making sure that the body does not interfere or collide
with any manufacturing equipment. The designer (or
design team) then verifies that the designed fixture
meets specified requirements for tolerances, stability
etc. In a typical production plant today, fixture design is
usually carried out in a CAD-environment by relying on
the designers experience or by using trial-and-error
methods.
Fig 2: An overview of the ART concept
To aid the designer in arriving at an optimal fixture
design research has been conducted into the
development of computer aided fixture design systems.
These systems for the design of modular or
standardized fixtures can basically be categorized into
three groups based on the degree of automation, i.e.
interactive, semi-automated or automated systems (the
latter is sometimes referred to as generative
systems)[10]. An interactive fixture design system
facilitates the design process by having informative user
interfaces. The user, based on his or her previous
knowledge, decides on datum points (interface points
between the product to be constrained and the fixture)
and elements to configure setup. Semi-automated
systems require certain inputs such as fixturing
surfaces or points from the user while automating other
tasks such as element selection. Fully automated
systems are based on different methods and algorithms
for generating the points and surfaces automatically
while also choosing the right corresponding fixture
elements. The automated systems have yet to be
developed to be able to generate detailed fixtures for
more complex parts[8]. The platforms for the systems
vary from Internet-enabled fixture design systems to
standalone systems.
2. THE ART TECHNOLOGY
As presented in [6] ART- Affordable Reconfigurable
Tooling, is a concept for flexible, reconfigurable fixturing
developed for the aircraft industry in collaboration with
SAAB Aerostructures, Sweden. The ART-concept fuses
modularity with reconfigurability by combining an
inaccurate modular framework, called BoxJoint, with
reconfigurable units, called Flexapods (see figure 2).
This combination makes it possible for the system to
conform to a large variety of different geometries since
the fixture can both be rebuilt and reconfigured.
Rebuilding means to physically detach or reattach
fixture components while reconfiguring implies that
some parts are adjustable. It also reduces the need for
customised parts leaving only the pick ups (the
interface parts between the fixture and the product) to
be product specific (se figure 3).
Fig 3: A generic model of the ART-concept (adapted
from [6])
2.1. BoxJoint principle
BoxJoint is a modular kit made up of steel beams
connected by patented joints. The joints utilize friction to
hold the beams together, eliminating the need for welds
and enabling seamless adjustment of the beams. The
joint is made up of fixing plates placed on either side of
the beam which are bolted together using standardized
bolts (see figure 4). Since all elements are standardized
they can be combined, making it easy to build suitable a
BoxJoint frame. A well defined method for the fixture
build process has been implemented that ensures
safety and optimizes robustness.
Fig 4: The BoxJoint Principle
2.2 Flexapods
The BoxJoint frame is robust but inaccurate. To come
to terms with the lack of accuracy without the iterative

process of measuring and adjusting, the BoxJoint
system is combined with a reconfigurable unit called the
Flexapod. Several different Flexapods have been
developed for use in aerospace assembly applications,
two examples are shown in figure 5, the Flexapod 6 and
the LiU Module. The latter is intended for complex
assembly where component access is a critical issue.
The Flexapod 6 has six links connected in parallel
between the top and the bottom plate and resembles a
parallel kinematic machine but without the actuators
and encoders. The links can be locked in position by a
hub-shaft connection mechanism and the resulting
device has six degrees of freedom. At the top of
Flexapod 6 interface called a Coromant Capto is
attached. The Coromant Capto interface serves as the
docking point for a positioning robot, for a metrology
probe and for the pick up interface that will hold the
part. The Capto system is robust and has a repetitive
accuracy of 2 μm and was originally designed for
holding cutting tools in CNC machines, hence they are
mass-produced and relatively cheap.
The LiU Module is a serial linked mechanism with
similar capabilities; six-degrees of freedom, a hub-shaft
connection mechanism and the Coromant Capto
interface. The joints are not only used for holding
beams together, but also for attaching Flexapods to the
framework structure. After attachment the Flexapods
are positioned either manually or by using a robot which
then docks to the Coromant Capto unit. Successful
trials have been conducted using a six-degree-of-
freedom measuring system to improve robot accuracy
when positioning a Flexapod [6]. In the manual case an
operator positions the Flexapod while getting feedback
from the measuring system via a screen. Other types
of Flexapods with less than six degrees of freedom for
other applications than assembly of airframe structures
are being researched.
3. DESIGN USING CONFIGURATORS
The ART-concept, with its discreet number of parts,
standardized elements and modularity makes
automation of certain steps of the fixture design process
possible. For this purpose, and to aid the tool designer,
“Configurators”, add on programs to an existing CAD-
system has been developed. The Configurators
presented in this paper may be described as an
interactive, semi-automated system, aiding the tool
designer when constructing the fixture but leaving the
option for adjustment. The CAD software utilized is
CATIA/DELMIA (Dassault Systemes) but the
Configurators may be applied to other 3D CAD
software. The main reason for using the Configurators
instead of conventional methods when designing a
fixture is to reduce lead-time. The tool designer may
need to quickly and accurately estimate tooling cost, for
example when receiving a quotation request from a
potential customer. For large complicated fixtures this is
both difficult and time consuming. The secondary
reason is to support the designer thus minimizing the
need for long experience. Also for modular tooling the
design process is more complex due to the multitude of
fixture parts and design constraints governing the
element selection. For example a joint has several
design constraints; the position of the joints edges
relative to beam centre and beam edge has to be
defined, the planes of the boxjoint plates has to be
aligned, as do the centre axes of the screws and holes
(se figure 3).
Fig 5: Different Flexapods, the Flexapod 6 and the LIU
Module
Fig 6: The CATIA/DELMI
A
CAD environment (right)
and the BoxJoint Configurator (left)
3.1 The Configurators
The Configurators are programmed in the Visual Basic
language and utilizes the APIs made available in the
CATIA/DELMIA package to interface with the software.
There are several types of Configurators that together
spans many aspects of the fixture design process. They
are presented below along with their main functionality:
• The BoxJoint Configurator - for creating the
BoxJoint frame (se figure 6)
• The Flexapod X Configurator - for calculating
desired Flexapod pose (X denotes the type of
Flexapod).
• FEA Configurator - for simplified FE analysis of
the BoxJoint frame
• LoS Configurator - for validating line of sight
for desired measuring equipment used when
positioning Flexapods

• CIK Configurator- for placing customer specific
pickups, generating BOMs and reconcile
fixture stock.
The Configurators are built to complement the
functionality of the CAD-software and serve as an aid
for the designer, helping in choosing beams, joints,
placement of fixture parts. The configurators may also
be used to carry out tasks and operations that are not
within the functionality of CAD-software. For example
the Flexapod 6 kinematic is not possible to simulate in
CATIA/DELMIA when this paper is written. Due to this
the Flexapod 6 Configurator also calculates the one
existing deterministic solution for the Flexapods 6’s leg
lengths to achieve desired pose The Configurators load
fixture parts from a part library into the 3D environment
and communicate the spatial placement to the CAD-
software based on the design constraints. The
database consists of different sizes of beams, joints,
Flexapods and customer added pick ups. When saving
a new fixture the paths to the right beams, joints,
Flexapods and pickups are linked to a single document.
This ensures easy administration and makes it possible
to use the Configurators with PDM-systems. To
exemplify this figure 7 describes the relationship
between the CAD-software and the BoxJoint and
Flexapod Configurators.
3.2 Design methodology
The design can be carried out in two directions, inside -
out or outside-in, starting with first placing the pickups
or constructing the BoxJoint frame (se figure 8). If
designing inside-out the process steps may be
described as:
1. Load the product into the 3D CAD
environment.
2. Examine product. Define datum points
(interface points between product and fixture).
3. Use the CIK Configurator to automatically
locate appropriate pickup on each datum.
4. Use Flexapod X (or Y) Configurator to insert
and place Flexapods. Attach Flexapod to pick
up, the Configurator calculates the correct
pose.
5. Insert appropriate start template of the
BoxJoint frame using The BoxJoint
Configurator. These templates are earlier
created fixture structures, useful when creating
fixtures, for example, within the same part
family.
6. Modify BoxJoint frame, adding, moving and
deleting parts, so that beams are in the vicinity
of the Flexapod base plates. Designer must
keep in mind the constraints of the
manufacturing process.
7. Attach Flexapod to the BoxJoint frame using
the Flexapod X Configurator. Built in
functionality ensures integer coordinates,
useful when fixture is built and measured.
8. Use FEA Configurator to evaluate structural
strength of the BoxJoint frame.
9. Modify frame for example by adding stiffeners
or choosing other types of beams to come to
terms with any problems with structural
strength. Iterate from step 8.
10. Use LoS Configurator to find appropriate
location for the metrology system used when
positioning the parts during physical build up.
11. Use CIK Configurator to generate Bill of
material.
12. Use CIK Configurator to reconcile what parts
are in stock and what parts have to be ordered
from supplier. Parts in stock are checked out.
When creating a new fixture the designer is prompted
to choose a template structure for the frame. The
template structure is loaded into the CAD environment
from the BoxJoint database. The designer may now add
joint boxes beams and adjust to fixture as needed. If the
designer chooses to insert a joint, the Configurator
prompts the designer to choose a placement beam.
Auto recognition chooses and places a correct joint on
the beam and the designer then slides the joint into
correct position. The same procedure is used when
adding a beam. The designer specifies a desired beam
length and is prompted to choose a joint for insertion.
Fig 8: The Configurators and there relation to the
ART-concept. Figure also describes the outside-in
and inside-out design approaches.
Fig 7: An o
v
erview of the Configurator approach

The Configurator auto recognises the dimensions of the
joint and inserts the correct beam.
The reconcile function is useful for investigation which
parts of previous fixture builds that may be re-used. It
also makes it possible to adjust the fixture to suit the
parts in stock (for example shortening a beam
correspond to beams in stock), thus eliminating any
unnecessary orders. When the designer checks out the
fixture parts from stock a BOM-list of parts that need to
be ordered is created.
If designing outside-in the steps are similar and
presented in short below:
1. Load product.
2. Define datum points
3. Insert appropriate start template of BoxJoint
frame using the BoxJoint Configurator.
4. Modify BoxJoint frame using the BoxJoint
Configurator.
5. Insert Flexapods using the Flexapod X
Configurator
6. Use FEA Configurator
7. Modify BoxJoint frame if needed using the
BoxJoint Configurator.
8. Iterate from step 6.
9. Use CIK Configurator to automatically locate
pick up.
10. Use Flexapod X Configurator to attach
Flexapods to corresponding pick ups. The
Configurator calculates the correct pose.
11. Use LoS Configurator.
12. Generate BOM with the CIK Configurator.
13. Reconcile stock using the CIK Configurator.
4. DESIGN CASES USING CONFIGURATORS
The Configurator approach has been tested and
evaluated in industrial applications ranging from
airframe structure assembly to commercial vehicle. The
two companies in the use cases are Volvo Wheel
Loaders and SAAB Aerostructures. For SAAB the
Configurators has been tested by tool designers when
creating fixtures for assembly of airframe structures.
SAAB sought a universal tooling solution that facilitated
simple tool design and shortened ramp up time for
equipment. A concurrent design process with several
iterative steps between product and tool design
emphasized the need for a methology that ensured that
initial tool design would be valid through to production
launch. The use of the ART-concept and the
Configurators has ensured a standardized methology,
shortened lead time and lessened the need for costly
tool design changes.
In the Volvo use case the ART-concept and
Configurators was employed for prototype
manufacturing. Previous to this Volvo did only utilize
simplified fixtures for prototype assembly. With ART,
the prototype fixture can be built to resemble the fixture
for full production, adding the benefits of production
concept validation, and the experience from the
prototype build is enhanced.
5. CONCLUSION
BoxJoint and the ART-concept were well suitable for
the Configurator approach due to its stringent design
criteria. To semi-automate the design steps but leave
room for interaction with the designer proved suitable
for the complex design tasks in the use cases. New
users with limited experience managed to create a
fixture design in very short time using the Configurator
add-ons. Also, using a multitude of different
configurators made it possible to customize an
appropriate solution for the application at hand. The
Configurators are currently being further developed and
will also be employed on different CAD-platforms.
4. REFERENCES
[1 Bi, Z.M. & Zhang, W.J. 2001, "Flexible fixture design
and automation: Review, issues and future
directions", International Journal of Production
Research, vol. 39, no. 13, pp. 2867-2894.
[2]Bone, G.M. & Capson, D. 2003, "Vision-guided
fixtureless assembly of automotive components",
Robotics and Computer Integrated Manufacturing,
vol. 19, no. 1, pp. 79-87.
[3]ElMaraghy, H.A. 2005, "Flexible and reconfigurable
manufacturing systems paradigms", International
Journal of Flexible Manufacturing Systems, vol. 17,
no. 4, pp. 261-276.
[4]Gandhi, M.V. & Thompson, B.S. 1986, "Automated
design of modular fixtures for flexible manufacturing
systems", Journal of Manufacturing Systems, vol. 5,
no. 4, pp. 243-252.
[5]Hargrove, S.K. & Kusiak, A. 1994, "Computer-aided
fixture design: a review", International Journal of
Production Research, vol. 32, no. 4, pp. 733-753.
[6]Kihlman, H. 2005, Affordable Automation for Airframe
Assembly - Development of Key Enabling
Technologies, Department of Mechanical Engineering.
[7]Mehrabi, M.G., Ulsoy, A.G. & Koren, Y. 2000,
"Reconfigurable manufacturing systems: key to future
manufacturing", Journal of Intelligent Manufacturing,
vol. 11, no. 4, pp. 403-419.
[8]Mervyn, F., Senthil kumar, A., Bok, S.H. & Nee,
A.Y.C. 2003, "Development of an Internet-enabled
interactive fixture design system", Computer-Aided
Design, vol. 35, no. 10, pp. 945-957.
[9]Nee, A.Y.C., Kumar, A.S. & Tao, Z.J. 2004,
Advanced Treatise on Fixture Design and Planning,
World Scientific Publishing Company,
http://www.ebrary.com.

[10]Senthil kumar, A., Fuh, J.Y.H. & Kow, T.S. 2000,
"An automated design and assembly of interference-
free modular fixture setup", Computer-aided Design,
vol. 32, no. 10, pp. 583-596.
[11]Shirinzadeh, B. 1995, "Flexible and automated
workholding systems", Industrial Robot: An
International Journal, vol. 22, no. 2, pp. 29-34.