fix_ijpr

int. j. prod. res., 2001, vol. 39, no. 13, 2867 ± 2894

Flexible ® xture design and automation: Review, issues and futuredirections

Z. M. BIy* and W. J. ZHANGy

The cost of designing and fabricating ® xtures can amount to 10± 20% of the totalmanufacturing system costs. To reduce manufacturing costs, a ® xture system isdesigned to be competent in ® xturing as many workpieces as possible. In massvolume production, this can be achieved by ® xturing a large quantity of the samekind of workpieces. In low-to-medium volume production, however, improve-ment of the ¯ exibility of ® xture systems becomes a favourable way to reducethe unit cost of product. This paper summarizes the latest studies in the ® eld of¯ exible ® xture design and automation. First, a brief introduction is given on thisresearch area. Secondly, taxonomy of ¯ exible ® xture design activities is presented.Thirdly, the ¯ exibility strategies based on the existing ¯ exible ® xture systems arediscussed. Fourthly, the contributions on design methodologies and veri® cationsare examined. Fifthly, advances on computer-aided design and see-up systems aresummarized. Finally, some prospective research trends are presented.

1. Introduction

1.1. The need for a Flexible Manufacturing System (FMS)Increasingly intensive global competition in manufacturing and changing con-

sumer demand are resulting in a trend towards greater product variety and innova-

tion, shorter product life-cycle, lower unit cost, higher product quality, and short

lead-time. Evolving from such a trend, both `market pull’ and t̀echnological push’

are forcing ® rms towards greater ¯ exibility. The case for ¯ exibility and automation is

reinforced further by crucial socio-economic issues such as the high cost of capital,the high cost of direct labour, and the shrinking skilled-labour pool. It becomes a key

dimension of ® rms’ competitive priorities. As a result, a great deal attention has been

directed towards the development of Flexible Manufacturing Systems (FMS) in the

past decades. An FMS ® lls the gap between high-volume transfer lines and a highly

¯ exible manufacturing situation, it is adopted to respond quickly, smoothly andcheaply 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.

1.2. Flexible Fixture System (FFS)

The ¯ exibility of a whole FMS is restricted by the ¯ exibility of any of its com-

ponents, including ® xture systems. The cost of designing and fabricating the ® xtures

in an FMS can amount to 10± 20% of the total system cost. Traditionally, the

International Journal of Production Research ISSN 0020± 7543 print/ISSN 1366± 588X online # 2001 Taylor & Francis Ltd

http://www.tandf.co.uk/journals

DOI: 10.1080/00207540110054579

Received March 2001.{ Advanced Engineering Design Laboratory, Department of Mechanical Engineering,

University of Saskatchewan, Saskatoon, S7N 5A9, Canada.* To whom correspondence should be addressed. e-mail: [email protected]

http://www.tandf.co.uk/journals

function of a ® xture 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 veri® cation pro-cesses. Custom-oriented dedicated ® xtures are not only time-consuming and costly

to build, but they also do not have the ¯ exibility to deal with parts or assemblies of

diŒerent shapes and sizes. To reduce the cost of a manufacturing system, the ® xture

system should be designed to be competent in ® xturing as many workpieces aspossible. In low-to-medium volume production, FFSs that are competent in ® xturing

diŒerent kinds of workpieces, become a prospective way of reducing the unit cost of

a product.

1.3. Some reviews on researches of FFSs

Fixture design and automation is an old topic in manufacturing engineering.

Thousands of technical papers were published in relevant journals and conferences.To our knowledge, there are eight review papers published in this ® eld, the latest one

appearing in 1996. A brief summary of these works is given in Table 1.

Two aspectsÐ design methodologies for determination of ® xture con® gurations

and the classi® cation of the existing ® xture systemsÐ are the main concerns. By

investigating recent research achievements in this ® eld, the authors have revealedsome limitations of these works. (1) Fixturing is always separated into some sub-

functions such as locating, supporting and clamping, the integrated implementation

of ® xturing behaviour is less noticed. (2) To design a ® xture con® guration, it always

means that the overall FFS is given. Some high-level issues, which are relevant to

selecting a suitable FFS for the family of workpieces, are not addressed. (3) Practical

issues rising from ® xture application environments are rarely considered.

1.4. Our observation

With the application of advanced technologies such as Artifact Intelligent and

robotic manipulators, the diŒerences between intelligent grippers and ¯ exible ® xture

systems become reduced. For example, the location of a workpiece can be performed

by visual identi® cation and supporting and clamping are often merged and carriedout by compliant vice or gripper. Passive elements in a ® xture system can be replaced

by active elements in order to achieve more ¯ exibility and automation. A ® xturing

process is traditionally regarded as a static process, but the dynamic ® xturing

methods are widely accepted in recent years. Moreover, ® xture-less operations

have been implemented in some manufacturing situations. Flexible strategies of

FFSs should be thoroughly studied to help develop novel FFSs.Manufacturing practice has also shown the problem of implementation of the

FFS concept. For example, in the case of modular ® xture systems, (1) the unaŒord-

able cost of some commercial modular ® xturing components and (2) the increased

level of knowledge to determine a ® xture con® guration and to assemble it into a

modular ® xture system. This further implies that commercial modular ® xturesystems are too general to be useful in various manufacturing environments. Some

application issues of ¯ exible ® xture systems need to be addressed.

Many methodologies were developed for optimal determination of ® xture con-

® gurations. They are competent in solving special classes of issues in design process.

However, a methodology guideline is lacking that can help the user to select a suit-able method and corresponding tools to solve various issues at diŒerent design

phases and aspects.

2868 Z. M. Bi and W. J. Zhang

1.5. Organization of paper

First, a taxonomy of issues in ¯ exible ® xture design is provided. Secondly, ¯ exible

strategies are discussed, which leads to a classi® cation of the existing FFSs. Thirdly,

the existing design methodologies and tools are examined. Finally, some prospective

directions for research are identi® ed.

2. Taxonomy of issues in ¯ exible ® xture designDesign of a ¯ exible ® xture system refers to two level tasks: the high-level task is

to determine the overall ¯ exible ® xture system based on the features of part families.

2869Flexible ® xture design and automation

Resources Concerns (issues, suggestions and remarks)

Grippo et al. Overview of research activities dedicated to develop viable(1988) universal ¯ exible ® xturing systems, highlighting the necessity for

an interdisciplinary research strategy in order to develop e� cientuniversal ® xture system

Hazen and Review of ® xture designs, ® xture analysis, planning, andWright (1990) automated ® xture assembly. Further studies were suggested:

(1) design of automated ® xture systems;(2) integration of planning, mechanical automation, and sensing

strategies

Trappy and Review of ® xturing principles, automated ® xture design, ® xtureLiu (1990) hardware design in 1980s. They revealed that comprehensive

automatic ® xture-design systems have not been completelydeveloped, and suggested (1) to construct a huge rule base coveringa su� cient domain in the expert system; (2) to study AutomatedFixture Design (AFD) software and the ® xture-hardware together.

Chang (1992) Discussion issues of ® xture planning for machining processes. Atentative classi® cation of ® xture components as well as a schemefor selecting the ® xture components for primary locating.

Liu and Strong Review of Fixture Design Automation. Further studies were(1993) suggested on:

(1) application of Al techniques, expert systems, feature designtechnique, group technology and 3D geometric reasoningtechniques to automatic ® xture design;

(2) integration of AFD with other systems;(3) development and exploitation of new locating principles for AFD.

Hargrove and Suggested new directions of research on computer-aided ® xtureKusiak (1994) design: (1) integration with other computer-aided engineering toolsHargrave (1995) for total ® xture design; (2) qualitative reasoning techniques can be

applied to ® xture design; and (3) integration with NC programmingschemes

Shirinzadeh Classi® cation FFSs from the viewpoint of ¯ exible strategies(1995)

Shimoga (1996) A survey of the existing grasp synthesis algorithms meant forachieving dexterity, equilibrium, stability, and dynamic behaviours.Prospective researches on this ® eld were (1) to simplify thecomputational complexities of the synthesis algorithm; (2) toimprove the precise sensing and control capabilities of the existinggrasping system.

Table 1. Summary of review papers on ® xture design and automation.

The low-level task is to determine a concrete ® xture con® guration, including ¯ exible

variables or assemblies based on the features of a special workpiece in the families. In

most previous works, the whole ¯ exible ® xture system is supposed to be given, and

only the low-level task is involved. Here, the low-level task is discussed ® rst, and the

high-level task of selection and designing of the whole FFS is mentioned in Section

2.6.

2.1. Design process in determining a ® xture con® guration

This design process refers to selecting the candidate elements, and to determining

their internal variables and external assembly based on ® xturing requirement sup-

posing 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 ® xture for a given product and for a

speci® c manufacturing operation. Generally, this procedure is as shown in ® gure 1.

Four design phases are involved: the description of the design problem, ® xture

analysis, ® xture synthesis, and con® guration veri® cation.


Fixture Analysis

Problem Description of Design a Fixture Configuration

Design parameters and

Variables

Fixture Verification · Form closure · Accessibility/Detachability · Deformation

Fixture Assembly and Running

Fixture Synthesis · Decomposition of the synthesis process · Determination of initial variables · Verification of candidates · Search strategies · Calculation reduction · Output of optimal Solution

Description of a Flexible

Fixture

Design restrictions

Design objectives

Description of Workpiece and Its Machining Process

Evaluation models · Stabiliablity · Equilibrium · Dynamic Behavior · Dexterity,……

Constriction Models · Form closure · Accessibility/Detachability · Deformation

Design Process

Figure 1. Process of determining a ® xture con® guration.

2.2. Description of design problem

A design problem can always be de® ned as an optimization problem. An opti-mization 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 modelling between the design variables and the

constraints, the evaluation modelling between the design variables and the designobjectives.

2.2.1. Design variables

Design variables are determined by the architecture of a given FFS. The conceptof variables represents a broad meaning. In an FFS, the selection of alternative

elements, the selection of the assembly between the elements, and adjustable par-

ameters within a modular element may all be de® ned 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 de® ned as design variables in some way. It is anon-trivial issue to de® ne the variables re¯ ecting these various design options.

2.2.2. Design constraintsThe function of a ® xture 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

veri® cation processes. This statement also provides the ® xturing 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 onthe 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 ® xture, it should ® rst 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 pre-viously assumed by the workpiece. After clamps are applied, the ® xture should

completely restrain the workpiece to counter any possible cutting forces and

couples in the machining stages.

. Processing stability: In favourable processing cases, where major cutting forcesare absorbed by the supporting and locating components, only small forces

need to be absorbed by the clamping components.

(2) Accessibility/detachability. The concept of ® xturing accessibility/detachabil-

ity covers the aspects of interference free conditions, and spatial geometricconstraint satisfaction. Two types of accessibility/detachability should be

considered. The ® rst is the reachability of an individual workpiece surface;

the second one is the easiness of loading and unloading the workpiece into a

® xture.

(3) Deformation constraints. Workpiece deformation during ® xture set-up andprocess operation is the most important consideration in the ® xture 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.

2.3. Fixture analysis

In ® xture 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. These models are used to verify whether a ® xture con® guration

satis® es the design requirements.

Kinematic analysis: refers to the kinematic models from the design variables to

kinematic constraints . It is necessary that the proposed ® xturing arrangement does

not interfere with the expected tool path, the ® xture does not restrict access to

features being machined, and that the ® xturing elements themselves can access

desired faces or the features for clamping. For correct location, the ® xturing ele-ments should completely specify the position and orientation of the part with respect

to desired datum surfaces, but should not over-determine the location.

Force analysis refers to the static models from the design variables to the static

constraints. Force analysis is concerned with checking that the forces applied bythe ® xtures are su� cient to maintain static equilibrium in the presence of cutting

forces.

Deformation analysis: refers to the tolerance models ranging from the design vari-

ables to workpiece deformation. It is the most computationally intensive step. The

concern is that a part may deform elastically and/or plastically under the in¯ uence ofcutting and clamping forces so that the desired tolerances will not be achieved.

Deformation is particularly a concern with ¯ exible parts and with parts in which a

great deal of material is removed. Hockenberger (1995) discussed the eŒect of

machining ® xture design parameters on workpiece displacement.

Evaluation models refers to how the ® xturing performance is evaluated. Thefollowing indices are often used to evaluate the performance of the con® guration

candidates:

. number of wrenches

. clamping forces

. workpiece equilibrium

. workpiece stability

. workpiece deformation

. ® xture dexterity

. ® xture set-up time

The evaluation models are used to obtain these performance indices.

2.4. Fixture synthesis

Fixture synthesis determines a set of design variables for a ® xture con® guration

that can satisfy the design constraints while achieving the best performances. For anFFS with a small number of design variables, the synthesis activity is relatively

simple using the models obtained from the ® xture analysis. However, ® xture syn-


thesis may become very complex if there are many design variables in an FFS.

Consider a modular ® xture system as an example, to reduce the calculation andimprove the design e� ciency, 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 ® xture con® guration,

and so on.

2.5. Design veri® cation

Fixture veri® cation is an integrated part of the design process and must allow for

the detection of any interference that may occur during the ® xture construction

(Shirinzadeh and Tie 1995). Veri® cation of a design solution is necessary for the

following reasons: (1) There are too many factors involved in the design process; it is

very di� cult to establish accurate analysis models. (2) Design constraints are con-sidered 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 veri® ed practicable for the

whole manufacturing system.Veri® cation or monitoring is also needed in the use of a ® xture system to justify

whether the system in a good condition. Choudhuri and Meter (1999) had analysed

the tolerance caused by machining ® xture locators, and Ceglarek and Shi (1996) used

pattern recognition to perform diagnosis of ® xture failure in autobody assembly.

2.6. 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 di� cult than the determination of a ® xture

con® guration, because the ® xturing 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 familyof 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 ® xtures are replaced by FFSs, and which is better: to buy com-

mercial FFS or to develop a special FFS for the family of workpieces.

To select, evaluate and design an FFS, more considerations should be included inthe evaluation models, such as cost, e� ciency, suitability, and lead-time. The analy-

sis process becomes most di� cult because there is an uncertain relationship with the

® xture requirements. Empirical methodologies are, in practice, applicable to the

overall process of selecting and evaluating.

3. Existing FFSs

Various FFSs have been developed; but only a few of them have become com-

mercial. Most FFSs are limited to special-purposes; however, they are ¯ exibleenough to accommodate one class of part family. Figure 2 shows a classi® cation

of these FFSs from the view of ¯ exible strategies.

3.1. Flexible strategies for ® xture systems

There are a number of ways to achieve ¯ exibility. The ® rst way is to make the® xture system contain many replaceable basic elements, which are called modular

elements. A ® xture con® guration is built by selecting modular elements and their


assembly; these elements are then connected to each other by standard connections.The bene® ts of modularized products were described by He and Kusiak (1996, 1997),

Rogers and Bottaci (1997), Kusiak and Larson (1995), and Newcomb et al. (1998).

The second way is to make some components contain internal variables that can be

adjusted to meet the diŒerent features of workpieces. The third way is to use phase-

change-like materials. In ® gure 2, FFSs are classi® ed into two types: Flexible Fixture

Systems with a Modular structure (MFFSs) and Flexible Fixture Systems with Singlestructure (SFFSs).

3.2. Flexible ® xture systems with modular structures

The concepts of interchangeability and modular ® xturing date back to the

Second World War. Modular ® xturing systems ® rst came into prominence in the

late 1960s and are mainly used in conjunction with NC machine tools. The extent of

their use was not widespread until the advent of multiple axis CNC machine tools

and Flexible Manufacturing Systems (FMSs). A modular ® xture kit consists of manyelements, and these elements belong to one of the following basic types: base plate,

locators, clamps and connections. Using the components from the kit one can

assemble a custom-oriented ® xture. Flexibility is achieved by selection of various

modules and assembly of the elements.

3.2.1. Classi® cation of FFSs with a modular structure

Table 2 shows four basic types of FFS with a modular structure and examples of

location identi® cation techniques.

3.2.2. Shortcoming of MFSsMFFSs are very popular in industry. They can widely accommodate various

changes of workpieces, i.e. in shape, size, and process, etc. However, this implies


Flexible Fixture Systems

T-Slot Modular Kits

Downel-Pin Modular Kits

Adaptive Clamp

FFSs with Single Structure (SFFSs)

Grid-Hole Modualr Kits

FFSs with Modular Structure (MFFSs)

Phase-change FFSs

FFSs Using Memory Metal

FFSs Using Pseudo Phase Materials

FFSs Using Authentic Phase-Change Materials

Reconfigurable modular FFSs

Figure 2. The classi® cation of ¯ exible ® xture systems.

that they are too general to be useful in some manufacturing environments. Only a

few of researchers have paid attention to the failures of MFFSs; however, manu-

facturing practice has shown the following problems of adopting modular ® xture

systems.

(1) Unaffordable cost: the initial cost of modular ® xture is often high.

(2) A large amount of knowledge: the increased level of knowledge required to

determine a modular ® xture system con® guration and to assemble it into a

modular ® xture system is dif® cult to obtain.


LocationType identi® cation Examples

Grid hole system Non-threaded hole Venlic Block Jig System, JapanYuasa Modular Flex System, USAKipp Modular FFS, Germany

T-slot or Tenon T-slot Eiwin Modular System, USA (® gure 3)system Warlton Unitool, UK

CATIC (China National AeronauticalTechnology Import and ExportCorporation) System, ChinaGridmaster System, UK

Dowel pin system Dowel or tapped Bluco Technik, Germany (® gure 4)hole MITEE-BITE Clamp

SAFE Fixture System, USAWrite Alu® x System, Germany

Recon® gurable Visual Sense Developed by Asada and Andre (1985)modular FFSs Joint encoder, et al. Developed by Shirinzadeh (1995)

Developed by Benhabib et al. (1991)

Table 2. Classi® cation of commercial MFFSs.

Figure 3. T-Slot Eiwin modular ® xture system.

(3) Limited ¯ exibility: only limited combinations are available from standard

® xture components to construct ® xture con® gurations for different work-

pieces. In many cases, dedicated ® xtures are needed for complex workpiecegeometry and operations.

(4) Limited ® xture performance: structural properties of modular ® xtures, such

accuracy, stiffness, stability, and convenience of loading and unloading, are

maintain.

(5) Schedule: there is a problem of how to utilize modular ® xture componentsef® ciently in production planning.

3.2.3. Set-up of MFFSsCon® gurations of MFFSs should be often modi® ed to accommodate the changes

of features of workpieces. Therefore, the set-up for a ® xture con® guration is an

important activity in the application of a MFFS. Empirical judgements are used

in the process of the set-up.

3.2.4. Sense-based set-up

Modular elements must have some degree of intelligence if they are required to

change automatically. One way is to use various senses. Tao and Kumar (1997)

developed an experimental ® xturing system for end-milling operations, in whichreal-time reaction forces on locators are measured and monitored through a data

acquisition system. Lu et al. (1997) designed a fast ¯ exible ® xture for two-dimen-

sional clamping, which is ® tted with sensors for automatically measuring the posi-

tions of the clamping surfaces and the vice opening. Karl et al. (1994) developed a

¯ exible automated ® xturing element, in which a hydraulic positioning mechanismwas designed without using a hydraulic servo valve. The device could achieve the

positioning accuracy within 0.001 inch. The modular elements developed by

Shirinzadeh and Tie (1995) were equipped with position and force sensors, and

the motion was controlled by hybrid servo systems.

Taylor et al. (1994) addressed the problem of planning two-® ngered grasps of un-modelled 3D objects using visual information. Barsky et al. (1989) proposed a robot

gripper control system using polyvinylidene ¯ uoride (PVDF) piezoelectric sensor.


Figure 4. Blucko Technik dowel ® xture.

Rodu and Fadi (1998) presented a visual approach to the problem of object grasping

and, more generally, to the problem of aligning and end-eŒector with an object.

3.2.5. Robotic ® xture assembly

The use of robotic systems is the main method in implementing the automation

of modular ® xture assemblies. The robot can con® gure ® xtures into a wide range ofworkpiece con® gurations without operator assistance. When coupled with ® xture

modules designed speci® cally for robot assembly, these systems show promise

(Youce-Toumi et al. 1989). In the recon® gurable modular systems developed by

Asade and Andre (1985) and Shirinzadeh and Tie (1995), the ® xture modules were

set up, adjusted and changed automatically by the assembly robot without human

intervention. Lim et al. (1989) used a gantry robot for the loading and unloading of

modular ® xtures, an automatic probe-changing coordinate-measuring machine forinspection. Ngoi et al. (1997) developed an automated ® xture set-up for inspection,

the system comprised a workplace with magazines, locators and a baseplate, the

SCORBOT-ER VII was used to automate the system. Yu and Goldberg (1998)

formalized robotic ® xture loading geometrically as a sensor-based compliant assem-

bly problem and gave a complete planning algorithm. Giusti et al. (1994) developed arecon® gurable assembly cell for mechanical products. This cell was composed of

three base plates, a pneumatic screwing unit, a hydraulic press and other devices

for the complete automation of the assembly operations. The handling functions of

the cell were performed by a six-axes robot. As shown in ® gure 5, Sela and Gaudry

(1997) developed a recon® gurable modular system for the ® xturing of thin-walled,¯ exible objects subject to a discrete number of point forces.

3.3. FFSs with a single structure

The con® gurations of this type of FFSs are unchangeable. However, some adjus-table variables are contained. Flexibility can be achieved by changing adjustable

variables to accommodate various shapes and sizes of workpieces. Two types are

included: the adaptive FFSs, phase-change FFSs.

3.3.1. Adaptive FFSs

An adaptive FFS achieves the ¯ exibility through its internal variables. Daniel et

al. (1998, 1999) designed a recon® gurable discrete die, which was being developed for


Figure 5. FFS for thin-walled parts by Sela.

aircraft fuselage parts, made by forming sheet metal or moulding composite

materials. As shown in ® gure 6, each pin in the die was a simple hydraulic actuator

® tted with an in-line NC solenoid valve to control its vertical position. Once the pinswere set, the entire matrix was clamped into a rigid tool. Brooke (1994) introduced

Fanc’s programmable ® xturing system, which could hold diŒerent platforms and

multiple models, change quickly, and do it for low cost. Wallack and Canny

(1994) designed an adaptive ® xture vice, which consisted of two ® xture table jaws

capable of translation in the X axis, As shown in ® gure 7, it was used for 2.5-

dimensional objects. Du and Lin (1998) developed a three-® ngered automated ¯ ex-ible ® xturing system. Most of grippers used in part assembly belong to this type. As

shown in ® gure 8, Kopacek and Kronreif (1994) developed a modular parallel grip-

per system. Chan and Lin (1996) developed ¯ exible grippers, where only one type of

modular element provided locating, supporting and clamping functions. As shown in

® gure 9, each element consisted of four ® ngers with eight degrees of freedom in orderto conform to any arbitrary workpiece surfaces. The eight motions of the four ® ngers

were controlled by one motor through the use of two transmission and clutch

systems. DiŒerent combinations of the multi® ngers could make diŒerent ® xture

recon® gurations for the product family. Causey and Quinn (1998) provided a guide-

line for designing various grippers. Smith (1998) developed a new concept of an

intelligent ¯ exible ® xture system.


The structure of pin

Figure 6. A recon® gurable discrete die.

Figure 7. A ® xture vice.

3.3.2. Phase-change FFSs

Flexible ® xturing based on the concept of material phase-change exploits the

ability of certain classes of material to change phase. Phase changes may be tem-

perature-induced, electrically induced or a combination of these Temperature-

induced phase-change ® xturing has traditionally been employed in encapsulation

for special-purpose precision machining, such as the milling of turban blades as

shown in ® gure 10 (Hazen and Wright 1990). Electrically induced phase-change

® xturing employs electro-rheological ¯ uids as the medium undergoing the change

of phase, Grippo et al. (1988) developed a prototype in ® gure 11, Pseudo phase

change ® xtures mimic the function of low melting point alloys. These alloys can

be `̄ uidized’ by compressed air, and when the air supply is shut oŒ; the alloys

become solid and hold the workpiece (Gandhi and Thompson 1985). Researchers

at MIT designed a set of comfortable clamps that use shape memory alloy wires for

motive force. As shown in ® gure 12, the clamps consisted of a 4 £ 4 array of ® ngers,

normally locked by the squeezing action of heavy-duty belleville springs.

Compliant mechanisms are mechanical devices that achieve motion via elastic

deformation. by adapting these mechanisms, some special ® xtures can be developed,


Figure 8. Gripper with two ® ngers.

Multi-Fingermodule as clamp

Cylinder

Workpiece

Multi-finger module as locators

Figure 9. CNC ¯ exible ® xture for assembly.


Workpiece

Compaction plate

Hydraulic clamp

Container wall

Air intake

Bed Material

Figure 10. The particulare ¯ uidized bed.

Removable module

E-R Valve

E-R fluid

Piston Array of E-R ‘ plug in’ module

Figure 11. Fixture made of electropheological ¯ uids.

Springs

Clamping Actuator Lever Conformable

Surface

Figure 12. MIT Conformable clamp made of shape memory alloy.

Frecker et al. (1997) discussed topological synthesis of compliant mechanisms using

multi-criteria optimization.

3.4. Robotics in ® xture-less assembly (RFA)

Robotic ® xture-less assembly refers to the performance of assembly tasks using

robots without the ® xtures. RFA is applicable to several industries, including auto-motive, aircraft, camera and photocopier manufacturing. The elimination of ® xtures

is expected to reduce greatly the costs and lengthy lead times associated with retool-

ing in these industries. General Motors of Canada is developing a RFA prototype, it

is being used for picking and accurate locating for a large number of complex-shaped

sheet metal (and sheet plastic) parts in the assembly processes. Bandyopadhyay et al.

(1993) designed a ®̀ xture-free’ machining centre for the machining of block-like

components.

4. Design methodologiesOnce the whole FFS is bought, selected, or produced, its running e� ciency

depends largely on the automation degree of design, and veri® cation of the ® xture

con® gurations. As mentioned in section 2, three basic activities are involved: (1)

description of the requirements and the constraints; (2) modelling in ® xture analysis;

and (3) search strategies in ® xture synthesis.

A description method has a great eŒect in choosing the appropriate tools for® xture analysis and synthesis. The geometry expression is the most direct method for

describing design requirements. It is often used in the detailed design of a ® xture

con® guration both for FFSs and dedicated ® xtures. The alternatives are a GT-based

code and feature-based method. To accommodate for computerization, the modular

® xture elements must be encoded (Lin and Huang 1997). Group Technology (GT)codes and feature-based descriptions have appeared in recent years with the increas-

ing usage of expert systems and rule-based systems in ® xture synthesis. However,

because the detailed description of design requirements might be di� cult, GT is only

suitable in the concept design of ® xture con® gurations.

4.1. Methodologies used in ® xture analysis

Fixture analysis builds relational models among the design variables, constraints,

and evaluation objectives. The optimal formula of the ® xture con® guration design is

not completed until all the constraint and objective models are established.

4.1.1. Geometry methodThe geometry-oriented approach is a well-accepted approach in which most of

the information required can be retrieved from CAD systems. MarkenscoŒand

Papadimitriou (1989) and MarkenscoŒet al. (1990) discussed the geometry of grasp-

ing and optimum gripping of a polygon using geometry methods. Trappey et al.

(1993) utilized the projective representation of a workpiece to ® nd a feasible ® xturecon® guration based on the 3-2-1 locating principle. Asada and Andre (1985)

employed analytic tools based on the workpiece geometry and the assembly opera-

tion required, and the ® xture con® guration could be changed automatically. Boema

and Kals (1988) established a CAD system used for automation of ® xture design and

set-ups. Considering the interference between ® xture modules in a recon® gurable® xture system, Wu and Rong (1998) presented a fundamental study of automated

® xture planning with a focus on geometric analysis. The initial conditions for mod-


ular ® xture assembly were established together with geometric relationships between

® xture components and the workpiece to be analysed. Willy et al. (1995) employedBoolean algebra in justify the possible ® xture con® gurations. Cai et al. (1996, 1997)

developed a variational method for robust ® xture con® guration design to minimize

workpiece resultant errors due to source errors. Using the proposed variations

approach, closed form analytical solutions were derived. Apley and Shi (1998) pre-sented an algorithm to detect and classify multiple ® xture faults based on least

squares estimation. Hwang et al. (1999) used kinematics to analyse the grasping of

a B-spline surface object by a multi-® ngered robot hand. Hong et al. (1996) applied

the spatial point contact constraint theory to develop a mathematical model of

¯ exible ® xturing systems.

4.1.2. Screw theory

Screw theory is based on the motion of a rigid body’s displacement in 3D space;

the force and the motion of a workpiece can be modelled as `wrenches’ and t̀wists’ .

To build some quality measures for evaluating a grasp, Chou (1989) presented amathematical theory for automatic con® guration of machining ® xtures for prismatic

parts with aid of screw theory, in which twists and wrenches are used to represent the

motion and force combinations. Screw theory was used to derive: (1) the minimum

number of contacts (locations and clamps); (2) the permissible motions due to a

contact set; (3) the reaction forces at special contacts; and (4) the clamping forces

required to balance cutting forces (Fuh and Nee 1994).

4.1.3. Finite Element Method (FEM)

Owing to its good capability of deriving the corresponding compressive force and

displacement, the FEM approach has been used in many de¯ ection-relatedresearches (Hou and Trappy 1997). Lee and Cuthowsky (1990) were the ® rst to

employ this method in Automated Fixture Design (AFD). De Meter (1998) pro-

posed the Fast Support Layout Optimization (FSLO) model, it utilized a FEM

model to characterize workpiece stiŒness. Solution of the FSLO model improved

an existing support layout by systematically altering the boundary conditions.

4.1.4. Rule-based or feature-based analysis

In a feature-based model, a typical approach is to represent a part with a single

set of features. In machining processes, diŒerent ® xturing parameters are required

when diŒerent sets of features are used for machining. Tseng (1998) developed afeature-based ® xturing analysis method to determine a good set of features that was

more suitable from the ® xturing point of view.

Dong et al. (1991) investigated the use of features for ® xture design, concentrat-

ing on the selection of locating elements and the identi® cation of locating surfaces

for workpiece positioning. The approaches of machining feature-based design andthe precedence matrix algorithm were combined to automate the process-planning

and ® xture design (Ozturk et al. 1996). By merging knowledge of manufacturing

methods and machine tool information with a special-purpose computer language,

Darvishi and Gill (1988) suggested developing, primarily for list processing and

symbolic manipulation, ground rules for a ® xture design approach. A ® xture processplanning system was implemented using a knowledge-based approach (Liou and

Suen 1992). Perremans (1996) described modular elements by means of form


features; this permitted the description of an arbitrary modular ® xturing system and

the use of the same logic to determine the assembly of modular elements.

4.1.5. Mechanical analysis methods

As well as the methodologies mentioned above, the mechanical analysis method

is very popular for modelling workpiece-® xture force interactions and deformations

of the workpiece. King et al. (1997) presented an analytical approach based on

kinematics, force and a minimization of potential energy method. Potential energywas regarded as a measure of excitability of a given physical state, the physical state

considered was the ® xturing con® guration of locators and clamps, and an optimal

algorithm was built to ® nd a con® guration with the minimum potential for pertur-

bation. Meyer and Liou (1997) developed a comprehensive methodology for hand-

ling the dynamic external force in a workpiece-® xturing system. Considering the

accurate and e� cient modelling of compliant ® xtures and grasps, Lin (1998) deriveda stiŒness matrix formula using the overlap compliance representation for quasi-

rigid bodies. It could incorporate a realistic nonlinear contact model, and could be

directly computed from CAD data on basic geometric and material properties of the

bodies. This formula was well-suited to automated planning algorithms. Mittall and

Cohen (1991) presented a dynamic modelling of the ® xture-workpiece system. Mostof force analysis approaches treated either the workpiece or the ® xture as a deform-

able elastic body; however, both of them were considered as deformable elastic

bodies in the mechanical analysis presented by Gui et al. (1996); a generalized

method to minimize the location deviation was proposed by using optimally deter-

mined clamping forces. Nguyen (1984) discussed constructing stable grasps usingmechanical modelling. Yeh and Liou (1999) developed the modelling of the response

frequency of a modular ® xturing system with multiple components in contact, and

showed it was possible to monitor the contact conditions based on a ® xture system’s

dynamic response frequency. Wu et al. (1997) considered surface contact modelling

for a ® xture± workpiece system. Shreyes and Melkote (1997) applied the mechanical

method to improving workpiece location accuracy through ® xture layout optimiza-tion.

4.2. Methodologies for ® xture synthesis

Two main considerations in ® xture synthesis are to simplify the synthesis processand to reduce calculations. In addition, search strategies in the feasible space of

® xture con® gurations are important.

4.2.1. Analytical methods

Analytical synthesis can only handle a small number of design variables. Two

possible situations might use analytical synthesis: (1) the determination of a ® xture

con® guration for the SFFSs with a few variables; (2) the determination of a fewinternal parameters for selected modular elements for MFFSs when the whole com-

plex design has been decomposed. Brost et al. (1996) presented an implemented

algorithm that accepted a polygonal description of the part silhouette, and e� ciently

constructed the set of all feasible ® xture designs that kinematically constrain the part

in the plane. Rong and Bai (1997) designed a Modular Fixture Element AssemblyRelationship Graph (MFEARG) to represent combination relationships between

® xture elements. Based on MFEARG, synthesis algorithms were developed to


search all suitable ® xturing unit candidates and mount them into appropriate posi-

tions on a base-plate with interference checking.

4.2.2. Rule-based reasoning

The use of acknowledge-based computer design scheme will help to introduce the

essential philosophy to ensure that the optimum design is achieved. Nee (1987)

applied AI in jig and ® xture design. Nnaji et al. (1988) and Nnaji and Alladin(1990) developed a framework for a rule-based expert ® xturing system for face

milling a planar surface on a CAD system using ¯ exible ® xtures. Roy et al. (1997)

addressed several implementation issues of a prototype AFM system. Such AFD

systems proposed a preliminary ® xturing con® guration by synthesizing the ® xture

design issues and reasoning about the necessary knowledge bases. Trappey and Liu

(1992) employed heuristic search techniques on the projected envelope of the work-piece to determine the locating and clamping points. King and Lazaro (1994)

presented a ® xture synthesis algorithm a hybrid (heuristic-cum-mathematical )

model to arrive at design decisions. Lin and Huang (1997) developed diŒerent

heuristic algorithms for ® xture element selection corresponding to diŒerent

functional requirements.

4.2.3. Genetic algorithm

Fixture Design is generally regarded as a complex multi-modal and discrete

problem. While other methods have di� culties dealing with it, the trials on GAs

have provided a viable alternative. Wu and Chan (1996) applied genetic algorithmsto the ® xture con® guration optimization: based on the information provided by the

veri® cation system, a genetic algorithm approach carries out the evaluation process

to determine the most statically stable ® xture con® guration among a large number of

candidates.

4.2.4. Case-based methodIn the CBR concept previous experience is stored as episodes in a case library.

Each episode, termed a case, records at least two kinds of information: problem

description and solution. When a CBR system faces a new problem, the most similar

previous case is retrieved and modi® ed to satisfy the new situation. Each case and the

input problem are indexed according to the index system. The similarity of each casecan then be evaluated by comparing the index of each case with the index of the

input problem. Sun and Chen (1995) developed a seven-digit code to represent and

classify the modular ® xtures. Using this index system and case-based method, the

designer can ® nd a rough sketch of this ® xture design.

4.2.5. Neural network algorithmsAfter network training, the ® xture mode of the workpiece can be inferred, and

selection of the ® xture elements can be completed. Ong and Nee (1996, 1998) applied

fuzzy theory to evaluate part ® xturability. Lin integrated the GT description of

modular ® xtures, neural networks, and the heuristic algorithm for ® xture synthesis

in his automatic ® xture design system. Genetic algorithms and neural networks canbe combined in a ® xture design system (Lin 1998); Kumar et al. (1992) applied this

integration methodology in conceptual design of ® xtures.


4.2.6. Blackboard-based design

Blackboard-based design systems are used in various other engineering applica-tions in which concurrent or cooperative problem solving processes are achieved.

Fixture design involves interdisciplinary knowledge, with the help of blackboard

architecture. The developed ® xture design system could perform lower level reason-

ing and allow the analysis to be integrated with the design process, so that reliabilityassessment can be accomplished when it could best aŒect the design (Roy and Liao

1998).

4.3. Summary of the methodologies in ® xture design

Fixture con® guration design can be separated into three phases: description of

design requirements, the ® xture analysis, and ® xture synthesis. Fixture analysis

involves the relational models among design variables, kinematic and dynamic con-strictions, and performance evaluation; while ® xture synthesis involves ® nding an

optimal solution for a given workpiece and its machining. Many methodologies have

developed for designing a ® xture con® guration. However, a suitable methodology is

very important for a given design requirement and design level to improve e� ciency,

and the harmony of the methodologies should be maintained. A brief summary onthe methodologies used in ® xture design is given in Table 3.

5. Computer-aided ® xture design and set-up system

A review is presented on computer-aided ® xturing design and set-up systems.

5.1. Architecture of computer-aided ® xture design and set-up systemSoftware architecture describes the system components and their relationships.

Bugtai and Young (1997) discussed the information model in an integrated ® xture

design support system. Nederbragt (1997) proposed a theoretical framework for the

design of a robotic ® xture. Fuh and Nee (1995) presented the architecture and details


Geometrymethod

p p p p p

Description Feature-of design based £ £

p p p

requirement GT codes £ £ £p

£

FiniteGeometry Screw element Rule-based Mechanics

Fixture analysis modelling theory method knowledge methods

Fixture Analyticalp p

£ £p

synthesis methodsRule-based £ £

p p£

Genetic £ £p p

£agorithm

case-based £ £ £p

£Neural £ £ £

p£

networks

£ means two methodologies are not compatible.pmeans two methodologies are compatible.

Table 3. Summary on the methodologies in ® xutre design.


Developersand resource Application Methodology

Pham and Lazaro Jigs and ® xture Expert CAD system(1990) ® nite element method

Nnaji and Alladin Face milling planar part, Rule-based expert system(1990) ¯ exible ® xtures

Young and Bell Two and half dimensional Geometric query, product(1991) prismatic arts model analysis

de Sam Lajaro and Machining ® xture Tolerance and sequentialKing (1992) operations analysis

Lim et al. (1992) part ® xturing in high-precision 3D modular ® xture design expertindustry, system

Trappy and Liu Workholding veri® cation Geometry analysis(1992)

Wolter and Trinkle Selection of ® xture point, Geometry method(1994) frictionless assemblies

King and Tolerance consideration, Knowledge-based systemsde Sa Lazaro (1994) general ® xture

Brost and Goldberg Polygonal parts, Geometric analysis(1994) modular ® xture

Mason (1995) General ® xture Finite element method

Willy et al. (1995) Modular ® xture Geometry and mechanic analysis

Sun and Chen (1995) Modular ® xture Cased-based reasoning, indexsystem

Perremans (1996) Prismatic parts, modular Feature-based description,® xture expert system

Cecil et al. (1996) Prismatic parts, general Integration methodology® xture

Shirnzadeh (1996) Interference detection, CAD-based heirarchicalrecon® gurable modular approach® xture system

Wu and Chan (1996) Optimal ® xture con® guration Generic algorithm

Rong (1996) Machining accuracy analysis Datum-machining surfaceRong and Bai (1997) design veri® cation relationship graph, matrix-

based reasoning

Jones et al. (1997) Automated assembly and Archidedes’ s approach® xture planning holdfast software

Ho and Ranky Flexible assembly system, Object-oriented modelling,(1997) recon® gurable conveyors heuristic searching

Roy et al. (1997) Prismatic parts, Geometry reasoning,modular ® xture object-oriented

Chou (1997) Complex part, general Fixturing feature analysis,® xture spatial inference analysis

Wu and Rong (1998) Dedicated ® xture Fixture structure analysis,geometry± element generator

Lin and Huang 3D workpieces, GT, neural networks, pattern(1997) modular ® xture classi® cation

Jeng and Gill (1997) Prismatic parts CAD-based approach

Table 4 (continued)

of the development of an interactive ® xture design and assembly (IFDA) environ-

ment. Chou (1997) presented a three-stage system architecture from a concurrentmanufacturing planning point of view. Jeng and Gill (1997) thought that arti ® cial

intelligence techniques should be used in conjunction with 3D solid CAD and data-

base systems at the commercial level for modular ® xture design, pricing and inven-

tory control. Greater emphasis should be put on the integration of the intelligent

® xture design system with the robotics ® xture assembly, knowledge-based processplanning system, and intelligent production scheduling system (Chou 1997).

5.2. Computer-aided ® xture design (CAFD)

Computer-aided ® xture design is mainly relevant to MFFSs and dedicated ® x-

tures. Many CAFD prototype systems have been developed; some CAFD systems,together with applications and using methods, are shown in table 4.

5.3. Computer-aided ® xture setup (CAFS) for MFFSs

One of the di� culties in the application of MFFSs is to plan and execute the set-

up of modular elements. The assembly location and sequence should be determined

for all modular elements in a CAFS system. Little attention has been paid to thisissue. Table 5 provides a summary of these works.

6. Some prospective research trends

Recent achievements in the development of ¯ exible ® xture systems, design meth-odologies, together with computer-aided design and set-up systems have been ex-

amined in this paper. Adaptation of FFSs can greatly reduce manufacturing costs in

low-to-medium volume products, and the design and set-up automation of FFSs can

reduce the lead-time of the product; however, the current design and automation

theories and technologies are not mature. The researches on these ® elds arepromising and challenging. It is our observation that three research aspects are

most promising.


Developersand resource Application Methodology

Tseng (1998) Prismatic machining parts, Feature-based analysis,general ® xture CAD/CAM integration

Brost and Peters 3D workpiece, modular Project geometry, mechanic(1998) ® xture and assembly pallets analysis

Ma et al. (1998) Modula ® xture Geometry modelling,Integration with CAD, system

Roy and Liao (1998) General ® xture Cooperative design, blackboardframework

Brown and Brost 3D prismatic parts, 3D geometric analysis(1999)

Kumar et al. (1999) Conceptual design of ® xture Genetic algorithm, neuralnetworks

Shreyes and Melkote Location accuracy, ® xture Mechanic analysis(1999) layout optimization

Table 4. Summary of recent developed CAFD systems.

6.1. Development of an autonomous ¯ exible ® xture systemIn today’s FFSs, especially for MFFSs, ® xture design, veri® cation, and assembly

largely depend on human activities. This situation has resulted in the low e� ciency,

unstable accuracy, long set-up time, and high cost of FFSs. Development and appli-

cation of autonomous ¯ exible ® xture systems can address this issue. The state of the

art in this ® eld shows the possibility of developing a autonomous ¯ exible ® xturesystem. `Autonomous’ means to implement the automatic running of an FFS with

the aid of ¯ exible elements and automatic servo control, arti® cial intelligence and

sensing. An AFFS should possess the following characteristics:

(1) have a large degree of freedom to accommodate the ® xturing feature vari-

eties of the workpiece family;(2) determine optimal ® xture con® guration based on design requirements auto-

matically;

(3) build the ® xture con® guration automatically by adjusting internal variables

or assembling modules;

(4) load the workpiece in the ® xture± workpiece system automatically;

(5) monitor ® xturing processes and adjust the con® guration dynamically toachieve the best manufacturing performance;

(6) unload the workpiece and reset the ® xturing system automatically.

6.2. Computer-aided ® xture design for manufacturing

Many researchers have realized the importance of integration CAFD systems

with other CAD software systems in a manufacturing system. However, their eŒortsshowed that they mainly concentrated on CAFD systems, and the integration was

limited to obtaining the ® xturing requirements from other CAD systems and sending

the result of the CAFD to CAPP systems directly; this is a single-direction connec-

tion. Most research works have neglected the eŒect of ® xture design on product

design and process planning, and the redesign of ® xtures has never been seriouslyconsidered. Future research on CAFD will put the emphasis on cooperation with the

other design systems, the connection should be bi-directional, and the CAFD process


Developersand Resource Application Methodology

Asada and Andre Sheet metal ® xturing Accessibility and detachability(1985) analysis

Lam and Lim (1989) Modular ® xture assembly Robotics oç ine programming

Youce-Toumi and Modular ® xture assembly Integration CAD systems,Bell (1989) kinematic analysis

Dai and Nee (1997) Assembly sequence Rule-based reasoning

Khan and Cegiarek Assembly for sheet metal Hierarchical group description,(1998) part ® xture fault diagnosis

Yu and Goldberg Module ® xture assembly Sense-based compliant assembly,(1998) geometrically reasoning

Table 5. Summary of computer-aided ® xture setup systems for MFFSs.

information should also be fed back to the other system as early as possible. In

considering the integration of the CAFD system with the other CAD manufacturingsystems, combination with advanced computer technologies, such as a WWW

browser, collaborative design system, integration architecture of various platforms,

becomes urgent.

6.3. Applied research

Practical issues also appear when ¯ exible ® xture systems are put into use. Not all

research has taken the issues seriously. However, the solutions for these issues are

vital to extend the application of FFSs.

6.3.1. Select or design a ¯ exible ® xture system for workpiece families

Any FFS has favourable ® xturing features in its application. An engineer has aresponsibility to select or design the most suitable FFS for the manufacturing part

family. Therefore, it becomes a meaningful issue to select, evaluate and design a FFS

for a part family optimally. This issue is more di� cult than the design of a ® xture

con® guration, because the ® xturing objects have uncertain requirements. To address

this issue, some qualitative considerations should be included in the evaluationmodels, such as cost, e� ciency, suitability and lead time.

6.3.2. From dedicated ® xtures to FFSs

Liu (1994) proposed a systematic design method that helped companies change

their dedicated ® xturing systems gradually into modular ® xturing systems. Thisresearch is very practical. Further research should be carried out, such as how to

design dedicated ® xture elements for evolving FFSs, how to improve the possibilities

of varieties of ® xture con® gurations from the systems, how to manage the dedicated

and ¯ exible ® xture hybrid systems, etc. The solutions to these issues can bring great

economic e� ciency to small-scale machining enterprises.

6.3.3. Multi-® xtures and multi-® xturing tasks schedule

In a real machining workshop, there may be various FFSs, dedicated ® xtures and

many workpieces. Dynamic scheduling methodology is needed to distribute the

limited ® xture resources for ® xturing workpieces. Some researchers have revealed

tooling management issues in manufacturing industry (Arun and Suresh 1996,Ebrahim and Liu 1995, Elon and Burdick 1998, Perera and Sharfaghi 1995).

However, they are mainly concerned with the scheduling issues of general machining

tools. The schedules of FFSs and their elements pose many special characteristics.

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