An integrated process planning and robust fixture design system 2

A rule/object-based approach was used to group the machining features into appropriate fixture set-ups, and suitable clamping, locating and supporting points are recommended.. A rule-bas

Literature Review

2.1 Introduction

In this research, four topics are addressed, viz., the definition of integrative features,

the feature-based approach, the optimization of set-up planning, and the robust fixture

layout In this context, feature-based approaches include integrative feature definition,

machining feature extraction and fixturing-feature-based approach Integrative

features are to be defined in Chapter 3 Research on machining feature extraction has

been well addressed, while fixturing-feature-based approach to a much lesser extent

Therefore, in this chapter, the previous research on fixturing-feature-based approach,

together with set-up planning and fixture layout are reviewed in detail After each

review section, a short discussion is made, followed by a summary at the end of this

chapter

2.2 Fixturing-feature-based Approaches

Fixtures are devices used to locate and hold workpieces in manufacturing operations

Fulfilling fixturing requirement during machining is as important as the operations

and tools in the manufacture of a part (Ong and Nee, 1994)

Feature-based technology is found to be feasible for the integration of

CAD/CAM/CAPP segments due to its ability to capture the designer’s intent from one

stage to the other of product development (Shah, 1990) It is important for achieving a

true integration of design and manufacturing stages during early product

development The manufacturing information needed from design models include the

retrieval of machining features with technological specifications (dimensions,

tolerances, etc.) with respect to the machining operations, and fixturing features which

are holes, recesses and facets on a part for providing places for locating and clamping

the workpiece during the machining processes

Research on machining feature extraction has been well addressed, while

fixturing-feature-based approach to a much lesser extent

Trappey et al (1990) developed an automatic fixture configuration system using a

projective spatial occupancy enumeration (PSOE) approach The PSOE approach uses

the 2D projection of a 3D object for the selection of locating and clamping positions

for a possible fixture configuration This research emphasized that PSOE could deal

with arbitrarily-shaped workpieces, and established several algorithms for searching

fixturing positions However, in using PSOE, one may lose the 3D geometric

information of the workpiece and lead to unrealistic solutions

A feature-based methodology for fixture design was developed to select fixturing

faces and elements efficiently by Dong (1991) He investigated the use of features for

fixture design, concentrating on the selection of locating elements and the

identification of locating surfaces for workpiece positioning

A feature-based classification scheme using a 3D solid modeler was presented It uses

a feature extractor and an object-oriented system shell (Nee et al, 1992) Its intended

use lies in variant fixture design as well as its association with machining operations,

machining environment, cutting tools and workpiece features The operations begin

with a solid model where machining features are extracted and grouped into set-ups

based on machining directions and tolerance factors A knowledge base is used to

infer the operations involved and the cutting tools for a giving machining

environment

Kumar et al (1992) developed a feature recognizer for extracting machining features

represented in a CAD model A rule/object-based approach was used to group the

machining features into appropriate fixture set-ups, and suitable clamping, locating

and supporting points are recommended The fixturing elements are then selected and

assembly sequences are planned A knowledge-based approach was used to reason the

clamping, locating and supporting faces for a set up

Fuh et al (1993) presented an approach to computer-assisted fixture planning,

emphasizing the integration of fixture planning with process planning A rule-based

approach was presented to determine planar locating and clamping surfaces of a

workpiece for a given machining operation on a three-axis vertical milling machine

Chou et al (1994) presented a method to identify fixturing features for a given

operation plan during the conceptual design stage of fixture planning Based on the

surface reasoning method, five common fixturing features are identified and defined

They are extreme surfaces, unobstructed surfaces, locating holes, corners and flanges

Fixture functions are also assigned to each fixturing feature, such as hole-location,

V-block, and 3-2-1 locations One design strategy is developed for each mentioned

fixture other than a general strategy for all types of fixtures The system can be used

for prismatic parts consisting of primarily flat and cylindrical surfaces

Roy and Sun (1994) presented a fixture configuration method using heuristic

algorithms for selecting the locating and clamping positions for a given workpiece in

an automatic fixture design (AFD) system (cutting force direction as the main factor

to determine the primary locating surfaces) It uses several geometric reasoning

mechanisms based on traditional fixture design principles, such as the 3-2-1 locating

principle and collision-free assembly

Form features have been used besides the planar surfaces in the research reported by

Ong and Nee (1994, 1996, 1997, 1998), where a methodology was presented for the

quantitative evaluation of the fixturing properties of features (both planar surfaces and

form features) on a part such as clamping, location and/or supporting features with the

use of fuzzy membership functions The system was designed for machining prismatic

parts on a 3-axis vertical machining center

Table 2.1 summaries the approaches, the types of fixturing features and workpieces

considered by other researchers

Table 2.1 Summary of fixturing-feature-based approaches in literature

Researchers Automatic Feature

Extraction Approach Types of Fixturing Feature WorkpieceTypes of

Trappey et al (1990) Projective spatial

occupation enumeration (POSE)

Planar surfaces

Arbitrarily-shaped Dong (1991) Design information

Prismatic, Rotational, Special

Kumar et al (1992) Rule/object-based Planar surfaces Prismatic

Fuh et al (1993) Rational/rule-based Planar surfaces

Cylindrical surface

Prismatic

Chou et al (1994) Surface reasoning Planar surface,

Form features Prismatic Rotational Roy and Sun (1994) Geometric reasoning Planar surface Prismatic

Ong and Nee (1994,

1996, 1997, 1998) Fuzzy membership functions Form features, Planar surfaces Prismatic

In general, feature-based design can be achieved through three methods:

 Automatic feature extraction

 Interactive feature definition

 Design-by-feature

Researchers mostly agree that an ideal feature-based system should provide an

environment for the design-by-feature approach, in combination with feature

extraction and interactive feature definition

Based on previous research, fixturing feature extraction is the popular approach while

design-by-fixturing-feature approach is less addressed A CAD model can usually

provide sufficient information for fixturing, either as geometric objects or with the

associated technical information Therefore, it is logical to have automatic extraction

of fixturing features from the CAD model in the first instance If there are insufficient

fixturing features for manufacturing purposes from the extraction results, additional

fixturing features should be defined or designed to ensure the manufacturing

processes are achievable Therefore, it is desirable to apply a hybrid of

fixturing-feature extraction with design-by-fixturing-fixturing-feature and interactive fixturing fixturing-feature

definition approaches

In this research, a hybrid fixturing-feature-based approach is adopted to obtain

fixturing features for the integration of design and manufacturing, which uses the

above-mentioned three approaches

2.3 Set-up Planning

Set-up planning is a function of both process planning and fixture design (Ong and

Nee, 1994) It should consider both design specifications and manufacturing

resources Design specifications include workpiece geometry, dimension, tolerance,

and features Manufacturing resources include available production equipment,

cutting tools, and fixtures A set-up plan which considers these two factors can ensure

the delivery of the product with not only high quality but also high throughput and

low cost

From literature, various set-up planning approaches have been applied to meet the

design specifications of workpieces in terms of tolerance analysis, precedence

constraint satisfaction, geometric data analysis, and tool access direction verification

Fuzzy sets theory was used by Ong et al (1994, 1996, 1997, 1998, 2000) to present

the geometrical, tolerance and fixturing relations, machining requirements, design

features, etc., in the set-up planning systems for manufacturability and fixturability

evaluation

Zhang et al (1995) proposed a hybrid heuristic-based and optimization approach, in

which various constraints other than tolerances in set-up planning are identified and

discussed

Precedence relationships among the features have been analyzed by Ong et al (2002)

to generate a precedence relationship matrix This matrix acts as the main constraints

for set-up planning optimization

Most of the research studies have adopted tolerance analysis as the main criterion in

set-up generation and sequencing

Boerma and Kals (1988) reported on the development of a computer-aided planning

system for the selection of set-ups and the design of fixtures in part manufacturing

The automated selection of set-ups is based on the comparison of the tolerance

relations between the different shape elements of the part A tolerance factor has been

developed to compare the effect of different tolerances The system selects the

positioning faces automatically and supports the selection of tools for positioning,

clamping and supporting the part

Zhang et al (1996) and Huang et al (1997) discussed the importance of set-up

planning in relation to tolerance control in process planning A graphical approach

was proposed to generate optimal set-up plans based on design tolerance

specifications

Wu and Chang (1998) described an approach that uses the tolerance specification in a

feature-based design system to generate set-up plans with explicit datum elements

The focus of this research is an automatic tolerance analysis approach for selecting

set-ups and datum for prismatic workpieces in the design system

Zhang and Lin (1999) introduced a systematic approach for automatic set-up planning

in CAPP The concept of “hybrid graph”, which can be transferred into directed graph

by changing any two-way edge into one-way edge, is introduced Tolerance relations

are used as critical constraints for set-up planning

Lin et al (1999) developed a variant CAPP system with tolerance charts to automate

the generation of operation illustration for aircraft components

Zhang et al (2001) employed an extended graph to describe a Feature and Tolerance

Relationship Graph (FTG) and a Datum and Machining Feature Relationship Graph

(DMG), which could be transferred to an analytical computer model, and a tolerance

decomposition model to partition a tolerance into interoperable machining errors

These could be used for locating error analysis or for feedback to the design stage for

design improvement

Tseng and Huang (2007) presented a multi-plant tolerance allocation model to

determine the working tolerance of each of the components by considering all the

feasible manufacturing operations of the available plants The primary objective is to

maximize the cumulative sum of the working tolerances

Hebbal and Mehta (2007) focused on the development of a formalized procedure for

automatic generation of feasible set-ups and selection of an optimal set-up plan for

machining the features of a given prismatic part The proposed work considers

simultaneously the basic concepts of set-up planning from both machining and

fixturing viewpoints in order to formulate feasible set-up plans

A few researchers have considered machine resources during set-up planning

Zhang et al (1999) proposed object-oriented manufacturing resources modeling

(OOMRM) and agent-based process planning (AAPP) OOMRM describes

manufacturing resource capability and capacity in an object-oriented manner, which

intends to encapsulate manufacturing system knowledge and the methods of using the

knowledge Based on OOMRM, an AAPP prototype is implemented as a

man-machine integrated process planning platform It supports an experienced

manufacturing engineer in mapping out a more reasonable and flexible machining

process

Ong et al (2002) presented a hybrid generative algorithm and simulated annealing

approach for set-up planning and reset-up planning in a dynamic workshop

environment

Cai et al (2008) proposed an adaptive set-up planning approach for various multi-axis

machine tools, focusing on kinematic analysis of tool accessibility and optimal set-up

plan selection

Since set-up planning can produce alternate set-up plans due to different

considerations between design specification and machine resources, the question of

optimization arises Different approaches have applied to deal with this problem

Zhang et al (1995) used a numerically exhaustive approach to select the best solution

from all the possible alternatives that satisfy the required constraints

Zhang et al (1996) and Huang et al (1997) proposed a graph-based theoretical

approach to represent the design specifications of a part The problem of identifying

the optimal set-up plan is transformed into a graph search problem

Zhang et al (1999) applied SA to set-up planning and Zhang (1997) used GA for the

optimization

Zhang et al (2001) presented seven set-up planning principles to minimize machining

error stack-up under a true positioning GD&T scheme assisted with the extended

graph approach

Ong et al (2002) presented a hybrid generative algorithm and simulated annealing

approach for set-up planning and reset-up planning in a dynamic workshop

environment Generated precedence relationship matrix acts as the main constraints

for the set-up planning optimization

Tseng and Huang (2007) presented a mathematical programming model to distribute

the components to the suitable plants to achieve the objective of minimizing

multi-plant manufacturing costs

Hebbal and Mehta (2007) focused on the development of a formalized procedure for

automatic generation of feasible set-ups and then to select an optimal set-up plan for

machining the features of a given prismatic part

An optimal tolerance assignment strategy is developed and implemented by Song et al

(2007) The optimization criteria are to minimize the manufacturing cost and cycle

time while maintaining product quality The cost model considers effective factors at

the machine level, part level, and feature level Optimization of tolerance assignment

plan with genetic algorithm is formulated The Monte Carlo simulation based

tolerance stack up analysis is employed to determine the satisfaction of design

tolerance requirements

Table 2.2 summaries the published research methodologies on set-up planning

considering both tolerances and machine resources