Computer Aided Process Planning (CAPP)

Computer Aided Process Planning (CAPP)

1. INTRODUCTION

Technological advances are reshaping the face of manufacturing, creating

paperless manufacturing environments in which computer automated process planning

(CAPP) will play a preeminent role. The two reasons for this effect are: Costs are

declining, which encourages partnerships between CAD and CAPP developers and

access to manufacturing data is becoming easier to accomplish in multivendor

environments. This is primarily due to increasing use of LANs; IGES and the like are

facilitating transfer of data from one point to another on the network; and relational

databases (RDBs) and associated structured query language (SQL) allow distributed

data processing and data access.

.With the introduction of computers in design and manufacturing, the process

planning part needed to be automated. The shop trained people who were familiar with

the details of machining and other processes were gradually retiring and these people

would be unavailable in the future to do process planning. An alternative way of

accomplishing this function was needed and Computer Aided Process Planning (CAPP)

was the alternative. Computer aided process planning was usually considered to be a

part of computer aided manufacturing. However computer aided manufacturing was a

stand alone system. Infact a synergy results when CAM is combined with CAD to

create a CAD/CAM. In such a system CAPP becomes the direct connection between

design and manufacturing.

Moreover, the reliable knowledge based computer-aided process planning

applicationMetCAPP software looks for the least costly plan capable of producing the

design and continuously generates and evaluates the plans until it is evident that non of

the remaining plans will be any better than the best one seen so far. The goal is to find a

useful reliable solution to a real manufacturing problem in a safer environment. If

alternate plans exist, rating including safer conditions is used to select the best plans

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1.1 COMPUTER AIDED DESIGN (CAD)

A product must be defined before it can be manufactured. Computer Aided

Design involves any type of design activity that makes use of the computer to develop,

analyze or modify an engineering design. There are a number of fundamental reasons

for implementing a computer aided design system.

a. Increase the productivity of the designer: This is accomplished by helping

the designer to visualize the product and its component subassemblies and parts; and

by reducing the time required in synthesizing, analyzing, and documenting the

design. This productivity improvement translates not only into lower design cost but

also into shorter project completion times.

b. To improve the quality of the design: A CAD system permits a more

thorough engineering analysis and a larger number of design alternatives can be

investigated. Design errors are also reduced through the greater accuracy provided

by the system. These factors lead to a better design.

c. To improve communications: Use of a CAD system provides better

engineering drawings, more standardization in the drawings, better documentation

of the design, fewer drawing error, and greater legibility.

d. To create a database for manufacturing: In the process of creating a the

documentation for the product design (geometries and dimensions of the product

and its components, material specification for components, bill of materials etc),

much of the required data base to manufacture the product is also created.

Design usually involves both creative and repetitive tasks. The repetitive tasks within

design are very appropriate for computerization.

1.2 COMPUTER AIDED MANUFACTURING (CAM)

By the time computer use in design began, numerical control technology (NC

technology) had matured to become cost effective for applications in machining. An

important in numerical control is part-programming. A part-program is simply a set of

statements comprehensible to the machine control unit (MCU) , that oversees slide and

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tool movements and other auxiliary functions. In the case of components with complex

geometries, part-programs had to carry out lengthy calculations for which it was logical

to use computers. This gave rise to machine control units (MCU’s) with built in

microprocessors- the building blocks of computers. The use of computers in extending

the applications of NC technology, especially to part-programming was earlier termed

Computer Aided Machining (CAM) and the associated technology was called Computer

Numerical Control (CNC). Later Computer Aided Machining became an acronym for

Computer Aided Manufacturing (CAM). Earlier Computer Aided Manufacturing used

to denote computer use in part-programming only. Today it means any non design

function of manufacturing that is computer aided.

1.3 CAD/CAM

As the use of computers in design and manufacturing broadened under CAD and

CAM, it became evident that certain tasks were common to both, eg:-both design and

manufacturing require data on tolerances. Part geometries created during CAD can

readily be saved in the database for latter use. The forward slash (/) between CAD and

CAM was meant to reinforce the shared functions of design and manufacturing.

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2. PROCESS PLANNING

The product design is a plan for the product and its components and

subassemblies. To convert the product design into a physical entity, a manufacturing

plan is needed. The activity of developing such a plan is called process planning. It is a

link between product design and manufacturing. Process planning involves determining

the sequence of processing and assembly steps that must be accomplished to make the

product. It is concerned with the engineering and technological issues of how to make

the product and its parts. What types of equipment and tooling are required to fabricate

the part and assemble the product. It involves determining the most appropriate

manufacturing and assembly process and sequence in which they should be

accomplished to produce a given part or product according to the specifications set forth

in the product design documentation. All the related information is documented on a

Route Sheet .The planning begins with engineering drawings, specifications, parts or

material lists and a forecast of demand. The scope and variety of processes that can be

planned are generally limited by the available processing equipment and technological

capabilities of the company or the plant.

Process planning is usually accomplished by manufacturing engineers. Based on

process planner’s skill, knowledge, and experience, the processing steps are developed

in the most logical sequence, to make each part.

The following are the list of many decisions and details usually included within the

scope of process planning.

Interpretation of design drawings: The part or product design must be

analyzed (materials, dimensions, tolerances, surface finishes etc) at the start of the

process planning procedure.

Processes and sequences: The process planner must select which processes are

required and their sequence. A brief description of all processing steps must be

prepared.

4


0010 5145 S/U COLLET 2.00 0.173ROUGH TURN M/C PER TAPE NO: LS982A 0 .440 DIA BY1.7500 LENGTTH

0 .300 DIA BY0.8120 LENGTTH 0 .275 DIA BY0.4375 LENGTTH FINISH 3/64 GROOVES (TYP) AND CHAMFERS 0.270 DIA.BY 0.375 LENGTH CHAMFER CUTOFF TO 1.9600015 1026 #2 CENTERS BOTH ENDS 0.25 0.0040020 9401 CARBURIZE AND HARDEN 0.500030 4063 S/U BETWEEN CENTERS 1.25 0.0983

GRIND OD HOLD CONCENTRICITYHOLD 0.4200 DIM. HOLD 0.2600 DIM.HOLD 0.2815 DIM. HOLD 0.2712 DIM.

0040 9501 BLAST TO CLEAN 0.0010050 9201 CHROME PLATE PER PRINT 0.380060 9805 FINAL INSPECT

ROUTE SHEET GENERATED BY MIPLAN

5

ORGANISATION FOR INDUSTRIAL RESEARCH, INC FACILITY-F1PART NUMBER:A63799

PART NAME:SHAFT, ARM

PLNG REV: 02 DWG REV:0

#3

MFG

ENG

Q/A

INSPECTIONS

PLANNRER: ADAMS

#2#1

FHB

PC

AH

OPERNO:

PIECE TIMES

PRJ#

MACHTOOL

OPER DESCRIPTION- ASSY INSTRUCTIONS

SET UP TIMES

OPR

S/O#

A34UB

ORDERQTY

MIN.QTY

SPECIAL INSTRUCTIONS / HANDLING:

½” DIA MS-500 H.R. STEEL (2” LENGTH)

DUEDATES PR#

CODE#: 1310-1181-2111-0000-0100-0000-0000-00

45D3 1000935 249

2


Equipment selection: In general, process planers’ must develop plans that

utilize existing equipment in the plant. Otherwise, the component must be purchased

or an investment must be made in new equipment.

Tools, Dies, Moulds, Fixtures and gauges: The process planner must decide

what tooling is required for each processing step. The actual design and fabrication

of these tools is usually delegated to a tool design department and tool room or an

outside vendor specializing in that type of tool is contracted.

Method analysis: Workplace layout, small tools, hoists for lifting heavy parts

even in some cases hand and body motions must be specified for manual operations.

The industrial engineering department is usually responsible for this area.

Work standards: Work measurement techniques are used to set time standards

for each operation.

Cutting tools and cutting conditions: These must be specified for machining

operations often with reference to standard handbook recommendations.

The results of planning are:

Routings which specify operations, operation sequences, work centers,

standards, tooling and fixtures. This routing becomes a major input to the

manufacturing resource planning system to define operations for production activity

control purposes and define required resources for capacity requirements planning

purposes.

Process plans which typically provide more detailed, step by step work

instructions including dimensions related to individual operations, machining

parameters, set-up instructions, and quality assurance check points

Fabrication and assembly drawings to support manufacture.

Manual process planning as mentioned earlier is based on a manufacturing engineer’s

experience and knowledge of production facilities, equipment, their capabilities,

processes and tooling. Process planning is a time-consuming process and the results

vary based on the person doing the planning.

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3. COMPUTER-AIDED PROCESS PLANNING ( CAPP)

Process planning translates design information into the process steps and

instructions to efficiently and effectively manufacture products. As the design process is

supported by many computer aided tools, computer aided process planning has evolved

to simplify and improve process planning and achieve more effective use of

manufacturing resources.

3.1 CAD/CAM INTEGRATION AND CAPP FEATURES

A frequently overlooked step in the integration of CAD/CAM is the process

planning that must occur. CAD systems generate graphically oriented data and may go

so far as graphically identifying metal etc to be removed during processing. In order to

produce such things as NC instructions for CAM equipment, basic decisions regarding

equipment to be used, tooling and operating sequence need to be made. This is the

function of Computer aided process planning. Without some elements of CAPP there

would be no such thing as CAD/CAM integration. The CAD/CAM systems that

generate tool paths and NC programs include limited CAPP capabilities or imply a

certain approach to processing.

CAD systems also provide graphically oriented data to CAPP systems to use to

produce assembly drawings etc. Further, this graphically oriented data can then be

provided to manufacturing in the form of hardcopy drawings or work instruction

displays. This type of system uses work instruction displays at factory workstations to

display process plans graphically and guide employees through assembly step by step.

The assembly is shown on the screen and as a employee steps through the assembly

process with a footswitch, the components to be inserted or assembled are shown on the

CRT graphically along with text instructions and warnings at each step.

If NC machining processes are involved, CAPP software exists which will select tools,

feeds, and speeds and prepare NC programs.

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3.2 COMPUTER-AIDED PROCESS PLANING : TYPES

Computer aided process planning systems are designed around two approaches.

These approaches are called:

1. Retrieval CAPP systems or Variant Approach

2. Generative CAPP systems or Generative Approach

Some Computer aided process planning systems combine the two approaches in what is

known as Semi Generative Approach.

1. Retrieval CAPP System or Variant Approach

The retrieval type is suitable for a family of parts. This system draws a standard

process plan and stores it in the database. Whenever a different part from the family is

to be processed, the standard process plan is retrieved and appropriately modified –

hence the retrieval to this system. The retrieval system relies on the concept of group

technology for part coding and classification. It is also compatible with the concept of

cellular manufacturing in which cells are designed and laid out for family-of-parts

production. In this type, as mentioned earlier a standard process is stored in computer

files for each part code number called the Route Sheet.

The retrieval CAPP system operates as given in figure 2 . Before the system can

be used for process planning, a significant amount of information must be compiled and

entered into the CAPP data files. This is referred to as the “preparatory step”. It consists

of the following steps:

i. Selecting an appropriate classification and coding scheme for the company

ii. Forming part families for the parts produced by the company

iii. Preparing standard process plans for the part families

Steps (ii) and (iii) continue as new parts are designed and added to the company’s

design database.

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GENERAL PROCEDURE FOR RETRIEVAL CAPP SYSTEMS

After the preparatory phase has been computed, the system is ready for use. For

a new component for which the process plan is to be determined, the first step is to

determine the GT code number for the part. With this code number a search is made for

the part family file to determine if a standard route sheet exists for the given part code.

If the file contains a process plan for the part, it is retrieved (hence the word “retrieval”

for this CAPP system) and displayed for the user. The standard process plan is

examined to determine whether any modifications are necessary. It might be that

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Derive GT code number for part

Search part family file for GT code number

Part family file

Process plan formatter

Edit existing plan or write new plan

Prepare standard process plans for part families

Selecting coding system and form part families

Standard process plan file

Retrieve standard process plan

New part design

Other application programs

Process plan (route sheet)


although the new part has the same code number, there are minor differences in the

process required to manufacture it. The user edits the standard plan accordingly. This

capacity to alter an existing standard process plan is what gives the retrieval system its

alternative name: “variant” CAPP system.

If the file does not contain a standard process plan for the given code number,

the user may search the computer file for a similar or related code number fro which a

standard route sheet does exists. Either by editing an existing process plan or by starting

from scratch the user prepares the route sheet for the new part. This route sheet becomes

the standard process plan for the new part code

The process planning session concludes with the process plan formatter, which

prints out the route sheet in the proper format. The formatter may call other application

programs into use. For eg: -To determine machining conditions for the various machine

tool operations in the sequence, to calculate standard time for the operations or to

compute cost estimates for the operations.

One of the commercially available Retrieval CAPP systems is MultiCapp, from

OIR, the Organization for Industrial Research. It is an online computer system that

permits the user to create new plans, or retrieve and edit existing process plans as

explained earlier.

2. Generative CAPP System or Generative Approach

The generative method of developing process plans involves starting from

scratch every time a different part is to be processed; no plans are available as the

baseline. The basic requirement for a generative process planning system is that the

given component model/drawing is to be interpreted in terms of manufacturability. Here

instead of retrieving and editing an existing plan contained in the computer database,

generative system creates the process plan based on logical procedures. In a fully

generative CAPP system the process sequence is planned without human assistance and

without a step of predefined plans.

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A generative CAPP system is usually considered part of the field of expert

systems, a branch of artificial intelligence. An expert system is a computer program that

is capable of solving complex problems that normally require a human with years of

education and experience. Process planning fits within the scope of this definition.

There are several ingredients required in a fully generative process planning system:

i. First the technical knowledge of manufacturing and the logic used by successful

process planners’ must be captured and coded into a computer program. In expert

systems applied to process planning, the knowledge and logic of human process

planners’ is incorporated into a so called “knowledge base”. The generative CAPP

system then uses that knowledge base to solve process planning problems (ie create

route sheets)

ii. Second ingredient in process planning is a computer compatible description of

the part to be produced. This description contains all the pertinent data and

information needed to plan the process sequence. Two possible means of providing

this description are:

a. the geometric model of the part that is developed on a CAD system

during product design and

b. a GT code number of the part that defines the part features in significant

detail.

iii. The third ingredient in a generative CAPP system is the capability to apply the

process knowledge and planning logic contained in the knowledge base to a given

part description. In other words, the CAPP system uses its knowledge base to solve

a specific problem – planning the process for a new part. This problem solving

procedure is referred to as the “inference engine” in the terminology of expert

systems. By using its knowledge base and inference engine, the CAPP system

synthesizes a new process plan from scratch for each new part it is presented.

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4. GROUP TECHNOLOGY

Group technology is a manufacturing philosophy in which similar parts are

identified and grouped together to take advantage of their similarities in manufacturing

and design. Similar parts are arranged into part families. Each family would possess

similar design and manufacturing characteristics. Hence processing of each member of a

given family would be similar and this results in manufacturing efficiencies. These

efficiencies are achieved in the form of reduced set-up times, lower in-process

inventories, better scheduling, improved tool control and the use of standardized process

plans. The design retrieval system is a manifestation of group technology principle

applied to the design function. To implement such a system some form of parts

classification and coding is required.

Part classification and coding is concerned with identifying the similarities

among parts and relating these similarities to a coding system. Part similarities are of

three types:

i. Design attributes (such as geometric shape and size)

ii. Manufacturing attributes (sequence of processing steps required to make

the part)

iii. Design and manufacturing attributes (combination of the design and

manufacturing attributes)

When implementing a parts classification and coding system most companies elect to

purchase a commercially available package rather than develop their own. The

following factors are considered in selecting a parts coding and classification system:

Objective

Scope and application

Costs and time

Adaptability to other systems

Management problems

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4.1 BENEFITS OF GROUP TECHNOLOGY

Product Design Benefits: When a new part design is required, the engineer or

draftsman can devote a few minutes to figure the code of the required part. Then

the existing part designs that match the code can be retrieved to see if one of them

will serve the function desired. The few minutes spent searching the design file

with the aid of the coding system may save several hours of the designers’ time. If

the exact part design cannot be found, perhaps a small alteration of the existing

design will satisfy the function. Use of the automated design-retrieval system

helps to eliminate design duplication and proliferation of new part designs. Other

benefits of group technology in design are it improves cost estimation procedures

and helps to promote design standardization. Design features such as inside corner

radii, chamfers, and tolerances are more likely to become standardized with group

technology.

Tooling and Set-ups: In tooling, an effort is made to design group jigs and

fixtures that will accommodate every member of a parts family. Work holding

devices are designed to use special adapters which convert the general fixture into

one that can accept each part family member. The machine tools in a GT cell do

not require drastic changeovers in set-up because of the similarity in the workparts

processed on them. Hence setup time is saved. It has been estimated that the use of

group technology can result in 69% reduction in setup time.

Materials Handling: Another advantage in manufacturing is a reduction in the

workpart move and waiting time. The group technology machine layouts lend

themselves to efficient flow of materials through the shop.

Production and Inventory Control: Grouping of machines into cells reduces the

number of production centers that must be scheduled. Grouping of parts into

families reduces the complexity and size of the parts scheduling problem. Because

of the reduced set-ups and more efficient materials handling with machine cells,

production lead times, work-in-process, and late deliveries can all be reduced.

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Employee Satisfaction: The machine cell often allows parts to be processed

from raw material to finished state by a small group of workers. The workers are

able to visualize their contributions to the firm more clearly. This tends to

cultivate an improved worker attitude and higher level of job satisfaction. Here

more attention tends to be given to product quality. Also the workers are more

responsible for the quality of work they accomplish.

Process Planning Procedures: The time and cost of process planning function

can be reduced through standardization associated with group technology.

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5. CAPP LITERATURE AND CAPP METHODOLOGY

The developed CAPP system consists of feature recognition module and other

modules for selecting machines, tools, machining parameters and optimization modules.

The feature recognition module has been developed by SolidWorks as modeling

software and features are interpreted using a programme written in Visual Basic 6.0.

The Oracle 7.3 has been used for database management. The highlight of the

system is that it has got excellent user interface by which user can interact with the

system at different levels while generating a process plan.

METHODOLOGY

Various modules used for generating process plan are described as follows:

Feature Extraction Module

The Solid modelling software plays an important role in providing features data.

The solid modelling software package used in this work is SolidWorks 98 Plus windows

based software. The software coding to extract feature along with its attributes has been

implemented in Visual Basic 6.0 and database is created in Oracle 7.3 as backend.

Using the Application Programme Interface (API) of SolidWorks 98 Plus, it is possible

to fetch any of the functions that are used for the modelling.

Blank Selection Module

After extracting the features information from the feature extraction module, the

next step is to find out the overall dimensions of the raw materials required. Overall size

of the component is required for the selection of the raw material from the raw material

database.

Once the component is modelled in SolidWorks modelling software, it is

possible to find out the overall size of the component directly from SolidWorks

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Application Program Interface. The system automatically adds the machining

allowances on each face. Provisions are also made so that the user can also customize

the machining allowances according to requirements.

Set-Up Planning

Once the features are recognized, the next step is to group the features into set-

ups. Set-Up planning deals with the selection and sequencing of the processes required

for generating the final shape of a component. It is a mixture of complex and inter-

related tasks. Set-Up planning also includes the orientation in which the component is to

be placed on the work-table for machining. It covers work-holding criteria as well.

Set-Up Planning Methodology

The input for this particular module comes from the feature extraction module.

All the features identified from the CAD model are given to this module for further

processing. Here, different features have different possible directions of approach. One

of the possible directions of approach is perpendicular to the location face. The user

decides this by interfacing with the system and entering the location face details for

each and every feature. Any feature, which lies in the particular orientation, is given the

location face accordingly.

The feature tree holds the information about its nodes, dimensions, location and

tolerance information. If the user selects a particular node, then information about the

feature can be viewed. The algorithm developed searches through all the features and

checks for each face the number of features that can be machined. The feasible faces are

then ordered accordingly and then grouped. The desired set-up planning is the one in

which maximum number of features like tolerances, location is to be given manually

prior to sep-up formation.

Routing

Routing is the method of directing the blank through various stages to get the

required feature. In this module the following information is generated.

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Possible route identification

Selecting the optimum route

Calculating the intermediate dimensions for the route selected

Possible Route Identification

The process selection is done taking into consideration the following details:

The geometric and technological information of the features.

Each and every feature and its associated accuracy and surface finish

requirement are taken into consideration.

Based on the surface finish requirement specified and the accuracy indicated, the

various possible routes available in the feature-process-route database are

identified.

The method adopted gives various options or routes for a given feature. An

exclusive feature-process-route table is maintained in the database and routes are

selected based on the accuracy that can be attained using the specific route for a given

feature.

A feature may have one or more number of routes and the user is allowed to

change the routes according to his requirement. A process route can be divided based on

the final finish requirement

Eg: - To mill a block of accuracy IT 8, may be done in the following two ways:

Rough Milling Semi-finish Milling Finish Milling

Rough Milling Semi-finish Milling Rough-grinding

Many routes are possible similar to these and should be considered before

selecting the final route. The dimensional variations that are allowable for the features

on a part to be machined will affect the operations, tools, and set-ups that are required to

machine the parts. Stringent tolerance specifications between features will require more

accurate operations, tools and additional set-ups for machining them. At this stage itself,

feature sequencing is carried out.

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Cutting Tool Selection

The main concerns in tool selection include tool type, material, geometry, and

tool dimensions. Selection of tool type is based mainly on operations to be carried out

and the machine tools involved. A criterion for selecting tools for rough machining, for

eg: - is to minimize the tool changes and to maximize the number of features machined.

Several other factors also influence productivity such as tool material and size.

The process generation module of the CAPP system decides the process to be

carried out to finish the part by means of feature process correlation. Based on the tool

process correlation the cutting tool is decided. The tool process correlation holds all the

information regarding tools and their operations. If the feature is a hole, then a drill is to

be used.

But based on the feature dimension, the best matching tool is to be selected from

the corresponding database, providing data like tool code, tool length, diameter, and

insert material, separate databases are developed and maintained for drill tools, milling

cutters, and grinding wheels. The tool selection is facilitated with dual methods, namely

automatic selection and manually.

Machine Selection For Individual Process

Machine selection is based on the blank size and process capability of the

machine. For each and every process one or more number of machines are possible.

This is added in a list where it is identified by a key. The list contains the following link

information:

Set-Up number

Feature name

Process

After selecting the various possible machines, redundant machines are eliminated and

only machines, which are unique, are identified and it is retained for further processing.

The information is available as a list structure where the set-up is the main link.

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6. CAPP APPLICATIONS: CASE OF METCAPP

SOFTWARE

MetCAPP is a knowledge –based process planning and cost estimating system

targeted at providing:

Improved productivity by reducing process time and variability

Reduced inventories

Consistent and higher product quality levels

MetCAPP provides the link between design and manufacturing floor. It provides the

ability to take CAD generated solid models and use them to generate process plans

based on the best :

Machines

Tools

Sequence of steps

Timing

Routing/cost combinations

Provision for alternate and concurrent operations

The technology modules are:

Feature recognition

Milling

Turning

Hole Making

The user effectively manages the system via the technology module manager as

illustrated in the figure.

MetCAPP, which is a knowledge-based manufacturing process planning and

cost estimation system, uses the Step Optimisation to recognize the machining steps, to

optimise the time and resources. The Step Optimisation should be used only after

calculating the steps for all features, either manually or by technology modules. Once

the optimisation is generated, any changes to the features are reflected in step

optimisation data.

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Though, the inclusion of a process plan in the problem requires optimisation

criterion. Process planning is connected with optimising the resources and processing

costs as well as

Fig 3. MetCAPP : TECHNOLOGY MODULE MANAGER AND TECH.

FEATURES

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Knowledge Based Machining

Technology Module Manager

Milling Module

Turning

Module

Hole making

Module

Manufacturing

Technology

Database

Rules

Features


processing time. Consequently, the object function used in process planning and its

optimisation is to minimize the number of rejects and the total processing costs, as well

as minimizing the time required to complete all operations.

Moreover, MetCAPP develops a process plan based on specific elemental

manufacturing features. This approach can be used to decompose a very complex part

into a number of separate manufacturing features. The process planner still has a full

control over the plan by determining the order in which the features are selected and

included in the process plan.

Process planning is the glue between product, process, and resources. It is

necessary ability to manipulate, view, and deliver multiple data formats and types.

Today’s solutions are, at best, short term. Systems should be designed to reflect this, or

allow incremental changes. MetCAPP Software analyses the manufacturability of

proposed design as follows:

1) Finding Machining Features:

Many aspects of feature recognition problem are still open and active areas of

research. Among these are: recognizing and representing interactive features,

incremental recognition of features, and incorporation of user customizable feature

classes. As an input, MetCAPP takes solid models (for instance from SolidWorks- a 3D

software) of a part P and stock S, along with tolerance specifications for P. The

tolerance specifications tell how much variation from the nominal geometry is

allowable in any physical realization of a part P.

An operation plan is a sequence of machining operations capable of creating P

from S. A workpiece is the intermediate object produced by starting with S and

performing zero or more machining operations. The machining operations in MetCAPP

currently considered are (end milling, side milling, face milling and drilling). Each

machining operation will create either a primary feature or a truncation of primary

features from P and S. MetCAPP generates F automatically from solid models of P and

S.

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2) Generating Feature Based Models (FBM’s):

A Feature Based Model (FBM) is any irredundant subset of features F such

that P can be produced from S by removing the features in F. Each operation plan O of

interest corresponds to an FBM, in the sense that each machining operation in O will

create either a feature in F or a truncation of a feature in F. Since each FBM is a subset

of , FBM can be generated using set-covering techniques.

3) Generating Operation Plans:

Each FBM can lead to several operation plans, of which some are better than

others. Thus, to generate operation plans from a given FBM, MetCAPP again does a

depth-first-branch-and-bound search. Due to various types of interactions (accessibility,

set-up, etc.) among the features in a FBM F, these intersections introduce precedence

constraints requiring that some features of F be machined before or after other features.

MetCAPP generates a total orderings on F consistent with the precedence constraints.

4) Operation Plan Evaluation:

Designers give design tolerance specifications to specify how far the design can

vary from its nominal geometry. To verify whether a given operation plan will satisfy

the design tolerances, MetCAPP must estimate what tolerances the operations can

achieve. Unlike typical approaches for computer-aided tolerance charting, which are

computationally very intensive and only consider limited types of tolerances, MetCAPP

evaluates the manufacturability aspects of a wide variety of tolerances.

In manufacturing planning the goal to be achieved is represented by a design

specification. In planning a sequence of machining operations it is physically

impossible to produce the exact nominal geometry of the design, so the objective is to

find any reliable plan which can produce an approximation of the design geometry that

satisfies various design tolerances. In addition, it is advantageous to have a highly

differentiated view of the production costs.

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THE BASIC APPROACH USED IN MetCAPP

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CAD SYSTEM

1.Identify Features

2.Generate FBM

3.Generate Plan

4.Evaluate Plan

5.Feedback

CAD models of the part P and stock S

Find all primary machining features for P. Call this set .

Generate the feature-based model F.

Generate an operation plan O for F. If O can satisfy P’s machining tolerances, then estimate its cost and time

Information about P’s manufacturability

Designer


6.1 THE MetCAPP FEATURES

Automated Feature Recognition: generates the flow of data from solid models

directly into MetCAPP. This module allows users to import 3D solid CAD models

into MetCAPP and then automatically analyzes the part to extract manufacturing

features for sequencing and process plan generation.

Process Documentation. MetCAPP’s report writer: allows users to merge texts

and graphics, including CAD drawings, photographs, electronic documents and

bar codes into a single document. This can be printed, sent to the floor

electronically or through Application Programme Interface (API) to other parts of

the IP system (eg:- NC tape generation, MRP or order entry)

Graphics. MetCAPP’s redline capability: allows the user to add layers of

annotations to a file without changing the original drawing/graphic. MetCAPP

supports over 40 different graphic file types for viewing, printing, and

redline/markup.

The MetCAPP Technology Modules: These contain rules and data to support a

wide range of features. These automatically select a process sequence, tools for

each step and speeds/feeds for each machining pass. The technology modules

evaluate the capabilities of the machine and utilize as much machine horsepower

as is available at the selected speed range.

Templates and Formulas: provide MetCAPP users the flexibility to define tasks

and work procedures specific to their operations. Recall and replication of these

on demand further enhances planner productivity.

Cost Estimating: Costed routings with accurate tooling, fixturing, and materials

provide the estimator with strong quotation support.

Group Technology: MetCAPP provides the ability to interrogate a standard

database of process plans and identify parts and assemblies by their

characteristics. This enables identification of similar parts for more rapid plan

generation as well as strong support for configuring products in order entry.

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6.2 MetCAPP BENEFITS

50% increase in process planner productivity.

40% increase in existing equipment’s capacity.

25% reduction in setup costs.

12% reduction in tooling requirements

10% reduction in scrap and rework

10% reduction in shop floor labor

6% reduction in work-in-process

4% reduction in material usage

These are in addition to the overall gains resulting from:

Connecting process planning to their CAD systems for swift feedback, rapid and

accurate data transfer, plus work-in-process images when desired.

Linking process planning to CAM systems

Linking process planning to Enterprise Planning System, MRP/ERP, yielding a

single point of entry.

Today process planning and workshop scheduling in industry are carried out as

sequential, non-collaborative tasks. This has several disadvantages and can be improved

by making these activities partly concurrent or at least collaborative. The integration or

collaboration of scheduling and process planning has been the focus of extensive

international research efforts in recent years. To achieve the overall goals is necessary to

be followed the specific technical objectives with consideration of reliability,

maintainability, and supportability. A reliable and user-friendly interface to standard

CAD systems used in industry is urgently required to close the gap in the CAD-CAM

information chain. The MetCAPP system highlights the requirements that a nowadays

CAPP system must meet, namely: flexibility, modularity, interoperability, autonomy,

and scalability. These have been selected based on weaknesses of the current available

CAPP systems.

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1) Automated transfer of geometry and technology information from CAD which

reduces the effort spent on entering CAD data into a CAPP system, and

2) Powerful CAPP modules for interactive, semi-automated and generative process

planning and full support for time and cost calculation.

Non-linear concept used, for instance in MetCAPP, is especially suited for production

environment like mechanical manufacturing of discrete parts of the pilot user.

The use of powerful CAPP tools that meet the performance requirements will

improve process-planning quality with respect to applied manufacturing technology and

accuracy of time and cost calculation. This will result in better utilization of the

available manufacturing resources, more exact product costing, improved product

quality and generally speaking towards a safer world. Collaboration between process

planning and scheduling will improve the logistical quality of process plans through

feedback of loading information

As a computer aided process planning system, enables the process engineer

and/or the cost estimator to more effectively and safely plan the manufacturing process.

MetCAPP users realize improved productivity in reduced process time and variability,

reduced inventories and higher product quality levels, and in general safer production.

Manufacturing can use MetCAPP for more reliable planning, estimation and shop floor

control of manufacturing operation leading towards reduction in number of poor parts

and hence guarantee better future performance, as well as improved competitiveness

and productivity in a safer environment.

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7. CAPP BENEFITS

Significant benefits can result from implementation of Computer Aided Process

Planning. In a detailed survey of twenty-two large and small companies using

generative type CAPP system, the following estimated cost savings were achieved.

58% reduction in process planning effort

10% savings in direct labor

4% savings in material

10% savings in scrap

12% savings in tooling

6% reduction in work in process

In addition there are intangible benefits as follows:

Process rationalization and standardization

Increased productivity of process planners

Reduced process planning and production lead time; faster response to

engineering changes

Greater process plan consistency, access to up to date information in a central

database

Improved cost estimating procedures and fewer calculation errors

More complete and detailed process plans

Improved legibility

Improved production scheduling and capacity utilization

Improved ability to introduce new manufacturing technology and rapidly update

process plans to utilize the improved technology

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8. CONCLUSION

CAPP is a highly effective technology for discrete manufacturers with a

significant number of products and process steps. Rapid strides are being made to

develop generative planning capabilities and incorporate CAPP into a computer

integrated manufacturing architecture. The first step is the implementation of GT or FT

classification and coding. Commercially available software tools currently exist to

support both GT and CAPP. As a result, many companies can achieve the benefits of

GT and CAPP with minimum cost and risks. Effective use of these tools can improve a

manufacturers competitive advantage too.

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REFERENCES

Mikell P. Groover & Emory W. Zimmres. Jr, “Computer Aided Design and

Manufacturing”, Prentice Hall Publication, 1994.

“CAPPturing Manufacturing”, The Machinist, May-June 2003

http://www.cimplex.com/metcapp.hmt

“Computer aided process planning based on Information Management”, Journal

of Materials Processing Technology 103 (2000) 120-127.

“Computer aided Process Planning”, Kenneth Crow, DRM Associates

“A Reliable Knowledge Based Computer aided Process Planning Application-

Case of MetCAPP Software”, Galia Novakova, PhD student, Polytechnical

University of Turin, Department of Production Systems and Economics

Mikell P. Groover “Automation Production Systems and Computer integrated

Manufacturing”, Prentice Hall India Pvt Ltd, 1997

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James.a.Rehg and H.W. Kraebber, “Computer Integrated Manufacturing”,

Pearson Education Asia, 2002

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Computer Aided Process Planning (CAPP)

Documents

computer aided design

better design

engineering design

time computer

design errors

later computer aided

manufacturing data

product design geometries