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Due to the rapid change of both economical and social life style of the world, the need and demands of products are also increasing. These results the growth of industries and the technologies used in industries to increase the productivity. In today’s industries, manufacturing of the product completely depends upon the design specification and process planning through which the product undergoes.
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 (CAPP) has evolved to simplify and improve process planning and achieve more effective use of manufacturing resources. To reduce the product lead time and relative cost associated with the product CAPP is introduced to meet the requirements of the customer and to produce the product both in high quality and quantity.
Computer-aided process planning is a link between design and manufacturing in a computer-integrated manufacturing (CIM) environment. Commonly used by manufacturing engineers, CAPP can be used to develop a product manufacturing plan based on projected variables such as cost, lead times, equipment availability, production volumes, potential material substitution routings and testing requirements.
The application of various methods of Computer Aided process planning (CAPP) has become one of the most important topics in manufacturing industries and particularly in the automotive industry. The application of various CAPP techniques practically covers the full product development cycle from the conceptual product design through the process planning up to the manufacturing phase of the production. CAPP techniques are widely used in sheet metal forming, for example to predict the formability, to determine the type and sequences of manufacturing processes and their parameters, to design forming tools, etc. The importance of the application of CAPP tools becoming more and more important as the manufactured parts are becoming ever increasingly complex.
WHAT IS PROCESS PLANNING?
Process planning is nothing but the bridge between design and manufacture of the product. Process planning translates the design specification of the product into a sequence of the operations or the machine instruction through which the manufacturing will be done to produce the final product.
Products and their components are designed to perform certain specific functions. Every product has some design specifications which ensure its functionality aspects. The task of manufacturing is to produce components such that they meet design specifications. Process planning acts as a bridge between design and manufacturing by translating design specifications into manufacturing process details. It refers to a set of instructions that are used to make a component or a part so that the design specifications are met, therefore it is major determinant of manufacturing cost and profitability of products. Process planning answers the questions regarding required information and activities involved in transforming raw materials into a finished product. The process starts with the selection of raw material and ends with the completion of part. The development of process plans involves mainly a set of following activities: -
• Analysis of part requirements
• Selection of raw work piece
• Selection of manufacturing operations and their sequences
• Selection of machine tools
• Selection of tools, tool holding devices, work holding devices and inspection equipments
• Selection of manufacturing conditions i.e. cutting speed, feed and depth of cut.
• Determination of manufacturing times
Process planning encompasses the activities and functions to prepare a detailed set of plans and instructions to produce a part. The planning begins with engineering drawings, specifications, parts or material lists and a forecast of demand. The results of the 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 checkpoints.
• Fabrication and assembly drawings to support manufacture (as opposed to engineering drawings to define the part).
INTRODUCTION OF CAPP
An important link of a Computer Integrated Manufacturing (CIM) system’s chain is the CAPP module, within which it consists of the preparation, design and coordination of the manufacturing processes. CAPP is the linkage between the Computer Aided Design (CAD) module and the Computer Aided Manufacturing (CAM) module, which confers it a special place in the CIM system. Thus, a performing CAPP module has to meet following requirements: -
•it must be capable of «understanding» and analyzing the parts’ characteristics;
•it must posses multiple knowledge about machine-tools, cutting tools and their
•it must posses the capability of analyzing the interdependencies between part-
Machining. Process - quality – cost .
•it must posses multiple calculus possibilities.
Starting from these requirements and after analyzing the information streams there
were defined the main objectives for conceiving a CAPP module which could be integrated in the chain of a CIM system.
The utilization of the CAPP module does not impose as an absolutely necessary
condition the existence of the previous process (CAD), neither that of the next process (CAM) and thus the existence of automated manufacturing systems .
If the goal is to conceive a CIM system, the two processes become absolutely necessary and the linkage between them is assured by CAPP.
Manufacturers have been pursuing an evolutionary path to improve and computerize process planning in the following five stages:
Stage I - Manual classification; standardized process plans
Stage II - Computer maintained process plans
Stage III - Variant CAPP
Stage IV - Generative CAPP
Stage V - Dynamic, generative CAPP
Manual Process Planning :
Prior to CAPP, The manual experience-based process planning was most widely used. It was mainly based on a manufacturing engineer's experience and knowledge of production facilities, equipment, their capabilities, processes, and tooling. The major problem with this approach was that it was time consuming and developed plans may not be consistent and optimum because different experts were doing the process planning differently. The feasibility of developed process plan was dependant on many factors such as availability of machine tools, scheduling and machine allocation etc.
Manufacturers attempted to overcome the problems of manual process planning by basic classification of parts into families i.e the production of a product is divided into several parts and similar parts are grouped into families and then developing somewhat standardized process plans for parts families (Stage I). When a new part was introduced, the process plan for that family would be manually retrieved, marked-up and retyped. While this improved productivity, it did not improve the quality of the planning of processes and it did not easily take into account the differences between parts in a family nor improvements in production processes.
Computer Aided Process Plans :
Computer-aided process planning initially evolved as a means to electronically store a process plan in computers once it was created Here more of the parts of process planning is done manually and some part of the planning process is done by the computer. During planning for a new product the existing plans made earlier can be retrieved and modified for a new part and print the plan (Stage II). Other capabilities of this stage are table-driven cost and standard estimating systems.
VARIANT COMPUTER AIDED PROCESS PLANNING :
This initial computer-aided approach evolved into what is now known as "variant"CAPP. However, variant CAPP is based on a Group Technology (GT) coding and classification approach to identify a larger number of part attributes or parameters. These attributes allow the system to select a baseline process plan for the part family and accomplish about ninety percent of the planning work. The planner will add the remaining ten percent of the effort modifying or fine-tuning the process plan. The baseline process plans stored in the computer are manually entered using a super planner concept that is, developing standardized plans based on the accumulated experience and knowledge of multiple planners and manufacturing engineers (Stage III).
The method through variants (VPP - Variant Process Planning) is conceptually based on
the idea that similar parts are being manufactured in similar ways. Therefore, one of the main components of the CAPP module is that of part coding, which uses the principle of group technology. In a consistent database we must search for the variant which is closest to the needed part. Creating and modifying the typified process is the job of the engineer.
The first activity carried out is that of coding, classifying and grouping the parts on
families, which represents the preparation state, followed by the production state, which refers to the usage of the CAPP module during actual production.
The database which was created during the preparation state undergoes a process of
continuous completing with new part types which will be machined. Their regrouping will be made based on the group technology, by modifying and adapting the configuration of the new part type as needed. The main role is that of the database and that of the knowledge base, which must be updated and improved constantly.
Coding, Grouping On
Setting the standard technological process plan for the family
Technological process optimization
It follows the principle that similar parts require similar plans. Therefore, the process requires a human operator to classify a part, input part information, retrieve a similar process plan from a database (which contains the previous process plans), and edit the plan to produce a new variation of the pre-existing process plan. Planning for a new part involves retrieving of an existing plan and modification. In some variant systems parts are grouped into a number of part families, characterized by similarities in manufacturing methods and thus related to group technology.
In comparison to manual process planning, the variant approach is highly advantageous in increasing the information management capabilities. Consequently, complicated activities and decisions require less time and labor. Also procedures can be standardized by incorporating a planner’s manufacturing knowledge and structuring it to a company specific needs. Therefore, variant systems can organize and store completed plans and manufacturing knowledge from which process plans can be quickly evaluated.
However, there are difficulties in maintaining consistency in editing practices and adequately inability to accommodate various combinations of geometry, size, precision, material, quality and shop loading. The biggest disadvantage is that the quality of process plan still depends on the knowledge background of a process planner.
Generative Computer Aided Process Planning :
The next stage of evolution is toward generative CAPP (Stage IV). At this stage, process planning decision rules are built into the system. These decision rules will operate based on a part's group technology or features technology coding to produce a process plan that will require minimal manual interaction and modification (e.g., entry of dimensions).
The principle based on the automatic realization of the machining process’ sequences,
without the operator’s decisional intervention is found in the generative method (GPP -
Generative Process Planning) .
To realize a generative CAPP module, it is necessary to have a knowledge base which
includes three main components:
•the parts’ description,
•the knowledge base and the database,
•the decisional logics with the calculus algorithms.
While CAPP systems are moving more and more towards being generative, a pure generative system that can produce a complete process plan from part classification and other design data is a goal of the future. This type of purely generative system (in Stage V) will involve the use of artificial intelligence type capabilities to produce process plans as well as be fully integrated in a CIM environment. A further step in this stage is dynamic, generative CAPP which would consider plant and machine capacities, tooling availability, work center and equipment loads, and equipment status (e.g., maintenance downtime) in developing process plans.
The process plan developed with a CAPP system at Stage V would vary over time depending on the resources and workload in the factory. For example, if a primary work center for an operation(s) was overloaded, the generative planning process would evaluate work to be released involving that work center, alternate processes and the related routings. The decision rules would result in process plans that would reduce the overloading on the primary work center by using an alternate routing that would have the least cost impact. Since finite scheduling systems are still in their infancy, this additional dimension to production scheduling is still a long way off.
Dynamic, generative CAPP also implies the need for online display of the process plan on a work order oriented basis to insure that the appropriate process plan was provided to the floor. Tight integration with a manufacturing resource planning system is needed to track shop floor status and load data and assess alternate routings vis-a-vis the schedule. Finally, this stage of CAPP would directly feed shop floor equipment controllers or, in a less automated environment, display assembly drawings online in conjunction with process plans.
The mixed or hybrid method, which combines the generative method and the method
through seems to be at the time being the best answer for the automation of the machining technology for companies which produce small series of similar parts, but using the group technology remains a problem for diversified parts. Because of the necessity to process and transmit a large amount of information and in order to realize the integration in the CIM system, for the conceiving of the CAPP module it was necessary to use besides of the variant and generative methods the object-oriented databases’ administration system (OODBAS), as well as the object-oriented programming
(OOP). The object-oriented databases’ administration systems (OODBAS) are part of the
products which allow the administration of the more and more complex information used in areas like Computer-Aided Design (CAD), Computer Aided Process Planning (CAPP),
programming engineering, information administration.
The object-oriented models were achieved in accordance to the requirements of
integration in a complex information system and to the functions of traditional databases’
administration systems. The main concept used is the object. It is described by a complex of properties which constitute the object’s attributes and methods (similar to the group technology).
The input information’s are graphical as well as alphanumeric. The CAPP modules
•graphical data related to the parts, which have the shape and dimensions as basic types of the object-oriented models;
•alphanumeric data, which contain technological information;
•methods base, which comprises optimization and calculation algorithms.
THE REALIZATION OF CAPP
Combining the three solutions presented above has led to the creation of a CAPP
module based mainly on the TechCIM program /2/, which allows the optimization of the
machining process according to specific algorithms, allowing also the intervention of the
engineer for the final determination of the machining sequences, by modifying the defined objects’ attributes, according to the access levels in databases. The TechCIM program for the automatised realizing of the machining process for cardanic transmissions uses the principles of group technology. The correspondence between surface coding and technical conditions for machining is achieved through the graphical files, realized in the CAD module. The processing of graphical information from CAD files is made together with that of the technological information, thus realizing as a first step an association between geometrical shapes and machining procedures. The program has been realized and structured in Microsoft Excel, because the corresponding database has been realized with the same program
After the surfaces-machining procedures association stage, there follows the
establishing of the final machining procedure of a surface based on the tolerance and
roughness conditions. Thus are being established the machining variants which are technically possible, according to the database with the technological equipment.
Based on the economic analysis, the optimal machining process is automatically being
determined and afterwards the technological documentation for the manufacturing is issued (matching parameters are being calculated, the necessary devices and cutting tools are being designed, the necessary time is set.
In realizing the CAPP module there have been used decision tables because, in this case, the decision processes can be expressed through algorithms, i.e. according to precise rules. A decision table is divided in two sectors: conditions and actions. In the upper sector there is a list of conditional parameters - conditions - whose characteristics depend on entities - actions - listed in the lower section. The rules indicate, for an accurate determination, which action is to be considered and under which circumstances. Thus, by creating a large number of rules, determinated for the specific situations of the production (endowment with technological equipment, personnel qualification, organization type, investments, etc.) there could be established the preliminary conditions for realizing the presentation model of the CAPP module.
Contrary to a relational model, the object-oriented model allows not only the
description of the static aspect of an application, regarding data and structure, but also the
description of the dynamic aspect, regarding the object behavior and communication.
The above diagram is the diagrammatic representation of Computer Aided Process Planning (CAPP)…
Generally CAPP uses generative approach in most of the industries. Generates process plans utilizes decision logic, formulae, manufacturing rules, geometry based data to determine the processes required to convert the raw materials into finished parts.
It develops new plan for each part based on input about the part’s features and attributes. Due to the complexity of this approach a generative CAPP system is more difficult to design and implement than a system based on the variant approach. But a generative CAPP system does not require the aid of a human planner, and can produce plans not belonging to an existing part family. It stores the rules of manufacturing and the equipment capabilities in a computer system.
The generative approach is complex and a generative system is difficult to develop. In comparison, the variant systems are better developed and mature than generative systems They are suitable for planning processes in mass or large production volumes. For planning discrete processes of manufacturing products of great diversity, generative systems are much more suitable than variant systems. However true generative systems are still to come although the earlier optimistic speculation made by researchers. Most CAPP systems in use now are either variant systems or semi-generative systems (with some planning functions developed with variant approach, others with generative approach). Proper combination of the two approaches can make an efficient CAPP system. First the system will check whether the process planning is possible for a new part by variant approach. If variant system is unable to identify the part to be of a previous group or family it will use generative technique for process planning. So both the variant and generative process planning approaches need further development in parallel.
The realization of the object-oriented database for the computer aided conception of
manufacturing processes makes it possible to use the object-oriented programming for
drawing out the technological documentation.
Beginning with the systemic approach of the mechanical machining process, figure 7,
by defining the subsystems which intervene in the manufacturing system there can be defined the objects for each program, e.g. for machine-tools, figure 8, each object being characterized by state, behavior, and identity.
The information about an object is to be accessed of modified only through the
multitude of tasks (methods) which define the object.
Now here is the small coding or process plans for the milling machine by using OOP concepts …..
. Gauge Dimensions
. Useful Work Way
. Rotation Range
. Feed range
. Cutting Tool Change
. Technical Conditions
(ENTITY RELATIONSHIP MODEL FOR TH EMACHINE TOOL)
From the object’s structure , there can be seen the information’s structure, as well as the tasks’ implementation :
APPLICATION OF CAPP IN AIRCRAFT ENGINEERING:
Aircraft structural parts make around 1% of the total aircraft components. At present, the production cycle of several components families is within the range of few hours. Conversely, the production of structural parts is mainly a human-made operation with manufacturing complexity increasing with the part’s morphology. The delicate thin elements and the presence of complex surfaces – mainly ruled surfaces which acquires the external aircraft body shape – sets the production time of mechanical structural parts ranging from few hours (for basic parts) up to over 20 days (for complex parts). The main loss of time is identified in the process planning field.
Thus Computer Aided Process Planning (CAPP) software is used in aircraft engineering
The new USIQUICK project and the resulting CAPP software functions and prototype give the air crafting technique easier.
ADVANTAGES OF CAPP AND FUTURE TRENDS:
CAPP has important advantages over manual process planning which includes;
• Reduced process planning and production lead-times
• Faster response to engineering changes in the product
• Greater process plan accuracy and consistency
• Inclusion of up-to-date information in a central database
• Improved cost estimating procedures and fewer calculation errors
• More complete and detailed process plans
• Improved production scheduling and capacity utilization
• Improved ability to introduce new manufacturing technology and rapidly update process plans to utilize the improved technology
Due to the introduction of CAPP the recent statistics shows the cost effectiveness of the manufacturing of the products as follows;
Significant benefits can result from the implementation of CAPP. In a detailed survey of twenty-two large and small companies using generative-type CAPP systems, the following estimated cost savings were achieved:
• 58% reduction in process planning effort
• 10% saving in direct labor
• 4% saving in material
• 10% saving in scrap
• 12% saving in tooling
• 6% reduction in work-in-process
In future CAPP will be introduced almost in all industrial applications like
1. tape automated bonding
2. digit length coding scheme
3. Agile Manufacturing Environment
4. steel metal forming
LIMITATIONS OF CAPP:
There are number of difficulties in achieving the goal of complete integration between various functional areas such as design, manufacturing, process planning and inspection. For example, each functional area has its own stand-alone relational database and associated database management system. The software and hardware capabilities among these systems pose difficulties in full integration. There is a need to develop single database technology to address these difficulties. Other challenges include automated translation of design dimensions and tolerances into manufacturing dimensions and tolerances considering process capabilities and dimensional chains, automatic recognition of features and making CAPP systems affordable to the small and medium scale manufacturing companies.
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 minimal cost and risk. Effective use of these tools can improve a manufacturer's competitive advantage.
1. Kenneth Crow ,DRM Associates (firstname.lastname@example.org)
2. Journals of Achievements in materials and manufacturing engineeringOccasional paper, volume 24. issue 1 , September 2007
3. Abo-Rayia, S., et. Al.,”Optimum conventional Computer Aided Process
Planning”,Computers and Industrial Engineering, Vol. 31, No. 1/2, pp. 177-180
Cherng, J.G., Shao, X.Y., Chen,Y., Sferro, P.R., ” Feature based part
modeling and process planning for rapid response manufacturing.”, Computers and Industrial Engineering, Vol.34, No.2, pp. 515-530,1998
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