GROUP TECHNOLOGY AND COMPUTER AIDED PROCESS PLANNING
Group technology (GT) is a philosophy that implies the notion of recognizing and exploiting similarities in three different ways:
1. By performing like activities together
2. By standardizing similar tasks
3. By efficiently storing and retrieving information about recurring problems
Large manufacturing system can be decomposed into smaller subsystems of part families based on similarities in design attributes and manufacturing features.
Concept of Group technology;
Group technology is a manufacturing philosophy in which similar parts are identified and grouped together to take the advantage of their similarities in manufacturing and design. Similar parts are arranged in to part families.
Advantages of group technology
· Product design benefits- 10 % reduction in the number of drawings
· Tooling and setup benefits – 69 % reduction of setup time.
· Materials handling benefits
· Production and inventory control benefits
· -70 % reduction in production time
· -62 % reduction in work in process inventories
· -82 % reduction in overdue orders
· Employee satisfaction
· Process planning procedures
Group technology (GT);
Group technology (GT) is a manufacturing philosophy to increase production efficiency by grouping a variety of parts having similarities of shape, dimension, and/or process route.
Group technology is a manufacturing philosophy in which similar parts are identified and grouped together to take the advantage of their similarities in manufacturing and design.
A part family is a collection of parts which are similar either because of geometric shape and size or because similar processing steps are required in their manufacture.
· Part configuration (round or prismatic)
· Dimensional envelope (length to diameter ratio)
· Surface integrity (surface roughness, dimensional tolerances)
· Material type
· Raw material state (casting, forging, bar stock, etc.)
· Operations and operation sequences (turning, milling, etc.)
· Batch sizes
· Machine tools
· Cutting tools
· Work holding devices
· Processing times
Benefits of Group Technology
Group technology, when successfully implemented, offers many benefits to industries. GT benefits can be realized in a manufacturing organization in the following areas:
1. Production design
2. Tooling and setups
3. Materials handling
4. Production and inventory control
5. Process planning Management and employees.
1. Benefits in product Design
The main advantages of GT for product design come in cost and time savings, because design engineers can quickly and easily search the database for parts that either presently exist or can be used with slight modifications, rather than issuing new part numbers.
A similar cost savings can be realized in the elimination of two or more identical parts with different part numbers. Another advantage is the standardization of designs. Design features such as corner radii, tolerances, chamfers, counter bores and surface finishes can be standardized with GT.
2.Benefits in Tooling and Setups
In the area of tooling, group jigs and fixtures are designed to accommodate every member of a part family. Also work holding devices are designed to use special adapters in such a way that this general fixture can accept each part family member. Since setup times are very short between parts in a family, a group layout can also result in dramatic reductions in setup times.
3.Benefits in material handling:
GT facilitates a group layout of the shop. Since machines are arranged as cells, in a group layout, the materials handling cost can be reduced by reducing travel and facilitating increased automation.
4.Benefits in production and inventory Control
GT simplifies production and planning control. The complexity of the problem has been reduced from a large portion of the shop to smaller groups of machines. The production scheduling is simplified to a small number of parts through the machines in that cell.
5.Benefits in Process Planning
The concept of group technology – parts classification and coding – lead to an automated process planning system. Grouping parts allows an examination of the various planning/route sheets for all members of a particular family. Once this has been accomplished, the same basic plans can be applied to other members, thereby optimizing the shop for the group.
6.Benefits to Management and Employees
It is understood that GT simplifies the environment of the manufacturing firm, which provides significant benefit to management.
· Simplification reduces the cumbersome paper work.
· Simplification also improves the work environment.
In the GT work environment, the supervisor has in – depth knowledge of the work performed and better control.
General methods used for part families;
1. Visual inspection,
2. Parts classification and coding system, and
3. Production flow analysis.
Production Flow analysis;
Production Flow analysis (PFA) is a method for identifying part families and associated machine groupings that uses the information contained on production route sheets rather on part drawings.
Various steps of PFA
1. Data collection
2. Part sorting and routing
3. PFA chart
Parts classification and coding system
1. system based on part design attributes
2. system based on manufacturing attributes
3. system based on design and manufacturing attributes
Code structures used in GT application;
· Attribute codes (or polycodes or chain type structure).
· Hierarchical codes (or monocodes or tree structure).
· Decision-tree codes (or hybrid codes or mixed codes).
Coding is the systematic process of establishing an alphanumeric value for parts on selected part features. Classification is the grouping of parts based on code values. This method is the most time consuming of the three methods, in parts classification and coding, similarities among parts are identified and these similarities are related in a coding system. Three categories of part similarities can be distinguished 1. Design attributes which
are concerned with part characteristics such as, geometry, size and material, and 2. Manufacturing attributes consider the processing steps required to make a part.3.system based on both attributes.
There are three basic coding structures
1. Hierarchical codes (or monocodes)
2. Attributes codes (or polycodes)
3. Decision tree codes (or hybrid codes)
Through more than 100 coding systems are available, the following coding systems are widely recognizes in industries
1. Opitz classification system 6. CUTPLAN system
2. DCLASS system 7. COFORM
3. CODE system 8. RNC system
4. MICLASS system 9. Part analog system
5.KK-3 system 10. Brish system.
Cellular manufacturing (CM) is an application of group technology in which dissimilar machines have been aggregated into cells, each of which is dedicated to the production of a part family.
The machines in a multi station system with variable routing may be manually operated, semi-automatic, or fully automated. When manually operated or semi automatic the machine groups are often called machine cells, and the use of these cells in a factory is called cellular manufacturing.
Design considerations guiding the cell-formation.;
· Parts/products to be fully completed in the cell.
· Higher operator utilization.
· Fewer operations than equipment.
· Balanced equipment utilization in the cell.
Types of cell design
1. Single machine cell
2. Group machine cell with manual handling
3. Group machine cell with semi-integrated handling
4. Flexible manufacturing system
Determining the best machine arrangement
Factors to be considered:
· Volume of work to be done by the cell
· Variations in process routings of the parts
· Part size, shape, weight and other physical attributes
Process Planning is the systematic determination of the methods by which a product is to be manufactured, economically and competitively.
Role of process planning
§ Interpretation of product design data
§ Selection of machining processes.
§ Selection of machine tools.
§ Determination of fixtures and datum surfaces.
§ Sequencing the operations.
§ Selection of inspection devices.
§ Determination of production tolerances.
§ Determination of the proper cutting conditions.
§ Calculation of the overall times.
§ Generation of process sheets including NC data.
Process planning techniques;
§ Manual approach
§ Computer aided process planning techniques
§ Retrieval type CAPP system (Variant type CAPP system)
§ Generative type CAPP system
1.Computer/Aided Process Planning;
§ CAPP refers to computer/aided process planning.
§ CAPP is used to overcome the drawbacks of manual process planning.
§ With the use of computers on the process planning one can reduce the routine clerical work of manufacturing engineers.
§ Also it provides the opportunity to generate rational, consistent and optimal plans.
Computer aided process planning system offers the potential for reducing the routine
clerical work of manufacturing engineers.
It provides the opportunity to generate routings which are rational, consistent and perhaps even optimal.
Retrieval type CAPP (Variant type) systems;
For each part family a standard process plan is established and stored in computer files and then it is retrieved for new work parts which belong to that family.
Because of the alterations that are made in the retrieved process plan, the CAPP system is known as variant system.
Generative CAPP system;
Generative process planning involves the use of computer to create an individual process plan automatically without human assistance.
The computer would employ a set of algorithms to progress through the various technical and logical decisions toward a final plan.
2. Variant or Retrieval approach;
A retrieval CAPP system, also called a variant CAPP system, has been widely used in machining applications. The basic idea behind the retrieval CAPP is that similar parts will have similar process plans.In this system., a process plan for a new part is created by recalling., identifying and retrieving an existing plan for a similar part, and making the necessary modifications for the new part.
In fact, the variant CAPP is a computer – assisted extension of the manual approach. The computer assists by providing an efficient system for data management, retrieval ,
editing and high speed printing of the process plans. The retrieval CAPP system has the capacity to alter an existing process plan. That’s why it is also known as variant CAPP
Procedure for using Retrieval CAPP system
A retrieval CAPP system is based on the principles of group technology (GT) and parts classification and coding. In this system, for each part family a standard process plan (i.e., route sheet) is prepared and stored in computer files. Through classification and coding, a code number is generated. These codes are often used to identify the part family and the associated standard plan. The standard plan is retrieval and edited for the new part.
Variant CAPP system procedure.
Step 1 :Define the coding scheme
Adopt existing coding or classification schemes to label parts for the purpose of classification. In some extreme cases, a new coding scheme maybe developed.
Step 2 :Group the parts into part families
Group the part families using the coding scheme defined in Step 1. based on some common part features. A standard plan is attached to each part family (see step 3) . Often, a number of part types are associated with a family, thereby reducing the total number of standard process plan.
Step 3: Develop a standard process plan for each part family based on the common features of the part types. This process plan can be used for every part type within the family with suitable modifications.
Step 4.: Retrieve and modify the standard plan:
When a new part enters the system, it is assigned to a part family based on the coding and classification scheme. Then the corresponding standard process plan is retrieved and modified to accommodate the unique features of the new part.
Advantages of Retrieval CAPP system:
· Once a standard plan has been written, a variety of parts can be planned.
· Comparatively simple programming and installation ( compare with generative
CAPP systems) is required to implement a planning system.
· Efficient processing and evaluation of complicated activities and decisions, thus reducing the time and labour requirements.
· Standardized procedures by structuring manufacturing knowledge of the process planners to company’s needs.
· Lower development and hardware costs. Draw backs of Retrieval CAPP system
· The components to be planned are limited to similar components previously planned.
· Maintaining consistency in editing is difficult.
· Experienced process planners are still required to modify the standard plan for the specific component.
3. Generative approach;
In the generative approach, an automatic computerized system is used to synthesize or generate each individual process plan automatically and without reference to any prior plan. The automatic computerized system normally consists of decision logic, formulas, technology algorithms and geometry based data to uniquely determine the many processing decisions required for generating process plans.
Unlike the retrieval CAPP no standard manufacturing plans are predefined or stored. Instead, the computer automatically generates a unique operation/ route sheet whenever the part is ordered. Thus the generative CAPP system automatically generates the process plan based on decision logics and pre-coded algorithms. The computer stores the rules of manufacturing and the equipment capabilities (not any group of process plans).
When using a system, a specific process plan for a specific part can be generated without any involvement of a process planner. The human role in running the system includes (i) inputting the GT code of the given part design, and (ii) monitoring the function.
Components of Generative CAPP system
The various components of a generative system are,
· A part description, which identifies a series of component characteristics, including geometric features, dimensions, tolerances and surface condition.
· A subsystem to define the machining parameters for example using look – up tables and analytical results for cutting parameters.
· A subsystem to select and sequence individual operations.
· Decision logic is used to associate appropriate operations with feautures of a component, and heuristics and algorithms are used to calculate operation steps, times and sequences.
· A database of available machines and tooling.
· A report generator which prepares the process plan report.
Advantages of Generative CAPP
The generative CAPP has the following advantages:
· It can generate consistent process plans rapidly.
· New components can be planned as easily as existing components.
· It has potential for integrating with an automated manufacturing facility to provide detailed control information.
4. Networking methods with necessary sketches;
Networking is a convenient technique for typing together the various islands of automations and in the process makes integration possible through high speed data exchange between different automated segments.
Networking of computers was initially adopted successfully by service sectors like banking, air lines and train reservation etc..,
Communication networks can be classified in four ways depending upon the physical separation of communicating devices.
1. Miniature – (<50m) such networks are concerned with the interconnection of multiple computational elements.
2. Small – (<500m) these are concerned with the interconnection of multiple computational units.
3. Medium – (<1km) these are concerned with the interconnection of multiple computational units. These are connected through a local area network or internet.
4. Large – (>1km) large networks involve connection of remote mainframes, networking of mini computer systems to a remote mainframe or terminals etc. it can be city wide or country wide or world wide. With internet becoming more and more popular, the intranet – internet – extranet technologies have found favor with manufacturing companies.
Network Wiring methods;
There are two basic ways by which three or more nodes can be incorporated in a network. These are point – to – point and multi drop.