CAM-UNIT-5

Production Planning and Control Introduction to production planning and control - Shop Floor control systems - Just in Time approach Emerging challenges in CAD/CAM, Product Data Management, - Product Modeling - Assembly and Tolerance Modeling. Production planning and control deals with preparing plans for manufacturing products, implementing these plans, monitoring progress to achieve these plai and taking corrective action if the original plans need modifications. The important functions of the production planning and control are given below. 1. Forecasting 2. Aggregate Production planning 3. Process planning 4. Estimating 5. Master Production Scheduling 6. Materials Requirements Planning (MRP) 7. Purchasing 8. Machine loading and scheduling 9. Dispatching 10. Expediting 11. Quality Control 12. Shipping and Inventory control The following diagram planning and control system shows the cycle of activities in a computer integrated production

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Demand Forecasting: Demand forecasting is used to predict the future sales activity of the firm’s products. Long range fb looks ahead 5 years or more iii advance and it is used to decide about plant constraining and equipment acquisition. Intermediate forecasting books ahead 1 or 2 years in advance and it is used to plan for long lead time materials and components. Short-term forecasting looks ahead 3 to 6 months in advance to decide about hiring of personnel, purchasing and production schedule Aggregate Production Planning: Once the demand of the products is known, an aggregate production plan can be prepared. It is based on the ‘Demand forecast’ and i used to increase (or) lower inventories, stabilize production over the planning horizon and allow for the launching of new products in the company’s product tine. Process Planning: We have already studied about the process planning. Process planning determines the sequence of operations required to produce a certain product. In manual process planning, the route sheets are prepared. The route sheets have a listing of operations and machine tools through which the part should be routed. Estimating: To fix a price of a product, the manufacturing lead time end production co should b arrived. The manufacturing lead time can be calculated by summing up the total time required to finish the job. The production cost is the sum of material cost, labor cost and overhead cost to produce the part. The data for arriving manufacturing Lead time and production cost are collected in the operation route sheets, purchase file and accounting records. Master Production Scheduling: The master production schedule is used to know how many number of units of each product-should be delivered and when should be delivered. The master production schedule is converted from the aggregate production plan. From the master production schedule, the purchase order for raw materials, and purchase order for components from outside vendors will be prepared. Also, the production schedule for the parts to be manufactured in the shop will also be prepared. Normally, the scheduling periods in the master production schedule may be of months, week (or) days. The quantities in master production schedule should not exceed the capacity of the plant. - Material Requirement Planning: (MRP) MRP is used to manage inventeries to issue orders for parts and materials. Based on the Master production schedule, the raw materials and components used are planned by MRP. The purchase of raw materials and production order of various components are ordered. All materials are planned so that they will be available whenever needed. MRP is an effective tool to minimize unnecessary inventory investment. It is very much useful for production scheduling and purchasing of materials MRP is a computerized method to process massive data required to convert master-production schedule into detailed schedule for raw materials and components. -

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The bill if materials show hat items and how many of new required. To assemble a finished product. The MRP analysis is performed to replenish the material the inventory .The important analysis used in inventory control is Economic Order Quantity (EOQ). The mathematical equation for calculating EOQ is

where D = Annual Demand Rate for the part (or) product. S = Setup cost (or) Ordering cost per order H = Annual Holding cost for the part to be carried in inventory. MRP can be calculated by breaking the master production schedule into dependent de minds using bill of materials. These demands Bill of materials is the details giving the components, sub assemblies and final issue (end products). The following figure shows the structure of BOM. Are consolidated to get gross requirements. Then, inventory on hand is deducted from the gross requirements. Also, purchased (or) manufacturing orders outstanding will be deducted and some safety stock will be added to arrive the net Requirements. -

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It shows that the components C5 and C6 make the sub assembly S3. The componentsC7 & C8 make the sub assembly S4. The sub assemblies S2, S3 and S4 are used to make final assembly end product P1. Purchasing: Only some components will be manufactured while others are purchased by using ‘make or buy’ deci some components will be purchased. Also, the raw materials for component to be manufactured will be purthased by the purchasing department. Materials will be ordered and received according to the time table given by MRP. Machine loading and scheduling Allocating the jobs to the individual machine is known as machine loading. Assigning starting dates and due dates for the parts to be manufactured in the factory is known as scheduling. According to Master production schedule, the scheduling is performed. Since the number of individual parts and orders to be scheduled are more; scheduling become complex. Each part has to go through different operation routes. Each part has to be processed through so many machines. When the number of machines is limited and the machines have different features and capacities, loading the machines will be difficult job. Each machine will have a so many job in queue waiting to be processed. With a use of computers, the machine loading and scheduling will become easy. We will study further about the machine loading and scheduling in shop floor control systems. Dispatching: Dispatching is the function concerned with issuing the individual orders to the machine operators. During this dispatching, the order tickets, route sheets, part drawings and job instructions will be given to the operator. The person doing the dispatching function is called dispatcher and some shops it is performed by the shop foreman. Expediting: Follow up is the very important functions for any production systems. Even though we have best plan, techniques and schedules, some things may go wrong. It should be expedited whether the actual progress of tell order is normal when compared with the production schedule. The person expediting these deviations is known as expeditor. The expeditor takes corrective action when any order falls behind the schedule. For this, Expeditor rearranges the sequence of order on a particular machine, requests the foreman to change the setup of a particular machine, and sends a particular part from one department to another to keep productions going without struck up. The machine breakdowns, non-availability of proper tool, delay in processing of parts in the previous departments, workers’ illness and so many reasons for things go wrong during production. So expedite job is very much important to make the corrective actions. Quality Control: The quality control is very much necessary to assure the quality of the product and its components to meet the standards specified by the designer. At each and every level in the manufacturing cycle, the product quality should be maintained. The materials and parts supplied by the vendors should be inspected to assure the quality. The parts manufactured inside the factory should be inspected at every stage of processing. Final quality inspection of final product should be performed to test its overall functional quality and appearance. Shipping and Inventory Control: -The finished product will either be shipped to the customers die (or) stocked as inventory. When it’s stocked in inventory, the financial investment should be carefully dealt with. Apart from the final products, the raw materials, purchased components and work-in-progress within the factory should be stocked in inventory. Too much inventory will make more expense and too little inventory will make possible stock outs. Hence, inventory control is linked with products sales and production department. The purpose of inventory control is to make sure that enough products of each type are available to the customers whenever they demand. Engineering and Manufacturing database: The computerized Engineering and manufacturing database have all the information needed to manufacture the components and assemble the products. The bill of materials, assembly list, part drawings, process route sheets, and other useful data are stored in this database. When there is a hanger in route sheet (or) in design (or) in drawing, it can be updated in the database. The design engineering and process planning provide information as input data to the engineering and 6 databases.

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Capacity Planning: MRP is used to plan the materials and components, while the capacity planning is used to determine the man power and machine capacities required to meet the production schedule. Capacity planning, often alters the master production schedule so that the master production schedule will not exceed plant capacity. So the master production schedule and plant capacity are adjusted each other to obtain balance. Plant Capacity means the maximum rate of output that the plant can produce under a given pirating conditions. The given cooperating conditions may be number of shifts (one shift, vow shifts. (or) three shifts per day), Number of working days per week, overtime, and n plaything levels. Normally, the capacity of the plant is measured as the output units of the plant. For karalla, in cement factory.. number of cement bags produced per day, in oil refinery - number of reels of oil per day, in steel mill - ton of steel produced per day, can be taken as the plant capacity. n job shop productions, labor hours (or) utilized machine hours can be taken. as a measure of ant capacity. Hence capacity planning is used to determine the man power (lab our) and (plant) machine paucity required to meet the master production schedule as well as long-term future production cads of the firm. The master production schedule is converted into material requirement planning MRR en the master production schedule and MRP are compared with plant capacity and liable man power. If they are not compatible with capacity, the adjustments are de either in the master production schedule (or)’in the plant capacity and man power sources. Capacity planning is done for short term and long term periods. The following factors are considered for short term planning. 1. Number of shifts per day: It can be increased (or) decreased according to the master production schedule. 2. Number of working days/week: 3. Lab our Overtime: If more work is to be completed, more overtime work will be done by the available workers. 4. Employment levels: Number of workers can be increased (or) decreased as per the production schedule. But often, we should not change the number of workers. We should give work to the workers continuously. 5. Inventory stock piling: When we give work continuously to the workers without lay off, there will be necessity stock the products in the inventory. To maintain steady employment all over the year including temporary slack periods, the inventory is necessary. 6. Subcontracting: Some jobs can be given to some other shops during busy periods to satisfy the customer’s need. The following decisions are taken during long term planning. 1. New plant construction 2. Buying more machines (or) more automatic machines (or) new types of machines to produce new product designs. 3. Increasing the plant capacity by purchasing running plants from other companies. 4. If the future need of the product is less, then close the existing plant (or) reduce the plant capacity. Cost plain and Control: Cost planning: Cost planning is used to determine the expected cost of manufacturing. For this, the standard cost is arrived. The standard cost is the sum of lab our cost, material cost and overhead cost. The following factors are considered to arrive the standard cost. 1. The bill of materials By using this, one can know the components and materials used in the product 2. Process route Sheets: It gives the various processes performed on the product and the production operations used for each component in the product. 3. Standard time; It gives the time required to complete each operation given in the route sheets. 4. Labour wages and machine rate: By this, one can translate the standard time into rate in terms of Rupees for each operation.

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The quotations for raw materials will give the price data and information about material cost from various suppliers, 6. Overhead cost: Overhead costs are arrived by accounting section. Cost Control: The cost control is used to find the actual cost of manufacturing and control the difference between the actual costs expected costs. There will be always differences between the standard costs calculated in cost planning and the actual costs occurred during productions. There are so many reasons for these differences. The raw material costs may be increased above the quotation rates the machine breakdown may occur, different lot sizes of :raw materials (or) purchased components may increase the production costs, the actual process may deviate from the original route sheets which all make the difference between the actual cost and standard cost calculated. The cost control function is to minimize these differences and preparing the report defining the actual costs and the deviation from the standard costs. The cost control collects the data on material costs from the purchasing department. It collects the data on Labour costs from the shop floor control system. Normally the overhead costs are excluded from consideration because they are no the actual expense of the product but they represent general story and corporate expenses. 5. Quotation for raw materials: Master production schedule: The master production schedule gives the details of what are the products to be produced, how many of each product should be produced and when the products should be delivered. The general format for the master production schedule is given here. Master production schedule. (Sample Month 8. Year 2003

It states that 50 units of Product 1, should be delivered on date 2/8/03. 100 units of Product I on 8/8/03 70 units of Product 3 on 9/8/03. Sources of Input data to MRP: 1. The master production schedule 3. Bill of materials 2. Requirements of service parts 4. Inventory record. The following figure shows the structure of MRP system.

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MRP output Reports: For planning and management of plant operations, the following MRP output reports are prepared. Primary output Reports: I. order release notice which will place the order according to MRP system 2. Reports showing planned orders to release in future periods. 3. Rescheduling notices, showing the changes in due dates for open orders. 4. Cancellation order notice to cancel the open order due to the changes in the master production schedule. 5. Inventory status report, Secondary output Reports: These reports include: I. Performance reports of various types, indicating costs, item image, difference between actual and planned lead times and other performance measures. 2. Exception reports which show the deviations from actual schedule and overdue orders. 3. Forecasting of inventory level for future periods. Benefits of MRP: The following are the benefits of MRP. 1. Customer service is improved. 2: Increased sales. 3. Reduction in sales price. 4. I in inventory level: Well designed, well managed MRP system influence the raw materials, purchased components and work in procuress inventories. 5. Reduction in set up costs. - 6. Reduction in Changeover costs. 7. Better machine utilization 8. Increased productivity Shop Floor Control System: The production planning and control systems are concerned with planning and controlling of manufacturing operations. The planning function deals with the development of the master - production schedule, capacity planning and MRP. The controlling function deals with the ‘Shop Floor Control (SFC)’. Functions of Shop Floor Control (SFC): 1. Periority control and assignment of shop orders. (Scheduling) 2. Maintain information on work-in-progress for MRP (Dispatching) 3. Monitor shop order status information (Follow up (or) Expediting) 4. Reporting production output data for capacity control purposes. Priority control and assignment of shop orders: (Scheduling) Priority control is concerned with time-phased planning of materials, work-in-progress and assembly of final product. The priorities for the jobs in the factory (shop) are determined on daily basis or weekly basis. This is known as scheduling. When the delivery date for one batch of product is moved forward because of high demand, the priorities should be given to that product. Priority control is concerned with in creasing priority for the fast moving jobs (high demand job) and decreasing the priority for the low -demand jobs. The priorities for the jobs in the shop are determined as a weekly (or) daily basis. Once these priorities are established, the work will be assigned to the machines in the factory. Maintain information on work-in-progress: (Dispatching) Shop Floor Control is also called as a method of controlling the work-in-progress in the factory. The main function of the SFC is to manage the parts and assemblies that are processed in the shop at that time. Information about the quantities and completion dates at various stages in the production sequence are compared with the MRP system had corrective actions are taken by adjusting the priority plan.. . Monitor shop order status: (Follow up (Or) Expediting) a It monitors work order status Ana provides information about the status of the orders in the shop. S1 order status information is checked by naked eyes (or) by using technology based methods. One of the important reporting documents in SFC is the Work Order Status Report. This action is known as Follow up (or) Expediting. This report will be updated every week depending on the nature of the product and the processes in the shop. This report is displayed in the computer monitor (or) print out may be taken. This report indicates the orders that were behind the schedule and significant incidents like machine breakdowns .The accuracy and

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latest version of work order status report are depending on the correctness and timeliness of the basic data collected in the shop. Production output data for Capacity Control: There is a difference between capacity planning and capacity control. The capacity planning is used to plan and determine the labour and equipment resources which are required to meet the master production schedule. On the other hand, the capacity control is a controlling function used to make adjustments in labour and equipment image to meet the production schedule to make these adjustments effectively, the capacity control function should have current (up to date) information as production rates and order status in the shop. Phases of SFC The following are the important phases of SFC 1. Order release 2. Order scheduling 3 Order progress. The computerized Shop floor system is shown in the following fig.

Order release: It is an official document for a job to he processed through the shop. This order release consists of: 1. Route sheet - used to list the sequence of operations and tools required. 2. Material requisitions - used to get necessary raw materials and components for assemblies from stock. 3. Job cards - used to report the labour for each operation on the route sheet. 4. Move tickets - used to move the parts between the machines. 5. Part lists - used for assembling the final product. Order Scheduling: Order scheduling will assign the or lets to the various machines in the factory. The data of the order release and priority control are the input to this phase. Order scheduling accomplishes the dispatching function in production planning and control. In dispatching function, order scheduling phase reports the job that should be done at each machine and certain details about the routing of the part. Order scheduling performs the machine loading and job sequencing functions, Allocating the orders to the machine is termed as machine loading. Determining the priority in which Jobs should be processed is termed as job sequencing.

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The jobs are assigned to machines. Since the total number of jobs exceeds the number of machines, each machine will have job in queue waiting to be processed. 20 jobs may be waiting for a particular machine. In what sequence, will the 20 jobs be processed? Answer to this question is job sequencing. For answering this question, priorities are established among the jobs in queue. The priorities are fixed according to some priority reifies. Highest priority should be given to the jobs with ‘earliest due date’ and” shortest processing time’. Priority can be given as a ‘first come - first serve’ basis. Higher priority should be given to the job having ‘least slack time’, Where slack time = (Remaining time until due date) - (Remaining process time)

Order progress: The order progress is used to accomplish the remaining functions of SFC: 1. To provide data for work in-progress status report. 2. Shop order status 3. Capacity control. Order progress collects data from shop floor and generates reports to assist production management. When an operator completes a particular process mentioned in the route sheet, it should be indicated in the order progress phase along with the data like piece count, scrap page, operator identification number, operation number, machine name (or) number and time of completion. The order progress phase maintains a transaction file reported as each of the uncompleted jobs. This file is known as open order file. The open order file contains the latest status of each job in the shop. The following are type of reports generated from open order file. 1. Work order status report: It shows the current stage of each job through the shop. 2. Progress report: It shows the pr of each job through the shop. 3. Exception report: It is designed to pinpoint the deviations from the production schedule, overdue jobs and other exception information. The above reports prepared daily to have better control over the jobs in the fr%hop floor. Gantt chart: Gantt Charts are used for scheduling. In this Chart, time is placed in the abscissa (x axis) and the machines are placed on the ordinate (y axis).

Each job is processed through some machines (or) all the machines according to the process route. Aparticular job is processed through particular machine at particular time. The duration for each process can be noted from time axis.

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The process route for each job is calculated by placing the job in the Gantt chart, until the Chart is full. When ‘scheduling the different jobs’ are completed at first attempt, there will be gap between each jobs. This indicates that particular machine is not utilized during that gap (particular time). - For example, machine 2 is idle up to first 8 minutes. The gaps can be filled up by rearranging the jobs. This procedure is continued until the gap between the various jobs is minimized to achieve acceptable result. Factory Data Collection System: • Factory data collections system (FDC) is used to collect data for monitoring order progress in SEC. It is a computerized system consists of paper documents, computers and automated devices in which the data collected are sent to the order progress phase to analyze and generate work order status rep and exception reports. The following are important data collected by the FDC system. I. Number of products (piece counts) completed at a certain machine 2. Number of parts scrapped (or) Number of parts to be reworked Centralized shop terminal: To reduce the clerical work into bare minimum, a computer (centralized terminal) is introduced in the shop floor. The centralized terminal is located near the centre of the plant. Each employee has to enter the order data. A computer operator is engaged to enter the data. No. of products produced, operation number, machine breakdown, product quality data etc are entered in the centralized computer for each job. Individual work centre terminal: For the convenience of the operator, individual terminal is located in the immediate work area attached to machine (or) processing equipment. This system is more useful because the operator can report status on the order more frequently. The order progress phase can be programmed to interpret the input data properly entered at each work centre. All individual work centre terminals are connected by LAN and hence the status report can be made more recent, current and updated. Voice data Input: This is a new sophisticated technology used to recognize the speech automatically and the speech will be converted into useful data. It is the easiest and quickest method of input. Most of these systems are speaker dependent ie the speech recognition processor recognizes only the unique voice patterns of the individuals who use the system. A reasonable vocabulary is needed to enter data. The employee is required to tell the information to the computer system (or) the computer operator. Then the data will be entered into the computer system. Just In Time Approach (JET approach) Just is a philosophy of manufacture based on planned elimination of all wastes and continuous improvement of productivity. JIT approach involves a continuous commitment to the pursuit of excellence in the manufacturing system design and operation. JJT seeks to manufacture 100% good products. SIT seeks to produce only required items, at the required time and in the required quantities. let seeks to achieve the following goals. 1. Zero defects 2. Zero setup time 3. Zero inventories 4. Zero handling 5. Zero breakdowns 6. Zero lead time 7. Lot size of one Zero defects: There is a belief that a certain level of defective product is unavoidable. But this is joint radiated by J approach which aims to eliminate, once and for all, the causes of defects and Isakson achieve excellence at all stages of the manufacturing process. It tells ‘Do it once and do it right. No excuse.’ Zero inventories: Normally, inventories are needed when there is uncertainty in supply of raw materials by the outside suppliers. Outside suppliers are not reliable and most of the time, they do not sup in time. Hence, a buffer stock is necessary, so inventory is essential to manage the uncertain availability of raw materials and unexpected customer order.

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But as per SIT approach, inventory is an evidence of poor design, poor coordination and operation of the production system. In JIT, there is no inventory, since there is no uncertain her and there are so many facilities to manage unexpected customer order. Zero Setup time and a lot size of one: If there is zero setup time, then there is no advantage in producing goods in batches. So a single product (a lot size of one) can be produced since setup time is zero. So the zero setup time and a lot size of one are interrelated. In inventory control, the EOQ (Economic Order Quantity) is as follows:

Where D = annual demand for the item. S = Setup cost or ordering cost per order H = annual holding cost of stock This EOQ is used to minimize the total cost of inventory by trading - off between the annual holding costs of stock and setup costs. Very large batches need high inventory cost (holding cost). Very small batches need lower inventory costs (holding costs) but involve a larger setup costs.. . . But, in JIT, the setup time is zero - So the setup cost is zero. So very smaller batch i.e.a lot size of one is economic in JIT approach. Zero lead time: Longer lead times make the manufacturing system to rely on forecasts and anticipation of customer orders. Short lead times make the manufacturing system to be adapted to shot-term fluctuations in market demand. Zero lead time approach makes the manufacturing system to operatic with greater flexibility than its competitors. Zero parts handling: . Manufacturing and assembling operations include a large -number of non-value adding activities as given below 1. Component feeding. 2. Component handling 3. Parts making 4. Part inspection And participation in achieving this high productivity. Kanban system is also used as an information system to monitor and control the production quantities at every stage of the manufacturing ing and assembly process Kanban system is normally used in a repetitive manufacturing environment. Emerging Chalienges in CAD/CAM The computer revolution continues at this moment in the industrial and the people in industries are only just really beginning to understand how to use the computers to the greatest effect. The opportunities of information technology come as a boon for the engineers. The current developments in CAD/CAM and in design and manufacture will become important in the future. The product data management allows all manner of product data to be collected and managed f globally distributed data bases. Also, the developments in product representation allow assembly and tolerance details. to be stored. The World-Wide-Web (www.com) makes enormous impact so that we can all share and use information. Product Data Management: In a modern manufacturing system, each department namely - production, design, manufacturing, purchasing, quality, market Management Information Systems (MIS) and accounting use data to describe the products from their point of view 1-lence, the product data management (PDM) is used to manage all of this product - related data for the various functions through the lifetime of the product. For example, an Engineering Change Order (ECO) in the design department will he requested by the Quality department which in turn, motivated by a complaint from a customer in the sales and marketing department. This ECO in turn makes changes in the manufacturing process parameters (or) changes in the purchasing department in terms of the specification of a buying component. The PDM system manages the product data to ensure that the implementation of ECO is performed correctly and that the data remains consist accurate and up to date using various computer applications.

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PDM is an extension of the Engineering Data Management Systems (EDMS) and is concerned with all of the documentation and data relating to a company’s products. PDM is a subject of Electronic Data Management (EDM). It is a general form for the computerized management of data of all sorts. PDM systems have two groups of functions. They are user Functions and Utility Functions. 1. Data vault (Information vault) and Document Management: The core of a PIJM system is the Data vault (or) information vault which is used for the storage and retrieval of the product defining information. This function manages the storage, security, access, version control etc of all product related data. This function of PDM system can integrate data from many different CAD/CAM systems and they can also reference data that is not computer based. 2. Process and Work Flow Management: This function manages and controls the business processes and workflows associated with definition, revision, sign-off and scheduling of documents. The process management captures the process sequences undertaken by different group within an organization and within its suppliers. It manages the flow of work between them. It uses this information to manage and direct the diverse tasks and the movement of data between the tasks. 3. Product Structure Management: This function provides complete bills of materials and bills of process data including planning bills. The configuration of a product describes the parts used in the product and their relationships to each other in the product structure. It controls the information relating to a product structure and its documentation over the life of the product from design to disposal. This function allows identification of different versions of a product and different issues of engineering data. It performs version control and variant control. The version control is used to control the variations in the product over time. For example, a manufacturer will replace an old model with a new version on the other hand, the variant control is used to control the variations of a product offered simultaneously. 4; Data search and Retrieval: This function provides software utilities to search for and retrieve data. It has search tools for locating and funding information based on values of attributes. This function facilitates to browse through graphical displays of the hierarchical structure of products such as the bill of material which shows the decomposition of the product into assemblies and subassemblies in the form of a hierarchical tree. The nodes of this tree are linked to the models describing individual parts (or) assemblies. 5. Project Management: PDM User functions include This function provides facilities for definition of breakdown, scheduling etc PDM Utility functions include 1. Data Communication and Notification: This functioning is used to handle all data communication between the various applications subsystems and also to external systems. 2. Data Transport: This function facilitates to move data between various applications, users and systems. Data transport and Data communication between them are used to cover Electronic mail (E-mail) and File transfer. 3. Data Translation: This function is used to allow translation of data among applications. For example, the CAD/CAM data may be translated into an appropriate format for a CNC programming application using IGES. 4. Image Services: This function is used to provide a viewing capability to review graphical images (CAD images) 5. System Administration: This function is used to system administration to set up and run the PDM system like access, control, authorizations, backups and archiving. The back up features are most important in situations that are most sensitive to system faults. By using this feature, the data may be copied in real time to alternative disk systems. Product Modelling: PDM (Product Data Management) integrates the geometric modeling, process planning and control, engineering analysis and, engineering data management systems. It allows the integration of diverse data structures into a common framework. The CAD/CAM is developed by the integration of the Ota generated

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throughout the product life cycle from specification to manufacture into a common logical framework. This is termed a product modelling. The product modelling has the aim of developing an integrated model to support all of the product life cycle aims. The following figure shows the range of product life cycle dealt with by a product modelling system.

The process of developing the product is important in product modelling. Product modelling should be able to support product development process changes throughout the product life, cycle and store all necessary information as product include data. The process information is used not only to guide the product development processes, but also to record the history of the product developer .it is used to provide a means to reconstruct the rationale behind design decisions. . Linear storage of the’ data model is necessary to allow manufacture. of spare parts and to allow data to be reused in the development of new products. In traditional CAD/CAM approach to product modelling, there are number of limitations as given below. 1. It is difficult to integrate specially manufacturing process with geometric models. 2. Various representations used in design, in process planning, in production control and so on have been developed independently and lack a common framework. 3. A number of life cycle issues are supported by little in the way of formal representations. To tackle above limitations, a formal approach to product modelling, based on the EXPRESS information modelling language, has been developed. And this is the basis of the STEP standard for the representation and exchange of product model data. Assembly and Tolerance Modelling: In EDMS and PDM systems, the bill of materials 4BOM) describes the main product structure. A ‘Bill of materials’ describes the product structure with a hierarchical assembly, subassembly and parts breakdown. The bill of materials describes the product with the parts which form the assembly. It also describes the spatial position of the parts to get final assembly. In tolerance modelling system, the structure of the ‘bill of materials’ combined with geometric models of the individual parts, we can explore the visual appearance and mass properties of assemblies and interference between parts.

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In assembly modelling system, we can explore how the assembly should be put together, how parts mate and move relative to other parts kinematically and dynamically. Also, we can explore how changes in one part affects others by using assembly modelling since it is I a richer model of relationships between the parts in the assembly. t The relationships in the assembly modelling system can be between subassemblies, parts (or) even elements of parts such as features (or) faces. The relationships describe ‘assembly constraints’ (also known as assembly relationships (or) mating relationships) between parts, mating relationships and other factors relating to how parts interact. It also describes the relationships between cylinders and holes sharing a common axis, degrees of fit (tight (or) loose), contacts (e.g. cylindrical face against a plane face), 1: mechanical transmission (e.g. mating gcars,.rack and pinion etc) and fastener constraints (e.g. screw threads) Relationships between elements are shown the following fig.

In this figure at subassembly level, one of the subassemblies (chassis) is fixed in world co and other subassemblies are located with respect to it. At part level, one of the parts is fixed (grounded) with respect to the subassembly coordinate system Also one feature face is fixed as base feature in each part. Tolerance modelling: Tolerances are used to specify the allowed variations in dimensions and position of parts. The tolerance modelling is closely related to the assembly modelling. The tolerances are functions of manufacturing process used to make the parts. There are two types of tolerances namely dimensional tolerances and geometric tolerances. The dimensional tolerances specify allowable deviations from an actual dimension. The geometric tolerances specify how far a part may deviate from a nominal position, attitude (or) shape. Geometric tolerances are widely used in industries, since they do not have ambiguities in their application and interpretation as in the case of dimensional tolerances. Tolerances are usually specified as Engineering Drawing using BSI,’ANSI (or) ISO standards.

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