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Effective scheduling with shop floor control

Effective scheduling with shop floor control

The shop floor control system is concerned with the release of production orders to the factory, monitoring and controlling the progress of the orders through the various work centers and gathering current information on the status of the orders. A typical shop floor control system consists of three phases namely, order release, order scheduling and order progress and the whole idea of shop floor control is aimed at a guarantee that the orders are processed and completed on time. A shop floor control system is an ideal tool of adjustments in a batch oriented plant for the management to undertake all the day today routine things that go wrong in production. These include absenteeism amongst workers, machine breakdowns, loss of raw materials, sudden change of orders, and many other unplanned complications. The management under above circumstances has to revise the priority of work orders on the information provided by the shop floor control system.

            The order release provides the documentation needed to process a production order through the factory which is sometimes called the shop packet consisting of the process plan, material requisitions, job cards, part lists and similar paperwork. In a modern factory, automated identification and data capture technologies are used to monitor the status of production order, thus rendering the above documents unwanted. The management can utilize a computer aided data collection system easily than in a manually recorded documentation. A lot of software to make this task handy has been made available in the open market, for example FACTIVITY.

            FACTIVITY is an advanced manufacturing execution system that performs data collection directly from the factory floor with touch screen computers located at the site of each critical process. This allows electronic access and retrieval of data and speedy adjustment of customer quantities, delivery dates and so on making the scheduling more effective.

            The order release module is driven by two inputs, firstly the authorization derived from the master schedule through material requirements planning and secondly the engineering and manufacturing data base which specifies the product structure and provides process planning information needed to prepare the various documents that accompany the order through the shop.

            Order scheduling module assigns the production orders to various work centers on the plant executing the dispatching function. A dispatch list indicates which production orders should be accomplished at various work centers. It also provides relative priorities of the different jobs, for example by showing deadlines of each job. This dispatch list guides their shop foreman in making work assignments and allocating resources to different jobs so that the master schedule can best be achieved.

            The order scheduling module is aimed to solve the problem of machine loading and job sequencing in production control. Machine loading is a question of queue or waiting time which refers to the loading of all machines in the plant. Since the total number of orders usually exceeds the number of work centers, each work center will experience a queue of orders waiting to be processed. The next question is what sequence should these jobs be processed. Job sequencing involves determining he sequence in which jobs will be processed through a given work center. At this point, priorities are established amongst the jobs in queue based on the following dispatching rules.

  1. first come first serve
  2. earliest due date
  3. shortest processing time
  4. least slack time
  5. lowest critical ratio

            The relative priorities of the different orders may again change over the time. Reasons behind the changes include lower or higher than expected demand for certain products, equipment breakdown that cause delays in production, cancellation of an order by a customer, defective raw materials that delay an order. The priority control mechanism reviews the relative priorities of the orders and adjusts the dispatch order accordingly.

            Order progress module in the shop floor system monitors the status of various orders in the plant. Its function is to provide information useful in managing the factory base don data collected form the factory.

            It is worthy to note some of the characteristics of a shop floor control system which helps effective scheduling in a firm of production. In essence, a shop floor control system has been defined in the APICS Dictionary as a system for utilizing data from the shop floor as well as data processing files to maintain and communicate status information on shop orders and work centers. The data exchanged in a shop floor control system is derived from the factory itself (first hand information). This meets a basic requirement of valid due dates on orders depending on proper priority as well as capacity panning. A very less room therefore exists for effective scheduling merely on the data imported outside. 

            The second point is the ability to establish and to reset the priority of work orders as and when necessary to do so. A shop floor control system makes the entire production process flexible in responding to sudden unexpected changes through ‘expansion’ or ‘contraction’ but still to get the work out on time. Through the use of dispatching rules, a job’s production lead time can be cut or increased as it goes through the shop. If a job is behind the schedule, its priority can be increased until it catches up the delay. Similarly a job can be slowed down if it is ahead of schedule. This is the function of the shop floor control system to provide information to management so that they can manage the production lead time dynamically. This in turn results effective scheduling.

            Waiting time is a phenomenon in any type of production or service prevailing in more or less magnitude. Waiting time is given a high consideration in any scheduling exercise. Bottleneck is a major cause of waiting time. Bottleneck is a work center that is fully utilized at a particular point of time while at least one job is waiting to be processed. The effect of bottleneck work centers on job scheduling substantially vary and a bottleneck work center will constrain the capacity of the entire shop. A shop floor control system enables identification of such bottlenecks in the production process so that the management can strategically work out a plan of action to schedule the bottleneck work center. This can include reduction of set up time, differing maintenance, adding other resources (more materials and labor) on a temporary basis.

A shop floor control system in addition to assigning priority provides actual output dates for capacity control purposes which help subsequent scheduling. It measures efficiency, utilization and productivity of manpower and machines which again again help effective scheduling for future jobs through various status and exception reports such as anticipate delay report, scrap report, rework report, performance summary, and shortage list and so on.

             

Reference

Chase, Richard B. (2001), Operations Management for Competitive Advantage (9th edition), Mc-Graw-Hill

Mikell P. Groover (2001), Automation, Production Systems, and Computer-Integrated Manufacturing, 2nd Edition, Prentice Hall

Schroeder, Roger G. (2004). Contemporary Concepts and Cases, Operations Management (2nd Edition). Mc-Graw-Hill

Kranjerski and Ritzman (1990), Operations Management, Strategy and Analysis, 2nd Edition

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