تويت
Just-in-time techniques, developed by the Japanese as a procedure to control in-process inventories and lead to continual improvement, requires that parts for production be produced or delivered as they are needed. Typically, materials are delivered to the floor when they are expected to be needed according to some planned schedule. Often the difference in time between when parts are thought to be needed and when they are needed is substantial and inventory costs and throughput times can soar.
Manufacturing resource planning (MRP II) procedures recommend that materials be released to the factory floor at that time that the production control system indicates the shop should be ready to receive them.
Production control includes the scheduling of production; the dispatching of materials, tools, and supplies at the required time so that the predicted schedules can be realized; the follow-up of production orders to be sure that proposed schedules are realized; the maintenance of an adequate inventory to meet production requirements at optimum cost; and the maintenance of cost and manufacturing records to establish controls, estimating, and equipment replacement. Consideration must be given to the requirements of the customer, the available capacity, the nature of the work that precedes the production to be scheduled, and the nature of the work that succeeds the current work being scheduled. Centered in the production control effort should be an ongoing analysis to continually focus on any bottlenecks within the plant, since it is here where increases in throughput time take place. Scheduling may be accomplished with various degrees of refinement. In low-production plants where the total number of hours required per unit of production is large, scheduling may adequately be done by departmental loading; e.g., if a department has a total of 10 direct-labor employees, it has 400 available work hours per week. Every new job is scheduled by departments giving consideration to the average number of available hours within the department. A refinement of this method is to schedule groups of facilities or sections, e.g., to schedule the milling machine section as a group. In high-production plants, detailed facility scheduling frequently is necessary in order to ensure optimum results from all facilities. Thus, with an 8-hour shift, each facility is recorded as having 8 available hours, and work is scheduled to each piece of equipment indicating the time that it should arrive at the work station and the time that the work should be completed. Scheduling is frequently done on control boards utilizing commercially available devices, such as Productrol, Sched-U-Graph, and Visitrol. These, in effect, are mechanized versions of Gantt charts, where schedules are represented by paper strips cut to lengths equivalent to standard times. The strips are placed in the appropriate horizontal position adjacent to the particular order being worked; delays are conspicuously marked by red signals at the delay point. Manual posting to a ledger maintains projected schedules and cumulative loads. The digital computer is successfully used as a scheduling facility. An adaptation of the Gantt chart, PERT (Program Evaluation and Review Techniques), has considerable application to project-oriented scheduling (as opposed to repetitive-type applications). This prognostic management planning and control method graphically portrays the optimum way to attain some predetermined objective, usually in terms of time. The critical path (CPM 5 Critical Path Method) consists of that sequence of events in which delay in the start or completion of any event in the sequence will cause a delay in the project completion. In using PERT for scheduling, three time estimates are used for each activity, based upon the following questions: (1) What is the earliest time (optimistic) in which you can expect to complete this activity if everything works out ideally? (2) Under average conditions, what would be the most likely time duration for this activity? (3) What is the longest possible time (pessimistic) required to complete this activity if almost everything goes wrong? With these estimates, a probability distribution of the time required to perform the activity can be made (Fig. 17.1.2). The activity is started, and depending on how successfully events take place, the finish will occur somewhere between a and b (most likely close to m). The distribution closely approximates that of the beta distribution and is used as the typical model in PERT. The weighted linear approximation for the expected mean time, using probability theory, is given by With the development of the project plan and the calculation of activity times (time for all jobs between successive nodes in the network, such as the time for ‘‘design of rocket ignition system’’), a chain of activities through the project plan can be established which has identical early and late event times; i.e., the completion time of each activity comprising this chain cannot be delayed without delaying project completion. These are the critical events.
Events are represented by nodes and are positions in time representing the start and/or completion of a particular activity. A number is assigned to each event for reference purposes.
Each operation is referred to as an activity and is shown as an arc on the diagram. Each arc, or activity, has attached to it a number representing the number of weeks required to complete the activity. Dummy activities, shown as a dotted line, utilize no time or cost and are used to maintain the correct sequence of activities.
The time to complete the entire project would correspond to the longest path from the initial node to the final node. In Fig. 17.1.3 the time to complete the project would be the longest path from node 1 to node 12. This longest path is termed the critical path since it establishes the minimum project time. There is at least one such chain through any given project. There can, of course, be more than one chain reflecting the minimum time. This is the concept behind the meaning of critical paths. The critical path method (CPM) as compared to PERT utilizes estimated times rather than the calculation of ‘‘most likely’’ times as previously referred to. Under CPM the analyst frequently will provide two timecost estimates. One estimate would be for normal operation and the other could be for emergency operation. These two time estimates would reflect the impact of cost on quick-delivery techniques, i.e., the shorter the time the higher the cost, the longer the time the smaller the cost.
It should be evident that those activities that do not lie on the critical
path have a certain flexibility. This amount of time flexibility or freedom is referred to as float. The amount of float is computed by subtracting the normal time from the time available. Thus the float is the amount of time that a noncritical activity can be lengthened without increasing the project’s completion date.
Figure 17.1.3 illustrates an elementary network portraying the critical path. This path is identified by a heavy line and would include 27 weeks. There are several methods to shorten the project’s duration. The cost of various time alternatives can be readily computed. For example, if the following cost table were developed, and assuming that a linear relation between the time and cost per week exists, the cost per week to improve delivery is shown
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