1:19 AM
Chapter 6
Production Activity Control
Introduction
· Production activity control (PAC) is responsible for executing the master production schedule and the material requirements plan. At the same time, it must make good use of labor and machines, minimize work-in-process inventory, and maintain customer service.
· The material requirements plan authorizes PAC: to release work orders to the shop for manufacturing, to take control of work orders and make sure they are completed on time, to be responsible for the immediate detailed planning of the flow of orders through manufacturing, and to manage day-to-day activity and provide the necessary support. The activities of the PAC system can be classified into planning, implementation, and control functions.
· The flow of work through each work center must be planned. PAC must ensure that the required resources are available to manufacture the components as needed and develop a load profile for each work center to ensure the timely completion of orders by the scheduled date.
· Next we implement the plan. PAC will gather the information needed by the shop floor to make the product and release orders to the shop floor as authorized by the material requirements plan (dispatching).
· Monitor the process and determine the necessary corrective action. PAC will rank the shop orders in desired priority sequence by work center and establish a dispatch list, track actual performance to plan and take corrective action by replanning, rescheduling, or adjusting capacity to meet delivery.
· Understand the characteristics and differences between flow, intermittent and project manufacturing .
Data Requirements
· To plan the processing of materials through manufacturing, PAC must have the following information: What and how much to produce. When parts are needed so the completion date can be met. What operations are required to make the product and how long the operations will take. What the available capacities of the various work centers are.
· PAC must have data, usually stored in databases, to drive the information systems. These database files are of two types, planning and control.
· The four planning files needed are the item master file, product structure file (bill of material file), routing file and work center file. The item master file contains all of the pertinent data related to each part number. The product structure or BOM file contains single-level BOM’s listing components and quantities needed to assemble a parent. It forms a basis for a “pick list”. A routing exists for each part number and consists of a series of operations and instructions required to make the item. The work center master file collects relevant data on a work center.
· The two control files are the shop order master file and the shop order detail file. Each active manufacturing order has a record in the shop order master file to monitor production performance for each shop order. The shop order detail file contains the performance record for each operation.
Order Preparation
· Once authorization to process an order has been received, PAC is responsible for planning and preparing its release to the shop floor. The order should be reviewed to be sure that the necessary resources are available. Material and capacity availability must be checked. Checking capacity availability is a two-step process. First, the order must be scheduled to see when the capacity is needed, and second, the load on work centers must be checked in that period.
Scheduling
· The objective of scheduling is to meet delivery dates and to make the best use of manufacturing resources. It involves establishing start and finish dates for each operation required to complete an item. To develop a reliable schedule, the planner must have information on routing, required and available capacity, competing jobs, and manufacturing lead times at each work center involved.
· Manufacturing lead-time is the time normally required to produce an item in a typical lot quantity and consists of five elements. The largest of the five elements is queue time. PAC is responsible for managing the queue by regulating the flow of work into and out of work centers. PAC must manage both the input of orders to the production process and the available capacity to control queue and work-in-process.
· Cycle time (throughput time) is the length of time from when material enters a production facility or operation until it exits.
· Forward scheduling assumes that material procurement and operation scheduling for a component start when the order is received, whatever the due date, and that operations are scheduled forward from this date. The result is completion before the due date, which usually results in a buildup of inventory (see figures 6.4 & 6.6).
· Backward scheduling (figures 6.4 & 6.6) schedules the last operation on the routing first and is scheduled for completion at the due date. Previous operations are scheduled back from the last operation.
· Infinite loading assumes infinite capacity will be available (figures 6.4 & 6.5).
· Finite loading assumes there is a defined limit to available capacity at any workstation (figures 6.6 & 6.7).
· In operation overlapping, the next operation is allowed to begin before the entire lot is completed on the previous operation. This reduces the total manufacturing lead times because the second operation starts before the first operation finishes all the parts in the order. Increased costs are possible from move costs and the impact of queue and lead-time for other orders.
· Operation splitting is the process of splitting orders into two or more lots and run simultaneously on two or more machines.
Load Leveling
· The load profile for a work center is constructed by calculating the standard hours of operation for each order in each time period and adding them together by time period (figure 6.10).
Scheduling Bottlenecks
· Bottlenecks are overloaded workstations where the required capacity is greater than the available capacity. It is a facility, function, department, or resource whose capacity is equal to or less than the demand placed upon it.
· Throughput is the total volume of production passing through a facility. Bottlenecks control the throughput of all products.
· Since bottlenecks control throughput.
Theory of Constraints and Drum-Buffer-Rope
· The fundamental concept behind Theory of Constraints, developed by Eliyahu M. Goldratt, is that every operation producing a product or service is a series of linked processes. Each process has a specific capacity to produce the given defined output for the operation, and that in virtually every case, there is one process that limits or constrains (bottleneck) the throughput from the entire operation. Focus on balancing the flow through the shop. The time lost at a nonconstraint is a mirage, and transfer batches do not have to be the same size as process batches.
· Once constraint has been identified, there is a five-step process that is recommended to help improve the performance of the operation. The five steps are: (1) identify the constraint, (2) exploit the constraint, (3) subordinate everything to the constraint, (4) elevate the constraint, (5) once the constraint is a constraint no longer, find the new one and repeat the steps.
· The scheduling system for Theory of Constraints is described as Drum-Buffer-Rope. The drum of the system refers to the “drumbeat” or pace of production. It represents the master schedule for the operation, which is focused around the pace of throughput as defined by the constraint. Since it is so important that the constraint never be “starved” for needed inventory, a “time” buffer is often established in front of the constraint. It is called a time buffer because it represents the amount of time that the inventory in the buffer protects the constraint from disruption. The analogy is that the rope “pulls” production to the constraint for necessary processing. The primary focus of the scheduling system is on effective management of the organization’s constraint to throughput and sales.
· Orders that do not have all of the necessary resources, tooling, material, and capacity, should not be released because they only cause excess work-in-process inventory and may interrupt work on orders that can be completed (see figure 6.11).
· A shop packet accompanies a shop order release to manufacturing. This packet may include the shop order, engineering drawings, bills of material, routing sheets, materials issue tickets or pick list, tool requisitions, job tickets for each operation to be performed, and move tickets that authorize movement of work between operations.
· Once work orders have been issued to manufacturing, their progress has to be controlled. To control progress, performance has to be measured and compared to what is planned. If what is actually happening (what is measured) varies significantly from what was planned, either the plans have to be changed or corrective action must be taken to bring performance back to plan.
· PAC must balance the flow of work to and from different work centers. This is to ensure queue, work-in-process, and lead times are controlled. The input/output control system is designed to balance the input rate in hours with the output rate. The input rate is controlled by the release of orders to the shop floor. The output rate is controlled by increasing or decreasing the capacity of a work center (see figure 6.12).
· To control input and output, a plan must be devised along with a method for comparing what actually occurs against what was planned. This information is shown on an input/output report (see figure 6.13). Cumulative variance is the difference between the total planned for a given period and the actual total for that period (Cumulative variance = previous cumulative variance + actual – planned). Planned and actual inputs monitor the flow of work coming to the work center. Planned and actual outputs monitor the performance of the work center. Planned and actual backlogs monitor the queue and lead-time performance.
· Operation sequencing is a technique for short-term planning of actual jobs to be run in each work center based on capacity and priorities. Control of priorities is exercised through dispatching. Dispatching is the function of selecting and sequencing available jobs to be run at individual work centers. The dispatch list is the instrument of priority control. It’s a listing by operation of all the jobs available to be run at a work center with the job listed in priority sequence.
· The ranking of jobs for the dispatch list is created through the application of priority rules. Some commonly used rules are: (1) first come, first served (FCFS), (2) earliest job due date (EDD), (3) earliest operation due date (ODD), (4) shortest process time (SPT), and (5) critical ratio (CR = (due date – present date) / lead time remaining).