25-01-2016, 03:58 PM
1. Introduction
• The time comes when plans must be put into action. 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:
o To release work orders to the shop for manufacturing.
o To take control of work orders and make sure they are completed on time.
o To be responsible for the immediate detailed planning of the flow of orders through manufacturing, carrying out the plan, and controlling the work as it progresses to completion.
o To manage day-to-day activity and provide the necessary support.
a. Planning
o The flow of work through each of the work centers must be planned to meet delivery dates, which means production activity control must do the following:
Ensure that the required materials, tooling, personnel, and information are available to manufacture the components when needed.
Schedule start and completion dates for each shop order at each work center so the scheduled completion date of the order can be met. This will involve the planner in developing a load profile for the work centers.
b. Implementation
o Once the plans are made, production activity control must put them into action by advising the shop floor what must be done. Usually instructions are given by issuing a shop order. Production activity control will:
Gather the information needed by the shop floor to make the product.
Release orders to the shop floor as authorized by the material requirements plan. This is called dispatching.
c. Control
o Once plans are made and shop orders released, the process must be monitored to learn what is actually happening. The results are compared to the plan to decide whether corrective action is necessary. Production activity control will do the following:
Rank the shop orders in desired priority sequence by work center and establish a dispatch list based on this information.
Track the actual performance of work orders and compare it to planned schedules. Where necessary, PAC must take corrective action by replanning, rescheduling, or adjusting capacity to meet final delivery requirements.
Monitor and control work-in-process, lead times, and work center queues.
Report work center efficiency, operation times, order quantities, and scrap.
o The functions of planning, implementing, and controlling are shown schematically
d. Manufacturing Systems
o The particular type of production control system used varies from company to company, but all should perform the preceding functions. However, the relative importance of these functions will depend on the type of manufacturing process. Manufacturing processes can be conveniently broken down into three categories:
1. Flow manufacturing.
2. Intermittent manufacturing.
3. Project manufacturing.
Flow manufacturing.
• Flow manufacturing. Flow manufacturing is concerned with the production of high-volume standard products. If the units are discrete (e.g., cars and appliances), the process is usually called repetitive manufacturing, and if the goods are made in a continuous flow (e.g., gasoline), continuous manufacturing. There are four major characteristics to flow manufacturing:
1. Routings are fixed, and work centers are arranged according to the routing. The time taken to perform work at one work center is almost the same as at any other work center in the line.
2. Work centers are dedicated to producing a limited range of similar products. Machinery and tooling are especially designed to make the specific products.
3. Material flows from one workstation to another using some form of mechanical transfer. There is little buildup in work-in-process inventory, and throughput times are low.
4. Capacity is fixed by the line.
• Production activity control concentrates on planning the flow of work and making sure that the right material is fed to the line as stated in the planned schedule. Since work flows from one workstation to another automatically, implementation and control are relatively simple.
Intermittent manufacturing.
• Intermittent manufacturing is characterized by many variations in product design, process requirements, and order quantities. This kind of manufacturing is characterized by the following:
1. Flow of work through the shop is varied and depends on the design of a particular product. As orders are processed, they will take more time at one workstation than at another. Thus, the work flow is not balanced.
2. Machinery and workers must be flexible enough to do the variety of work. Machinery and work centers are usually grouped according to the function they perform (e.g., all lathes in one department).
3. Throughput times are generally long. Scheduling work to arrive just when needed is difficult, the time taken by an order at each work center varies, and work queues before work centers, causing long delays in processing. Work-in-process inventory is often large.
4. The capacity required depends on the particular mix of products being built and is difficult to predict.
• Production activity control in intermittent manufacturing is complex. Because of the number of products made, the variety of routings, and scheduling problems, PAC is a major activity in this type of manufacturing. Planning and control are typically exercised using shop orders for each batch being produced. Our discussion of PAC assumes this kind of environment.
Project manufacturing.
• Project manufacturing usually involves the creation of one or a small number of units. Large shipbuilding is an example. Because the design of a product is often carried out or modified as the project develops, there is close coordination between manufacturing, marketing, purchasing, and engineering.
2. Data Requirements
• To plan the processing of materials through manufacturing, PAC must have the following information:
o What and how much to produce.
o When parts are needed so the completion date can be met.
o What operations are required to make the product and how long the operations will take.
o What the available capacities of the various work centers are.
• Production activity control must have a data or information system from which to work. Usually the data needed to answer these questions are organized into databases. The files contained in the databases are of two types: planning and control.
a. Planning Files
o Four planning files are needed: item master file, product structure file, routing file, and work center master file.
Item master file.
• There is one record in the item master tile for each part number. The file contains, in one place, all of the pertinent data related to the part. For PAC, this includes the following:
o Part number, a unique number assigned to a component.
o Part description.
o Manufacturing lead time, the normal time needed to make this part.
o Quantity on hand.
o Quantity available.
o Allocated quantity, quantities assigned to specific work orders but not yet withdrawn from inventory.
o On-order quantities, the balance due on all outstanding orders.
o Lot-size quantity, the quantity normally ordered at one time.
Product structure file (bill of material file).
• The product structure tile (bill of material file) contains a list of the single-level components and quantities needed to assemble a parent. It forms a basis for a “pick list” to be used by storeroom personnel to collect the parts required to make the assembly.
Routing file.
• The routing file contains a record for each part manufactured. The routing consists of a series of operations required to make the item. For each product, this file contains a step-by-step set of instructions describing how the product is made. It gives details of the following:
o The operations required to make the product and the sequence in which those operations are performed.
A brief description of each operation.
o Equipment, tools, and accessories needed for each operation.
o Setup times, the standard time required for setting up the equipment for each operation.
o Run times, the standard time to process one unit through each operation.
o Lead times for each operation.
Work center master file.
• The work center master file collects all of the relevant data on a work center. For each work center, it gives details on the following:
o Work center number.
o Capacity.
o Number of shifts worked per week.
o Number of machine hours per shift.
o Number of labor hours per shift.
o Efficiency.
o Utilization.
o Queue time, the average time that a job waits at the work center before work is begun.
o Alternate work centers, work centers that may be used as alternatives.
b. Control Files
o Control in intermittent manufacturing is exercised through shop orders and control files that contain data on these orders. There are generally two kinds of files: the shop order master file and the shop order detail file.
Shop order master file
• Each active manufacturing order has a record in the shop order master file. The purpose is to provide summarized data on each shop order such as the following information:
o Shop order number, a unique number identifying the shop order.
o Order quantity.
o Quantity completed.
o Quantity scrapped
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o Quantity of material issued to the order.
o Due date, the date the order is expected to be finished.
o Priority, a value used to rank the order in relation to others.
o Balance due, the quantity not yet completed.
o Cost information.
Shop order detail file.
• Each shop order has a detail file that contains a record for each operation needed to make the item. Each record contains the following information:
o Operation number.
o Setup hours, planned and actual.
o Run hours, planned and actual.
o Quantity reported complete at that operation.
o Quantity reported scrapped at that operation.
o Due date or lead time remaining.
3. Order Preparation
• Once authorization to process an order has been received, production activity control is responsible for planning and preparing its release to the shop floor. The order should be reviewed to be sure that the necessary tooling, material, and capacity are available. If they are not, the order cannot be completed and should not be released.
• Tooling is not generally considered in the material requirements planning (MRP) program, so at this stage, material availability must be checked. If MRP software is used, it will have checked the availability of material and allocated it to a shop order so no further checking is necessary. If MRP software is not used, production activity control must manually check material availability.
• If a capacity requirements planning system has been used, necessary capacity should be available. However, at this stage, there may be some differences between planned capacity and what is actually available. When capacity requirements planning is not used, it is necessary to determine if capacity is available.
• 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 and loading are discussed in the next two sections.
4. Scheduling
o 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 (MLT) at each work center involved.
a. Manufacturing Lead Time
o Manufacturing lead time is the time normally required to produce an item in a typical lot quantity. Typically, MLT consists of five elements:
1. Queue time, amount of time the job is waiting at a work center before operation begins.
2. Setup time, time required to prepare the work center for operation.
3. Run time, time needed to run the order through the operation.
4. Wait time, amount of time the job is at the work center before being moved to the next work center.
5. Move time, transit time between work centers.
o The total manufacturing lead time will be the sum of order preparation and release plus the MLTs for each operation. Figure 6.3 shows the elements making up manufacturing lead time. Setup time and run time are straightforward, and determining them is the responsibility of the industrial engineering department. Queue, wait, and move times are under the control of manufacturing and PAC.\
o The largest of the five elements is queue time. Typically, in an intermittent manufacturing operation, it accounts for 85%—95% of the total lead time. Production activity control is responsible for managing the queue by regulating the flow of work into and out of work centers. If the number of orders waiting to be worked on (load) is reduced, so is the queue time, the lead time, and work-in-process. Increasing capacity also reduces queue. Production activity control must manage both the input of orders to the production process and the available capacity to control queue and work-in-process.
o A term that is closely related to manufacturing lead time is cycle time. The ninth edition of the APICS Dictionary defines cycle time as the length of time from when material enters a production facility until it exits. A synonym is throughput time.