Ch 22 - Workcenter Scheduling Flashcards
What is a schedule?
Keep in mind that workflow equals cash flow, and scheduling lies at the heart of the process. A schedule is a timetable for performing activities, utilizing resources, or allocating facilities.
What is a manufacturing execution system?
Operations scheduling is at the heart of what is currently referred to as Manufacturing Execution Systems (MES). An MES is an information system that schedules, dispatches, tracks, monitors, and controls production on the factory floor. Such systems also provide real-time linkages to MRP systems, product and process planning, and systems that extend beyond the factory, including supply chain management, ERP, sales, and service management. A number of software specialty houses develop and implement MESs as part of a suite of software tools.
What is a workcenter?
Often referred to as a job shop, a process structure suited for low-volume production of a great variety of nonstandard products. Workcenters sometimes are referred to as departments and are focused on a particular type of operation.
The workcenter may be a single machine, a group of machines, or an area where a particular type of work is done. These workcenters can be organized according to function in a workcenter configuration or by-product in a flow, assembly line, or group technology cell (GT cell) configuration. Many firms have moved from workcenters to GTs.
When a job arrives at a workcenter—for example, the drilling department in a factory that makes custom-printed circuit boards—it enters a queue to wait for a drilling machine that can drill the required holes. Scheduling, in this case, involves determining the order for running the jobs, and also assigning a machine that can be used to make the holes.
A characteristic that distinguishes one scheduling system from another is how capacity is considered in determining the schedule.
Scheduling systems can use either infinite or finite loading. Infinite loading occurs when work is assigned to a workcenter simply based on what is needed over time. No consideration is given directly to whether there is sufficient capacity at the resources required to complete the work, nor is the actual sequence of the work as done by each resource in the workcenter considered. Often, a simple check is made of key resources to see if they are overloaded in an aggregate sense.
A finite loading approach actually schedules in detail each resource using the setup and run time required for each order. In essence, the system determines exactly what will be done by each resource at every moment during the working day. If an operation is delayed due to a part(s) shortage, the order will sit in the queue and wait until the part is available from a preceding operation.
Another characteristic that distinguishes scheduling systems is whether the schedule is generated forward or backward in time.
Forward scheduling refers to the situation in which the system takes an order and then schedules each operation that must be completed forward in time. A system that forward schedules can tell the earliest date that an order can be completed.
Conversely, backward schedulingstarts from some date in the future (possibly a due date) and schedules the required operations in reverse sequence. This tells the latest time when an order can be started so that it is completed by a specific date.
Example of MRP system:
A material requirements planning (MRP) system is an example of an infinite loading, backward scheduling system for materials. With simple MRP, each order has a due date sometime in the future. In this case, the system calculates parts needs by backward scheduling the time that the operations will be run to complete the orders. The time required to make each part (or batch of parts) is estimated based on historical data. The scheduling systems addressed in this chapter are intended for the processes required to actually make those parts and subassemblies.
Thus far, the term resources has been used in a generic sense. In practice, we need to decide what we are going to actually schedule. Commonly, processes are referred to as either machine limited or labor limited.
In a machine-limited process, equipment is the critical resource that is scheduled. Similarly, in a labor-limited process, people are the key resource that is scheduled. Most actual processes are either labor limited or machine limited but, luckily, not both.
The following functions must be performed in scheduling and controlling an operation:
- Allocating jobs, equipment, and personnel to workcenters or other specified locations. Essentially, this is short-run capacity planning.
- Determining the sequence of order performance (that is, establishing job priorities).
- Initiating performance of the scheduled work. This is commonly termed the dispatching of jobs.
- Shop-floor control (or production activity control) involving: a. Reviewing the status and controlling the progress of jobs as they are being worked on. b. Expediting late and critical jobs.
Example of workcenter scheduling process:
At the start of the day, the scheduler (in this case, a production control person assigned to this department) selects and sequences available jobs to be run at individual workstations. The scheduler’s decisions would be based on the operations and routing requirements of each job, the status of existing jobs at each workcenter, the queue of work before each workcenter, job priorities, material availability, anticipated job orders to be released later in the day, and workcenter resource capabilities (labor and/or machines).
What are the objectives of workcenter scheduling?
The objectives of workcenter scheduling are to
(1) meet due dates,
(2) minimize lead time,
(3) minimize setup time or cost,
(4) minimize work-in-process inventory, and
(5) maximize machine or labor utilization. It is unlikely, and often undesirable, to simultaneously satisfy all of these objectives.
For example, keeping all equipment and/or employees busy may result in having to keep too much inventory. Or, as another example, it is possible to meet 99 out of 100 of your due dates but still have a major schedule failure if the one due date that was missed was for a critical job or key customer.
The process of determining the job order on some machine or in some workcenter is known as sequencing or priority sequencing. Priority rules are the rules used in obtaining a job sequence.
Priority rules can be eg. first come first served, shortest operating time, earliest due date first, random order, last come, first served etc. All the rules can be calculated before deciding how to prioritze the orders and scheduling the sequence of work to be done.
The following standard measures of schedule performance are used to evaluate priority rules:
- Meeting due dates of customers or downstream operations.
- Minimizing the flow time (the time a job spends in the process).
- Minimizing work-in-process inventory.
- Minimizing the idle time of machines or workers.
Scheduling job priorities is just one aspect of shop-floor control (now often called production activity control). The APICS Dictionary defines a shop-floor control system 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 workcenters.
What are the major functions of shop floor control?
∙ Assigning the priority of each shop order.
∙ Maintaining work-in-process quantity information.
∙ Conveying shop-order status information to the office.
∙ Providing actual output data for capacity control purposes.
∙ Providing quantity by location by shop order for WIP inventory and accounting purposes.
∙ Measuring efficiency, utilization, and productivity of manpower and machines.
The basic tools of shop-floor control are:
- The daily dispatch list, which tells the supervisor which jobs are to be run, their priority, and how long each will take.
- Various status and exception reports, including
a. The anticipated delay report, made out by the shop planner once or twice a week and reviewed by the chief shop planner to see if there are any serious delays that could affect the master schedule.
b. Scrap reports.
c. Rework reports.
d. Performance summary reports giving the number and percentage of orders completed on schedule, the lateness of unfilled orders, the volume of output, and so on.
e. Shortage list. - An input/output control report, which is used by the supervisor to monitor the workload–capacity
What is the input/output control from a manufacturing planning and control system point of view?
Input/output (I/O) control is a major feature of a manufacturing planning and control system. Its major precept is that the planned work input to a workcenter should never exceed the planned work output. When the input exceeds the output, backlogs build up at the workcenter, which in turn increases the lead time estimates for jobs upstream.
The control process would entail finding the cause of upstream problems and adjusting capacity and inputs accordingly. The basic solution is simple: Either increase capacity at the bottleneck station or reduce the input to it.
