Chapter 15

Finite Loading

Finite loading is an operational schedule with start and finish times for each activity. It does not allow you to load more work than can be done with the available capacity. The finite loading schedule shows how a company plans to use available capacity at each work center. In a manufacturing company, the schedule shows the jobs to be done at a particular work center if the work center uses a set number of production hours each day. For example, if the work center can build 50 wire assemblies per hour and the company needs 1000 wire assemblies, the job will take 20 hours of capacity at that work center. In a service organization, a doctor's office is a good example. To spend ten minutes with each patient, the doctor can have six patients scheduled per hour. Figure 15.4 is an example of finite loading. Note that no work center is assigned more work than it is able to handle. The disadvantage of finite loading is that it tends to break down over the long term: problems arise and the schedule slips, causing jobs to be rescheduled. Finite loading is why you may have to wait at the doctor's office.

Shop loading techniques include infinite and finite loading.

Infinite loading schedules include jobs without capacity constraints. Finite loading loads jobs up to a predetermined capacity level. Loading can be done using forward or backward scheduling. Forward scheduling starts the job as soon as possible, whereas backward scheduling works back from the due date.

Two kinds of work scheduling or work loading are infinite loading and finite loading

Infinite loading schedules work without regard to capacity limits. They let you know how much capacity you need to meet a schedule.

Monitoring Workflow

Input/output control is a capacity-control technique used to monitor workflow at individual work centers. Input/output control monitors the planned inputs and outputs at a work center against the actual inputs and outputs. Planned inputs are based on the operational schedule, whereas planned outputs use capacity-planning techniques. Actual input is compared with planned input to ensure that enough work enters the measured work center. A work center cannot process items that have not yet arrived. Actual output is used to identify possible problems in the work center, such as an equipment problem or unexpected absences.

Infinite Loading

Manufacturing companies can use infinite loading according to a proposed master production schedule (MPS). A service organization like a law firm can use infinite loading to identify the resources needed to complete the proposed case load. Infinite loading identifies uneven workloads and bottlenecks. Figure 15.3 is an example of infinite loading. We can see from the chart that the shop has enough capacity in periods 4 and 7 but not enough capacity in periods 5 and 8. In this way, we identify time periods when capacity is either poorly used or inadequate and change the schedule to level the resource requirements.

The procedure for using TOC consists of the following steps.

Step 1 Identify the System's Bottleneck(s). At Akito's Flowers, identify the floral arrangers as the bottleneck. Step 2 Exploit the Bottleneck(s). For the floral shop, take orders ahead of time to make sure there is always a buffer of orders for the arrangers to work on. This prevents idle time at the bottleneck resource Step 3 Subordinate All Other Decisions to Step 2.Schedule nonbottleneck resources to support the maximum use of the bottleneck. For Akito's Flowers, have the clerk transfer orders to the arrangers every ten minutes at the start of the day to make sure the bottleneck is fully used. You may have to arrive early or stay late to be sure the orders are processed and waiting for the arrangers to arrive first thing each day. Step 4 Elevate the Bottleneck(s).If after Steps 1 through 3 the bottleneck is still a constraint, then consider increasing the capacity of the bottleneck. At Akito's Flowers, add another floral arranger. Step 5 Do Not Let Inertia Set In.Although the floral arrangers may improve their throughput, check to see whether new constraints have developed. If so, work on increasing throughput.

THEORY OF CONSTRAINTS

The theory of constraints (TOC) is an extension of OPT. According to the TOC, a system's output is determined by three kinds of constraints: internal resource constraint, market constraint, and policy constraint. An internal resource constraint is the classic bottleneck discussed in the previous section. A market constraint results when market demand is less than production capacity. Since companies do not want excess inventory buildup, the market determines the rate of production. Policy constraint means that a specific policy dictates the rate of production (for example, a policy of no overtime).

When scheduling bottleneck systems, the basic principles of OPT apply.

The theory of constraints expands OPT into a managerial philosophy of continuous improvement.

Backward Scheduling

With backward scheduling, you begin scheduling the job's last activity so that the job is finished right on the due date. To do this, you start with the due date and work backward, calculating when to start the last activity, when to start the next-to-last activity, and so forth. Figure 15.5 gives an example of backward scheduling. Backward scheduling shows you how late the job can be started and still be finished on time. When you are using backward scheduling and forward scheduling together, a difference between the start time of the first activity indicates slack in the schedule. Slack means that you can start a job immediately but you do not have to do so. You can start it any time up to the start time in your backward schedule and still meet the due date.

Forward Scheduling

With forward scheduling, processing starts immediately when a job is received, regardless of its due date. Each job activity is scheduled for completion as soon as possible, which allows you to determine the job's earliest possible completion date. Figure 15.5 shows an example of forward scheduling. The job is due at the end of week 10, but it can be finished as early as the end of week 7. With forward scheduling, it is not unusual for jobs to be finished before their due date. The disadvantage to finishing a job early is that it causes an inventory buildup if items are not delivered before the due date.