The ordering party sets the latest acceptable completion date and sometimes the earliest acceptable start date for a production order. The planner must establish start and/or due dates of the operations as well as the latest possible start date and the earliest possible completion date in advance, in order to obtain an initial estimate of feasibility and in preparation for work center loading and the setting of reservation dates for components.
For this, time management divides the lead time into meaningful time elements that can be measured or estimated relatively simply. Planners make use of the order of the operations (sequence or network of operations) of the product to be manufactured. Each operation has an operation time, and there are interoperation times before and after the operation. In addition, there are administrative times for each partial order and for the order in its entirety.
In job shop production, unproductive interoperation times make up the major proportion of total lead time. Simple models for estimating transportation times allow sufficiently precise estimates to be made without expending a lot of time and effort on data management. However, it is difficult to determine the adequate size of buffers or queues at the work centers. Statistical analysis of queues as the effect of random load fluctuations yields useful information with regard to reducing wait times: High loading as well as long or highly varied operation times lead to long wait times. This underlines the conflict between the entrepreneurial objectives of “low costs” and “short lead time” as set out in Section 1.3.1.
Scheduling management starts out from the dates set by the ordering party and calculates the other dates required for determining feasibility, loading capacity, and reserving components. The following list shows the scheduling techniques discussed in the chapter (for sequences as well as directed networks of operations), comparing data input with data output:
Forward scheduling:
- Input: earliest order start date, lead-time-stretching factor
- Output: earliest order completion date, earliest start and completion dates for each operation, earliest reservation date for each component
Backward scheduling:
- Input: latest order completion date, lead-time-stretching factor
- Output: latest order start date, latest start and completion date for each operation, latest reservation date for each component
Central point scheduling:
- Input: central point date, lead-time-stretching factor
- Output: latest order start date and earliest order completion date; latest start and completion date for each operation as well as latest reservation date for each component before the central point, earliest start and completion date for each operation as well as earliest reservation date for each component after the central point
Probable scheduling:
- Input: earliest start date and latest completion date for the order
- Output: lead-time-stretching factor, probable start and completion date for each operation, probable reservation date for each component.
Splitting and overlapping are techniques frequently used to reduce lead time. Their incorporation into the lead time formula, as well as the attempt to include other effects, reveals the limits to lead time estimation. Not all time elements can be estimated accurately, and only a modest degree of complexity can be expressed as a formula. Moreover, there are unforeseen factors that can always arise during actual production. On the other hand, planners must have a fair idea of cumulative lead time so that they can set it in relation to the customer tolerance time. With this, in the short term, the basic decision can be made to accept or decline an order. In the medium term, it allows planners to sketch out a possible load profile for the work centers along the time axis.
Course 13: Sections and their intended learning outcomes
Course 13 – Time Management and Scheduling
Intended learning outcomes: Present the elements of time management. Explain in detail knowledge on buffers and queues. Disclose scheduling of orders and scheduling algorithms. Describe splitting and overlapping.
13.1 Elements of Time Management
Intended learning outcomes: Describe the order of the operations of a production order, operation time and operation load, the elements of interoperation time, administrative time, and transportation time.
13.2 Logistic Buffers and Logistic Queues
Intended learning outcomes: Explain wait time, buffers, the funnel model, and queues as an effect of random load fluctuations. Present conclusions for job shop production. Produce an overview on logistic operating curves.
13.3 Scheduling of Orders and Scheduling Algorithms
Intended learning outcomes: Describe the manufacturing calendar and the calculation of the manufacturing lead time. Differentiate between Backward Scheduling and Forward Scheduling. Explain network planning, central point scheduling, the lead-time stretching factor, and probable scheduling. Present scheduling of process trains.
13.4 Order Splitting, Order Overlapping, and Extended Scheduling Algorithms
Intended learning outcomes: Explain order or lot splitting, and overlapping. Present an extended formula for manufacturing lead time and extended scheduling algorithms.
13.5 Summary
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13.6 Keywords
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13.7 Scenarios and Exercises
Intended learning outcomes: Assess queues as an effect of random load fluctuations. Calculate examples for network planning, backward scheduling, forward scheduling, the lead-time stretching factor, and probable scheduling.
13.8 References
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