Intended learning outcomes: Describe a sequence of operations. Differentiate between a directed and an undirected network of operations. Identify a synchronization point.
In materials management, lead time (see Section 1.1.6 and Section 1.2.3) is a basic attribute of both manufactured and purchased products. With this data, the start date of a production or procurement order — starting from the due date — can be calculated, and rudimentary scheduling can be performed.
The value for lead time can be a value based on prior experience. However, for effective planning, particularly of production orders, such more or less arbitrary values are often not precise enough:
- Some components do not need be
reserved for the start date of an order, as they are only needed for a later
operation. - For exact capacity planning, we
need to know the point in time at which the work center will be loaded by work
to be executed and thus a start date for each operation.
For a detailed calculation of manufacturing lead time, the essential elements are attributes of the bills of material and routing sheets. We can develop the process plan from these elements (see also Figure 1.2.3.3). Manufacturing lead time is the sum of the three different time elements that are defined in Section 1.2.3:
- Operation
time (see Section 13.1.2) - Interoperation
time (see Section 13.3.1) - Administrative time (see Section 13.1.4)
Lead time calculated on the basis of the lead times for individual operations is only an estimated value, since — especially for interoperation times — it is dependent on assumed average values. In this case, lead time calculation does not take into account the definite capacity utilization of work centers, which can dramatically affect wait time estimates (see also Section 13.2.). However, the “normal” lead time calculated in this way is accurate enough for several planning methods, and especially for rough-cut planning.
Lead time calculation is based on the order of the operations of the routing sheets.
A sequence of operations is the simplest order of operations. It is illustrated in Figure 13.1.1.1. In this simplest case, lead time is merely the sum of the time elements.
Fig. 13.1.1.1 A sequence of operations.
Besides the simple sequence of operations, there are also more complex structures, which can be portrayed as networks.
- In a directed network of operations, no operations are repeated. We can identify the operations in
ascending order (in a semiorder). Lead time corresponds to the longest path
through the network. - In an undirected network of operations, sequences of operations within the network may be repeated. In this case, we can calculate lead time only if we
know the number of repetitions or other constraints.
Figure 13.1.1.2 shows a typical example. In a directed network of operations, the lead time corresponds to the longest path through the network.
Fig. 13.1.1.2 A network of operations.
A process plan for multistage production, such as in Figure 1.2.3.3, corresponds to a directed network if a joint start event links together the open arborescent structure at the left.
A synchronization point is a link between the routing sheet and the bill of material, and thus between time management and materials management.
In Figures 13.1.1.1 and 13.1.1.2, circles designate the synchronization points at transitions between individual operations. At these points, we may channel in goods taken from a warehouse, directly procured, or taken from another, synchronous production order. At the same time, the circles represent an intermediate stage of the manufactured product. This can also be a partially completed product stage stocked as an in-house item. This means that these points in time on the time axis are also the planning dates for the necessary components.
Course section 13.1: Subsections and their intended learning outcomes
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.
13.1.1 Sequence of Operations and Network of Operations — The Order of the Operations of a Production Order
Intended learning outcomes: Describe a sequence of operations. Differentiate between a directed and an undirected network of operations. Identify a synchronization point.
13.1.2 Operation Time and Operation Load
Intended learning outcomes: Present the simplest formula for operation time using a graphic representation. Explain the simplest formula for operation load.
13.1.3 The Elements of Interoperation Time
Intended learning outcomes: Identify the elements of interoperation time. Differentiate between technical wait time and nontechnical wait time. Describe transportation time.
13.1.4 Administrative Time
Intended learning outcomes: Describe the need and the various kinds of administrative time.
13.1.5 Transportation Time
Intended learning outcomes: Explain the transportation times matrix. Disclose a robust approximation of transportation time.
