Integral Logistics Management — Operations Management and Supply Chain Management Within and Across Companies

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,

Time management is the observation, control, and manipulation of time elements. Time elements are the duration of operations, inter­operation times, and administration times.

In typical job shop production, the focus is on inter­operation times, since they make up more than 80% of the total lead time. However, in line production, observation of the duration of the operations themselves is also of particular interest.

13.1.1 The Order of the Operations of a Production Order

In materials management, lead time (see Sections 1.1.2 and 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 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 In this simplest case, lead time is merely the sum of the time elements.

Fig.       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 shows a typical example. In a directed network of opera­tions, the lead time corresponds to the longest path through the network.

Fig.       A network of operations.

A process plan for multistage production, such as in Figure, 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 and, 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.

13.1.2 Operation Time and Operation Load

Operation time is the time required to carry out a particular operation. It is defined in Section 1.2.3 as the sum of setup time for machines and tools and the run time for the actual order lot.[note 1301] The latter is the product of the number of units produced (the lot or batch) and the run time for a unit of the lot produced (the run time per unit). The simplest formula for operation time occurs when run times are scheduled serially following the setup time, as in Figure

Figure shows the formula for operation time as a graphic representation.

Fig.       The simplest formula for operation time (graphic representation).

The formula for calculating operation time becomes more complicated when we include special effects such as splitting or overlapping. See also Section 13.4.

Operation load is the work content of the operation, measured in the capacity unit of the work center used for the operation. In Section 1.2.4, we saw that operation load is the sum of the setup load— the work content that is independent of batch size — and the run load for the actual order lot.[note 1302] The latter is the product of the number of units produced (the lot or batch) and the run load per unit for a unit of the lot produced.

Figure shows the formula for the operation load shown in the simplest case. Compare here the formula given in Figure

Fig.       The simplest formula for operation load.

Often, the capacity unit for the work center used for the operation is a unit of time. In this case, setup time and run time are generally identical with setup load and run load. There are, however, instances in which the operation time bears no relationship to the operation load.

  • For subcontracted operations, e.g., a cost unit may be chosen as the capacity unit.
  • For operations with an extremely complicated execution or for purely fictitious “waiting operations,” which have no influence upon the load of a work center or upon manufacturing costs, the chosen operation time must be different from the operation load.

If the inter­operation times exert the dominant influence on total lead time, scheduling does not require exact knowledge of the operation time. For purposes of capacity management, however, planners need the exact value of the operation load to gain a meaningful load profile for a work center. If they are now able to derive the operation time from the operation load, they can calculate the precise operation time as well as the operation load.

13.1.3 The Elements of Inter­operation Time

Inter­operation time occurs before or after an operation (see definition in Section 1.2.3). Figure shows the elements of inter­operation time:

Fig.       The elements of inter­operation time.

  • Technical wait time after an operation describes the time required to complete testing, a chemical reaction, a cool-down period, or other things. It is an attribute of the operation. As is true of the operation itself, it is not generally possible to shorten this wait time, for example, to accelerate the order.
  • Nontechnical wait time after an operation is the wait time incurred before the lot is collected for transport. It is dependent on the work center and can be an attribute of this object or be included in transportation time.
  • Transportation time, also called move time, move time, or transit time, is the time needed to transport the lot from the current work center to the work center that will carry out the subsequent operation. This time is dependent on both work centers. There are various techniques for determining this time (see Section 13.1.5).
  • Nontechnical wait time before an operation is made up of the so-called queue time, that is, the amount of time a job waits at a work center before setup or work is performed on the job. This includes preparation time for the operation, as long as it is not counted as a part of the actual setup time. This time is dependent on the work center and is an attribute of that object (see Section 13.2).
  • Technical wait time before an operation is made up of the operation-specific preparation time, such as a warm-up process, which does not yet load the work center. In practice, this time is of minor significance. It is an attribute of the operation.[note 1303]

All components of inter­operation time, with the exception of technical wait times before and after the operation, are “elastic”: We can lengthen or shorten them depending on the load at the work center and the order urgency (compare Section 13.3.6). Therefore, the values specified in the master data are only average values, and they can fluctuate widely.

13.1.4 Administrative Time

Administrative time is the time needed to release and complete an order (see definition in Section 1.2.3).

Administrative time at the beginning of an order is required for order release. This comprises availability control, decision making as to type of procurement, and the preparation time that the production or purchasing office needs for the order. It is also a lead time for the data or control flow (i.e., without flow of goods).

Buffer times added to this administrative time wherever possible will serve to control fluctuations in the effective loads of work centers. This will keep the capital-intensive lead time for goods as short as possible. Schedulers can use the play resulting from this buffer to move the entire order forward or backward on the time axis, according to the load of the work centers at the time of order release.

In addition, schedulers should plan administrative time for coordination purposes for each partial order. This time can also include a “normal” stock issue time for components, as long as it has not already been accounted for in the routing sheet as an independent operation, called “stock issue,” for example.

Similarly, at the end of each partial order, there is administrative time that generally includes time to place the completed order in stock or to prepare it for shipping. This time may also include a “normal” control time, provided that schedulers do not want to account for this in the routing sheet as an independent operation, called “final control,” for example.

13.1.5      Transportation Time

There are different techniques to determine transportation time between work centers (also called move time or transit time):

  • Simple, but inexact: As a scheduling rule, planners use one single time that is not dependent on the work centers.
  • Exact, but complex: A matrix of transportation times contains an entry for every combination: “preceding work center Û following work center.” This matrix should be maintained in the form of a table in a separate entity class. It is a square matrix containing zeros on the diagonal. If it is not dependent on the direction of the transport, the matrix will be symmetrical (see Figure The difficulty with this technique lies in maintaining the two-dimensional table, since the number of work centers and the transportation times are continually changing.

Fig.       Transportation times matrix.

An efficient compromise between these two extremes is to use an approximation based on an analysis of transportation times, and that experience has been shown to be reliable, as in Figure

Fig.       Approximation of transportation time.

  • Within a plant, planners define a fictitious center and assume that each shipment must pass through this center. With this, the transporta­tion time from one work center to another becomes the sum of the transportation time from the first work center to the fictitious center and the transportation time from the fictitious center to the other work center. As a result, you only have to register two attributes for every work center, and their values are not dependent on the other work centers.

This approximation is reliable, because the loading and unloading of the means of transportation comprise the greatest portion of transportation time. Actual transportation time from one work center to another varies little in relation to this.

  • Between the fictitious centers of two plants, planners assume an additional transportation time. Again, for production facilities in the same region, this approxi­mation is reliable, because loading and unloading of the means of transport make up most of the additional move time. In relation, the actual transportation time between the plants varies little.
  • Characterizing plants by the attribute “region” will distinguish among plants in differing geographic areas. This allows differen­tia­tion among regional and interregional or even national and international shipments.

Course 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.

  • 13.6 Keywords


  • 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


Print Top Down Previous Next