*Intended learning outcomes: Produce an overview on lead time scheduling. Explain forward scheduling and backward scheduling. Describe a simple algorithm for backward scheduling.*

For every production order, the planner should know the load of each operation and the point in time at which the work center will be loaded. To determine these factors, planning uses lead-time scheduling techniques.

Inlead time scheduling,a schedule is developed by calculating the lead time. This calculation includes the duration of all operations, interoperation times, and administrative times. Thelatest dateis a date that we cannot exceed in execution and control of operations. Similarly, we cannot allow a date to fall before theearliest date. Aset dateis set “externally” and cannot be changed by means of the scheduling algorithm.

The two most important scheduling techniques are the following:

Backward scheduling, orback scheduling, begins with the set (that is, thelatestacceptable)completion datefor the order (that is, theorder due date), and calculates — for each operation — the latest (acceptable) completion date (that is, theoperation due date) and the latest (possible) start date (that is, theoperation start date), as well as thelatest (possible) start datefor the order.

Forward schedulingbegins with the set (that is, theearliestacceptable)start datefor the order and calculates the earliest (acceptable) start date and the earliest (possible) completion date for each operation, as well as theearliest (possible) completion datefor the order.

Figure 13.3.3.1 illustrates the two principles.

**Fig.
13.3.3.1** Forward
scheduling and backward scheduling.

Figure 13.3.3.2 shows the simplest algorithm for backward scheduling (the algorithm for forward scheduling has a similar structure):

- The order of the operations is assumed to be a sequence of operations.
- The production order consists of one single partial order.
- All n operations are included in the lead time scheduling; that is, the order has not yet begun.
- The interoperation times are weighted with a factor of 1; that is, they are assumed to be “normal.”

The formal description of this scheduling task is as follows:

- Take a production order consisting of one partial order with n operations i, 1 ≤ i ≤ n, and m components j, 1 ≤ j ≤ m, as given. The operation numbers stand in a semiorder; if, for example, i
_{1}< i_{2}, then operation i_{1}is performed before operation i_{2}.

- Beginning with the set (that is, the latest acceptable) order completion date, we calculate the following “latest” dates:
- Start and completion dates for the individual partial order
- Start and completion dates for the individual operations
- Reservation dates (= start date) for the components
- Start date for the order, with an exception message if it is earlier than a set (earliest) start date

As data specifications, the following notations are used:

- x = order, partial order, or one position in the partial order (component or operation)
- LCD[x] = latest completion date for x
- ECD[x] = earliest completion date for x
- LSD[x] = latest start date for x
- ESD[x] = earliest start date for x
- OT[i] = operation time for operation i
- INTBEF[i] = interoperation time before operation i
- INTAFT[i] = interoperation time after the end of operation i
- INTTEC[i] = technical interoperation time after operation i
- ADMPORDBEG = administrative time for the partial order at the beginning
- ADMPORDEND = administration time for the partial order at the end

Remarks:

- For comparing the date attributes with one another, we will use the standardized “ISO” format, that is, YYYYMMDD.
- A date is calculated either by the scheduling algorithm or given as a set date. We distinguish the latter from the former by the addition of (set), for example, LCD(set)[x].

**Fig. 13.3.3.2** Simple algorithm for
backward scheduling.

## Course section 13.3: Subsections and their intended learning outcomes

##### 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.

##### 13.3.1 The Manufacturing Calendar, or Shop Calendar

Intended learning outcomes: Present characteristics of the manufacturing calendar, or shop calendar. Explain an example of a manufacturing calendar.

##### 13.3.2 Calculating the Manufacturing Lead Time

Intended learning outcomes: Identify definitions for the elements of operation time. Present the lead time formula and the start date as a function of completion date. Differentiate between manufacturing lead time, cycle time and throughput time.

##### 13.3.3 Backward Scheduling and Forward Scheduling

Intended learning outcomes: Produce an overview on lead time scheduling. Explain forward scheduling and backward scheduling. Describe a simple algorithm for backward scheduling.

##### 13.3.4 Network Planning and CPM — Critical Path Method

Intended learning outcomes: Explain network planning and the critical path method (CPM). Present an example of a scheduled network. Describe a network algorithm for backward scheduling.

##### 13.3.5 Central Point Scheduling

Intended learning outcomes: Explain central point scheduling. Describe several possible solutions in a directed network of operations.

##### 13.3.6 The Lead-Time-Stretching Factor and Probable Scheduling

Intended learning outcomes: Produce an overview on order urgency and slack time. Differentiate between forward, backward, and probable scheduling. Explain the role of the lead-time-stretching factor in probable scheduling. Describe the equation for recalculation of lead-time-stretching factor.

##### 13.3.7 Scheduling Process Trains

Intended learning outcomes: Differentiate between reverse flow scheduling, forward flow scheduling, and mixed flow scheduling.