Understanding Operational Definition
After you have calculated the number of labor and machine operations that are required for each process in the line design, the next step is to create an operational definition. The operational definition enables you to simulate various options for structuring each process within the line design. After you have arrived at a satisfactory solution, you can return to the sequence of events and line design for which you calculated the operational definition and adjust them, based on the calculation.
In the operational definition, you group the tasks from the sequence of events that the line design is based on into operations based on the number of operations that are calculated for the mixed-model line design. You calculate the operational definition using one product that represents the other products that go through the same process.
When you create an operational definition using the Define DFM Operation Definition program (PF30L950), you retrieve all existing line designs and select the one for which you want to create an operational definition. The system retrieves the processes from the selected line design, including the following values:
Process or cell.
Takt time.
Actual time weighted for machine and labor.
Calculated and designed number of machine and labor resources.
Work content selection.
To calculate an operational definition for the line design, specify a representative item number for each process. When you enter the representative item, the system calculates the sequence-of-events total time, as well as the operational index. This value serves as a guideline for grouping tasks into operations. You can also enter a tolerance percent value that enables you to group tasks to an operation using a time frame that is greater than the calculated operational index by the defined percentage. The system uses the operational index and the tolerance percentage to determine how many tasks can fit into one operation.
The operational definition calculates operations like this: The system retrieves all the tasks from the sequence of events for the process. The calculation assigns tasks to operations in the order that they appear on the sequence of events, accumulating the task times until the operational index is reached. After you add a task time that causes the cumulative task time to exceed the operational index value, the system assigns this task to the next operation. If tasks are repeated, the system can assign repetitions to different operations, if necessary. The last operation receives all the remaining tasks regardless of whether the accumulated task time is greater than the operational index. You can group tasks that should be performed together to avoid moving them to another operation. The system groups them to the operation of the first task in the task group.
When you calculate an operational definition for the first time, the calculation is automatically based on the actual time values from the sequence of events. Actual time from the sequence of events equals task time multiplied by the task repetition value. For overlapping tasks, the system uses the time of the longest of the overlapping tasks. More than two tasks can overlap. After you have calculated the operational definition, the system displays the last calculation date. If you have already calculated an operational definition and want to calculate it again after making changes, the system automatically selects the operational definition as the calculation basis. When the system recalculates the operational definition, the operational index is recalculated to account for the changes that you made. The system does not recalculate the number of operations.
In addition to assigning tasks to operations, the system calculates in-process kanbans (IPK) as part of the operational definition. In-process kanbans represent in-process inventory and are used to signal demand, resolve imbalances, and indicate where and when to flex the operational definition. The system calculates in-process kanbans by operation. It becomes 1 by default if the actual time by operation is less than or equal to Takt time. The IPK calculation is based on the line design type and sequencing capability.
The in-process kanban for a dedicated line is calculated like this: IPK = I × C ÷ Takt, where I represents imbalance and C represents cycle of imbalance.
Imbalance is calculated by subtracting Takt time from operational actual time. The calculation of the cycle of imbalance depends on the line design type and sequencing capability. The following table illustrates how the calculations differ:
Dedicated Line |
Mixed Model Line |
|---|---|
C = HE ÷ AT |
|
The operational definition displays the in-process kanban at the appropriate task. You can manually change the calculated value.
After you have calculated the operational definition, the system enables you to review the calculated results and manually balance operations in the Operational Definition Maintenance program (PF30L951). You can move tasks or delete them, for example, non-value added tasks. You can also add new tasks. When you have completed this step, you recalculate the operational definition with the operational definition as the calculation basis.
Based on the operational definition, you can create operation method sheets. Operation method sheets are detailed graphical work instructions that you can attach to each item and operation combination. To create operation method sheets, create media objects as attachments.