In Figure 6.3, a more detailed process map is shown as a network of connected activities. Each operation is quantified using its average and standard deviation of cycle time measured in seconds, shown in parentheses as (mean, standard deviation). After this type of process description is created, the initial takt time and the key operational metrics listed at the
Process mapping current process.
bottom of Figure 6.2 are estimated from data collected from the process. Figure 6.3 shows three NVA operations located at operations B, D, and F. An initial goal should be to eliminate NVA operations, and then, over time, improve the efficiency of the remaining operations through simplification and standardization. Business value-add operations are needed for regulatory and compliance reasons or technical constraints. Examples include audit, certain types of testing and additional operations that are not value-added but must be done.
The takt time of this network is shown in Table 6.5. It is calculated as the daily demand on the system, which is 1,000 units, divided by the available time, which is 7 hours or 25,200 seconds after breaks have been subtracted from the total time of 8 hours. The takt time for this example is 25.2 seconds per unit. In other words, every 25.2 seconds, one unit is produced and exits the process.
The next step is to add the cycle times at each operation to estimate the total time to complete one unit. In this example, it takes 290 seconds to complete one unit. Dividing the total time to complete one unit by the takt time shows that the process requires 11.5 people or workstations with each producing at the takt rate. Note that the 11.5 must be rounded up to 12 because you can’t have half a person or half a workstation. This rule is correct if every operation takes exactly 25.2 seconds to complete. In practice, however, this is not usually true because some operations take longer or
Calculating Takt Time and the Minimum Number of Workstations
less than exactly 25.2 seconds. Table 6.5 shows that the operational cycle times range between 10 and 90 seconds. This means we must balance the work tasks within each operation across the 12 people so that no person is allocated work with a completion time exceeding 25.2 seconds or too little work. But this may not be possible if the operations are not adjacent. As an example, we must use 14 rather than 12 people because some operations cannot be combined and have idle time.
A situation shown in Figure 6.4 is a more likely scenario where some operations cannot be combined and additional people are assigned to operations, as seen in process D, resulting in idle time. But an initial balancing requires all operations or groups of operations have a takt time of less
than 25.2 seconds to meet demand. The calculated efficiency of the current operational balance is 83% of the calculated optimum because we must use 14 people versus 12 people. The team would immediately remove all NVA operations (i.e., B, D, and F) and rebalance the process with fewer resources. Over time, various types of process waste would also be eliminated, such as process scrap and rework, maintenance issues, non-standardization, and other waste. Reducing process waste will “lean out” the process to consistently meet the takt time of 25.2 seconds, but with less resource utilization.
Figure 6.5 shows how value adding work is separated from non-value adding work. Value adding work is needed by a customer, is physically changed by an operation and done right the first time. What is value adding in one industry may not be in another. Logistics is an example, where the movement of materials is value adding as a core competency; whereas in most of other industries movement of materials is a form of process waste, unnecessary movement of material and information are minimized. Eliminating the non-value adding components shown in Figure 6.5 helps reduce wasted resources in a process which also helps
Simplifying a process.
stabilize the takt time i.e. fewer operations reduces overall lead time variation.