Once a cell takt time has been determined, it is now a matter of comparing several aspects of the process and the takt time in order to design a balanced cell. The operational elements (machine time, man time, and setup time) of each product are examined with relation to takt time. Machine time is com- pared to takt time in order to determine if the fixed cycle time of any piece of equipment is greater than the takt time. If this is so, action must be taken to change the available time, off load, reduce the cycle time, change processes, add equipment, split demand, etc. If the operation remains greater than takt time, it will need to be balanced with in-process Kanban inventory and/or additional shifts.
Man time is compared to takt time to address two opportunities: (1) autonomation and (2) workload balance. The first opportunity, autonoma- tion, means equipment does not need to be watched in case something goes wrong. Autonomation equipment will automatically shut off when an abnor- mality is discovered, thereby allowing the operator to do other value-added work. This opportunity is invaluable for increasing productivity and quality. The second opportunity, workload balancing, has to do with examining the individual work elements of each operation and determining if they can be reduced, shifted, resequenced, combined, or eliminated. This effort to bal- ance the workload to takt time is a main enabler for achieving one-piece flow and minimizing manufacturing lead-times.
Setup times are almost always greater than takt time and need to be addressed as part of the cell design process. By comparing setup time to takt time, one has a greater appreciation as to how far setups need to improve in order to create a flexible work environment. The initial stake in the ground is to plan on setting up each high-volume product every day and then to schedule the product mix to run accordingly. If this cannot be accomplished, then plan to run 2 to 3 days’ worth at a time and hold the excess inventory until the customer or customer cell asks for it (never allow this to extend past more than a one week’s run). It will become very clear, very quickly, why setup reduction is so important, when the supplier cell has to physically Chapter 06 Page 71 Saturday, August 12, 2000 1:37 AM
72 Lean Manufacturing: Tools, Techniques, and How To Use Them
hold the excess inventory until the customer cell asks for it through a Kanban. Once each of these three operational elements is determined for each product, they are compared to the overall takt time of the cell. This information is placed on a loading chart for each individual product SKU (Figure 6.11).
From this point, it is a matter of generating ideas and looking for cell design solutions that will balance the cell workload for all parts and takt time. By reviewing the actual work elements and either improving the operations or shifting the work content, the cell can become more balanced compared to the takt. This is accomplished much more easily in an assembly environ- ment than in a fabrication environment, but it can be done in both.
When the operations are balanced to takt time, it is possible to take advan- tage of a one-piece flow approach to workflow instead of running in large batch quantities. With one-piece flow, the manufacturing lead-time, level of inventory, and feedback on quality issues are far superior to that of a batch- and-queue system. In a batch-and-queue system, individual pieces are com- pleted at an operation and sit waiting in queue until the entire batch is complete, at which point they are moved to the next operation in sequence and wait in queue for other orders to be completed that arrived there first before moving forward. In the one-piece flow approach, products are passed one piece at a time from operation to operation with a first-in/first-out (FIFO) priority. Product manufacturing lead-times are now only as long as the total of all the takts they had to get through. For example, five operations each with a takt of 1.0 minute require a manufacturing lead-time of five minutes. Another significant benefit to one-piece flow is the impact on quality. There
Figure 6.11 Loading Chart
Manufacturing Flow Element 73
are fewer units in flow to rework or scrap; if there is a defect found, the feedback is almost instantaneous and corrective action is taken on the spot, not several weeks later.
Once we know the cycle time for the process and we know the designed takt time, we can take the known cycle time and divide it by the takt time to determine the maximum staffing requirements for the cell. For instance, the cycle time from the example above was 5.0 minutes. If takt time for that process were 2.5 minutes, then the required staffing would be two operators. Actual head counts will vary with changes in required daily demand, which is why cross-training and operator flexibility are so important in supporting one-piece flow.