Theory of Constraints Handbook - James Cox Iii [116]
When an item is released to the floor, it is released into a buffer—constraint, shipping, or assembly buffer depending on the shop’s configuration. Buffers are sized so that each batch or order should arrive at the buffer in time to maintain the buffer approximately half-full. The buffer is actually divided into three regions9, each representing one-third of the buffer length. Region I (Red) consists of the oldest batches, which should be processed soon; Region II (Yellow) represents intermediate batches, about half of which should be in the buffer; and Region III (Green) represents the most recently released material, which is generally expected to still be en route to the buffer. If material released into the shop and under the control of the buffer has not yet reached the buffer it is called a buffer hole. Simatupang (2000) provides a good description of Buffer Management activities.
There is a person called the buffer manager whose responsibility is to steer material into the buffer on time. Holes in the Green Region of the buffer require no action. If a hole moves into the Yellow Region, the buffer manager will locate the item and remind the workstation holding the batch that it is soon due in the buffer. If a hole reaches the Red Region, the buffer manager will expedite the batch through the station holding the material and any stations between the batch’s location and the physical buffer. The buffer manager will also note the location of the expedited batch and the reason for delay for prioritizing future improvement efforts. Gardiner et al. (1992) state that 90 percent of orders should require no expediting if the buffer is properly sized. The buffer size is dynamic—if too much expediting is occurring, the buffer can be made bigger; if virtually no expediting is occurring, the buffer can be made smaller. Because of Buffer Management and dynamic buffer sizing, the initial size of the buffer is not that critical—if it is initially the wrong size, Buffer Management activities will quickly reveal that fact and the buffer can be resized.
When a job is released to the shop floor, its paperwork should show the due date of the job in the buffer toward which it is moving. The supervisor of each workstation can use this information as an aid in sequencing jobs. The buffer manager of the buffer involved has a sequenced list of jobs due in the buffer, which he can use in determining the location of holes in the buffer and to decide whether to begin investigative action or expediting.
Tseng and Wu (2006) describe Buffer Management in a modified system employing five buffer regions rather than three: early arrival zone, ignored zone, mentioned zone, expediting zone, and delayed zone. The three middle zones correspond to the normal three regions of the buffer, while the first zone represents material released to the shop too early and the fifth zone represents material not processed in time. Simatupang (2000) describes how Buffer Management can be used to direct the application of preventive maintenance activities.
Schragenheim and Dettmer (2001) describe a variation on Buffer Management called the “red-line control mechanism,” which collects data on jobs that are about to be late and assists managers in determining the stability of the shop floor.
Buffer Sizing
One of the questions that must be addressed in establishing a DBR system is, “What size should the buffers be?” Goldratt has suggested to