|
|
|
Move batching delay |
Example
Principle (Move Batching): WIP and flow time increase proportionally in the batch size used to move parts between processes within a production process.
Motivation
It is very common to move parts between stations in batches. The primary reason is limited material handling capacity. For instance, castings might travel from a machining operation to a finishing operation by forklift on pallets. In theory the forklift could move the castings one-at-a time, but since this would require many trips by the forklift, it might be deemed more practical to move them in batches. However, whenever parts are moved in batches it induces a delay and hence flow times are inflated. Moreover, the larger the move batch size, the greater the inflation of flow time.
The following figure illustrates the mechanics of how batching causes delay. Here we consider two stations, both of which process parts one-at-a-time. However, the parts are moved between the stations in batches of 4. As a result, the first part in the batch must wait until all the other parts in the batch are completed before it can move to the next station. The last part in the batch is able to move downstream immediately after it completes processing. The average time a part waits is between this maximum and minimum delay and is called wait to batch time. Similarly, when the batch reaches the second station (i.e., the front of the queue of batches) the first part immediately gets processed, but the last part must wait for all its batch-mates to be processed before it can begin. The average delay experienced by the jobs is called wait to unbatch time. Both wait-to-batch and wait-to-unbatch time increase with the size of the move batch.
|
|
|
Move batching delay |
Example
Two Workstation Line: For a two machine line with move batching, we can derive a mathematical formula for the flow time as a function of batch size. Since both wait-to-batch and wait-to-unbatch time increase linearly in the number of parts in the batch, it is not surprising that the additional delay induced by batching increases linearly in the batch size. The relationship is shown graphically in the following diagram.
Graph of the linear relationship between the move batch size and the flow time