One of the inevitable consequences of 5S activity is that you find out that you don’t need some of the things you have, and that you don’t have some of the things you need. More often than not, you also find things you have forgotten about. While doing a bit of spring cleaning in the archived articles and old article drafts I found an old request to explain the tsurube system.
Tsurube is the Japanese word for “well bucket”. Tsoo-roo-bay. It is an application of the pull system, a material replenishment method used when there is the middle process step is physically separated from the upstream and downstream processes by lead-time due to distance and/or batch processing. A “bucket” (container of unprocessed material) is sent down the “well” (the remote process) and at the same time a full “bucket” (processed parts) is returned up from the well. Get yourself to a village well with a pulley, rope and two buckets if the explanation is not clear. Or you can click on the 17 second animation below which illustrates this well bucket concept.
The features of a tsurube system are that the standard work in process quantity (two buckets) maintains continuous flow through a batch or outside process. Transportation of materials is never empty in either direction and the arrival of the bucket to the well process is a signal to produce. It is in effect an instance of a non-card type production instruction kanban.
A simple calculation of the standard work in process (SWIP) quantity within a tsuube system is as follows:
SWIP = (lead time / takt time) x 2
Lead-time = 480 min
Takt time = 10 minutes/piece
(480 min / 10 min) x 2 = 96 pieces
96 pieces (2 buckets of 48) are needed to maintain continuous flow
Lead time is the the time for “the well” process return “the bucket”. This includes all transport time to the process, waiting time, actual processing time, and time to transport back to the customer process. A safety factor can be built in for variability in transportation time, yield, or processing time. For all practical purposes this lead time is “order to receipt” and should be measured and validated rather than relying on estimates in the material planning system. Adjustments should be made as conditions change.
The quantity in any given bucket is at most is half of the number above. There should always be one lot of material at or in the furnace (depending on lead time and how often the material is run) and some combination of unprocessed and processed parts at the cell (customer process).
A very typical application of the well bucket system is to maintain one-piece flow through a machining line that has a heat treatment process midway through the process. It is often impractical to move a furnace into the cell due to size, sharing of equipment with other cells, safety, or set up times driving up lot sizes. Instead, we leave space where the furnace would be at the appropriate location in the cell and create a location for material outgoing and material incoming. The combined work in process quantity within the two locations should equal one furnace load. When the outgoing load is full
Due to the fact that it is impractical for the people working in or supporting the cell to physically go pull the materials and drop off the unprocessed materials, the material delivery is often done by the upstream (furnace in example above) process. Ideally a material handler working in support of the cell would go to the supplier process just before the materials are about to run out, drop of the unprocessed parts and return with a full load.
Although the name is a bit awkward in Japanese (doesn’t have the good old “bon-bon” ring like kanban) the tsurube system is still one of my favorites for its elegance and practical use in situations where monuments and outside processes threaten to shut down your one-piece flow.