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07/19/2012 02:48 PM
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Can someone explain to me how a conveyer on an assembly line can be defined as Inventory in a Value Stream Map? If an automated assembly line uses a pallet and a conveyer to move from one station to another to do separate operations how is that Inventory?
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07/20/2012 10:13 AM
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Hi Daniel,
Transporting product does not add value. The product might be moving, but it is still work in process inventory.
Don
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07/20/2012 10:13 AM
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The parts on the pallet / conveyor should be listed on your VSM as Work in Process inventory. Your WIP should be defined in the VSM accordingly.
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07/20/2012 10:24 AM
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The conveyor itself is not inventory, but the material sitting on top of it is. The assumption is that if there is a conveyor somewhere, chances are there is WIP sitting on it.
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07/20/2012 10:24 AM
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I can agree with parts being on the pallet being WIP. What symbol should be used to denote this operation? I it a triangle with an "I" inside it? If so I don't agree. The parts are only being moved a few feet in some cases.
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07/20/2012 10:47 AM
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Daniel,
Short answer: It's inventory because there are parts there!
Longer answer: It could depend on the logic of your conveyors. If the downstream machine is in some fault condition and cannot accept the next part, what do your conveyors do? In my experience, even very short conveyors have some type of buffering logic built in. (even if only 1 part) So, the upstream machine is complete, and sends the part out to the conveyor. The conveyor checks the downstream machine to see if it's ready to accept the part. If not, the part is held on the conveyor until the downstream machine is ready. The upstream machine only stops if the conveyor itself is full. This is a push of parts from upstream to downstream. If the end machine on your line had a fault for several part cycles, your potential buildup of WIP in the line would grown from N (number of machines, assuming 1 part per) to 2N (really, 2N-1). Putting aside warehousing, buffering, etc., you have double the potential WIP! Also, with parts exposed and possibly waiting, is there an opportunity for them to be damaged while on the conveyors? Has anyone added a manual buffer as a workaround to extended downtime, pulling parts off the conveyor to be stacked in some temporary storage? There are a lot of potential issues, even with a single part waiting on a conveyor.
Now, if your conveyor logic is that the upstream machine releases a part only when the downstream machine is clear, and the conveyor never has a part waiting on it, then parts are flowing through the line and you may have an argument to make.
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07/20/2012 11:48 AM
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Going to agree with Don here, a conveyor is transportation just like sitting on a truck. If your map is at the process level, and the conveyor is between processes, then it is inventory. If you are mapping the building level, then the conveyor has goods that are being 'worked' on and not sellable yet, then it can be WIP.
Unless, of course, the conveyor is holding something (like a car) so you can work on it. Then it is WIP while you are working on it.
Dan
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07/23/2012 11:45 AM
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Hi dan
Your question is interesting but if we think lean principles then it is easy to answer.
1 I assume that your conveyor moves a certain pitch and stops. then you do assmbly at varous stations, then conveyor moves for a pitch. In this case conveyor movement is transportation loss. Also the material between two stations is an inventory.
2. There is another way of looking to this is moving conveyor is part of process time where you can consider the material on the conveyor while moving is WIP and there is no inventory. In the process time then you have VA and NVA. Moving conveyor would be NVA.
3.
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07/23/2012 11:45 AM
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I agree with the other comments that this is inventory especially as you are moving pallet by pallet implying some level of batching to build up to full pallets rather than true one-piece flow.
On the VSM, I'd show it as a FIFO lane, not traditional inventory.
When you do your lead time ladder for a FIFO lane, assume that there is as much inventory in the FIFO lane as it can hold so that your lead time ladder shows the worst case - and therefore the greatest opportunity if you can eliminate it.
Phil
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07/27/2012 11:26 AM
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The symbol would be the triangle, and normally I would write the WIP COUNT inside the triangle on the map. The i just indicates "inventory" for training purposes.
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