References // One-Piece-Flow-System
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| One-Piece-Flow-System |
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| One-Piece-Flow-System |
Assembly lines following the principle of one-piece flow
One piece flow is a concept from the Toyota production system. It refers to production facilities in which manufacturing is based on single workpieces which "flow" through the entire production system without any intermediate storage or butter. One piece each is processed continuously within the production flow until it is complete. During this process, the workpiece moves from one workstation to the next without any interruption. Necessary assembly and testing operations are carried out at these stations where the workpiece concerned is also clamped in different positions.
One-piece flow renders batch manufacture superfluous since this production system is flexible enough to work efficiently even when processing different single workpieces following one after the other. The main advantage is obtained by omitting inventory management within the production system. But in practice this ideal is only achieved in an approximate way in most cases.
In its pure form, a one-piece flow would mean that all operators participating in an assembly process are proficient in all operations involved, and also carry out these operations, thus being responsible for the overall process and for the resulting product. To enable this, the stations of an assembly process are mostly arranged within a cell. An employee starts at the first station, and moves through the entire cell by following the product from station to station until he or she reaches the last station. That's where the product is inspected normally. Then, the operator moves back to the first workstation, and starts his or her process again. To allow this, the start and the end point of such a cell should be as close as possible. For this reason, U-shaped assembly lines are used very often. They are also called U-lines or Chaku-Chaku lines in Japanese.
The advantages of this method include:
• Minimum flow times
• Small stock-in-trade
• Minimum floor space required
• High quality
• Small capital expenditure (compared to transfer system assembly solutions)
• High flexibility in terms of variants
• High flexibility to cope with fluctuations in production
• Long-term flexibility regarding new tasks for the assembly facility
• Retooling flexibility for manufacturing different workpiece types having similar process sequences (use of tool changing devices etc.)
Limits of this method:
• Impossibility to meet the qualification requirements for the operators in every case (replacement for holidays, illness, etc.) so that inconsistencies in quality cannot be excluded.
• High personnel expenses will be incurred if a one-piece flow system is utilised permanently in a magnitude nearing 100% of its planned capacity limit. This is especially true when the potential for retooling flexibility is not utilised for covering a bigger workpiece range.
Degree of automation:
The principle of one-piece flow was conceived primarily for manual workstations where employees carry out assembly operations by hand under sensory monitoring. But fully automatic machines are also used to an increasing extent. In this case, the operator only needs to place the workpieces into the fixtures, and remove them after assembly or testing. During automatic processing at a station, an operator will change workpieces in one or more other stations within the cell. A U-shaped arrangement is also an advantage in this case as it allows the control of multiple machines located at different U-line positions within short walking distances.
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