One-Piece Flow
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One-piece flow is the ideal state where items move through processes one at a time without batching, waiting, or accumulation.
Definition
One-piece flow is the ideal state where work moves through the value stream one unit at a time, with each unit proceeding immediately from one value-adding step to the next without waiting, batching, or queue time. In one-piece flow, lead time approaches the sum of value-adding cycle times because there's no waiting. One-piece flow immediately exposes problems (quality issues affect only one unit), minimizes work-in-process, and dramatically reduces lead time. Achieving one-piece flow requires addressing all obstacles: changeover time, equipment reliability, quality variation, and process synchronization.
Examples
A traditional machining department processed parts in batches, with weeks of lead time. Reorganizing into cells with machines arranged in process sequence achieved one-piece flow. Parts moved from operation to operation immediately, reducing lead time from three weeks to three hours.
Key Points
- One-piece flow is the ideal; practical constraints may require small batches
- Achieving one-piece flow requires solving underlying problems, not just rearranging work
- One-piece flow exposes problems immediately—there's no buffer to mask issues
- Lead time in one-piece flow equals the sum of cycle times
Common Misconceptions
One-piece flow means everyone works faster. One-piece flow often allows slower pace because there's no expediting or crisis response to delays. The dramatic lead time improvement comes from eliminating waiting, not working faster.
One-piece flow is impossible in our industry. While true one-piece flow may not be achievable everywhere, movement toward smaller batches and better flow is always possible. The ideal guides improvement direction even if never fully reached.