August 1, 2005

By Darren Dolcemascolo

One-Piece flow is one of the most important principles of lean manufacturing. Yet, many people still do not understand what it truly means to achieve one-piece flow. Let us begin by discussing terminology. There are several basic terms used to describe one-piece flow. The most common are as follows:

One Piece flow

Single Piece Flow

Continuous Flow

Make One - Move One

Flow Manufacturing

Each of the above terms describes the same key element of the Toyota Production System, illustrated in the diagram below. As you can see, "Flow" is a shown as a key element of the pillar, "Just-in-Time."

Thus, one-piece flow is a tool that will help a manufacturer achieve true just-in-time manufacturing. That is, the right parts can be made in the right quantity at the right time. In the simplest of terms, one-piece flow means that parts are moved through operations from step-to-step with no WIP in between either one piece at a time or a small batch at a time. This system works best in combination with a cellular layout in which all necessary equipment is located within a usually U-shaped cell in the sequence in which it is used. To achieve true one-piece flow, equipment must have basic stability:

Highly capable processes. Processes must be able to consistently produce good product. If there are many quality issues, one-piece flow is impossible.

Highly repeatable processes. Process times must be repeatable as well. If there is much variation, one-piece flow is impossible.

Equipment with very high (near 100%) uptime. Equipment must always be available to run. If equipment within a manufacturing cell is plagued with downtime, one-piece flow will be impossible.

One-piece flow is usually associated with low-mix, high-volume manufacturing environments. However, one-piece flow also lends itself to high-mix, low-volume environments. It is usually achieved by creating mixed model or group technology cells, in which a number of products run through a particular cell utilizing one-piece flow. Although every organization has unique challenges, one-piece flow can be achieved through proper application of the principle.

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