Toyota, which has been perfecting Lean for 50 years, organizes its production areas into U-shaped “Cells which offer several advantages:
- Part travel distances are as short as possible, with no need to move the completed WIP of one activity to the incoming queue area for the next activity; they are the same. In many cases the part moves directly from one workstation to the next one, without resting between them. Often the part is automatically unloaded from the machine to allow unhindered access to it by the next part.
- One worker can operate more than one machine, if his attention is not needed there for the duration of the machine’s activity.
- Additional workers can be added when more capacity is needed. In this case additional duplicate work stations are manned or each worker operates fewer machines than at normal capacity. The amount of effort required from each person remains the same, independent of volume, though each machine may work at a higher capacity utilization.
- Just as cells can be turn on – or off – to meet demand, they can be operated for more hours in periods of peak demand, at a predictable production rate.
- Workers are located close enough to communicate with or help one another. This makes the work more enjoyable but is especially important for training new employees and in correcting errors. Injury data shows that people are also less likely to lift an item beyond their capacity if assistance is immediately available.
- It is much easier to see job status when all processes can be seen from one position (the open end of the "U"). Toyota has refined work cells to where incoming material starts at one end of the "U" and finished material comes out the other end, eliminating the need to count partially complete material to calculate percent complete.
- Flexibility of scope; if a machine or an activity is unnecessary for a particular product it is simply bypassed.
- Flexibility of product: attention to shorter changeover times allows parts that require changes in setups to be produced in smaller batches, reducing lead time.
There are a number of prerequisites to cellular layouts:
- Work station layout must be efficient; no one should have to leave the area for tools or materials, and the area should be free of unnecessary tools or material.
- The equipment must be dependable; if each of ten machines is only 10% reliable, the cell may never function.
- Machine setup and operation must be simple if operation of multiple machines by a flexible or cross-trained work force is to be expected. Standard Operations sheets posted in the cell can help.
- Smaller and/or lower capacity machines are preferable to larger ones. Some machines may be impossible to move due either to size or to a need for them elsewhere. A program of Plant and Equipment purchases that support cellular layout is a necessity. Equipment size and orientation should be considered; Toyota and others jointly develop equipment with their vendors, and typically have operator access at the narrow side of the machine to allow for tighter machine spacing.
- Tool breakage detection should be specified in equipment purchases and retrofitted where possible. In-house design of such equipment by try-storming is very effective.
- Changeover time reduction must become a focus, to allow multiple part numbers or expedited orders to be produced with minimal disruption.