Production levelling

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Production levelling is key to reducing the Mura waste and to the development of production efficiency in the Toyota Production System and Lean Manufacturing.

On a production line, as in any process^[1], fluctuations in performance increase waste. This is because equipment, workers, inventory and all other elements required for production must always be prepared for peak production. This is a cost of flexibility. If a later process varies its withdrawal of parts in terms of timing and quality, the range of these fluctuations will increase as they move up the line towards the earlier processes. This is known as demand amplification.

To prevent fluctuations in production, even in outside affiliates, it is important to try to keep fluctuation in the final assembly line to zero. Toyota's final assembly line never assembles the same automobile model in a batch. Production is leveled by making first one model , then another model, then yet another^[2]. In production levelling, batches are made as small as possible in contrast to traditional mass production, where bigger is considered better. When the final assembly process assembles cars in small batches then the earlier processes, such as the press operation, have to follow the same approach. Long changeover times have meant that economically it was sound to punch out as many parts as possible. In the Toyota Production System this does not apply. Die changes (changeovers) are made quickly (SMED) and improved even more with practice. In the 1940's it took two to three hours, in the 1950's it dropped from one hour to 15 minutes, now it takes three minutes^[3].

[edit] Implementation

Even Toyota have not moved to the final stage in this journey which is single piece flow; indeed they recommend following their journey rather than trying to jump into an intermediate stage. The reason Toyota advocate this is that each production stage is accompanied by adjustments and adaptations to support services to production, if those services are not given these adaptation steps then major issues can arise.

Implement Green stream/Red stream or Fixed sequence, fixed volume to establish the entry and exit criteria for products from these streams and establish the supporting disciplines in the support services. The cycle established will produce every product every cycle (EPEC).
Faster fixed sequence with fixed volume keep the streams the same but use the now established familiarity with the streams to maximise learning and improve speed of production (economies of repetition). This will allow the shortening of the EPEC cycle so that the plant is now producing every product every 2 weeks instead of month and then later every week. This may require support services to speed up as well.
Fixed sequence with funfixed volume keep the stream sequences the same but now phase in allowing actual sales to influence volumes within those sequences. This affects inbound componentry as well as support services.
Unfixed sequence with fixed volume the stream sequences, and EPEC, can now be gradually flexed but move to small fixed batch sizes to make this more manageable.
Unfixed sequence with unfixed volume finally move to true single piece flow and pull by reducing batch sizes until they reach one.