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Production leveling, also known as production smoothing or – by its Japanese original term – heijunka (平準化),[1] is a technique for reducing the mura waste and vital to the development of production efficiency in the Toyota Production System and Lean Manufacturing. The general idea is to produce intermediate goods at a constant rate, to allow further processing to be carried out at a constant and predictable rate.
Ideally production can easily be leveled where demand is constant but in the real world where actual customer demand appears to fluctuate two approaches have been adopted in lean: Demand leveling and production leveling through flexible production.
On a production line, as in any process,[2] 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.[3] In production leveling, 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 1940s it took two to three hours, in the 1950s it dropped from one hour to 15 minutes, now it takes three minutes.[4]
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Leveling by volume or by product type or mix
Production leveling can refer to leveling by volume, or leveling by product type or mix, although the two are closely related.
Leveling by volume
If for a family of products that use the same production process there is a demand that varies between 800 and 1200 units then it might seem a good idea to produce the amount ordered. Toyota's view is that production systems that vary in the required output suffer from mura and muri with capacity being 'forced' in some periods. So their approach is to manufacture at the long-term average demand and carry an inventory proportional to the variability of demand, stability of the production process and the frequency of shipments. So for our case of 800-1200 units, if the production process were 100% reliable and the shipments once a week, then the production would be 1000 with minimum standard inventory of 200 at the start of the week and 1200 at the point of shipment. The advantage of carrying this inventory is that it can smooth production throughout the plant and therefore reduce process inventories and simplify operations which reduces costs.
Leveling by product
Most value streams produce a mix of products and therefore face a choice of production mix and sequence. It is here that the discussions on economic order quantities take place and have been dominated by changeover times and the inventory this requires. Toyota's approach resulted in a different discussion where it reduced the time and cost of changeovers so that smaller and smaller batches were not prohibitive and lost production time and quality costs were not significant. This meant that the demand for components could be leveled for the upstream sub-processes and therefore lead time and total inventories reduced along the entire value stream. In order to simplify leveling of products with different demand levels a related visual scheduling board known as a heijunka box is often used in achieving these heijunka style efficiencies. Other Production leveling techniques based on this thinking have also been developed. Once leveling by product is achieved then there is one more leveling phase, that of "Just in Sequence" where leveling occurs at the lowest level of product production.
The use of production leveling as well as broader lean production techniques helped Toyota massively reduce vehicle production times as well as inventory levels during the 1980s.
Implementation
Even Toyota have not yet moved to the final stage in this journey for all their processes 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). This is a specific form of Fixed Repeating Schedule. Green stream products are those with predictable demand, Red stream products are high value unpredictable demand products.
- 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 on repeating every week. This may require support services to speed up as well.
- Fixed sequence with unfixed 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. This is a more generalised form of Fixed Repeating Schedule.
- 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.
References
- ^ Businessknowledgesource.com
- ^ Taguchi loss function
- ^ Toyota Production System, Taiichi Ohno, Productivity Press, p 126
- ^ Toyota Production System, Taiichi Ohno, Productivity Press, p 127