Technique for reducing the mura (unevenness) which in turn reduces muda (waste)
Production leveling, also known as production smoothing or – by its Japanese original term – , is a technique for reducing the mura (unevenness) which in turn reduces muda (waste). It was vital to the development of production efficiency in the Toyota Production System and lean manufacturing. The goal is to produce intermediate goods at a constant rate so that further processing may also be carried out at a constant and predictable rate.
Where demand is constant, production leveling is easy, but where customer demand fluctuates, two approaches have been adopted: 1) "demand leveling" and 2) "production leveling" through flexible production.
To prevent fluctuations in production, even in outside affiliates, it is important to minimize fluctuation in the final assembly line. Toyota's final assembly line never assembles the same automobile model in a batch. Instead, they level production by assembling a mix of models in each batch and the batches are made as small as possible.
Production 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 1,200 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 800–1,200 units, if the production process were 100% reliable and the shipments once a week, then the production would be with minimum standard inventory of 200 at the start of the week and 1,200 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. 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 hasn't reached the final stage in this journey, single-piece flows, across all of their processes; indeed they recommend following their journey rather than trying to jump into an intermediate stage. The reason Toyota advocates 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.
Demand leveling.
Demand leveling is the deliberate influencing of demand itself or the demand processes to deliver a more predictable pattern of customer demand. Some of this influencing is by manipulating the product offering, some by influencing the ordering process and some by revealing the demand amplification induced variability of ordering patterns. Demand levelling does not include influencing activities designed to clear existing stock.
Historically demand leveling evolved as subset of production levelling and has been approached in a variety of ways:
Implementation.
If it is accepted that a large part of demand variability in high volume products can be substantially caused by sales and ordering process artifacts then analysis and leveling can be attempted.
The use of long delay supply chains to reduce manufacturing costs often means that production orders are placed long before customer demand can be realistically estimated. The much later arrival of forecast product demand volumes makes demand leveling irrelevant since the issue has now switched to disposal at best price possible products that are already created and possibly paid for. Demand leveling has only proven possible where build times have been made relatively low and production has been made relatively reliable and flexible. Examples of these are fast airborne supply chains (e.g. Apple iPod) or direct to customer selling through web sites allowing late customisation (e.g. NIKEiD custom shoes) or local manufacture (e.g. Timbuk2 custom courier bags).
Where actual build-delivery times can be brought within the same scale as customer time horizons then effort to modify impulse buying and make it somewhat planned can be successful. Reliable, flexible manufacturing will then mean that low stock levels (if any) do not interfere with customer satisfaction and that incentives to sell what has been produced eliminated.
Where demand follows a predictable pattern, e.g. flat, then regular deliveries of constant amounts can be agreed with variances in actual demand ignored unless it exceeds some agreed trigger level. Where this cannot be agreed then it can be simulated and the benefits gained through frequent deliveries and a market location.
The predictable pattern does not have to be flat and may, for example, be an annual pattern with higher volumes at particular periods. Here again the deliveries can be agreed to follow a simplified but similar pattern, perhaps one delivery volume for six months of the year and another for the other six months.
References.
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