Lean manufacturing or lean production, often simply “lean”, is a systematic method for waste minimization (“Muda”) within a manufacturing system without sacrificing productivity. Lean also takes into account waste created through overburden (“Muri”) and waste created through unevenness in work loads (“Mura”). Working from the perspective of the client who consumes a product or service, “value” is any action or process that a customer would be willing to pay for.

Lean manufacturing makes obvious what adds value, by reducing everything else (which not adding value). This management philosophy is derived mostly from the Toyota Production System (TPS) and identified as “lean” only in the 1990s.[1][2] TPS is renowned for its focus on reduction of the original Toyota seven wastes to improve overall customer value, but there are varying perspectives on how this is best achieved. The steady growth of Toyota, from a small company to the world’s largest automaker,[3] has focused attention on how it has achieved this success.

Overview

Lean principles are derived from the Japanese manufacturing industry. The term was first coined by John Krafcik in his 1988 article, “Triumph of the Lean Production System”, based on his master’s thesis at the MIT Sloan School of Management.[4] Krafcik had been a quality engineer in the Toyota-GM NUMMI joint venture in California before joining MIT for MBA studies. Krafcik’s research was continued by the International Motor Vehicle Program (IMVP) at MIT, which produced the international best-selling book co-authored by Jim Womack, Daniel Jones, and Daniel Roos called The Machine That Changed the World.[1] A complete historical account of the IMVP and how the term “lean” was coined is given by Holweg (2007).[2]

For many, lean is the set of “tools” that assist in the identification and steady elimination of waste. As waste is eliminated quality improves while production time and cost are reduced. A non exhaustive list of such tools would include: SMED, value stream mapping, Five S, Kanban (pull systems), poka-yoke (error-proofing), total productive maintenance, elimination of time batching, mixed model processing, rank order clustering, single point scheduling, redesigning working cells, multi-process handling and control charts (for checking mura).

There is a second approach to lean manufacturing, which is promoted by Toyota, called The Toyota Way, in which the focus is upon improving the “flow” or smoothness of work, thereby steadily eliminating mura (“unevenness”) through the system and not upon ‘waste reduction’ per se. Techniques to improve flow include production leveling, “pull” production (by means of kanban) and the Heijunka box. This is a fundamentally different approach from most improvement methodologies, and requires considerably more persistence than basic application of the tools, which may partially account for its lack of popularity.[5]

The difference between these two approaches is not the goal itself, but rather the prime approach to achieving it. The implementation of smooth flow exposes quality problems that already existed, and thus waste reduction naturally happens as a consequence. The advantage claimed for this approach is that it naturally takes a system-wide perspective, whereas a waste focus sometimes wrongly assumes this perspective.

Both lean and TPS can be seen as a loosely connected set of potentially competing principles whose goal is cost reduction by the elimination of waste.[6] These principles include: pull processing, perfect first-time quality, waste minimization, continuous improvement, flexibility, building and maintaining a long term relationship with suppliers, autonomation, load leveling and production flow and visual control. The disconnected nature of some of these principles perhaps springs from the fact that the TPS has grown pragmatically since 1948 as it responded to the problems it saw within its own production facilities. Thus what one sees today is the result of a ‘need’ driven learning to improve where each step has built on previous ideas and not something based upon a theoretical framework.

Toyota’s view is that the main method of lean is not the tools, but the reduction of three types of waste: muda (“non-value-adding work”), muri (“overburden”), and mura (“unevenness”), to expose problems systematically and to use the tools where the ideal cannot be achieved. From this perspective, the tools are workarounds adapted to different situations, which explains any apparent incoherence of the principles above.

Also known as the flexible mass production, the TPS has two pillar concepts: Just-in-time (JIT) or “flow”, and “autonomation” (smart automation).[7] Adherents of the Toyota approach would say that the smooth flowing delivery of value achieves all the other improvements as side-effects. If production flows perfectly (meaning it is both “pull” and with no interruptions) then there is no inventory; if customer valued features are the only ones produced, then product design is simplified and effort is only expended on features the customer values. The other of the two TPS pillars is the very human aspect of autonomation, whereby automation is achieved with a human touch.[8] In this instance, the “human touch” means to automate so that the machines/systems are designed to aid humans in focusing on what the humans do best.

Lean implementation emphasizes the importance of optimizing work flow through strategic operational procedures while minimizing waste and being adaptable. Flexibility is required to allow production leveling (Heijunka) using tools such as SMED, but have their analogues in other processes such as research and development (R&D). However, adaptability is often constrained, and therefore may not require significant investment. More importantly, all of these concepts have to be acknowledged by employees who develop the products and initiate processes that deliver value. The cultural and managerial aspects of lean are arguably more important than the actual tools or methodologies of production itself. There are many examples of lean tool implementation without sustained benefit, and these are often blamed on weak understanding of lean throughout the whole organization.

Lean aims to enhance productivity by simplifying the operational structure enough to understand, perform and manage the work environment. To achieve these three goals simultaneously, one of Toyota’s mentoring methodologies (loosely called Senpai and Kohai which is Japanese for senior and junior), can be used to foster lean thinking throughout the organizational structure from the ground up. The closest equivalent to Toyota’s mentoring process is the concept of “Lean Sensei,” which encourages companies, organizations, and teams to seek third-party experts that can provide advice and coaching.[9]

In 1999, Spear and Bowen[10] identified four rules which characterizes the “Toyota DNA”:

All work shall be highly specified as to content, sequence, timing, and outcome.
Every customer-supplier connection must be direct, and there must be an unambiguous yes or no way to send requests and receive responses.
The pathway for every product and service must be simple and direct.
Any improvement must be made in accordance with the scientific method, under the guidance of a teacher, at the lowest possible level in the organization.