History of Lean. Basic principles
 The concept of lean manufacturing first became widely known with “The Machine That Changed the World” and “Lean Thinking” By James P. Womack. These books are interesting from the standpoint of developing lean principles, but they do not show how these principles can be applied to other companies. “The Toyota Way” by Jeffrey Liker offered a more detailed description of the basic principles of lean and how to apply them. The book uses a pyramid model to show Toyota's approach. The base of the pyramid is a management that builds its decision making based on the long term, even if it has to incur financial expenses in the short term. The second layer relates to ensuring the right processes, namely the production flow should be leveled, be adjusted to the pulling system, there should be visual management to make problems immediately visible, and everything should be standardized. The next layer refers to relationships. The corporate culture of Toyota and every lean organization is built on mutual respect and growth when challenging tasks are set and results are achieved together. The apex of the pyramid refers to problem solving and continuous improvement when tools such as Kaizen and Genchi genbutsu are used.   Figure  SEQ Figure \* ARABIC 2. The Toyota Way model according to Jeffrey Liker Overall, Womack and Jones formulated 5 basic principles of the lean organization:  Specify value from the standpoint of the end customer by product family. Identify all the steps in the value stream for each product family, eliminating whenever possible those steps that do not create value. Make the value-creating steps occur in tight sequence so the product will flow smoothly toward the customer. As flow is introduced, let customers pull value from the next upstream activity. As value is specified, value streams are identified, wasted steps are removed, and flow and pull are introduced, repeat this process again and continue it until a state of perfection is reached in which perfect value is created with no waste.   center16873Figure  SEQ Figure \* ARABIC 3. 5 principles of Lean Production. Source: [Lean Enterprise Institute.]0Figure  SEQ Figure \* ARABIC 3. 5 principles of Lean Production. Source: [Lean Enterprise Institute.] Figure  SEQ Figure \* ARABIC 3. 5 principles of Lean Production.  Source: [Lean Enterprise Institute.] Figure  SEQ Figure \* ARABIC 3. 5 principles of Lean Production.  Source: [Lean Enterprise Institute.]  When companies identified value for their customers and all the steps that create it, ones have to focus on creating flow and searching for processes that do not bring any value. According to Liker (Liker, 2004), organizations should focus on eliminating or at least reducing the following three M’s from their processes and the way of work:  Muda, waste  Mura, unevenness Muri, overburden  One of the founder of TPS – Taiichi Ono was an adept at fighting with unproductive waste (muda), that is, reducing any activity that consumes resources but does not add value to the end consumer. To do this, the first step is to identify the operations that add value and minimize cost for the rest of operations. Taiichi Ono identified seven main groups of waste (Ohno, 1988, p. 42-43). The eighth group was formulated by Jeffrey Liker. This type of waste has also become canonical for lean manufacturing. Here are main groups of waste:   Overproduction. Production of items for which there was no order leads to excess inventory and generates losses such as excess labor and storage space, as well as transportation costs. Waiting. Workers who watch automated equipment work are idle waiting for the next operation, tools, parts, etc., or simply sitting idle due to missing parts, delays in machining, equipment downtime and lack of capacity. Excessive transportation or relocation. Moving work in progress over long distances, creating transportation inefficiencies, and moving materials, parts, and finished goods to and from the warehouse. Over-processing. Unnecessary operations in the machining of parts. Inefficient machining due to poor tool quality or poor design that leads to unnecessary movements and defects. Losses caused by excessive quality requirements. Surplus Inventory. Excess raw materials, work in progress or finished goods increase lead times, cause product obsolescence, result in damaged finished goods, transportation and storage costs, and cause delays and procrastination. In addition, excess inventory prevents detection of problems such as production imbalances, delivery delays, defects, equipment downtime and long changeovers. Excessive movement. All the unnecessary movements that employees have to do in the process of work: searching for what is needed, having to reach for tools, parts, etc. or to deal with their packing. This also includes walking. Defects. Production of defective parts and correction of defects. Repairs, rework, waste, product replacement and inspection result in wasted time and effort. Unrealized creative potential of employees. Loss of time, ideas, skills, opportunities to improve and gain experience due to inattentive employees who managers don't have time to listen to. Muri - overloading of people or equipment. In a sense, it is the opposite of muda. Muri forces a machine or person to operate at its maximum capacity. Overloading people threatens their safety and causes quality problems. Overloading equipment leads to accidents and defects (Liker, 2004). Mura is the result of the first two M. At times there is more work in normally functioning production systems than specialists and equipment can handle, and sometimes there is not enough work. The cause of the unevenness is an improper schedule or fluctuating production volumes caused by internal problems, such as downtime, missing parts, or defects. Muda is the result of Mura. Uneven production levels cause the need to match the available resources (equipment, materials, people) to the maximum amount of production, even if in fact its average level is much lower (Liker, 2004).  When implementing lean production tools, every organization focuses specifically on reducing muda, without giving proper attention to mura. As Liker points out, this is a wrong approach because without an evenly balanced flow, any waste reduction is meaningless. The concept that requires a production flow leveling is called Heijunka. The realization of this principle is a condition for the elimination of mura, which, in turn, is necessary for the elimination of muri and muda. Eliyahu M. Godratt's “The goal: A process of ongoing improvement” perfectly describes the process of creating a leveled production flow. One of the results of leveling is a reduction in batches and an increase in the number of models produced. Indeed, many tools are used in the leveling process, not only from the lean philosophy, but from the general principles of operations management, however, Goldratt, Liker, Womack and many other lean specialists point that SMED method is an essential tool in creating a leveled production flow which in turn is a necessary step to reduce other types of waste and create a lean organization. 
The main problem of the lean organization is the reduction of waste. The main types of waste are:  Muda - waste of time, material and skills.  Muri - waste of time, material and skills.  Overburden - waste of time, material and skills.  The main way to reduce waste is to create a flow and reduce the waste. The first step is to identify the activities that create value and minimize cost for the rest of activities. The second and the most important step are to reduce the waste of muda and mura.