The objective of lean is to create the most value for the customer while consuming the least amount of resources to design, build, and sustain the product. Companies will gain improvements from lean when they redesign their value streams by applying the following principles:
- Specify value from the standpoint of the customer
- Identify the value stream for each product or service-line family
- Make value flow toward the customer
- Produced based on the pull of the customer, and
- Strive continually to approach perfection.
The objective of these lean principles is to create the best possible system, from concept to consumer using the current financial and resource constraints to provide the most value to the customer. Once the value stream is designed, or redesigned, improvements can be made by implementing lean tools and techniques appropriate to the particular situation. The lean tools and methods are briefly discussed in this article.
Lean tools and methods:
1. Standardized Work
Standardized work is the safest, easiest, and most effective way of doing the job that we currently know, but the purpose of standardized work is to provide a basis for improvement on that job. The goal should be to optimize the utilization of people instead of machines because the flexibility of people provides more benefits than machine utilization. The lean system of standardized work is based on human movement. Standardized work provides many benefits such as: process stability, clear stop and start points for each process, organizational learning, audit, and problem solving, employee involvement, poka-yoke, and kaizen (continuous improvement). It also provides a basis for training.
At Toyota, the supervisor determines the components of standardized work, but at most other companies, this determination is usually made by the Industrial Engineering staff. Toyota has set it up this way because of their belief that the supervisor has a better knowledge of the performance of workers. Toyota’s model for elements of standard operations is depicted in Figure 1.
|Figure 1: Elements of standard operations (Source: Toyota Production System: An Integrated Approach in Just in time, Nocross, GA: Engineering and Management Process)|
To use standardized work, a process must be stable without continuous line stoppages and slowdowns. Lean activities support stability. For example, 5s and TPM discussed in earlier sections support machine stability and safety. Standardized work involves three elements, which are the baseline against which any given process can be accessed: takt time, work sequence, and in-process stock. Standardized work is relayed to the operators through standard operations sheets and charts that define standard work.
2. Takt Time and Cycle Time
Takt time or cycle time is the time needed to manufacture one unit of a product to customer demand, measured as the elapsed time between the completion of one unit and the completion of the next. The word is German describes a stroke in beating 34 time. The takt time reveals the demand frequency, or how frequently a product needs to be produced, tact time is calculated as follows:
Takt time =
Daily amount of the product required by customer
This calculation enables understanding of production briefly. For example, if takt time is 1 minute, we should see a product moving past every minute. This understanding allows for quick countermeasures to get the line moving properly again.
3. Work Sequence
The work sequence is the standard operations routine or the order in which the work is done in each process and represents the current best way known to accomplish the task. At Toyota, pictures and drawings depicted how to do the job right with such information as proper posture, how the hands and feet should move, how to hold tools, and critical quality and safety issues.
4. In-Process Stock
In-process stock or standard quantity of work in process is the minimum number of unfinished work pieces required for an operator to complete the process. Work cannot progress without this certain number of pieces on hand. The standard quantity held should be kept as small as possible because this will reduce holding costs as well provide a visual control for checking product quality because defects are more evident.
5. Production Capacity Chart
This chart determines the capacity of machines in a process. Production capacity for a given machine is calculated by the following formula:
Production capacity =
Process time + (Set up time/Parts produced between setup)
Kanban is the Japanese word for card or communication. Kanban as applied to lean manufacturing is a stocking technique using containers, cards, and electronic signals to make production systems respond to real needs and not predictions and forecasts. A kanban is a major component of JIT production. Three types of kanbans are mainly used: withdrawal kanban, production ordering kanban, and supplier kanban.
A withdrawal kanban specifies the kind and quantity of a product in which the subsequent process should withdraw from the preceding process. A production ordering kanban, sometimes called in-process or production kanban, specifies the kind and quantity of a product in which the preceding process must produce. A supplier kanban or subcontractor kanban is used for making withdrawals from a vendor like a part or materials supplier. The supplier kanban includes instructions, which request the delivery of the supplier is product. Figure 2. provides a visual depiction of the kanban pull system.
|Figure 2. Kanban Pull system (Source: Implementing a mixed model kanban system: The lean replenishment technique for pull production, Portland, OR: Productivity Press)|
In order to achieve JIT production, Toyota specifies certain rules in regard to the use of kanbans that must be followed.
👉Rule 1: The subsequent process should withdraw the necessary products from the preceding process in the necessary quantity at the necessary point in time.
- Any withdrawal without a kanban is prohibited
- Any withdrawal greater than the number of kanbans is prohibited
- A kanban should always be attached to a product
👉Rule 2: The process should produce its products in the quantities withdrawn by the subsequent process
👉Rule 3: Defective products should never be convened to the subsequent process
👉Rule 4: The number of kanbans should be minimized.
👉Rule 5: Kanbans should be used to adapt to small fluctuations in demand
In order to determine the number of kanbans needed for any given process, first, a demand analysis and a capacity analysis must be conducted. Demand analysis determines the current daily demand for each process, which can be done using historical order patterns but ideally with current booked orders. Capacity analysis determines the actual capacity for the particular product. This information is used for the calculation of the actual number of kanbans required by the system.
Number of Kanbans =
Daily Demand is the current quantity level of daily demand for a component. This number must be recalculated often as demand varies over time. Order frequency represents the frequency at which the consuming process will place orders to the supplying process for a component. This number is expressed in days. Lead time is an estimate of how long the consuming process will need to wait for a product once replenishment has been authorized. Safety time is allotted to compensate for the impact of waste on the supplying process.
This number is also expressed in days. Container quantity is a standardized number of units of each product that a container will hold. Of all the elements of the kanban equation, the container size has the most freedom for change.
Another calculation, which is needed for kanbans, is the determination of the run line. The run line is the number of kanbans that need to accumulate in order for the production of that component to be authorized. Run line is calculated as follows:
Run line =
After the number of kanbans and the run line for each item has been determined, the maximum and average amount of inventory can be calculated as well as the production lot size for each item.
- Maximum Inventory = Number of kanbans * Container Quantity
- Average Inventory = Daily Demand (1/2 Order Frequency + Safety Time)
- Lot Size = Run Line Value * Container quantity
In order to obtain these numbers used to determine the implementation strategy for kanbans, Vatalaro & Taylor suggest conducting a value stream mapping exercise.
A ‘supermarket’ is a kanban stock point. Like an actual supermarket, a small inventory is available for one or more downstream customers inside a process who come to the supermarket to pick out what they need. The upstream work center then replenishes stocks as required. Supermarkets are used when a one piece or continuous flow is impractical, and the upstream process must operate in batch mode. The ‘supermarket’ reduces overproduction and limits total inventory.
The term kaizen is often mentioned in the application of lean manufacturing. It simply means, “Change for the good of all”, in Japanese and is used as an improvement tool. Kaizen is the starting point for all lean initiatives. Kaizen is a team approach to quickly tear down and rebuild a process layout to function more efficiently. Quality in Toyota’s just in time manufacturing system was based on the kaizen continuous improvement concept.
This approach is used to create trial and error experiences in eliminating waste and simplifying processes, and this approach is repeated over and over again to continuously look for problems and solutions. A Kaizen Blitz is a term used to describe when a process is quickly changed to eliminate activities that have no value.
9. Just-in-Time Production
Just in Time (JIT) production means producing the right item, at the right time, and in the right quantity. Anything else is muda or waste. JIT consists of many other lean tools such as: kanban (card or signal), heijunka (production leveling), SMED or quick machine changeovers, visual management, and having a stable process which is a benefit of many different lean tools such as 5s, TPM, and standardized work.
10. Jidoka or Root Cause Analysis
Jidoka is a Japanese word comprised of three Chinese characters, ji-do-ka. The first, “ji” is the worker. If there is something wrong or a defect, the worker must stop the line. “Do” refers to the motion to stop the line and the “ka” means action. Taken all together jidoka is defined by Toyota “automation with a human mind.” This implies that workers and machines have the intelligence to identify errors and take quick countermeasures for correction.
The goal of jidoka is to prevent defects. The first use of jidoka was in the textile industry in 1902 when Sakichi Toyada, the founder of Toyota, invented a loom that would stop automatically if any threads snapped. This invention allowed for the creation of automated looms where a single operator could handle many looms at a time.
This new idea also introduced the concept that it was all right to stop production in order to find out the root cause of a defect. Shigeo Shingo developed and extended the jidoka concept, which is in contrast to W. Edwards Deming statistical process control (SPC). The difference is that SPC shows how many defects will be produced, but jidoka’s goal is to prevent defects through 100 percent inspections. To achieve this goal, Shingo developed the concept of poka-yoke.
11. Production Leveling, Smoothing or Heijunka
Heijunka or Production Smoothing is Toyota’s means for adapting production to variable demand by distributing the production volume and mix evenly over time. Production leveling also determines the schedule of personnel, equipment, and materials. The goal is to have as little quantity variance in the production line as possible. At Toyota, there are two Phases of the leveling process: smoothing the total production quantity and the smoothing of every model’s production quantity. The goal is to produce the same amount of products every period.
12. SMED or Quick Machine Changeover
Single Minute Exchange of Dies (SMED) is a series of techniques developed by Shigeo Shingo for reduction in production changeover time to less than ten minutes. ‘One-touch setup’ applies to a changeover taking less than a minute and ‘zero set-up’ are changeovers that happen instantaneous. Shingo has compiled this methodology into his book entitled “A Revolution in Manufacturing: the SMED System”.
13. Total Productive Maintenance
Total Productive Maintenance (TPM) is another component of visual management, which works especially well with the 5s organizational system. As discussed earlier, one pillar of 5s is shine in which cleaning is used as a form of inspection. The goal of this is to eventually train the operators to look after the equipment in their workstation. Total productive maintenance assigns basic maintenance work such as: inspection, cleaning, lubricating, tightening, etc., to the operator.
This frees up the technicians or maintenance team for productive maintenance, which includes higher value-added activities such as: equipment improvement and overhauls, training, etc. Just as in safety the target is zero incidences, in TPM the target is zero breakdowns. The key measure of TPM is machine effectiveness, which is availability, performance efficiency, and overall equipment effectiveness (OEE).
- Availability = (loading time1 – down time)/ loading time
- Performance efficiency = (net operating time – lost time) / net operating time
- OEE = availability * performance efficiency * quality rate
Accurate data is essential. It is not time wasted to measure and record machine performance. Accurate equipment records are essential in order to identify potential problems.
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