The main cell and process performance indicators are linked to initial takt-time calculation. The term takt-time comes from the GermanTaktzeit, meaning rhythm or time marked by a metronome. Takt-time sets the sales rhythm, and proves to be the best tool to achieve “tight/tense” flow from customer demand to delivery. To reduce waste and WIP, cells and processes need to “pulse” at this frequency.
Obviously, to do so, cells must be redesigned, balanced and improved where inefficient, according to the methods discussed in Chap.6. Calculating takt-time is very simple and can be done using the following formula:
Takt-time¼ time available daily=daily product demand
Example of Takt-Time Calculation. A customer requires 950 products of a certain code every day. The company only has one shift, thus takt-time should be calculated as follows:
Takt-time¼ 8 hours=950 products ¼ 0:00842 hours In this case takt-time should be converted into seconds:
Takt-time¼ 0:00842 3600 ¼ 30:3 seconds
Thus the final cell, the one in direct contact with delivery, cannot have takt-time less than 30.3 s, or rather, the process has to supply an output of one code of that certain product every 30.3 s. The final cell receives the products from another upstream cell, which produces two components assembled in the final cell; the takt-time for each of these components is around 15.15 s.
This concept can also be applied to services; for example, a laboratory can set the takt-time for withdrawals. Similarly, the front office of a public authority can set takt-time in terms of customers/citizens attended to per procedure.
Once organization has been improved and waste has been removed during a Kaizen workshop, takt-time can be set and workers need to receive training to raise awareness regarding its importance. Management, too, has to change outlook, moving from a vague monthly image towards a detailed hourly focus on all processes.
Product/process flow should be like a raging river that flows towards the sea (i.e.
the customer) without encountering any type of obstacle. If any obstacles turn up unexpectedly, they need to be removed immediately before they cause a flood (and a lot of damage).
Nowadays there are many relatively cheap solutions to automatically monitor takt-time and signal any variations.
Many companies use the famous Andon lights and sirens in the traffic-light version, which immediately signal if takt-time is not being kept. In the past few 120 7 Lean Metric, Lean Accounting and Value Stream Accounting
years many companies have adopted displays, which are connected to specific software and display continuously updated vital cell information. Figure7.2is a photo of a 46-inch display placed in a central position within the production line of a worldwide car manufacturer.
The display is connected to a computer that communicates with the plant ERP database. The picture in Fig.7.2was taken at the end of a shift and shows a positive daily result.
The monitor also shows an important indicator for both health and safety management and staff motivation:
Number of days without injuries.
If many operations are carried out automatically, members of staff do not need to waste time by recording data; it also permits all data to be saved for future reference. The main benefit remains, however, the speed of data analysis that allows quick action when problems crop up. If computers, software or similar tools are not available, however, it is always best to fall back on good common sense and traditional methods.
A one-piece-flow Kaizen workshop team managed to completely transform a cell by calculating takt-time, and by training all workers to strive to achieve targets and immediately react when defects occur that could shift the process away from the target. The team, however, did not have a computer with a large monitor and thus preferred to use the board shown in Fig.7.3as a temporary solution.
Every day a staff member appointed as the “Heijunka planner” sets a takt-time target for each hour (PROG. column) based on production scheduling and Heijunka leveling; in the photo the takt-time target for the whole day is 35. The third column contains the amount of acceptable products produced; if any products are deemed unacceptable, then the amount needs to be recorded in the sixth column together with a description of the problem.
Fig. 7.2 “Andon” automatic daily cell performance control display
This method could probably be improved as time passes, however workers consider it to be the most simple and intuitive system to control takt-time and defects, and indicators show great improvement in performance. Because the system works quite well, the team should carefully evaluate whether buying a new monitor, computer and so on is really necessary. The most important aspect that needs to be considered is whether a monitor will provide easier access for workers. The Heijunka planner (who has the same function as the milkman, see Chap.6) requires 5 min each morning to prepare the board and 5 min at the end of the day to record the new data: a small investment that definitely pays off.
Other important indicators are also used within the cell, and they too require prompt collection and analysis.First Time Through (FTT) or First Time Yield (FTY) is another important indicator used by Lean organizations. The indicators can be used to measure product quality and efficiency.
FTT or FTY¼ ðTotal units processeddefectsÞ=Total units processed FTY measures the percentage of products processed that do not need to be reprocessed, fixed, and so on. If a cell produces, for example, 100 products in an hour, and out of these 100 products two are defects, then FTY is 98%. FTY can be calculated for single activities, cells or machines within a process; if a cell has multiple stations, FTT or FTY of the whole cell is equal to the product of the single values.
FTT cell¼ FTT1 FTT2 FTT3 FTT4 ¼ 0:80 0:98 0:99 1
¼ 0:78 ð78%Þ Fig. 7.3 Heijunka board
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Obviously FTT does not need to be calculated for every small process; it should only be calculated for the main processes involved. An accurate analysis of the Value Stream Map can help identify where this indicator should be applied.
The data collected through these indicators can be recorded on data sheets hour-per-hour or day-by-day. The boards that record the progress of process performance should also contain graphs to help workers understand FTT progress and the importance of process quality.
Many Lean organizations also use theMorning Market method to quickly solve problems linked to defects. The Morning Market was introduced by the Japanese consultant Masaaki Imai in the bookGemba Kaizen. The concept involves solving problems in the morning before commencing production, when problems are still
“fresh” like the fish caught the day before. The Japanese refer to the Morning Market asAsaichi, Japanese for the early morning fish market.
The Morning Market basically consists of a table placed conveniently within the cell, where the following data is recorded every morning:
• Date of when the defect was detected;
• Process involved and process supervisor;
• Product/service number/code;
• Brief description of the problem;
• Containment action;
• Defect type, for example a¼ known cause, b ¼ unknown cause or repetitive defect;
• Any action taken;
• Root cause, if known;
• Closure date.
The following Table7.1is an example of a Morning Market or Asaichi board.
The table should be dynamic, where defects can be left open for up to several days.
The table is filled in every morning and allows the Asaichi team (3–4 process members) to:
• Quickly detect defects;
• Connect to other processes thanks to the actions listed in column number 7;
• Keep causes in mind and test the efficiency of solutions;
• Save time in data registration, avoiding cumbersome paperwork like ISO 9001.
Table 7.1 Example of Asaichi board content Detection 13/10 Lapping P456A Unacceptable 100 %
control
B Problem
solving with UT
OEE is another indicator used by many Lean organizations of excellence. OEE measures the overall performance of machines by combining three types of measurements:
OEE¼ Availability Efficiency Quality where
Availability¼ ðTotal time availabledowntimeÞ=Total time available
¼ Actual working time=total time available
Downtime can be caused by many different problems, including breakdowns, set up, repairing, and adjustments. These causes are among the famous six big equip-ment losses that will be discussed in the following section.
If, for example, the total time available is 8 h (480 min) and total downtime builds up to 80 min, then availability will be calculated as follows: 400/
480¼ 83.33 %.
Efficiency (often referred to as speed) is the production rate of a machine, and every machine is designed to have an ideal production rate. If a machine has, for example, an ideal production rate of 100 products per day but only produces 90, efficiency is 90 %.
Efficiency¼ actual production rate=ideal production rate
To avoid overproducing and creating WIP, the ideal production rate cannot be equal to the maximum production rate the plant can reach. Target production should respect takt-time, and if target production is equal to maximum plant production it will only be of advantage for plant schedule.
Quality calculation is the same as the FTT or FTY calculation that has already been discussed.
Quality¼ FTY ¼ Total acceptable units processed=total quantity processed The indicator measures the percentage of components without defects produced.
OEE is certainly one of the most important indicators for Lean organizations, as many companies that have applied Lean can confirm. Both workers and managers need to be familiar with OEE so that they can then apply corrections and preventive action to keep respecting takt-time and avoid creating waste. OEE is especially important because not only does it give a general indication of plant progress but it also provides information regarding the availability, efficiency and quality of elements. We recommend analyzing these three elements separately, rather than as a whole, for the reasons that will now be explained.
The following Table7.2is an example of data collected for two different product codes that are produced within the same cell.
Code 2 has a higher OEE and excellent machine availability; it has, however, poor performance in terms of defects.
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OEE is often linked to TPM and maintenance that can be carried out by workers (see Chap.6) and can, in this case, be used to measure the benefits obtained thanks to TPM. In addition to the traditional use in TPM, OEE can also be used to respect takt-time. Takt-time for a cell has, for example, been set at 200 s and the cell contains several assembly points and one single plant with a 100-second cycle time.
The plant, however, only has 90 % OEE, and thus slows production frequency down to 90 s (100 s cycle time 90 % OEE). Ideally machines should have an OEE of 100 % but in reality low OEE can cause slower cycle times.