Chapter 8. Quality Maintenance. Part 3

Oct

2017

Posted in Instructor Training, Quality No comments

8. Quality Maintenance in Manual Work Processes (a Case Study)

This case study looks at how QM can be developed in an assembly process, and it comes from a company that makes automatic transmission systems for cars. Figure 8.21 shows how the company defined the quality assurance rate in its assembly process.

8.1 Step 1 : Identify Existing Situation

(1) Investigate defect situation

Figure 8.22 relates to one of the assembly lines for front-engine front-wheel-drive transmissions, and shows the level of defects created in that line and carried over to subsequent processes. The company was relying on pre-shipment performance inspections to prevent these defects from being passed on to the end customer.

The firm therefore instigated a company-wide QA accreditation scheme to reward excellence in the field of quality. Their aim was to build quality into the product through their processes, and the initiative was very successful. Here, we look in detail at how this was applied to the assembly line, and what results it achieved.

(2) Create Process Quality Assurance Rate Evaluation Table

To develop their programme for recognising and rewarding excellence, the company drew up an evaluation table based on forecasting the occurrence of defects and assessing the QA rate of the assembly process. Firstly, each assembly operation was listed, with the actual and predicted deficiencies for that process, and the quality assurance (checking) procedures used (see Table 8.10).

8.2 Step 2 : Restore

(1) Evaluate QA levels for each process and calculate process QA rate

The company took the Process Quality Assurance Rate Evaluation Table, or ‘QA Matrix’, as they called it, and noted down what checks were used and what level of assurance was established for each of the deficiencies (QA items). They then made an overall judgement about each item and what assurance rate it had actually achieved (see Table 8.10) This process of investigation revealed a process QA rate of 87.2%.

(2) Ensure the rules are followed and assess the results

The QA figure of 87.2% included points that were not being implemented fully, and the company decided at this stage to re-educate the workers about the ideal conditions, so that they would really take them on board. Although this reduced quality fluctuations, the QA level was still low in many cases, so the company embarked on an improvement cycle while sustaining the current situation.

8.3 Step 3 : Analyse Causes

(1) Investigate current work

The company undertook a detailed investigation of the work procedures and methods currently used, looking at the areas where the assurance level was not good enough. In doing this, they took account of the importance of each of the transmission system’s functions.

(2) Analyse correlation between assurance levels and defects

By studying the likelihood of a defect occurring in each of the work procedures, and the assurance level for each work operation, the team were able to identify the precise mechanisms behind defects. In practical terms, what they did was to work with the operators to investigate the situations in which defects arose, and then analyse the differences between the standard work procedures and the work actually carried out. This allowed them to discover exactly why conditions were not being observed. (See Figure 8.23).

8.4 Step 4 : Eradicate Causes

(1) Propose and implement improvements

Having established the potential defects and the mechanisms behind them, the company then devised improvement proposals aimed at nipping these in the bud. With help from technical staff and maintenance personnel, a total of twenty improvements were implemented on the line. The key aim was to find improvements that would minimise the number of operations relying on workers’ judgement and powers of concentration and make it impossible for them to carry out their work other than in the standard manner. Figure 8.23 (1) and (2) illustrates specific examples of some of these improvements.

Figure 8.23 (1) Improving Operator-Dependent Assurance Modes

Figure 8.23 (2) Improving Operator-Dependent Assurance Modes

(2) Check results

These improvements helped to raise the quality assurance rate for the assembly process to 95.3%, meaning that fewer potential defects were passed on to later processes (see Figure 8.24). The system had been shown to be effective – by identifying potential defects and nipping them in the bud, the company was able to improve the QA rate of the process.

Figure 8.24 Confirmation of Results

8.5 Step 5 : Establish Conditions

(1) Revise Quality Assurance Rate Evaluation Table

The QA Rate Evaluation Table was revised each time the team completed a particular improvement.

(2) Revise standards

Making improvements meant changing the operational benchmarks, and therefore the team revised all of the task guidelines and other work standards appropriately. These changes affected, for example, the order in which components were assembled, the way components were arranged, and the way jigs and tools were used.

8.6 Step 6 : Maintain Conditions

The company wanted to make sure that the new situation was faithfully sustained after the accreditation for excellence had been achieved, so they instituted a system of daily checks to ensure that the employees were maintaining the equipment and work area in the proper condition, and organising and observing the job site control forms and standards properly. They also wanted to confirm that the work standards had been established and were being obeyed, and that if any defects occurred, they would be dealt with painstakingly, and reliably prevented from happening again.

8.7 Step 7 : Improve Conditions

Factors such as the arrangement of the components, the difficulties of using a particular jig or tool, or the inclusion of strenuous operations that require twisting or stretching, may not directly cause defects, but they can be a contributing factor.

Therefore, after restoring the situation at Step 2, Company B sought to improve difficult tasks to make them easier to carry out correctly. In conjunction with this, they instituted tagging and detagging of such tasks as part of their Autonomous Maintenance programme.

9. P-M Analysis in Eight Easy Steps

9.1 Step 1 : Clarify Phenomenon

To identify exactly what a phenomenon consists of, we cannot rely on simply stratifying the data in a Pareto chart. We also need to make a careful study of the genbutsu (the actual objects and materials) that we have homed in on. In the example in Figure 8.25, we should observe, for instance, that there are too many process defects, that most of these are coil defects, and that voltage resistance defects are the principal cause of these coil defects. Ultimately, our close study of the genbutsu relating to the defect shows us that the windings are too thick and touch the outer case, causing the voltage resistance to fall. This is the only way we can pinpoint the problem to be solved.

Figure 8.25 Using Pareto-diagram Stratification to Observe the Genbutsu

9.2 Step 2 : Do Physical Analysis of Phenomenon

Our physical analysis concentrates on the points of interaction between the product and the process, and it requires a full understanding of the principles and parameters involved (see Figure 8.26). We express our results in the ‘ABCD’ format, where A stands for the equipment, B stands for the product, C stands for the physical quantity involved, and D stands for the deviation in this physical quantity that brings about the phenomenon. In other words, we try to answer the question, ‘What physical quantity (C) connecting the equipment (A) and the product (B) deviates in what way (D) in order to produce the phenomenon?’

Figure 8.27 shows a physical analysis of the ‘thick windings’ phenomenon.

Figure 8.26 Understanding the Contact point and the Principles of Processing

Figure 8.27 Physical Analysis Using ABCD Diagram

9.3 Step 3 : Identify Contributing Conditions

We must identify the conditions in each functional part (unit) of the machinery that bring about the phenomenon. This requires us to draw a simple sketch of the equipment mechanisms, like that in Figure 8.28, from which we can work out the function of each unit. Any function that affects the physical quantity (C) is a contributing condition (see Table 8.11).

Figure 8.28 Simple Mechanism Diagram (Compact Turntable Winder)

Table 8.11 Unit Function Investigation Table

9.4 Step 4 : Study 4-M Correlations

In Step 3, we identified the contributing conditions at the unit level. Here, we look for correlation between these conditions and the 4-Ms, by picking out every conceivable cause at the sub-assembly (primary) level and component (secondary) level (see Table 8.12).

Table 8.12 P-M Analysis Table

9.5 Step 5 : Revise Optimal Conditions (Standards)

Determining the optimal conditions means setting standard values we can use to judge whether a state is normal or abnormal. Chronic problems often arise at the border between the normal and abnormal zones, so we must pay particular attention to this ‘grey area’.

9.6 Step 6 : Investigate Measurement Methods

When considering measurement methods, it is important to start from the standards set at the unit level. We should choose methods that will help us to consolidate the checks required to sustain the improvements.