1. The Need for Quality Maintenance
The only way we can achieve our goal of zero quality defects is to discard the reactive, ‘shutting the stable door after the horse has bolted’ type of approach, where we only examine quality after it has already been built into the product. Instead, we must set our production processes up in such a way that they cannot produce any quality defects and then ensure that the processes always operate under those conditions. In Quality Maintenance, this is called ‘establishing and maintaining zero-defect conditions’. In other words, to build quality into the product by means of the process, as quality experts have been urging us to do for decades, we must switch from controlling the outputs of our processes (the quality of our products) to controlling the causes of quality (the process and equipment conditions themselves).
If we really want to create a system that ensures perfect quality, we must prevent defects from happening at all, rather than just screening them out after they have happened. To do this, we must build a system for controlling the causes of quality – in other words, we must establish and maintain the ‘zero-defect conditions’ mentioned earlier. ‘Controlling quality through its causes’ is the essence of Quality Maintenance.
2. What is Quality Maintenance?
Quality Maintenance is based on the idea of maintaining perfect equipment in order to maintain perfect quality (i.e. 100% good products). Essentially, it involves:
- Establishing zero-defect conditions in order to create equipment that does not produce any quality defects, and checking and measuring those conditions periodically,
- Preventing quality defects by keeping those conditions within a standard range of values,
- Predicting the possibility of quality defects by monitoring trends in the measured values, and taking preventive action.
(See Table 8.1 and Figure 8.1)
Table 8.1 The Definition of Quality Maintenance
Establish zero-defect conditions in order to create equipment and processes that do not produce any quality defects
Check and measure those conditions periodically
Daily Checks / Periodic Checks
Prevent quality defects by maintaining those conditions within a standard range of values
Preventive Quality Maintenance
Predict the possibility of quality defects by monitoring trends in the measured values
Trend Monitoring / Predictive Maintenance
Take preventive action
Figure 8.1 An Illustration of the Definition of Quality Maintenance
2.1 Establish Zero-Defect Conditions in Order to Create Equipment and Production Processes that Do Not Produce Any Quality Defects
To consistently produce 100% good product, we must specify clearly which conditions relating to the 4 Ms (machinery, materials, men/women, methods) have to be controlled, together with the range of values they must be restricted to.
2.2 Check and Measure Conditions Periodically
If we want to identify potential causes of equipment under-performance, and rectify them before an unwanted result actually occurs, then we must check the specified conditions routinely (either daily or at longer intervals) to confirm that they remain within their permitted ranges.
2.3 Prevent Quality Defects by Keeping Conditions within Standard Ranges
To avoid defects, we need to take preventive action, such as restoring basic operating conditions to prevent accelerated decline of equipment functions.
2.4 Monitor Trends and Predict Possible Occurrence of Quality Defects
A ‘zero-defect condition’ is defined as a condition that will not cause quality defects if kept within its standard range. We therefore need to understand the mechanisms of equipment deterioration, and estimate the rate at which our equipment deteriorates, so that we can predict when a condition is likely to exceed its standard range.
2.5 Take Preventive Action
Losses arising from equipment malfunction must be prevented before they occur. This requires us to halt the equipment and take corrective action whenever the occurrence of a defect has been predicted, or restore functions systematically during periodic servicing.
3. The Basic Philosophy of Quality Maintenance
To prevent quality defects due to equipment or process conditions, we have to establish the conditions under which the equipment or process will not produce any defects. To do this, we must fully co-ordinate our quality assurance activities and equipment management activities, and identify how the product’s quality characteristics are affected by materials, methods and equipment precision. Establishing zero-defect conditions involves identifying the exact group of causes behind any possible defects, and then setting ranges for the material conditions, method conditions and equipment precision, so as to ensure 100% good product. The central philosophy of Quality Maintenance is to sustain and control the established conditions, and consequently achieve zero defects, and this approach relies on having operators who are fully conversant with their equipment and have been trained in Autonomous Maintenance and other relevant skills (see Figure 8.2 ).
Achieving perfect quality requires an important shift in attitude – from the conventional approach, where defects are only discovered when the product is inspected, and action is taken after the event, to an approach in which each condition affecting quality is measured periodically, and preventive action is taken before that condition exceeds its permitted range.
This chapter looks at how to establish, standardise and improve zero-defect conditions for materials, machinery and methods.
Figure 8.2 The Basic Approach to Quality Maintenance
3.1 The 4 Ms – the Determinants of Quality
The key to ensuring quality (i.e. production standards) on the shop floor is achieving optimal conditions for the 4 Ms (Men/Women, Machinery, Materials, Methods) (see Table 8.2 The 4-M Conditions – The Determinants of Quality).
Improving quality means establishing optimal conditions for the 4 Ms, raising the quality assurance capability (Cp) of the process until it stabilises at a high level, and setting work standards and inspection standards to maintain that capability. Sustaining quality, on the other hand, means faithfully applying the work standards and inspection standards that maintain the optimum conditions we have set. Figure 8.3 illustrates this relationship.
Figure 8.3 The 4 Ms – The Determinants of Quality
Table 8.2 The 4-M Conditions – The Determinants of Quality
3.2 Raising the Quality Assurance Capability (Cp)
* Raising the Cp value (see Table 8.3) means minimising any variations in the quality characteristics.
* An effective approach for eliminating variations is one based on eradication of minor imperfections in the machinery, jigs and tools, measuring equipment, materials and work environment.
Calculating the process capability index
If a standard has upper and lower limits, the process capability index, Cp (or Cpk), is given by the formula
Table 8.3 Assessing Quality Levels
4 Prerequisites for Promoting Quality Maintenance
If we want to build quality into the product by means of the equipment, then the following prerequisites must be satisfied.
4.1 Eliminate Forced Deterioration, to Achieve Stable Equipment Subject to Natural Deterioration Only
4.2 Ensure that All Staff, from Managers through to Operators, Are Equipment Experts
(1) Eliminating forced deterioration
If equipment is in a state of forced deterioration, any attempts to control its precision will be frustrated, because the components that determine that precision will have short, highly-variable lifetimes, making checking very difficult. This makes it essential to eliminate forced deterioration through Autonomous Maintenance, thereby extending and stabilising component lifetimes.
(2) Developing an equipment-competent workforce
Both managers and the operators who actually work with machinery must have excellent equipment skills, as well as the ability to sustain the required conditions. To achieve zero defects, production must be carried out in a situation where people and machines are in perfect balance. For this, operators must be trained so that they can immediately spot any abnormality in the causal system which they think might lead to a defect, and know how to deal with that abnormality promptly and accurately.
To ensure that the people in the Operations Department have this level of competence, it is vital to implement Autonomous Maintenance, Focused Improvements, machine diagnosis techniques, skills training, and other related initiatives.
Promoting Quality Maintenance also involves some other important issues: (3) Activities for achieving zero equipment failures
The most important aspect of Quality Maintenance is eliminating sudden or gradual equipment failures. A particularly constructive approach is to identify the exact correlations between product quality characteristics and equipment conditions (modules and individual components), and then develop and apply diagnostic techniques that expose any deterioration in these conditions.
(4) MP design of new products and new machinery
It is important to establish a system that allows defect-free products and equipment to be created right from the initial design stages. ‘MP design’ (maintenance prevention design) is discussed in the chapter on Early Management.
Figure 8.4 shows the relationship between Quality Maintenance and the other TPM pillars.
5. Developing Quality Maintenance (A 10-Step Procedure)
Figure 8.5 illustrates the overall process by which Quality Maintenance is carried out, while Table 8.4 shows a 10-step procedure for developing Quality Maintenance, based on the process shown in Figure 8.5. This procedure requires us to verify the defect phenomena; investigate the processes that give rise to defects; investigate and analyse the conditions relating to the 4 Ms; plan action to correct lapses in these conditions and restore the situation; analyse and assess situations where the conditions required to achieve good products are unclear; carry out quality improvements to fix deficiencies in these conditions; establish zero-defect conditions for the 4 Ms; consolidate the checking methods to make the conditions easy to maintain and monitor; establish standard values for checks; revise the standards relating to the 4 Ms; and monitor trends. Through these QM activities, we can achieve and sustain zero quality defects (see Figure 8.5).
To achieve this, we must develop the abilities of our people (the first of the 4 Ms) through Autonomous Maintenance and skills training.
In developing Quality Maintenance, wider-ranging and more technically demanding issues should be handled by a project team led by a unit manager, whilst comparatively straightforward issues can be dealt with principally by line team leaders, who will establish zero-defect conditions and implement Autonomous Maintenance activities focusing on sustainment.
Each step of the development procedure is described below with reference to a real-life example of a PVC resin drying process. Despite every effort to make improvements, quality defects do sometimes still arise, and Quality Maintenance is a powerful approach for eliminating them.
Figure 8.5 Carrying Out Quality Maintenance
Table 8.4 The Development of Quality Maintenance (a 10-step procedure)
5.1 Step 1 Verify the Existing Situation
In this step, the existing situation is analysed to establish baseline values and targets for the Quality Maintenance project and plan for its smooth implementation.
The specifications for the product in question are confirmed, and all quality characteristics and defect modes that may affect these specifications are identified. A flow chart for the processes that determine the quality of the product is then created, and the defect situation and phenomena are studied and stratified. The financial costs of the defects themselves, together with the associated complaints and inspections, should be calculated and made known to everyone concerned.
(1) Confirm quality standards and characteristics
On the basis of the product specifications, product characteristics, production standards and inspection standards, we identify the quality characteristics that must be maintained.
(2) Create flow diagram of individual processes that determine product quality
First of all, a flow chart of the individual processes that determine product quality should be created (see upper part of Figure 8.6).
In addition, the equipment systems’ mechanisms, functions, processing principles, sequences, etc. should be identified, and the control items (standards and methods) that have been established for maintaining quality in each individual process should be surveyed, and entered in the process control flowchart (see lower part of Figure 8.6).
(3) Investigate defect situation and phenomena, and stratify
Here, we seek to understand the circumstances under which a defect occurred during processing. We stratify the defect phenomena and identify the particular processes in which they occurred.
Figure 8.6 Example of Process Flow Diagram and Control Items
5.2 Step 2 Investigate the Processes where Defects Occur
In this step, we draw up a QA matrix (see Figure 8.7) that analyses the relationships between the individual processes and defect modes identified in Step 1. This allows us to identify the processes in which defects that impair quality occur, and understand what kinds of defect will arise if certain equipment or method conditions are not observed. At the same time, we analyse past records of defect modes and their seriousness.
Figure 8.7 Example of QA Matrix
QA Matrix For Ordinary PVC Drying Process
5.3 Step 3 Investigate and Analyse 4-M Conditions
Here, we study the drawings, standards, and actual equipment and materials with the aim of identifying the 4-M conditions (relating to materials, machinery, methods, and checks) under which the defect modes in each of the individual steps in the QA matrix would not occur. As Table 8.5 illustrates, we identify the conditions that must be observed in order for no defects to be produced, and work out where they are deficient, by assessing whether there are clear standards for them, whether these standards are being observed, and whether any necessary standards have been omitted. It is generally found that required conditions and/or standards have not been specified at all, or are unclear and are left to the subjective judgement of the operators who are supposed to implement them.
Table 8.5 Example of QA Matrix. 4-M analyses of defect modes of medium process feeds indicate that many instances of X, Y and Z occur in machinery and methods (people), so solutions to abnormalities are needed in these areas.
5.4 Step 4 Plan Action to Correct Deficiencies
Here, we identify any lapses in the 4-M conditions for each process as found in Step 3, and list them in a Deficiencies Chart. We then work out how to correct these deficiencies (see Table 8.6). If the actions required to correct a particular deficiency are immediately obvious, then someone can be assigned to do them right away. If not, however, we go to Step 5 and work out how to proceed.
Table 8.6 Chart of Countermeasures for Deficiencies in the 4 Ms
List of Abnormalities in Ordinary PVC Drying Process
FMEA of Abnormalities in Ordinary PVC Drying Process
5.5 Step 5 Analyse Situations where the Conditions for Building in Quality Are Unclear
In Step 5, we employ methods such as P-M Analysis, FMEA and design of experiments, to find ways of correcting deficiencies listed in the Deficiencies Chart at Step 4 that did not seem to have an obvious solution (where the conditions required for building in quality are hard to specify). Figure 8.8 shows an example of P-M Analysis.
Figure 8.8 Example of P-M Analysis of Ordinary PVC Drying Process
5.6 Step 6 Eliminate Flaws in 4-M Conditions
In this step, we actually implement the improvements proposed in Step 5. The results are then assessed at prescribed intervals to ensure that they satisfy the product’s designed quality characteristics (see Figure 8.9).
Figure 8.9 Eliminating Flaws in 4-M Conditions through Kaizen
5.7 Step 7 Finalise 4-M Conditions
On completing Step 6, we revise the conditions and standards identified in Step 3 (Investigate and analyse 4-M conditions) in order to establish zero-defect 4-M conditions (see Figure 8.10)
Figure 8.10 Finalising 4-M Conditions
5.8 Step 8 Consolidate Checking Methods
Each of the 4-M conditions established in Step 7 must be controlled in order to ensure 100% quality. However, they are usually very numerous, and the time required to check all of them can strain available resources. To make them manageable, they must be fixed (made so that they cannot change with time and do not need to be checked so often) and consolidated (grouped together to reduce their number) in accordance with the procedure shown in Figure 8.11.
(1) Consolidating static precision checks
Starting at the top with the quality defect phenomenon, we must identify the entire causal system, branching right down to the basic underlying causes, and standardise the condition checks as close to the top level as possible, bearing in mind the nature of the equipment and whatever easy measurement procedures are currently available.
(2) Consolidating dynamic precision checks
Measuring vibration is an easy and effective way of checking many of the conditions that lead to surface defects such as imperfect finishing, and it allows us to integrate a large number of checks. (See Table 8.7)
Figure 8.11 Basic Approach to Consolidating Checks
Table 8.7 Example of Check Consolidation in Chemical Plant
5.9 Step 9 Determine Standard Values for Checks
To ensure that the checks can always be carried out easily and reliably, it is important to use a Quality Maintenance Matrix (a QM Matrix) to detail the relationships between the product’s quality characteristics and the standard precision values for each part of the machinery. This matrix, sometimes also called a Quality Check matrix, is an essential tool for making certain that everyone understands what checks are required, and when, where, how, by whom and why they should be carried out. Of course, we must also introduce improvements at this stage to increase the reliability of checks, simplify them and reduce the number of people required (see Figure 8.12).
Figure 8.12 Example of QM Matrix
Ordinary PVC Drying Process Quality Maintenance Matrix
Chapter 8. Quality Maintenance. Part 2
Develop production plant personnel with the highest skills and competency standard that al…