Chapter 4. Focused Improvement. Part 4

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12. Reducing Speed Losses

12.1 Some Common Issues Associated with Speed Losses

(1) Equipment specifications are unclear

In some cases, owing to insufficient thoroughness at the design stage, the equipment specifications themselves are unclear and the equipment is operated too fast, resulting in quality defects and breakdowns. Sometimes the opposite happens, and the equipment is operated slower than necessary. This is particularly common with older equipment or equipment developed in-house. In either case, a thorough review of the rated speeds should be undertaken.

(2) Equipment is not operated at the specified speed, even though it could be

Sometimes, equipment is not operated as fast as it could be because it was slowed down at some time in the past to cope with a spate of quality problems or mechanical difficulties, and has been operated at a lower speed ever since. In some cases, the true causes were not investigated properly, the problems were never solved, and it was concluded that nothing could be done and the situation would simply have to be accepted. Another reason why equipment may have to be operated slower than necessary is that slight defects and forced deterioration have been neglected, despite the fact that the rated speed could easily be achieved by dealing with them. In yet other cases, equipment is not operated at the specified speed because of a problem that could not be solved when it occurred but could easily be solved with today’s level of technology and management.

(3) Not enough effort is made to uncover the causes of reduced speed

In some cases, quality and mechanical problems will gradually worsen as the speed of a machine is increased above its present level, while in other cases, they appear all at once over a certain speed. The causes of such problems are present in the equipment even when it is operating at lower speeds; it is just that they are not apparent at these speeds. When the speed is increased, the causes are magnified, eventually becoming serious enough to create real problems. This means that increasing the speed of a machine is one way of exposing its deficiencies. However, few companies are brave enough to deliberately speed up their machines in order to see what would be revealed. Even when they do so, they rarely make enough effort to track down the causes of the resulting quality defects, breakdowns and other problems. These causes usually fall into one of the following categories:

  • Teething problems left unresolved because of insufficient debugging
  • Mechanical or system faults
  • Problems with day-to-day management
  • Lack of precision

The direct causes of the problems and other associated factors should be identified and steps taken to eliminate them. Doing so will help to raise technical standards

(4) Insufficient attention is paid to ‘air-cutting’ time and idling time

‘Air-cutting’ time is the time wasted when a machine tool is ready for action (cutting, grinding, etc.) but is not actually doing any useful work. In the case of a grinding machine, for example, a work cycle starts with setting the workpiece in position, advancing the grindstone quickly at first, and then advancing it slowly as it approaches the workpiece. The grindstone is cutting air as it is being advanced, and it does not do any real work until the sparks begin to fly.

Idling time is the time lost when a machine has completed one operation and is preparing for the next. This usually lasts for at least 0.5 – 2.0 seconds, so the total time wasted can be very large if the machine goes through a complex cycle of operations. Designers tend to build a lot of idling time into the sequence to provide a margin of safety and ensure that the machine’s timing is correct. Most companies do not pay enough attention to air-cutting and idling time and in many cases do nothing about it, despite the fact that reducing it could bring significant benefits, especially to high- volume factories.

The reason why these losses remain hidden is that air-cutting and idling times are very short in a high-speed machine, and tools and techniques for measuring time losses of under a second are not in widespread use. Nevertheless, companies need to use instruments such as visicorders (oscillograph recorders) to measure these times and make people aware of the losses.

(5) Insufficient attention is paid to actuating and rotating speeds

If the time taken for automated equipment to complete each of its actions is observed, it will often be seen that many of the motions it performs are slow and wasteful. Cutting speeds are also often slower than they need to be. In many cases, this is because some problem occurred in the past, but nothing was done to identify and eliminate its root causes. Instead, the equipment was slowed down to avoid quality defects and has been operated at that speed ever since. In other cases, nothing is done to speed the equipment up because the people managing it do not have enough experience to judge what its optimal speed should be. Since the equipment is working, albeit after a fashion, it can be difficult to determine whether or not its current speed of movement is fast enough, but improvements should be sought in any case, without concluding that the current speed is the best.

Common problems include:

  • Actuating times vary
  • Machines judder while moving
  • Machines are slow because cutting/processing conditions are inadequate
  • Machines are slow because they are old
  • Machines are slow because they are inaccurate
  • Machines are slow because they have been neglected

Problems like this must be taken seriously and solved as a matter of urgency.

(6) Theoretical analysis is weak
People usually try to raise the speed of a machine by increasing it gradually and seeing what happens. Although this method is not wrong, there is a limit to how high the speed can be raised, and it is difficult to proceed without knowing what that limit might be. More analysis needs to be done to determine theoretical speed limits. Even if an entirely accurate answer cannot be obtained, theoretical considerations based on certain assumptions will allow some kind of limit to be set.
In the case of an injection moulding machine, for example, the cooling time is usually the determining factor that controls the overall cycle time. The theoretical minimum cooling time can be calculated (or at least estimated) from the thermal
capacity of the system, taking into consideration the input and output temperatures of
the cooling water, its flow rate, the diameter of the pipes, the layout of the pipework (and the possibility of changing it), the thickness of the die, the material from which the die
is made, and so on. The theoretical value should then be compared with the actual value, and the system should be investigated to see whether any improvements can be made to raise the cooling capacity and bring the cooling time closer to the theoretical minimum.

12.2 Strategies for Reducing Speed Losses

(1) Identify the true cause of the problem

When attempting to raise the speed of a machine, tests should be conducted to see what kind of problems occur in practice. The speed should be raised by about 50% and items such as the following should be checked:

  • How did the Cp value change?
  • Was there any change in the rate of occurrence of each type of quality defect?
  • Were any new types of quality defect generated?
  • How long did the cutting tools last?
  • How much did the equipment vibrate?
  • How many minor stops occurred?
  • Was there any change in the behaviour of the materials handling equipment?
  • How much did the materials handling equipment vibrate?
  • Was there any vibration or change in sound during processing?

Raising the speed of a machine usually magnifies its defects, increasing the rate of occurrence of minor stops and quality problems. These should be analysed and solved in the normal way. In the case of minor stops, this means observing them happening (or, if this is not possible, observing the behaviour of the products) until the causes are identified or at least narrowed down. In the case of quality defects, it means analysing the equipment’s mechanisms and the construction and functions of its components until the specific causes are pinpointed.

(2) Check the effectiveness of the machine’s motions
Figure 4.34 shows one approach to increasing equipment speed. This approach incorporates the following strategies:

  • Reduce the ‘air-cutting’ time
  • Reduce the idling time between motions
  • Increase the machine’s speed of movement
  • If possible, perform motions in parallelWhen attempting to reduce the idling time (the time wasted between motions), the first step is to measure it accurately using an instrument such as a visicorder. Such measurements often show that the idling time is surprisingly long (usually in the order of 1.5 to 2.5 seconds). The challenge is to minimise this without causing mechanical problems. If the overall cycle time is short, a reduction of as little as 0.7 to 1.0 seconds in the idling time will be of great benefit.Increasing the machine’s speed of movement means reducing the time it takes to accomplish those movements. When doing this, the time for which the equipment is actually adding value should be measured. Three examples will be given to illustrate this:
  • Grinding machines: the time interval between the loading of the workpiece and the appearance of the first spark can be monitored with a visicorder to see from the change in power consumption how much of it is value-adding and how much is not.
  • Hydraulic loading and unloading devices: lost time can be calculated by comparing the theoretical value (obtained from the relationship between the weight of the workpiece, the distance moved vertically and horizontally, the diameter of the hydraulic cylinder and the capacity of the pump) with the actual value.
  • Checking the possibility of parallel operations: operations conventionally performed in series should be examined to see whether there is any possibility of performing them in parallel. In many cases, sensors can be installed and the sequencer reprogrammed to allow one operation to start while another is still going on.Regardless of which of the above three approaches is taken, the equipment’s mechanisms, the construction and functions of its parts, its timings, and the cycle line diagram prepared when the equipment was designed should all be examined carefully to determine whether any improvement is feasible. If an improvement is considered possible, it should be tested in practice. Any problems that arise should then be analysed carefully in order to identify and eliminate their root causes. Table 4.21 shows a procedure for reducing speed losses.

13. Reducing Quality Defects

13.1 Some Common Issues Associated with Quality Defects

(1) They are often abandoned after unsuccessful attempts to cure them

The causes of chronic quality defects are notoriously difficult to pin down. As a result, teams often attempt solutions without understanding the true causes of the problem; the situation consequently fails to improve, and the attempt is abandoned. Various excuses are put forward for giving up, such as that it is impossible to find a solution without knowing the cause, or that the equipment concerned is so badly designed that there is no way the problem could be fixed, but generally speaking the following mistakes are made:

1 Taking the wrong approach: When faced with a problem, we always seem to assume that there must be a definite cause, and focus our efforts on finding it. This approach inevitably leads us to make up our minds what the cause must be, or at least to narrow it down to a few possibilities, and concentrate on eliminating those. However, as explained earlier, chronic quality defects are due to multiple causes that differ from occasion to occasion, so tackling only a few of them is not usually very effective. What needs to be done is to eliminate anything that could reasonably be suspected of being implicated in the problem, but this is not usually practised thoroughly enough.

2 Studying the phenomenon from a blinkered technical viewpoint (a mistake often made by qualified engineers): A problem common to many companies is that when expert engineers investigate a phenomenon, they tend to restrict their thinking to their particular area of expertise. This leads them to complicate the issue; they fail to find a solution to what is really a relatively simple problem, and eventually abandon the attempt.

Some quality defect problems can be solved by the application of a specific engineering technology, but in most cases, this is insufficient. The way in which the machine is treated on a daily basis by the people responsible for operating and maintaining it must also be included in the equation. A single problem phenomenon is governed by a multitude of factors on the production floor, any of which could change at any time and give rise to the problem. The rate of quality defects after a changeover, for example, will depend on how the changeover is done, how the adjustments are made, how the parts are installed, how the processing conditions and clearances are set, and so forth. Engineers need to develop their ability to observe the equipment itself, the way in which the shop floor works, the way in which changeovers and adjustments are carried out, and so on. They need to be able to spot the variables in the work and the equipment.

When much time and effort has been spent on introducing a specific technology but the results are inconsistent or not as good as expected, the problem usually lies at the interface between the technology and the way in which it is handled from day to day.

Many more problems could be solved if tackled by a combination of the shop-floor and engineering approaches, rather than by the engineering approach alone.

(2) Possible causes are not identified or analysed adequately

Another common problem when tackling quality defects is lack of skill at identifying the relevant factors (the control points) and lack of skill at analysing them once they have been identified. The first of these is due to inadequate observation and analysis of the problem phenomenon, mis-application of analytical techniques, making deductions based solely on past experience, or, as discussed earlier, overlooking relevant factors by jumping to conclusions about the causes. The second is due to managers’ and engineers’ inability to see exactly what is going on with the equipment and the process; that is, their inability to recognise deficiencies (deviations from the optimal) and understand their effect on the problem phenomenon. Because of this inability, deficiencies or their warning signs go unnoticed, and nothing is done about them because they remain undefined. The result is uncontrolled chronic defects. The only way to improve a situation like this is to do a comprehensive review and deep analysis of all possible relevant factors.

13.2 Sporadic Defects and Chronic Defects

A sporadic defect is a sudden undesirable change that requires some action (such as replacing a worn jig) to restore the status quo, while a chronic defect is a prolonged undesirable situation that requires some action to alter the status quo. In the first case, we are trying to maintain the existing situation, while in the second we are trying to break out of it. Being different in nature, sporadic and chronic defects require different approaches: the former is the joint responsibility of operators and managers and requires restorative action with the aim of getting the results back to what they were before, through comparison with existing standards and by checking control points; the latter is a management responsibility and requires ameliorative action to break through and change the situation with the aim of achieving new policy-led targets, by revising the standards and control points and setting new ones where necessary.

Sporadic defects occur when existing control points and causal factors change suddenly; they are resolved by restoring the variable factors to their original state. Chronic defects, on the other hand are not reduced by complying with the original control points or causal factors; completely different ones have to be established and adhered to. This requires breakthrough thinking. Existing causal factors must be controlled much more strictly, new control methods must be introduced to ensure that deficiencies are not overlooked, and other as-yet uncontrolled causal factors must be investigated.

13.3 Strategies for Reducing Chronic Quality Defects

(1) Stabilise causal factors

A distinction should be drawn between causal factors and true causes. Causal factors are anything that might conceivably affect the problem phenomenon, while true causes are anything that has actually been proven to or deduced to produce it, either directly or indirectly. True causes are therefore a subset of causal factors. Causal factors may be variable, semi-fixed or fixed. Variable causal factors are by definition liable to change at any time. They are difficult to fix and, even if fixed, are unlikely to remain so for long; they include the changes resulting from product changeover, parts replacement, work by a different operator, and component deterioration, for example. Stabilising a causal factor means fixing it so that it cannot change.

Semi-fixed and fixed factors are distinguished for convenience according to the length of time for which they remain constant. Semi-fixed factors are those that remain unchanged for a period of at least 6 months to 2 years provided that the equipment has been properly restored and all the parts that need to be replaced have been replaced (as mentioned previously, if the static and dynamic precision of a machine and its associated fixtures are restored to their original level, they should stay at that level for a reasonably long time). Fixed factors are those which, provided they are properly restored, remain unchanged for a period of 5 to 6 years; for example, misalignment in the installation of a machine, insufficient dynamic rigidity, and so forth. Once corrected, factors like this tend to stay corrected for a long time.

When something goes wrong on a production floor, a great many factors could be implicated in the problem. Because these factors are not fixed, the work is being carried on amid great instability, with everything in flux. This is why it is so important to reduce the number of variables by stabilising the causal factors one by one, making them fixed or at least semi-fixed. Some variables relate to people (e.g. problems due to poor cleaning, problems due to poor methods of disassembly and reassembly, problems due to poor changeover methods, or problems due to inadequate setting of processing conditions), while others relate to hardware (e.g. problems due to failure to maintain jig or machine precision).

Different people tend to perform the same job (operation, changeover, adjustment, etc.) in slightly different ways. People-related factors are often variable precisely because it is so difficult to ensure that everyone does everything in exactly the same way. In contrast, equipment-related factors are often variable because the precision of the equipment is not properly maintained. They can be turned into semi-fixed factors by restoring the equipment, because, once restored, it will stay that way for some time. Nevertheless, semi-fixed factors will eventually revert to being variable if left to themselves, and this is why it is necessary to establish some means of monitoring them to see whether they are changing or not.

In summary, the important thing is to examine each causal factor in turn, compare it with its ideal condition, and restore it in such a way that it remains fixed for as long as possible. The fewer variable factors acting in a situation, the easier it is to reduce the level of quality defects.

(2) Do comparative studies

When trying to reduce quality defects, it is important to compare the equipment or process where things are going wrong with other equipment or processes where things are not going wrong in order to try to identify any significant differences. Many simple quality problems remain unsolved because this is not done. Comparative studies are useful for determining (both quantitatively and qualitatively) where, how, to what extent and why the differences between acceptable and unacceptable products arise. There are three basic methods of carrying them out:

  1. Compare the results (i.e. the products)
    Compare how defective products differ from good ones in terms of their shape, size, functions, etc., and investigate how the defects vary over time and with their location on the product.
  2. Compare the processes
    Compare the shapes, sizes, surface roughness, etc. of machines, jigs, tools, dies and other equipment producing defective products with those producing good ones. When doing this, it is particularly important to develop methods of measuring apparently unquantifiable attributes.
  3. Compare the effects of replacing parts
    With an assembled product, the effects of interchanging product parts thought to be relevant to the defect should be investigated. Machine parts, jigs and tools should also be replaced to see what happens.When making comparisons, factors such as the following should be examined:

When doing this, the following points should be noted:

  • Increase analytical precision – devise ways of identifying even the slightest differences
    When things appear identical to the naked eye, it is important to look for slight variations by using a magnifying glass, a microscope, etc. The key is to highlight small differences that were not recognised in the past in order to identify the normal condition exactly. Using a higher level of analytical precision makes it possible to recognise significant differences. It is particularly important to analyse unquantifiable shape differences, since unquantifiable factors are often the biggest problem.
  • Develop new methods of measurement
    It is often necessary to develop new measurement techniques in order to detect significant differences, such as degrees of surface roughness that do not show up in the dimensions, localised uneven wear, etc. Surface roughness gauges and microscopes with projectors can be particularly useful.

(3) Apply the ‘zero’ approach (use P-M Analysis)
There are three principal reasons why product quality defects remain unsolved:

  • Insufficient analysis of the phenomenon
  • Inadequate grasp of causal factors
  • Poor identification of deficiencies in the causal factors

1 The conventional approach to improvement
The QC story format (establish reason for selecting topic – set targets – analyse current situation – analyse causal factors – take corrective action – check results – standardise – decide on future issues) is a widely-used improvement approach for reducing breakdowns and quality defects on the shop floor because it is so easy to apply. The cause-and-effect (fishbone/Ishikawa) diagram is also a commonly-used improvement tool when the target is to bring problems down to 1/2 – 1/3 of their current level. The thinking behind these approaches, however, is priority-based; in other words, they are targeted at the biggest problems, the biggest defects, and the improvements that will bring the biggest results. When tackling quality defects, for example, the idea is to start by identifying the biggest defect, analyse its causes, and eliminate the causes that have the greatest effect on the problem.
While there is nothing inherently wrong with this approach, and it can be very successful in reducing defect rates by as much as 1/2 or 2/3, it very rarely succeeds in reducing them to zero. It is effective when the defect rate is at a high level of 5%-10%, but ineffective when the defect rate is at a low level of 1% or less.

2 The ‘zero’ approach to improvement

The basic principle behind reducing chronic quality defects to zero is to avoid prioritising. Acting only against the causal factors that have the biggest impact on the quality defect in question, or correcting only the larger deficiencies, is the wrong way to go about eliminating chronic quality defects once and for all. With many chronic quality defects, it is often impossible to determine how much each causal factor contributes to the defect, or which is the true cause. This is why the priority-based approach of focusing on a small number of what are thought be the most important factors is ineffective. With chronic quality defects, it is necessary to act equally against all causal factors that could conceivably influence the problem, regardless of how great or small their influence might be. Any deficiencies must be identified, regardless of their size, and every single one must be corrected.

In short, the keys to eliminating chronic quality defects are:

(a) Review all causal factors
The fact that a quality defect is occurring means either that some of the causal factors that should be controlled have been overlooked, or that there are deficiencies in some of the causal factors that are being controlled. Solving the problem therefore requires a comprehensive review of all the causal factors. This requires the application of P-M Analysis, based on a sound understanding of the concept behind it.

(b) Investigate everything about each causal factor
Every single causal factor in the list generated by P-M Analysis or fishbone analysis should be investigated to determine whether or not it is deficient in any way. Although a comprehensive investigation like this may be difficult because of the time and resources required, it must be done.

There is an inevitable tendency, if two or three deficiencies thought to be of great or moderate significance are detected during the course of the investigation, to focus on these and forget about the rest. This will not work at all. Naturally, it is important to take action against large or medium-sized deficiencies, because they have a commensurately large effect on the problem; but they are not the end of the story. There may also be deficiencies in other causal factors. It is of the utmost importance to investigate every single causal factor in order to find out whether or not it contains any deficiency, because it is impossible to achieve zero quality defects if only the larger deficiencies are considered. Experience has shown that the defect rate may drop temporarily if this is done, but will soon revert to its original level.

(c) Correct every single deficiency
The following should be identified as deficiencies:

  • A discrepancy between a parameter and the current official standard.
  • A discrepancy between a parameter and a provisional technical standard, if noofficial standard exists.
  • Failure of a component to function in the most desirable way, even if no officialstandard has been set.
  • Borderline cases that might or might not be a deficiency.In addition, the optimal state of assemblies and individual components should be identified (mainly from the functional standpoint) and any deviation from this should be treated as a deficiency.When measurements are performed in order to identify deficiencies, there are often problems with the way the measurements are taken, the way reference surfaces are established, and so on. When searching for deficiencies, we should always ensure that the most appropriate measurement technique is used. When deficiencies have been identified, they should all be eliminated, regardless of how much they contribute to the quality defect. It is essential to go back to the basics and get everything into its best possible condition.

(d) Correct deficiencies en masse
The conventional approach once a number of deficiencies have been identified is to correct them one by one, checking the result each time. For example, if 10 deficiencies have been found, the effect on the result is checked a total of 10 separate times. Although this is method is not wrong per se, the causes of quality defects are often unclear, making it impossible to determine whether correcting a particular deficiency has been effective or not.

In the zero approach to eliminating quality defects, we correct all the deficiencies together; if there are 10 deficiencies, for example, we correct all 10 at the same time. As discussed in the section on the characteristics of chronic quality defects, their causes fall into one of two patterns: multiple causes acting one at a time, or complex combinations of causes. When the influence of each individual deficiency on the quality defect is slight, the relationship between any particular corrective action and the result it produces is weak, so a difference is more likely to be seen in the result after all the deficiencies have been eliminated. Some people complain that this approach makes it difficult to ensure that the solution is sustained, because it is impossible to know which corrective actions have produced the result. However, clear cause-and-effect relationships are usually very hard to determine in the case of chronic defects, and there is not a lot of point in trying to do so. If eliminating the deficiencies in the causal factors cures the quality defect, it is sufficient to take steps to keep the causal factors in their optimal state; this will ensure that the solution is sustained.

(e) If there is no improvement, repeat the exercise
Sometimes, the hoped-for result is not achieved even after a P-M Analysis or fishbone analysis has been conducted and all the deficiencies have been eliminated. There are several possible reasons for this, for example:

  • Some causal factors have been overlooked.
  • The identification of the deficiencies was inadequate (insufficiently rigorous, resulting in some deficiencies being missed).
  • There are problems with the established standard values.
  • There are problems with the measurement methods (unsuitable techniques, unsatisfactory selection of reference surfaces, etc.).
  • There are problems with the way in which the deficiencies were eliminated.Although it requires a lot of commitment to redo the analysis, it is important not to give up but to apply the approach exhaustively. Doing so will provide the opportunity to rectify the above problems and ultimately reach a satisfactory solution.Table 4.22 contrasts the conventional and zero improvement approaches, while Table 4.23 shows the steps involved in reducing quality defects/establishing quality maintenance.

Table 4.23 Procedure for Reducing Quality Defects

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