MISTAKE PROOFING | Six Sigma and Beyond: Design for Six Sigma, Volume VI

DEFINITION

Mistake proofing by definition is a process improvement system that prevents personal injury , promotes job safety, prevents faulty products, and prevents machine damage. It is also known as the Shingo method, Poka Yoke, error proofing, fail safe design, and by many other names .

THE STRATEGY

Establish a team approach to mistake proof systems that will focus on both internal and external customer concerns with the intention of maximizing value. This will include quality indicators such as on-line inspection and probe studies.

The strategy involves:

Concentrating on the things that can be changed rather than on the things that are perceived as having to be changed to improve process performance
Developing the training required to prepare team members
Involving all the appropriate people in the mistake proof systems process
Tracking quality improvements using in-plant and external data collection systems (before/after data)
Developing a "core team" to administer the mistake proof systems process This core team will be responsible for tracking the status of the mistake proof systems throughout the implementation stages.
Creating a communication system for keeping plant management, local union committee, and the joint quality committee informed of all progress ” as applicable
Developing a process for sharing the information with all other departments and/or plants ” as applicable
Establishing the mission statement for each team and objectives that will identify the philosophy of mistake proof systems as a means to improve quality

A typical mission statement may read: to protect our customers by developing mistake proofing systems that will detect or eliminate defects while continuing to pursue variation reduction within the process.
Developing timing for completion of each phase of the process
Establishing cross-functional team involvement with your customer(s)

Typical objectives may be to:

Become more aware of quality issues that affect our customer
Focus our efforts on eliminating these quality issues from the production process
Expose the conditions that cause mistakes
Understand source investigation and recognize its role in preventing defects
Understand the concepts and principles that drive mistake prevention
Recognize the three functional levels of mistake proofing systems
Be knowledgeable of the relationships between mistake proof system devices and defects
Recognize the key mistake proof system devices
Share the mistake proof system knowledge with all other facilities within the organization

DEFECTS

Many things can and often do go wrong in our ever-changing and increasingly complex work environment. Opportunities for mistakes are plentiful and often lead to defective products. Defects are not only wasteful but result in customer dissatisfaction if not detected before shipment.

The philosophy behind mistake proof systems suggests that if we are going to be competitive and remain competitive in a world market we cannot accept any number of defects as satisfactory.

In essence, not even one defect can be tolerated. Mistake proof systems are a simple method for making this philosophy become a daily practice. Simple concepts and methods are used to accomplish this objective.

Humans tend to be forgetful, and as a result, we make mistakes. In a system where blame is practiced and people are held accountable for their mistakes and mistakes within the process, we discourage the worker and lower morale of the individual, but the problem continues and remains unsolved.

MISTAKE PROOF SYSTEM IS A TECHNIQUE FOR AVOIDING ERRORS IN THE WORKPLACE

The concept of error proof systems has been in existence for a long time, only we have not attempted to turn it into a formalized process. It has often been referred to as idiot proofing, goof proofing, fool proofing, and so on. These terms often have a negative connotation that appears to attack the intelligence of the individual involved and therefore are not used in today's work environment. For this reason we have selected the term "mistake proof system." The idea behind a mistake proof system is to reduce the opportunity for human error by taking over tasks that are repetitive or actions that depend solely upon memory or attention. With this approach, we allow the worker to maintain dignity and self-esteem without the negative connotation that the individual is an idiot, goof, or fool.

TYPES OF HUMAN MISTAKES

Forgetfulness

There are times when we forget things, especially when we are not fully concentrating or focusing. An example that can result in serious consequences is the failure to lock out a piece of equipment or machine we are working on. To preclude this , precautionary measures can be taken: post lock out instructions at every piece of equipment and/or machine; have an ongoing program to continuously alert operators of the danger.

Mistakes of Misunderstanding

Jumping to conclusions before we are familiar with the situation often leads to mistakes. For example, visual aids are often prepared by engineers who are thoroughly familiar with the operation or process. Since the aid is completely clear from their perspective, they may make the assumption (and often do) that the operator fully understands as well. This may not be true. To preclude this , we may test this hypothesis before we create an aid; provide training/education; standardize work methods and procedures.

Identification Mistakes

Situations are often misjudged because we view them too quickly or from too far away to clearly see them. One example of this type of mistake is misreading the identification code on a component of a piece of equipment and replacing that component with the wrong part. To prevent these errors , we might improve legibility of the data/information; provide training; improve the environment (lighting); reduce boredom of the job, thus increasing vigilance and attentiveness.

Amateur Errors

Lack of experience often leads to mistakes. Newly hired workers will not know the sequence of operations to perform their tasks and often, due to inadequate training, will perform those tasks incorrectly. To prevent amateur errors , provide proper training; utilize skill building techniques prior to job assignment; use work standardization.

Willful Mistakes

Willful errors result when we choose to ignore the rules. One example of this type of error is placing a rack of material outside the lines painted on the floor that clearly designate the proper location. The results can be damage to the vehicle or the material or perhaps an unsafe work condition. To prevent this situation , provide basic education and/or training; require strict adherence to the rules.

Inadvertent Mistakes

Sometimes we make mistakes without even being aware of them. For example, a wrong part might be installed because the operator was daydreaming . To minimize this , we may standardize the work, through discipline if necessary.

Slowness Mistakes

When our actions are slowed by delays in judgment, mistakes are often the result. For example, an operator unfamiliar with the operation of a fork lift might pull the wrong lever and drop the load. Methods to prevent this might be : skill building; work standardization.

Lack of Standards Mistakes

Mistakes will occur when there is a lack of suitable work standards or when workers do not understand instructions. For example, two inspectors performing the same inspection may have different views on what constitutes a reject. To prevent this , develop operation definitions of what the product is expected to be that are clearly understood by all; provide proper training and education.

Surprise Mistakes

When the function or operation of a piece of equipment suddenly changes without warning, mistakes may occur. For example, power tools that are used to supply specific torque to a fastener will malfunction if an adequate oil supply is not maintained in the reservoir. Errors such as these can often be prevented by work standardization ; having a total productive maintenance system in place.

Intentional Mistakes

Mistakes are sometimes made deliberately by some people. These fall in the category of sabotage . Disciplinary measures and basic education are the only deterrents to these types of mistakes.

There are many reasons for mistakes to happen. However, almost all of these can be prevented if we diligently expend the time and effort to identify the basic conditions that allow them to occur, such as:

When they happen
Why they happen

and then determine what steps are needed to prevent these mistakes from recurring ” permanently.

The mistake proof system approach and the methods used give you an opportunity to prevent mistakes and errors from occurring.

DEFECTS AND ERRORS

Mistakes are generally the cause of defects. Can mistakes be avoided? To answer this question requires us to realize that we have to look at errors from two perspectives:

Errors are inevitable: People will always make mistakes. Accepting this premise makes one question the rationale of blaming people when mistakes are committed. Maintaining this "blame" attitude generally results in defects. Also, quite often errors are overlooked when they occur in the production process. To avoid blame, the discovery of defects is postponed until the final inspection, or worse yet, until the product reaches the customer.
Errors can be eliminated: If we utilize a system that supports (a) proper training and education and (b) fostering the belief that mistakes can be prevented, then people will make fewer mistakes. This being true, it is then possible that mistakes by people can be eliminated.

Sources of mistakes may be any one of the six basic elements of a process:

Measurement
Material
Method
Manpower
Machinery
Environment

Each of these elements may have an effect on quality as well as productivity. To make quality improvements, each element must be investigated for potential mistakes of operation. To reduce defects, we must recognize that defects are a consequence of the interaction of all six elements and the actual work performed in the process. Furthermore, we must recognize that the role of inspection is to audit the process and to identify the defects. It is an appraisal system and it does nothing for prevention. Product quality is changed only by improving the quality of the process. Therefore, the first step toward elimination of defects is to understand the difference between defects and mistakes (errors):

Defects are the results.
Mistakes are the causes of the results.

Therefore, the underlying philosophy behind the total elimination of defects begins with distinguishing between mistakes and defects. Examples of mistakes and defects are shown in Table 5.4.

Table 5.4: Examples of Mistakes and Defects
Mistake	Resulting Defects
Failure to put gasoline in the snow blower	Snow blower will not start
Failure to close window of unit being tested	Seats and carpet are wet
Failure to reset clock for daylight savings time	Late for work
Failure to show operator how to properly assemble components	Defective or warped product
Proper weld schedule not maintained on welding equipment	Bad welds, rejectable and/or scrap material
Low charged battery placed in griptow	Griptow will not pull racks resulting in lost production, downtime, etc.

MISTAKE TYPES AND ACCOMPANYING CAUSES

The following categories with the associated potential causes are given as examples, rather than exhaustive lists:

Assembly mistakes
- Inadequate training
- Symmetry ( parts mounted backwards )
- Too many operations to perform
- Multiple parts to select from with poor or no identification
- Misread or unfamiliar with parts/products
- Tooling broken and/or misaligned
- New operator
Processing mistakes
- Part of process omitted (inadvertent/ deliberate )
- Fixture inadequate (resulting in parts being set into incorrectly)
- Symmetrical parts (wrong part can be installed)
- Irregular shaped/ sized part (vendor/supplier defect)
- Tooling damaging part as it is installed
- Carelessness (wrong part or side installed)
- Process/product requirements not understood (holes punched in wrong location)
- Following instructions for wrong process (multiple parts)
- Using incorrect tooling to complete operations (impact versus torque wrench)
Inclusion of wrong part or item
- Part codes wrong/missing
- Parts for different products/applications mixing together
- Similar parts confused
- Misreading prints/schedules/bar codes etc.
Operations mistakes
- Process elements assigned to too many operators
- Operator error
- Consequential results
Setup mistakes
- Improper alignment of equipment
- Process or instructions for setup not understood or out of date
- Jigs and fixtures mislocated or loose
- Fixtures or holding devices will accept mislocated components
Assembly omissions ” missing parts
- Special orders (high or low volume parts missing)
- No inspection capability (hidden parts omitted)
- Substitutions (unexpected deviations from normal production)
- Misidentified build parameters (heavy duty versus standard)
Measurement or dimensional mistakes
- Flawed measuring device
- Operator skill in measuring
- Inadequate system for measuring
- Using "best guess" system
Processing omissions
- Operator fatigue (part assembled incorrectly/omitted)
- Cycle time (incomplete/poor weld)
- Equipment breakdown (weld omitted)
- New operator
- Tooling omitted
- Automation malfunction
- Instructions for operation incomplete/missing
- Job not set up for changeover
- Operator not trained/improper training
- Sequence violation
Mounting mistakes
- Symmetry (parts can be installed backwards)
- Tooling wrong/inadequate
- Operator dependency (parts installed upside down)
- Fixtures or holding devices accept mispositioned parts
Miscellaneous mistakes
- Inadequate standards
- Material misidentified
- No controls on operation
- Counting system flawed/operating incorrectly
- Print/specifications incorrect

SIGNALS THAT ALERT

Signals that "alert" are conditions present in a process that commonly result in mistakes. Some signals that alert are:

Many parts/mixed parts
Multiple steps needed to perform operation
Adjustments
Tooling changes
Critical conditions
Lack of or ineffective standards
Infrequent production
Extremely high volume
Part symmetry
Asymmetry
Rapid repetition
Environmental
- Housekeeping
- Material handing
- Poor lighting
- Foreign matter and debris
- Other

Ten of the most common types of mistakes are:

Assembly mistakes	Processing mistakes
Inclusion of wrong part or item	Operations mistakes
Setup mistakes	Assembly omissions (missing parts)
Measurement mistakes	Process omissions
Mounting mistakes	Miscellaneous

APPROACHES TO MISTAKE PROOFING

As we already mentioned, any mistake proofing system is a process that focuses on producing zero defects by eliminating the human element from assembly. There are two approaches to this ” see Figure 5.7.

Figure 5.7: Approaches to mistake proofing.

Reactive systems (defect detection)

This approach relies on halting production in order to sort out the good from the bad for repair or scrap.
Proactive systems (defect prevention)

This approach seeks to eliminate mistakes so that defective products are not produced, production downtime is reduced, costs are lowered , and customer satisfaction is increased.

Major Inspection Techniques

Figure 5.8 shows major inspection techniques. Source inspection utilizing mistake proofing system devices is the most logical method of defect prevention.

Figure 5.8: Major inspection techniques.

Mistake Proof System Devices

Mistake proof system "devices" are simple and inexpensive. There are essentially two types of devices used:

Detectors (sensors) ” to detect mistakes that have occurred or are about to occur
Preventers ” to prevent mistakes from occurring

Devices Used as "Detectors of Mistakes"

When used as detectors (sensors), these devices:

Provide prompt feedback (signals) to the operator that a mistake has occurred or is about to occur
Initiate an action or actions to prevent further mistakes from occurring

Devices Used as "Preventers of Mistakes"

When used to prevent mistakes, these devices prevent mistakes from occurring or initiate an action or actions to prevent mistakes from occurring.

EQUATION FOR SUCCESS

To be successful with a mistake proofing initiative one must keep in mind the following equation:

Source investigation + Mistake proofing = Defect free system

However, to reach the state of defect free system, in addition to signals and inspection we must also incorporate appropriate sensors to identify, stop, and/or correct a problem before it goes to the next operation. Sensors are very important in mistake proofing, so let us look at them little closer.

A sensor is an electrical device that detects and responds to changes in a given characteristic of a part, assembly, or fixture ” see Figure 5.9. A sensor can, for example, verify with a high degree of accuracy the presence and position of a part on an assembly or fixture and can identify damage or wear. Some examples of types of sensors and typical uses are:

Welding position indicators: Determine changes in metallic composition, even on joints that are invisible to the surface
Fiber sensors: Observe linear interruptions utilizing fiber optic beams
Metal passage detectors: Determine if parts have a metal content or mixed metal content, for example in resin materials
Beam sensors: Observe linear interruptions using electronic beams
Trimetrons: Exclude or detect preset measurement values using a dial gauge (Value limits can be set on plus or minus sides, as well as on nominal values.)
Tap sensors: Identify incomplete or missing tap screw machining
Color marking sensors: Identify differences in color or colored marking
Area sensors: Determine random interruptions over a fixed area
Double feed sensors: Identify when two products are fed at the same time
Positioning sensors: Determine correct/incorrect positioning
Vibration sensors: Identify product passage, weld position, broken wires, loose parts, etc.
Displacement sensors: Identify thickness , height, warpage, surface irregularities, etc.

Figure 5.9: Functions of mistake-proofing devices.

Typical Error Proofing Devices

Some of the most common mistake proofing devices used are:

Sensors
Sequence restrictors
Odd part out method
Limit or microswitches, proximity detectors
Templates
Guide rods or pins
Stoppers or gates
Counters
Standardized methods of operation and/or material usage
Detect delivery chute
Critical condition indicators
Probes
Mistake proof your mistake proof system

and so on