Mistake Proofing

What do we mean by mistake proofing? As the very name implies: It refers to the application of tools, devices, pictures or other ideas to a process or product to make it impossible or difficult to make a mistake in that process or process.

It is mostly believed that this is the only way to achieve zero defects. Several other definitions are also given:

• Any mechanism that either prevents a mistake from being made or makes the mistake obvious at a glance.
• A technique that prevents errors from being converted into a defect.
• “The causes of defects lie in worker errors, and defects are the results of neglecting those errors. It follows that mistakes will not turn into defects if worker errors are discovered and eliminated beforehand”[Shingo 1986].

Another commonly used term is “Poka-Yoke”, which is Japanese for mistake proofing. The idea of mistake proofing originates from the fact that defects in a product are due to errors in the process/product.  It has to be understood that in most processes or product usage, humans are directly involved and quite often there are going to be errors due to negligence, mishandling, forgetfulness, physical factors, natural events, and similar other factors.  These errors are going to result in defects. Inspecting and identifying these defects is a major concern.

Why Do We Want to Mistake Proof?

It is somewhat obvious why you want to mistake proof your manual assembly operations – people make mistakes. Why people make mistakes and errors on the simplest of tasks is not completely understand. There are some ideas however, that make sense. Most tasks are simply too mundane for our minds. Because of that, most people tend to daydream.

It might be:

“What’s for supper?”, “What will I wear to the game?”, “Why did that stock price go up?”, “Did I forget to turn off the coffee maker?”, or whatever.

Because of these random thought patterns, we tend to lose track of what we’re doing. Have you ever driven through an intersection and two blocks down the road wondering, if the light was red or green? All of us have done this at one time or another. But when you think about it, that was a situation that could have been life threatening.

Using mistake proofing, some production facilities have operates for months without a single detection of the defect.  Unlike traditional inspection methods, which Deming describe as ineffective, mistake-proofing can eliminate mistakes and defects at their source.

Mistake Proofing Approach 

The important characteristics of a mistake proofing approach are as follows:

• Inspection is autonomous, occurring without intervention.
• Simple, usually inexpensive, devices.
• Simple and quick procedures.
• Minimal disruption to workers (low “hassle-factor”)
• Low cost to implement into process.
• Large impact on quality of production.
• Focus on:
  • Eliminating the causes of errors or defects.
  • Enabling inexpensive and fast inspection of product or service..

Where Does Mistake Proofing Apply?

Now that we know what mistake proofing is all about, where is it applicable? Literally speaking, the idea is applicable to any task or day-to-day event where mistakes are likely to happen.

As discussed earlier, mistakes are possible in processes such as assembly line, services, etc. These mistakes can be attributes to the negligence, mental state, and forgetfulness of workers.

Mistake proofing devices can be applied to overcome such problems. The mistake does not occur or is too evident for the worker to avoid it.  In general, mistake proofing can be broken down into two areas:

Processes and product usage:

Processes:

Eliminate or minimize manufacturing or assembly process errors and service errors.  Some examples include:

• Special fixtures ensure proper part alignment or orientation.
• Components that are similar in size or shape may be color coded for easier identification.
• Redundancies are built into assembly processes: transmission oil fill operation: transmissions are weighed dry, filled with a measured amount of oil, then weighed again to verify proper oil fill.
• Elimination of fasteners (retaining rings, screws).
• Use of symmetric or bidirectional components (can be assembled forward or backward).

“Jiffy Lube” oil change stations:  The attendant shows the customer the oil level on the dipstick prior to completing service.  This ensures that the oil has indeed been replaced and filled.

Product Usage:

Eliminate or minimize problems or damage from improper product usage – usage beyond design intended levels.

• Tamper-proof packaging
• Engine rev limiter
• Fuses / circuit breakers

Control system prevents inadvertently shifting your transmission out of “park” without first depressing the brake pedal.

Mistake Proofing Strategies

The proper mistake proofing strategy is critical in any quality assurance system.  The strategy chosen should coincide with the type of product, process, or system under consideration.  Proactive strategies seek to actively eliminate or prevent mistakes from occurring, while reactive strategies alert the system that an error has occurred and minimize damage. The four strategies are as follows:

1. Elimination: The most desirable strategy.  The “mistake” is design out of the product or process up front by selecting technologies, materials, processes, geometries, etc. that will eliminate the likelihood of potential errors. This is a proactive strategy.

Using aluminum instead of magnesium reduces likelihood of severe fire during machining.

2. Prevention: Reduces chances of mistakes but does not eliminate possibility of errors.  This is a proactive strategy.

Color coding components for proper assembly

3. Detection:Provides warning if an error occurs.  This is a reactive strategy.

Alarms, warning lights

4. Loss Control: Reacts to minimize or limit damage once a mistake has occurred.  This is a reactive strategy.

Safety glasses, seat belts

Case Study Example:  Mistake proofing a building from fire

Examples of Mistake Proofing

Refer to the following link for a list of mistake proofing examples and their classifications:

The file is groups according to example type (for a car, etc.), applications (process and product usage) and strategies (elimination, prevention, detection, loss control).

Comparison of Inspection Techniques

1 – Judgement Inspection
2 – Informative Inspection
3 – Source Inspection

Judgment inspection

involves sorting the defects out of the acceptable product, sometimes referred to as “inspecting in quality.”

Informative inspection

Uses data gained from inspection to control the process and prevent defects. Traditional SPC is a type of informative inspection. Both successive checks and self-checks in ZQC are also a type of informative inspection. Successive checks were the response to the insight that improvements are more rapid when quality feedback is more rapid. Work-in-process undergoes many operating steps as it is move through a manufacturing facility. Often inspections are conduct at intermediate stages in the process.

Most often inspections may not occur soon enough after production to give the best information necessary to determine the cause of the quality problem. So that, it can be prevent in the future. By having each operation inspect the work of the prior operation, quality feedback can be given on a much timelier basis. Successive checks are having the nearest downstream operation check the work of the prior operation.

Each operation performs both production and quality inspection. Effective poka-yoke devices make such an inspection system possible by reducing the time and cost of inspection to near zero. Because inspections entail minimal cost, every item may be inspect. Provided that work-in-process inventories are low, quality feedback used to improve the process can be provided very rapidly.

Checking

While successive checks provide rapid feedback, having the person who performs the production operation check their own work provides even faster feedback. Self-checks use poka-yoke devices to allow workers to assess the quality of their own work. Because they check every unit produced, operators may be able to recognize what conditions changed that caused the last unit to be defective. This insight is use to prevent further defects, self-checks are prefer to successive checks whenever possible.

Since the main difference between successive checks and self-checks is which workstation performs the inspection, in this research we do not distinguish between the two types of informative inspection. Both successive and self-checks provide information “after the fact.”

Source inspection

Determines “before the fact” whether the conditions necessary for high quality production exist. Usually, the occurrence of a defect is the result of some condition or action, and that it is possible to eliminate defects entirely by pursuing the cause.

With source inspection, poka-yoke devices ensure that proper operating conditions exist prior to actual production. Often these devices are also design to prevent production from occurring until the necessary conditions are satisfying us.

Source inspection, self-checks, and successive checks are inspection techniques use to understand and manage the production process more effectively. Each involves inspecting 100 percent of the process output.

In this sense, zero quality control is a misnomer. These inspection techniques are intend to increase the speed with which quality feedback is receive. Source inspection is intend to keep defects from occurring. Self-checks and successive checks provide feedback about the outcomes of the process. Self-checks and successive checks should be use when source inspection cannot be done or when the process is not yet well enough understood to develop source inspection techniques.

Mistake Proofing Guidelines

• Mistake proof at the earliest possible opportunity. Focus on elimination at the design phase (proactive).
• Tools for mistake proofing include QFD, Concurrent Engineering, FMEA (both Process and Design), Failure Tree Analysis.
• Use a cross-functional team when discussing mistake proofing opportunities (for both design and process).
• It is easier to mistake proof new products than to develop devices or procedures for existing products.
• Identify measures (scrap, rework, warranty, FMEA, customer returns) that will help pinpoint potential problems that could be solved by error proofing.  Re-address the tools when necessary.

Benefits

The benefits of mistake proofing are somewhat obvious. Eliminating mistakes, from the most general point of view reduces cost.

Below is a list of the most common benefits:

1. More competitive pricing due to:
   a. Reduced production time.
   b.  We can Reduce scrap and rework at production lines.
   c. Savings in labor cost.
   2. Higher profit.
   3. Higher quality.
   4. Better reliability.
   5. Customer satisfaction.
   a. Mistake proofing prevents product dissatisfaction and yields product satisfaction.
   b. Products and processes that are mistake tolerant are always preferring by customers, (idiot proof).
   6. Meeting deadlines.
   7. More robust product and process performance.