Mistake Proofing Overview (Part 2 of 2)

Mistake Proofing On the Job

Shutdown: Effective in both prediction and detection. An example of a shutdown device is a camera that will not function when there is not enough light to take a picture. The meter predicts the picture will not take, and it shuts down. Some clothes dryers shut down when they detect an overheating situation.

Control: A control device can make pending errors impossible. Erroneous items cannot be used in the process. When gas stations introduced unleaded gasoline, the nozzle on the leaded pump was designed to be too big to fit into an unleaded tank, thereby preventing error. A fruit orchard assures that only the biggest apples pass on to its customers by passing the apples through a sizer. Apples that are too small fall through and are sent to a discount outlet.

Warning: A warning device predicts when something is about to go wrong. It can also sound immediately when something does go wrong. Seat belt buzzers are warning devices used for prediction. Smoke detectors detect a hazardous situation.

Contact Methods: Contact with a part highlights errors. Electrical outlets have been mistake proofed to assure proper polarity. It is impossible to put a plug in an outlet incorrectly.

Fixed Value Methods: Errors are detected through counting. Many boxes should be shipped with four different hazardous material warning labels. When labels were on separate rolls, errors in shipping often occurred. By placing all labels on a common roll and aligning them for easy reference, the worker easily knew when a label was missed.

Motion Step Methods: Detect errors by motion, or the lack of it. Seven screws in various sizes were inserted in the final assembly of a CD-ROM drive. Often a screw would be forgotten. The seven Different screws were put into bins with photo-electric switches. When a screw is removed, the beam is broken. The part cannot move on to the next operation until the beam is broken on all seven bins.

As you begin to recognize the techniques and methods that apply to mistake proofing, it is easy to see how the principal has been applied. Limit switches are commonly applied to assure that machines are not operated beyond their design capacity or that they are operated in a safe manner. Guide pins are routinely used to assure that jigs and molds go together in the correct configuration. At the end of this module are eight mistake proofing challenges. How can the problems be solved?

When You Can't Mistake Proof

Some strategies can be employed to minimize mistakes, or make them easier to detect. Consider the following:
• Colors and color coding. Credit card receipts are almost always designed so the customer gets the yellow copy and the merchant gets the white copy.
• Use of shapes. Stop signs are always octagonal.
• Auto-detection. Word processors come with spell checkers. These reduce errors considerably.
• Make it easy to do it right. Use checklists. Have effective data collection forms that follow the process. Design work flows with fewer hand-offs.

5 Steps to Mistake Proofing

There are five steps to mistake proofing:
1. Identify problems. Look at customer returns, defective parts analyses, and error reports. Do a FMEA.

2. Prioritize problems. Look at problem frequency and cost. Identify wasted materials, rework time, detection time, and detection cost. Consider inspection costs.

3. Seek out the root cause. Drive down until you truly understand the source of the error. Correct mistakes at their source.

4. Create solutions. Make it impossible to do it wrong. Perform a cost benefit analysis to see how long the investment in the solution will take to repay itself. Be creative in solution generation.

5. Measure the results. Have errors been eliminated and what is the impact?
There is no doubt that mistake proofing offers several advantages when it is designed into the product. No formal training programs are required to implement it or to use the devices. It eliminates many inspection operations, and it relieves operators from repetitive tasks that take away from their creativity and value added activities. It results in defect-free work. It provides immediate action when problems do arise.

Mistake Proofing Challenges

Look at the following situations. What mistake proofing methods or techniques might you consider applying?
1. ATM customers complain that they never know the right way to insert their ATM card, and it usually takes a couple of tries to get it right.

2. A local bottling company wants to make sure that all bottles are filled with exactly the right amount of soda.

3. A large pharmaceutical company is trying to develop a way to assure elderly people living alone take their prescription medicine at the right time and in the correct dosage.

4. A manufacturer of home power tools wants to make sure that customers are wearing eye and hand protection before using their equipment.

5. A consumer electronic company has had several customer complaints lately that their instructions were missing from their product.

6. Nine different tools are needed to change a jig and die. Often, in the middle of changeover, the set-up operator realizes a tool is missing and has to stop to look for the tool.

7. An easy-to-assemble furniture manufacturer has received several complaints about holes not being tapped for all screws. Because the furniture is made from hardwood, the customers have to drill the holes themselves.

8. A contact lens solution manufacturer received complaints from distributors that some customers found empty boxes with their shipments. The company wants to make sure no more empty boxes are shipped.