In traditional manufacturing environments, quality inspections usually occur at receipt of goods and through sampling after a product's final assembly. In lean manufacturing environments, inspection (and product rework) are accommodated at any point in a product's life, as it is being produced.

FACTS AND CONCEPTS

• This is accomplished by determining equipment (or other manufacturing) parameters that produce good parts, then rigorously monitoring and adjusting each operation to meet these parameters.
• Employees are empowered to stop the equipment (or the equipment is built to stop on its own) when the process drifts outside the parameter limits.
• Often combined with this is a program to error-proof each manufacturing step (called Poka-Yoke). This involves equipping machines or workstations with devices to assure that parts can only be made the correct way.

ZERO QUALITY CONTROL

Product defects hurt the company's reputation with its customers and waste valuable resources in scrap and rework. Companies that pursue low-inventory production no longer have a large buffer to absorb quality defects. To keep production moving smoothly, it is especially important to prevent defects

Mistake proofing is an effective quality assurance approach that prevents defects by catching errors and other nonstandard conditions before they actually turn into defects. The mistake-proofing system known as Zero Quality Control ensures zero defects by inspecting the processing conditions for 100 percent of the work, ideally just before an operation is performed. If an error is discovered, the process shuts down and gives immediate feedback with lights, warning sounds, and so on.

Zero Quality Control Elements

• Source inspection to catch errors before they become defects
• 100 percent inspection to check every workpiece, not just a sample
• Immediate feedback to shorten the time for corrective action
• Poka-yoke (mistake-proofing) devices to check automatically for abnormalities

POKA-YOKE

Because people can make mistakes even in inspection, mistake-proofing often relies on sensing mechanisms called Poka-Yoke, which check conditions automatically and signal when problems occur. Poka-Yoke devices include electronic sensors such as limit switches and photoelectric eyes, as well as passive devices such a positioning pins that prevent backward insertion of a workpiece. Poka-Yoke devices may use counters to make sure an operation is repeated the correct number of times.

The key to effective mistake-proofing is determining when and where defect-causing conditions arise and then figuring out how to detect or prevent these conditions, every time. Shop floor people have important knowledge and ideas to share for developing and implementing poka-yoke systems that check every item and give immediate feedback about the problem.