Quality Inspection Process
Many of today’s parts manufacturing companies are heeding Deming’s recommended practices of continuous improvement. They set a decrease in their dependence on quality inspections as one of their goals, and seek to accomplish that goal by steadily improving the reliability of their manufacturing processes so as to build quality into their products. The need for quality inspection and quality inspection services has not yet gone away however. The need for Quality Inspection (QI) services is likely to be with us as long as buyers depend on more than a single supplier or offshore suppliers whose manufacturing techniques cannot always be easily monitored.
QI services will continue to provide value, by saving buyers and suppliers the time and effort that might otherwise have to be devoted to retooling, rework, repair, or replacing discarded parts or assemblies.
Types of Inspections
A quality inspection (QI) is normally carried out by an independent party; but this service can be conducted internally when the inspectors, who may be on the supplier’s payroll, are dedicated to serving the customer or buyer. Depending how and where parts are to be used, and the risks associated with using a defective part, or a wrong part, QI can take on many forms:
Inspection of parts-producing machines and processes. These are pre-production QI services that tell the buyer, among other things, the kinds and quality of raw materials that will be used, and can include a detailed review of production processes to ensure that corners are not being cut for the purpose of speeding up production or making a manufacturing process easier.
During production inspections (DUPRO) provide a measure of average product quality early on. This is a very useful, yet under-rated QI service. It can predict the possibility of a need for rework, or a need to take corrective actions, as well as providing data for look-ahead planning.
Final random inspections are conducted after parts have been packed for shipping. This QI service is typically performed to ensure that parts suppliers are not playing games. Final inspections can also be used to verify that the correct parts are being shipped.
Platform inspections occur after parts have been shipped, but before they are put to use. These inspections can be totally random, but often involve 100% of the shipped parts. Once the parts pass inspection, the probability of a part being put to use is defective or incorrect is zero, or close to zero.
Piece-by-piece inspection in the factory is another QI method that can ensure the number of defective parts will be close to zero. Piece-by-piece QI services are often used for complex parts, and they are typically performed by inspectors who are not on the parts manufacturer’s payroll.
QI services can also be performed by a company that has been charged with establishing and managing a buyer’s inventory, especially when parts from multiple suppliers need to be stored.
QI Methods and Tools
The two most common methods of parts inspection involve manual measurements, such as calipers and/or gauges, and the use of coordinate measuring (CMM) systems, in which physical geometrical characteristics of a part are measured by other than purely mechanical methods. CMMs often find their greatest used for complex parts, where manual measurements may be difficult, cumbersome, or even impossible.
There are many types of CMMs in use today. They are either manually or computer controlled. They typically involve a probe that is attached to the third moving axis of a stationary machine. Common probe types include mechanical, optical, laser, and white light measuring devices. CMMs can be free-standing, portable, or handheld, with the latter often using optical triangulation methods that allow complete freedom of movement around the part being inspected.
A basic CMM system consists of a coordinate measuring machine, a probing system (arm or laser/optical), and a data collection and reduction system; typically consisting of a desktop computer and the appropriate applications software.
CMM machines are currently in use that can provide precise measurement results to 5 places, and down to the nanoscale level.
Gauges and calipers have by no means become obsolete. They are capable of making very accurate measurements, they enjoy wide usage, and they likely will continue to do so.
While it is still desirable to have third-party inspectors conduct QI tasks, modern equipment featuring digital data reduction and reporting techniques has made it easier for buyers to accept quality inspection results that are performed at supplier facilities, and by the supplier.
Deming’s advice of building quality into a part, remains an objective, but even if do so effectively can become a standard practice, the need for quality inspection of parts and manufacturing processes is unlikely to disappear.