Inspection and testing
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Inspection and testing
Industrial activities which ensure that manufactured products, individual components, and multicomponent systems are adequate for their intended purpose. Inspection and testing are the operational parts of quality control, which is the most important factor to the survival of any manufacturing company. Quality control directly supports the other factors of cost, productivity, on-time delivery, and market share. Therefore, all quality standards needed to produce the components of a product and perform its assembly must be specified in a manner such that customers' expectations are met. Global competitive pressures force manufacturing companies to become more customer-oriented and focused in terms of offering higher-quality products and services. See Quality control
Inspection and testing are performed before, during, and after manufacturing to ensure that the quality level of the product is within acceptable design standards.
Whereas inspection is the activity of examining the product or its components to determine if they meet the design standards, testing is a procedure in which the item is observed during operation in order to determine whether it functions properly for a reasonable period of time. See Design standards
In statistical quality control, inferences are made about quality based on a sample taken from the population of the items. The sample of items is generated randomly from the population. Each item in the sample is inspected or tested for certain quality characteristics. For example, the diameter of a cylindrical part is measured after the turning operation that generated the part is completed.
The objective of statistical quality control is to determine when the process has gone out of statistical control, so that corrective action can be taken. The two principal techniques in statistical quality control are acceptance sampling and control charts. In acceptance sampling, a sample is taken from a batch of parts and, depending on the number of parts that pass the inspection or testing, the batch is accepted or rejected. A control chart is designed to be a simple graphical technique to monitor and control a single characteristic of the process output. The objective is to obtain an estimate of the principal parameter that describes the variability of this characteristic and then to use a test of hypothesis to determine if the process is in control. A control chart contains three horizontal lines: the central line represents the mean of the process output, and the upper and lower control limits indicate extreme statistical values of the process output. If a measured value of the output is outside these limits, the process is out of control and needs to be examined to determine the reason. The natural tolerance limits (±3 standard deviations) are normally used to specify the upper and lower limits in a control chart. See Control chart
Sensor technologies for automated inspection can be divided into two broad categories: contact and noncontact inspection methods. Contact inspection methods involve the use of a mechanical probe or another device that makes contact with the object being inspected. The purpose of the probe is to measure or gage some physical dimension of the object. These methods are used predominantly in the mechanical manufacturing industries. Coordinate measurement machines, flexible inspection systems, and inspection probes represent the high end of this technology. Noncontact inspection methods involve the use of a sensor located at a certain distance from the object to measure or gage the desired features. Two significant advantages of noncontact inspection methods are shorter inspection times and avoidance of damage to the object. Noncontact inspection methods include machine vision, electrical field techniques, radiation techniques, and ultrasonics.