Quality Control Quick Guide to Precision Measuring Instruments
Quality control (QC) A system used to ensure products and services are produced to meet the quality standards of the producer and the customer. Process quality control Activities to reduce variation in product quality and output by a process and maintain reductions. Process improvement and standardization are promoted through these activities.
Histogram A diagram that divides the range between the maximum and the minimum measurement values into several bins and shows the number of values (frequency) in each bin in the form of a bar graph. The histogram shows the spread, whether the center of the distribution is on target, and the shape of the data. A bell-curve-shaped distribution is ideal since it represents the normal distribution that is assumed in all statistical calculations. Process capability The ability of a process to consistently produce output that meets specified requirements once any causes of variation are eliminated and the process is in a state of statistical control. Process capability compares the natural variation of the process to the allowed tolerance. Process capability index (PCI or Cp) The index value is calculated by dividing the tolerance of a target characteristic by the process capability (6 ). The value calculated by dividing the difference between the mean (X) and the standard value by 3 may be used to represent this index in cases of a unilateral tolerance. The process capability index assumes that a characteristic follows the normal distribution.
A
Statistical process control (SPC) Process quality control through statistical methods.
Population The entire batch, lot, or product stream in a defined time frame. A sample of this population is used to estimate process parameters.
Lot Collection of product produced under the same conditions.
Sample An item (or items) of product taken out of the population to analyze and monitor the process.
Note : If a characteristic follows the normal distribution, 99.74% data is within the range ±3 from the mean.
Sample size Number of items in the sample.
Bilateral tolerance
USL: Upper specification limit LSL: Lower specification limit
Bias Value calculated by subtracting the true value from the mean of measurement values when multiple measurements are performed. Variance / Precision Variation in the measured values in relation to the mean for that characteristic. Standard deviation is usually used to represent the dispersion of values around the mean.
Unilateral tolerance ... If only the upper limit is stipulated
USL-LSL Cp = — 6
Unilateral tolerance ... If only the lower limit is stipulated
USL-X Cp = — 3
X-LSL Cp = — 3
Noncompliance
Measurement values
Bias
Variance / Precision
Tolerance (USL-LSL)
A-31
Powered by FlippingBook