Quality Control in Automation

The changing role of industry, the trend towards automation, the in creased cost of raw materials and labour rates all emphasize the need for improved quality of manufacture which will lead to better reliability, reduced maintenance costs and incidence of failure, as well as greater consumer confidence. Many countries, including Japan, are noted for their high quality of manufacture. This has helped them monopolize the world radio and television market and achieve great in-roads into the motor industry. If other economies are to compete effectively against such competition, then improvements in quality and performance of products are necessary. The need for control of quality embraces many fields from design, manufacture, function testing and inspection, through to an understand ing of sampling procedures and sampling theory, embracing both employee and management responsibilities to the maintenance of quality. The aim of this book is to give an understanding of industrial quality control. The techniques described have been selected so that they may be applied to industrial problems. It is hoped and expected that these methods will lead to improvements in quality and economics of manufac ture, thus leading to increased productivity and profitability. 9 1 The Need for Quality Control The major goals for quality control are manifold and include a need to determine the real functional tolerances required for engineering specifications and to provide a plan for control of the quality of the results of a process related to time. This is normally achieved by statistical or control chart methods.

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1: The Need for Quality Control.- Control considerations.- 2: Process Variability.- Dimensional error.- Common errors.- Establishing and controlling process capability.- Collecting data.- 3: Probability Theory.- Empirical probability.- Theoretical probability.- Addition law of probability 2.- Multiplication law of probability.- Multiple sampling of large batches.- Binomial expansion.- Sampling small batches.- Law of permutations.- 4: Location and Scatter.- Measures of location.- Calculation of mean, median and mode.- Relationship between mean, median and mode.- Measures of scatter.- Calculations of measures of scatter.- 5: Useful Distributions.- Hypergeometric distribution.- Binomial distribution.- Poisson distribution.- Distribution of continuous data.- Normal (Gaussian) distribution.- 6: Measurement and Control of Workpiece Size.- In-process measurement.- Post-process measurement.- Advantages of process control.- Control and pre-control charts.- Modified control charts.- Control charts for proportion defective: P charts.- d Charts, sampling the number of defects within one sample.- Interchangeability of P and d charts.- 7: Sampling Inspection.- Single sampling inspection plan.- Producer and consumer risks.- Double sampling schemes.- Sequential sampling.- AOQL: a basis for acceptability sampling.- 8: Statistical Tolerances.- Variability of a single dimension.- Variability of stacked dimensions.- Unequal tolerance applications.- Assembly condition of clearance.- Acceptable levels of defective assemblies.- Implementation of statistical tolerances.- The implications of statistical tolerancing.- 9: Confidence Limits and Tests of Significance.- Unbiased estimates of population parameters based on sample observations.- Standard deviation of the distribution of the sum or difference of two variates.- Estimate of population standard deviation based on two samples.- Distribution of sample means.- Student's 't'distribution.- Single-tailed or double-tailed tests.- Variance ratio (F) test (Fisher's ratio).- Selection of levels of significance.- Confidence limits for populations whose standard deviation is known.- The t-test for a single mean.- The t-test for the means of paired observations.- The t-test for the difference of the means of two independent samples.- Test of variance and difference of mean of two independent samples.- 'Chi-square' test.- Contingency tables.- 10: Analysis of Variation.- Variance between sample averages.- Calculation of control charts from the residual variance estimate.- 11: Cumulative Sum Charts.- Decision making with 'V' masks.- Design of cusum schemes.- Design of 'V' masks.- 12: Reliability.- Product life and failure characteristics.- Life characteristic curve.- Exponential distribution model.- Effect of working environment.- Additivity of failure rate.- Failure in parallel systems.- Reliability life tests.- Weibull probability distribution.- 13: Pareto Analysis.- Developing a pareto diagram.- Reassessment after process or design change.- 14: Variability when Sampling Gaussian and non-Gaussian Populations.- Random sampling from Gaussian populations - uncorrelated.- Random sampling from Gaussian populations - correlated.- Variation in non-normal surfaces.- 15: Effect of Quality on the Economics of Assembly Processes.- Assembly machines.- The cost of quality.- Optimization of parts quality.- Manually assembled products.- 16: Selection and Effectiveness of Quality Engineers and Inspectors.- Selection of quality assurance managers.- Selection of quality engineers.- Selection of semi-skilledinspectors.- Selection of skilled inspectors.- Effectiveness of inspectors.- Inspector assessment.- Implications of inspector inefficiency.- 17: Automated Inspection.- A simple measuring system.- Types of control system: in-process measurement.- Errors introduced by measuring systems.- Gauging systems.- Application of in-process measurement.- Post-process gauging.- System specification.- Examples of hardware associated with an automated low cost gauging system.- 18: Visual Inspection.- Principle of the television camera.- Charge coupled devices.- Components of a vision system.- Monochromatic lighting.- Achievement of binary images from video pictures.- Evaluation of the component.- Orientation.- Simple optical systems.- System specification.