GB852437A - Improved statistical quality-control system - Google Patents

Abstract

852,437. Registering output of machines. HRABAK, J. July 23, 1958 [July 30, 1957; July 31, 1957; Dec. 21, 1957; June 10, 1958], No. 23781/58. Class 106 (4). [Also in Groups XXIII and XXXV] Quality control apparatus, Fig. 2, comprises evaluating means S E in which are compared numbers (R s , R rU , R rL ) predetermined statistically and numbers S, r occurring in a sample n in three predetermined categories defined e.g. by a particular dimension deviating from a desired mean #X by more or less than the standard deviation of a normal frequency distribution so that, in the notation of Fig. la (which shows such a distribution lying wholly within tolerance limits T L , T U ), if in any sample n, Z + Z- (= r) is greater than R rU there has been an undesirable upward shift of the mean, or if less than Rr L an undesirable downward shift, while if Z + + Z- (= S) is greater than R S there has been an undesirable dispersion from the mean X. When the predetermined limits are passed retroactive control of the production machine may take place or coloured lamps may give signals to inform an operator what corrective action to apply to the machine. One embodiment, Figs. 4 and 5 (not shown), is a modification of apparatus with lamps of three colours which flash in different combinations and sequences according to the category of a gauged dimension of the workpiece. This apparatus is adapted to count the frequency of occurrence of the different colours and compare these with appropriate limits. Another embodiment, Fig. 6, uses a running sample of n and has a distributer D passing pulses to a sequence of memory units in a device M and a cancelling unit F to remove information from the first unit when information is fed to the (nr1)th so that only the most recent n workpieces produced influence the evaluator E. Suitable memory and cancelling units, Figs. 8, 10 (not shown), distributer, Fig. 9 (not shown), and comparators Figs. 11, 12 (not shown) are described using telephone-type selector switches, relays and resistances. The quality control system involves only counting and comparing, eliminating complex calculating means and may be used in the control of a centreless grinding machine or rolling mills producing sheet iron.