A unified method of inspection and quality control

Abstract

It has been shown earlier that the mean and range charts used for S.Q.C. can be replaced by a single chart showing the largest and smallest values of samples taken from the machine at regular intervals. This chart designated as L-S chart has been found to be economical and efficient in many respects. The present paper shows that by modifying the sampling procedure, the S.Q.C. chart, besides providing information on the machine setting, tool wearing etc. for controlling the quality of the products collected during sampling intervals and thus enables us to dispense with 100 percent stage inspection which is in practice in the ordnance factories. The modification consists in taking one or two jobs from the machine at regular intervals and another four jobs at random from the products manufactured during every interval. The samples so selected are plotted on a chart against the specification limits. The collections during the different intervals will be accepted if all the four jobs are within the specification limits; otherwise screening will be done. Condition of the machine is judged mainly from the jobs taken from it. The experimental investigations conducted at Gun and Shell Factory, Cossipore show that this procedure works as effectively as the L-S chart and cent per cent inspection