Study on the Variable Quality Control Limits in Small-Batch Manufacturing Process

In this paper, a new method which uses variable control limits is presented for quality control in small-batch manufacturing process. First, the high false alarm probability, which is caused by applying traditional quality control chart to small-batch manufacturing process, is analyzed, and the relationship between the number of samples and the probability of false alarm is found out. Then, the normal distribution is used to approximate the distribution of sample standard deviation, and the variable control limits coefficient is calculated approximately according to the characteristics of the normal distribution, and the variable control limits are achieved. At last, a manufacturing process is carried out. The results of the experiment show that the probability of false alarm can be maintained steadily regardless of the number of observation, and the variable control limits can be applied to the quality control in small-batch manufacturing process.