Statistical Quality Control

 

Statistical Quality Control**

Meaning:

S.Q.C. means planned collection & effective use of data for studying causes of variations in quality either as between processes, procedures, materials, machines etc. or over periods of time. By statistical quality control, we mean the various statistical methods used for the maintenance of the quality in manufactured product.

Purpose of SQC:

1) To provide a basis for a better understanding of the variations that exists in quality characteristics.

2) To help directly or indirectly to improve quality.

3) To help in separating the assignable causes from the chance causes.

4) Better uniformity of quality.

5) Better utilization of raw materials.

6) More efficient use of equipment.

7) Less scrap and rework, hence lowering costs.

8) Better inspection.

9) Improved producer-consumer relations.

Quality of Product:

Quality product means good or excellent product. In industry, a quality product is one that fulfils customer’s expectations. These expectations are based on the intended use and the selling price of the product. In other words, quality must be judged in customer satisfaction. Quality means fitness for use.

Quality is inversely proportional to variability. Quality improvement is the reduction of variability in processes and products. Quality of a product is related with the following:

i) Quality of material: Material of good quality will result in smooth processing thereby reducing the waste & increasing the output. It will also give better finish of end products.

ii) Quality of manpower: Trained & qualified personnel will give increased efficiency due to the better quality production through the application of skills & also reduce production cost & waste.

iii) Quality of machines: Better quality equipment will result in efficient working due to lack or scarcity of breakdowns & thus reduce the cost of defectives.

iv)Quality of management: A good management is imperative for increase in efficiency, harmony in relations, growth of business & markets.

Types of Control:

1) Process Control: Process controls are quality systems based on preventing defects by controlling and monitoring manufacturing processes. Because no defective product is produced, these processes can achieve much higher quality levels. Processes must be rigorously characterized, understood, and controlled for this system to be effective. Here we want to ensure that the proportion of defective items in the manufactured product is not too large. This is achieved through the technique of control charts.

2) Product Control: Product controls are quality systems that focus on sorting and isolating defective product. Because the process generates defective product, efforts are made to identify, sort, and segregate them. Management also must be prepared to accept the greater costs that come with a quality system founded on product controls. In order to have quality system based on controlling product, robust systems for material control, calibration, maintenance, and rework (as applicable) are required.

SHEWHART CONTROL CHART:

Control chart are most commonly used methods of statistical process control which monitors the stability o fa process. It is a powerful graphical tool of detecting assignable causes of variation. This technique was developed by Dr. Walter A. Shewhart in 1924.Control chart consists of three lines: a) a central line b) upper control limit c) lower control limit. The central line denotes the expected value of the statistics such as length, diameter etc. of a product. Then the line corresponding to the limits prescribed are drawn. One lying above the central line and indicating the upper limit of the statistic is called the upper control limit. The other line lying below the central line and indicating the lower limit of the statistic is called the lower control limit.