In establishing & maintaining quality control, it is essential to realize that error; failures to meet the standards always have a tendency to occur. Moreover, they tend to incite or provoke others of their own kind and so make a bad matter so worse. The number of such errors and failures can be reduced and held at a reasonable minimum only by foresight and continuing cash.
image source: principalforensicservices.com
The most recent contribution to the methods and practice in quality control and inspection is the application of statistical theory. Juran has outlined the relationship theory and problems in this field.
ADVERTISEMENTS:
Satistical theory bears the same relationship to problems in its field that strength of material theory bears to problems in its field. For many years, structures were designed without the use of strength of materials theory.
Similarly, many engineering problems frequently handle hither to without the use of statistical methods can be handled in a more scientific way of these methods.
In the field of quality control, the problems susceptible of statistical analysis are those of standardization, specification and inspection; thus it includes problems both of quality of design and quality of conformance.
The principle applications of the statistical method to quality control are the following:
To problems of quality of design:
ADVERTISEMENTS:
1. To assist in determining the essential characteristics of vital raw materials and finished products.
2. To determine to what extent variations in essential characteristics affect the value of a finished product.
3. To compare different methods of measuring the essential characteristics of finished product.
4. To establish a continuous, vigilant observations of finished product which will show any significant change in essential characteristics?
ADVERTISEMENTS:
5. To determine whether significant variations in essential characteristics of finished product are due to variations in raw materials, or in manufacturing processes, or can be assigned to pure chance within statistically determined limits of probability.
6. To give warning of any hitherto under recognised source of variations in the essential characteristics of a finished product, by nothing that the allowable limits of pure chance have been exceeded.
7. To judge between proposed methods for lessening or eliminating the variations in essential characteristics of finished products.
Benefits/Advantages of Statistical Quality Control:
Benefits & advantages of statistical quality control are as follows:
1. It provides a means of detecting error at inspection.
2. It leads to more uniform quality of production.
3. It improves the relationship with the customer.
4. It reduces inspection costs.
5. It reduces the number of rejects and saves the cost of material.
6. It provides a basis for attainable specifications.
7. It points out the bottlenecks and trouble spots.
8. It provides a means of determining the capability of the manufacturing process.
9. It promotes the understanding and appreciation of quality control.
Principles of (Statistical) Quality Control:
The principles that govern the control of quality in manufacturing are:
1. Control of quality increases output of salable goods, decreases costs of production and distribution, and makes economic mass production possible.
2. The quality of manufactured goods is variable with an upward trend under conditions of competitive manufacturing.
3. The conformance of finished product to its design specifications and standards should be accomplished by avoiding the making of non-conforming materials rather than by storing the good from the bad after manufacturing is completed.
Rice has pointed out that it is necessary to fit the organisation and procedure of quality control to the situation in each plant. To do this, certain fundamental principles of statistical Quality Control must be adhered to for success in applying these techniques to manufacturing processes. Rice has stated the principles of statically quality control:
1. Variability exists in every repetitive operation, statistical methods enable management to determine what the expected or chance variability of the process is, and thus isolates the excessive variations due to an assignable cause from those due to chance. These may then be studied for the cause and corrective steps taken.
2. Wherever like products are turned out in quantity, statistical quality control techniques are applicable.
3. A state of statistical quality control, in which an operation produces articles that remain consistently within their range of chance variation, so that no assignable or findable cause is present, is not usually found where statistical control techniques have not been used.
4. Quality must be built into a product. It cannot be introduced through inspection. In the words of W. C. Deming “not how much product, but how much acceptable product is what counts.”
5. A state of control must be established at a satisfactory quality level before maximum efficiency in the operation can be obtained. The controlled process to be satisfactory must produce a satisfactory average product as well as are that does not vary from the average more than would be expected by chance.