Sunday, June 8, 2008

CONTROL CHART

These are graphical methods to distinguish between avoidable and unavoidable variations of process output. There are various control charts but basically all are similar, each will have control line (Average Line) denoting the expected behavour and an outer limit called the upper control limit within which deviations are permitted from the mean value. In control charts whole production line is divided into a no. of homogenous sub groups.







Y



Upper Control


Control Line


Lower Control Line


O X
1 2 3 4 5 6 7 8 9 10
Rational Sub Groups

Type of Control Charts:

1. Control Chart far Variables: Such control charts are used to measure those quality characteristics which can be measured and expressed in specific units e.g. dimensions physical characteristics like density etc. such charts are further divided on to parts:
(a) Mean Chart or X Chart: This chart is used for analysis and controlling the mean value of a variable such as size or strength of materials.

Y
End Limit
Upper Control

Control Line

Lower Control Line


O X



(b) Range Chart: This chart is used for analyzing and controlling the variability of a variable with the help of standard deviation and range of a particular quality control variable.

Y

End Limit
Upper Control

Control Line

Lower Control Line


O X


2. Control chart for Attributes: Attributes are such parameters which can’t be measured in specific units such as cracks scratches, air bubbles, defective knots in clothes. This chart is used for measuring defects are known as control chart for attributes. These are of two types:

(a) ‘P’ Chart: It is the percentage of defectiveness it is based on binomial distribution.

(b) ‘C’ Chart: It is used for controlling the number of defects per sample unit. It is used where chances of defects are high while accuracy rate is low. It is based on POISSION Distribution e.g. Textiles, Automobiles, Electronic Equipments etc.

QUALITY ASSURANCE

The responsibility of quality control department has increased tremendously in most of the industries that resulted in increase information requirements. So quality assurance involves new product developments, modernization of methods, reliability of the product, so that customers can be served with the quality and duration can be removed from the production process to enhance quality.

According to the ANST/ASQC, “Quality assurance are all those plans and systematic actions necessary to provide confidence that a product of service will satisfy given need.”

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