Unit-3 Control chart Presented by N.vigneshwari

Today’s topic  Control chart

Variation-The axiom of manufacture Before describing the various control charts, it is appropriate to have the knowledge of process variations. One of the axioms or truisms of manufacturing is that no two objects are ever made exactly alike. When variations are very small, it may appear that items are identical; but precision instruments will show differences Four sources of variations: There are four factors that contribute to these variations. They are processes, materials, operators and miscellaneous factors. The source of miscellaneous variations include environmental factors such as heat, light, radiation and humidity. Types of variations: There are two kinds of variations. They are 1.Assignable causes of variations 2.Chance causes of variations

Assignable causes of variations Assignable causes of variations are larger in magnitude and can be easily traced and detected. The reasons for assignable causes of variation are due to: (a) Differences among machines (b) Differences among materials (c) Differences among processes (d) Differences in each of these factors overtime, and (e) Differences in their relationship to one another The prime objective of a control chart is detecting assignable causes of variation by analyzing data (say in length, diameter, weight of a part).

Chance (or random) causes of variations  Chance causes of variations are inevitable in any process. These are difficult to trace and control even under best conditions of production. All occur at random  Random variables cannot be avoided. They are caused by factors such as human variability from one operation cycle to the next, minor variations in raw materials, and machine vibration.

Distinction between chance and assignable causes of variation  When one chance causes are present in a process, the process is considered to be in control  When an assignable causes of vibration are also present, then the variation will be excessive and the process is classified as out of control i.e., beyond the expected normal variation.  Therefore, the objective of control charts is to restrict the chance causes of variation by detecting and eliminating the assignable causes

Control charts  A control chart is a statistical technique for controlling the quality of a product being manufactured. It was first devised by Dr.Walter A.Shewart after whose name these charts are also called shewart charts. The main advantage of a control chart is that it can predict the rejects when they are likely to occur, which enables corrective action to be taken before a defective product is actually produced. It is based upon the fact that variability does exist in all the repetitive processes  A control chart is a graphical representation of the collected information. The information may pertain to measured quality characteristics or judged quality characteristics of samples. It detects the variation in processing and warns if there is any departure from the specified tolerance limits.  In other words, control chart is a device which specifies the state of statistical control, second a device for attaining statistical control, and third, a device to judge whether statistical control has been attained. The control limits on the chart are so placed as to disclose the presence or absence of the assignable causes of quality variation. This makes possible the diagnosis and correction of many productions troubles and often brings substantial improvements in product quality and reduction of spoilage and rework.

 Moreover, by identifying certain of the quality variations as inevitable chance variations, the control chart tells when to leave the process alone and thus prevents unnecessarily frequent adjustments that tend to increase the variability of the process rather than to decrease it.  With the help of a control chart it is possible to find out the natural capability of a production process, which permits better decisions on engineering tolerances and better comparisons between alternative designs and also between alternative production methods.

Characteristics of control charts A control chart is a time-ordered diagram to monitor a quality characteristic, consisting of: 1. A nominal value, or centre line, the average of several past samples 2. Two control limits used to judge whether action is required, an upper control limit (UCL) and a lower control limit (LCL)

Objectives of control charts 1.Control charts are used to analyze a process with a view to one or more of the following objectives: To secure information to be used in establishing or changing specifications or in determining whether a given process can meet specifications To secure information to be used in establishing or changing production procedures. Such changes may be either elimination of causes of variation or fundamental changes in production methods that may be called for whenever the control chart makes it clear that specifications cannot be met with present methods. 2. They are used to provide a basis for current decisions during production as to when to hunt for causes of variation and take action intended to correct them, and when to leave a process alone. This is nearly always one of the purposes of any control charts for variables.

Types of control charts  Control charts for variables  Control charts for attributes

Control charts for variables  The quality characteristics which can be measured and expressed in specific units of measurement are called variables  Control charts based upon measurements of quality characteristics are called as control charts for variables Types of variable control charts : The most commonly used variable control charts are  X bar or average charts  R or range charts, and  Standard deviation chart

Control charts for attributes  Where the nature of product is such that the quality characteristic cannot be measured quantitatively, the items are classified only defectives and non- defectives at the time of final inspection. There can be a number of factors responsible for defining any item to be defective and the separate record for each cause may be out of question. Types of attributes control charts used are:  P chart  Np chart  C chart  U chart

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