What is control chart
It helps to determine whether or not a process is operating consistently or if a special cause has occurred to change the process mean or variance.
The control chart have a centre line that represents the average value of the quality characteristic corresponding to the in-control state.Two other horizontal lines, called the upper control limit (UCL) and the lower control limit (LCL).
→Control Chart in 7QC tool is a type of run chart used for studying the process variation.
Use of control chart
To find, is there any change in location of process average.
To find,is there any change in the spread of the process.
To find, is there any change in shape.
History of control chart
The control chart was invented by Walter A. Shewhart working for Bell Labs in the 1920 so this is called Shewhart control charts.
What is Variability?
variability is define as the net result of many factors , which are constantly affecting the process
Cause of variability:
Common cause are those cause which randomly affect the system, they are always present and built in to the process it self. this is also called as "inherent cause".
Special cause are those cause which represent non-random variation in process , which can be identified to a particular cause.
special variation might be caused by one or more of the following
Man-Setup (Speed, Feeding), accuracy, training, Experience etc.
Machine- Accuracy, Calibration, wear etc.
Material- Different material , mixing accuracy etc.
Measurement- Repeatability, Precision, accuracy, calibration etc.
Environment- Temperature , humidity etc.
Types of data
There are basically two type of data
These types of data are often referred to as discrete data.There are two kinds of attributes data:yes/no type of data and counting data.
A quality characteristic that is measured on a numerical scale is called a variable. Examples
include dimensions such as length or width, temperature, and volume
Types of control chart
Control chart for variable
The x̅ and R control charts are widely used to monitor the mean and variability of variables
Step to Make x̅ ,R Chart
Collect the data
Calcuate the average of sub group
Calculate the over all average
Calculate the range
Calculate the average of range
Calculate the control limit
data plot in graph
x̅ ,R Control Chart Formula
X is the individual value
n is the sample size
X bar is the average of reading in a sample
R is the Range (the difference between largest and smallest value in each sample)
R bar is the average of all the ranges.
UCL is Upper control limit
LCL is Lower control limit