Control charts

Control Charts
Control Charts are use to distinguishes between specialcause or common-cause of variation that is present in a process. There are two basic types of control charts:-

Variables

-

Quantitative data (Measured)

Attributes

-

Qualitative data (Counted)

Variable Control Charts
Use actual measurements for charting
Types:-

Average & Range charts
Median & Range charts
Average & Standard deviation charts
Individual & Moving Range charts
Run Charts

Attribute Control Charts
Use pass/fail or go/no-go judgment

Types :p - chart np - chart

c - chart u - chart

OBJECTIVES OF VARIABLE CONTROL CHARTS

For quality improvement.
To determine the process capability.
For decisions in regard to product specifications.

For current decisions in regard to the production process.
For current decisions in regard to recently produced items. Average & Range charts ( and R)
Guidelines for subgroup sizes (n):-

1.

As n increases the CL become closer to central line.

2.

As n increases the inspection cost per subgroup increases. 3.

Distributions for averages of subgroups are nearly normal for n = 4

4.

If n = 10 use the s -chart instead of R-chart

5.

n = 5 commonly being used in industry

6.

For destructive test use n = 2 but increase the sampling frequency

Formulae for calculating Control Limits (CL)
Average Chart:-

UCLx  x  A2 R
LCLx  x  A2 R

A2 = Constant
N

x

x i 1

N

N

i

R

R i 1

N

Range Chart:-

UCLR  D4 R
LCLR  D3 R
D4 & D3 = Constants

Example

1
2
3
4

1
5.55
5.65
5.6
5.45

2
5.55
5.4
5.45
5.55

3
5.55
5.6
5.6
5.7

4
5.55
5.55
5.55
5.55

5
5.55
5.6
5.65
5.6

6
5.6
5.65
5.55
5.6

7
5.65
5.55
5.6
5.65

8
5.75
5.8
5.6
5.85

9
5.65
5.55
5.65
5.75

10
5.7
5.75
5.8
5.85

11
5.6
5.65
5.45
5.4

12
5.45
5.5
5.45
5.6

Mean
Max
Min
Range

5.56
5.65