© ABSL Power Solutions 2007 © STM Quality Limited STM Quality Limited STATISTICAL PROCESS CONTROL TOTAL QUALITY MANAGEMENT Introduction to S.P.C.

© STM Quality Limited STM Quality Limited What is meant by Statistical Process Control?

© STM Quality Limited STM Quality Limited This is a control system which uses statistical techniques for knowing, all the time, changes in the process. It is an effective method in preventing defects and helps continuous quality improvement. Statistical Process Control (S.P.C.)

© STM Quality Limited STM Quality Limited What does S.P.C. mean? Statistical: Statistics are tools used to make predictions on performance. There are a number of simple methods for analysing data and, if applied correctly, can lead to predictions with a high degree of accuracy.

© STM Quality Limited STM Quality Limited What does S.P.C. mean? Process: The process involves people, machines, materials, methods, management and environment working together to produce an output, such as an end product.

© STM Quality Limited STM Quality Limited What does S.P.C. mean? Control: Controlling a process is guiding it and comparing actual performance against a target. Then identifying when and what corrective action is necessary to achieve the target.

© STM Quality Limited STM Quality Limited S.P.C. Statistics aid in making decisions about a process based on sample data and the results predict the process as a whole.

© STM Quality Limited STM Quality Limited The Process PeopleMachinesMaterial ManagementMethodsEnvironment Output

© STM Quality Limited STM Quality Limited S.P.C. is statistical analysis of the predictability and capacity of a process to give a uniform product. Definition

© STM Quality Limited STM Quality Limited The Aim of S.P.C. - Detection Strategy Detection: This focuses on identification of problems after production, by 100% inspection or by customer complaints. It is a historically-based strategy.

© STM Quality Limited STM Quality Limited The Aim of S.P.C. - Detection Strategy Detection Drawbacks: Production is already made. Customer dissatisfaction. Inflated costs - rework; inspection. Repetitive problems. Neglected improvements.

© STM Quality Limited STM Quality Limited The Aim of S.P.C. - Prevention Strategy Prevention: This focuses on in-process production and identification of problems through analysis of process capability. It is a future-orientated strategy.

© STM Quality Limited STM Quality Limited The Aim of S.P.C. - Prevention Strategy Prevention Benefits: Improved design and process capability. Improved manufacturing quality. Improved organisation. Continuous Improvement.

© STM Quality Limited STM Quality Limited The S.P.C. has to be looked at as a stage towards completely preventing defects. With stable processes, the cost of inspection and defects are significantly reduced. S.P.C. as a Prevention Tool

© STM Quality Limited STM Quality Limited Assesses the design intent. Achieves a lower cost by providing an early warning system. Monitors performance, preventing defects. Provides a common language for discussing process performance. The Benefits of S.P.C.

© STM Quality Limited STM Quality Limited Process Element Variable Examples Machine………………………….Speed, operating temperature, feed rate Tools………………………………..Shape, wear rate Fixtures…………………………..Dimensional accuracy Materials…………………………Composition, dimensions Operator…………………………Choice of set-up, fatigue Maintenance…………………Lubrication, calibration Environment…………………Humidity, temperature Process Variations

© STM Quality Limited STM Quality Limited No industrial process or machine is able to produce consecutive items which are identical in appearance, length, weight, thickness etc. The differences may be large or very small, but they are always there. The differences are known as ‘variation’. This is the reason why ‘tolerances’ are used. Process Variations

© STM Quality Limited STM Quality Limited Common causes are the many sources of variation that are always present. A process operates within ‘normal variation’ when each element varies in a random manner, within expected limits, such that the variation cannot be blamed on one element. When a process is operating with common causes of variation it is said to be stable. Stability

© STM Quality Limited STM Quality Limited The process can only be termed ‘under control’ if it gives predictable results. Its variability is stable over a long period of time. Process Control

© STM Quality Limited STM Quality Limited Graphs and charts have to be chosen for their simplicity, usefulness and visibility. They are simple and effective tools based on process stability monitoring. They give evidence of whether a process is operating in a state of control and signal the presence of any variation. Process Control Charts

© STM Quality Limited STM Quality Limited Data Interpretation Consider these 50 measurements

© STM Quality Limited STM Quality Limited Data Interpretation As a set of numbers it is difficult to see any pattern. Within the table, numbers 30 and 37 were outside the tolerance – but were they easy to spot? A way of obtaining a pattern is to group the measurements according to size.

© STM Quality Limited STM Quality Limited Data Interpretation – Tally Chart The tally chart groups the measurements together by size as shown. The two parts that were out of tolerance are now easier to detect (36.38mm).

© STM Quality Limited STM Quality Limited Tally Chart - Frequency The tally chart shows patterns and we can obtain the RANGE mm to 36.38mm. The most FREQUENTLY OCCURRING size is 36.35mm

© STM Quality Limited STM Quality Limited Tally Chart - Information The tally chart gives us further information: The number of bores at each size; The number of bores at the most common size; The number of bores above and below the most common size (36.35mm) - number above 36.35mm is 7+6+2=15 number below 36.35mm is =19

© STM Quality Limited STM Quality Limited Histogram We can redraw the frequency chart as a bar chart known as a histogram:

© STM Quality Limited STM Quality Limited Smoothed Frequency If we now draw a smooth curve through the top of each box we get a bell-shaped pattern:

© STM Quality Limited STM Quality Limited The bell-shaped pattern is fairly typical of most industrial processes. There is a central value at the highest point of the curve and the pattern of results spread out equally on both sides of the central value. The further we move from the central value, the fewer values we will find. This bell-shaped pattern is known as the ‘NORMAL DISTRIBUTION’. Distribution

© STM Quality Limited STM Quality Limited The bell-shaped pattern known as ‘Normal Distribution’ is one that we would expect to see where the process is running in a stable condition. Where you work in a manufacturing environment, why not perform the exercise on 50 parts for yourself and see if your process is in a stable condition! Normal Distribution

© STM Quality Limited STM Quality Limited S.P.C. is a very effective method in preventing _______. Self-test (1 of 9) defects S.P.C. uses statistical techniques for knowing _______________. process variation

© STM Quality Limited STM Quality Limited Controlling a process is guiding it and comparing ________________ against a target and identifying when and what ______________ is necessary to achieve the target. Self-test (2 of 9) actual performance corrective action

© STM Quality Limited STM Quality Limited The process may involve a combination of people, machines, ________, methods, management and ___________ working together to produce an output such as an end product. Self-test (3 of 9) materials environment

© STM Quality Limited STM Quality Limited Self-test (4 of 9) uniform predictability S.P.C. is a statistical analysis of the __________ and capacity of a process to give a _______ product.

© STM Quality Limited STM Quality Limited Self-test (5 of 9) historically already made Disadvantages of 100% detection inspection are that production is ___________ and that it is __________ based.

© STM Quality Limited STM Quality Limited Self-test (6 of 9) future quality An advantage of prevention inspection is that _______ is improved. It is a strategy that is _____ orientated.

© STM Quality Limited STM Quality Limited Self-test (7 of 9) variation identical No industrial process is able to produce consecutive items which are _______. There are always differences, known as _______.

© STM Quality Limited STM Quality Limited Self-test (8 of 9) stable predictable The process can only be termed ‘under control’ if it gives __________ results. Its variability is ______ over a long period of time.

© STM Quality Limited STM Quality Limited Self-test (9 of 9) normal histogram A frequency chart drawn with bars is known as a _________. A smooth curve drawn in a bell-shaped pattern is known as the ______ distribution curve.