statistical process control (spc)

Statistical Process Control (SPC)

QUALITY CIRCLE FORUM OF INDIAVADODARA CHAPTER

612, Bluechip Business Centre, Near Vadodara Stock Exchange, Sayajigunj, Vadodara – 390 005

Telefax:0265-2361184, Phone: 0265-6596140E-mail : [email protected]

PRESENTED BY

QCFI/SPC-Eng/SCP/1 of 28/12-12-2006


1. OBJECTIVES OF SCP

1.1 General Objectives : One or more of the following objectives• To increase knowledge about the process• To Steer a process to behave in the desired way• To reduce variation in final product• To improve performance of a process



1.2 Specific objectives:Considerations :• The market being served• Nature of the product• Process technology• Customer needsEffective implementation of SPC reduces cost and increase profile How?


Statistical Process Control (SPC)1.2 Specific objectives: (Contd...)

• By managing the process more economically aiming at greater consistency and improvement.• By reducing variation around a target value in either a final product or process output parameter. • By transferring variation in an in-process product

parameter to a controllable or manipulated process variable. • By providing signals and evidence of how a process is behaving and how it is likely to behave.



• By assessing and quantifying what quality and consistency level the process is currently capable of producing. • By identifying when and where to look for special

causes and / or make preventive process adjustments and when not to. • By pointing to potential root causes or failure modes and their sources. • By identifying sources of poor yield and variability

and detecting special causes for the same. QCFI/SPC-Eng/SCP/5 of 28/12-12-2006


• By increasing speed of detection and reduce trouble shooting costs.• By controlling and / or reducing common cause variation through process design changes and other system changes to procedures. • By increasing knowledge of how the system of causes affects the process, to help make improvements to the process. SPC IS A STRATEGY FOR DEFECT PREVENTION


Statistical Process Control (SPC)2. CONCEPTS :

In implementing SPC we need to understand following concepts / terms • Process• Variation• Distribution• Stability• Common causes & special causes• Process control• Process capability


Statistical Process Control (SPC)2.1 Process :

Transformation Activities

People

Equipment

Materials

Methods

Documents /Specifications

Products /Services

A process is a set of interrelated or interacting activities which transform inputs into outputs.


Statistical Process Control (SPC)2.1 Process : (Contd...)

• Process performance has to be monitored, to help timely corrective actions to control the process.

• Information about process performance can be had from process characteristics e.g. Temperature, Cycle time, Feed rate etc.• Target values for process characteristics are to be set and monitoring is to be done to see whether actual

process confirms to these targets, to what extent. • Timely action is then taken if needed.


Statistical Process Control (SPC)2.1 Process : (Contd...)

• Action on the process Vs Action on the output

Action on the process is generally most economical for defect prevention strategy. This helps in

stability and control of variation of process output within acceptable limits.

Action on the output is generally least economical when used to defect and to correct out of

specification product.QCFI/SPC-Eng/SCP/10 of 28/12-12-2006

Statistical Process Control (SPC)2.2 Variation :

Basic to process control is the concept ofvariation.

Not two products or characteristics areexactly alike because process contains many sources of variability.

The difference among products may be large or immeasurably small, but they always

exist. QCFI/SPC-Eng/SCP/11 of 28/12-12-2006

Statistical Process Control (SPC)2.2 Variation: (Contd...)

Example: A machined shaft diameter; potential sources of variation.

Machine : Clearances, Bearing wear Tool : Strength, Rate of wear Material : Diameter, HardnessOperator : Part feed, Speed accuracy of centeringMaintenance : Lubrication, Replacement of worn out

: parts Environment : Temperature consistency of power

supply.


Statistical Process Control (SPC)2.2 Variation : (Contd...)

Variation over time :

• Some sources of variation in the process cause short term piece to piece difference.

• Other sources of variation tend to cause changes in the output only over a long period of time either

gradually or stepwise or irregularly.

Hence time period and conditions over which measurement are taken will affect total variation.


Statistical Process Control (SPC)2.2 Variation : (Contd...)

View of Variation:• From requirements point of view, products

within specification limits are acceptable and those beyond specification limits are not acceptable. • To manage any process and reduce variation it must be traced to sources. The causes of variation are distinguished as COMMON causes and SPECIAL causes. QCFI/SPC-Eng/SCP/14 of 28/12-12-2006

Statistical Process Control (SPC)2.3 Distribution :

While individual measured values may all be different, as a group they tend to exhibit a pattern. This is called DISTRIBUTION. It can be desired by:• Location (Process level or centering)• Spread or dispersion (Range of values from smallest to

largest.)• Shape (Pattern of variation, whether symmetrical or skewed etc.)Any changes either in location or spread or dispersion or shape indicates changes of process performance. No change in these implies stability and therefore repeatability of the process.


Statistical Process Control (SPC)2.3 Distribution : (Contd...)

Changes In Distribution – Location

A: Original Process B: Process Level Shifted

A B

Shift in Process Level QCFI/SPC-Eng/SCP/16 of 28/12-12-2006


A

B

Change in Process Spread / Range

2.3 Distribution : (Contd...)

Changes In Distribution – SpreadA: Original Process B: Increase in spread with same location



A

B

Change in Shape – Pattern of Variation

2.3 Distribution : (Contd...)

Changes In Distribution – ShapeA: Original Symmetrical PatternB: Pattern is skewed



When distribution remains unchanged over a period of time, then process is said to be stable and Repeatable (When only common causes are present in the process).

2.4 Stability :

Time Target


Statistical Process Control (SPC)2.5 Common Causes :

They are minute and many and individually may not be measurable.The resulting pattern under the influence of common causes is called “State of Stastical Control” or just “In Control”.The advantages of maintaining a state of statistical control are: • Variation is restricted.• Process is repeatable. • Quality of future production can be predicted.


Statistical Process Control (SPC)2.5 Special Causes :

When they occur, they make the overall process distribution change. Unless they are arrested they will continue to affect the process output in unpredictable ways.

Changes in process pattern due to special causes can be either detrimental or beneficial. When detrimental they need to be identified and eliminated. When beneficial they need to be continued by making them a permanent part of the process.


Statistical Process Control (SPC)2.6 Process Control :

This is the state where only common causes are present. The proof of this is when the pattern of variation conforms to the statistical normal distribution.

It involves continuous monitoring of the process for special causes and eliminating them.

Evidence of special causes is provided by the changes in process distribution.


Statistical Process Control (SPC)2.7 Process Capability :

The ability of a process to produce within the variation permitted by tolerance is called process capability.

The process control and process capability are different.

• Process control is basic and is required for stability of a process.

• Process capability is containment of common cause variation substantially within the range provided

by the tolerance. QCFI/SPC-Eng/SCP/23 of 28/12-12-2006


2.7 Process Capability : (Contd...)

LSL USL

Process is in Control (Stable) and Capable QCFI/SPC-Eng/SCP/24 of 28/12-12-2006



LSL USL

Process is in Control but not satisfactory (Capable)




LSL USL

Process is capable but not in control because process level is not properly centered.


Statistical Process Control (SPC)2.7 Process Capability : (Contd...)

LSL USL

Process not in control and not capableQCFI/SPC-Eng/SCP/27 of 28/12-12-2006


Process Capability

Process In Control

Process Out of Control

Capable Acceptable Not acceptable

Not capable Not acceptable Not acceptable
