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Tema 3: Control Estadstico de Procesos (SPC)- IN ENGLISH

Tema 3: Control EstadTema 3: Control Estadstico de Procesos (SPC)stico de Procesos (SPC)-- IN ENGLISHIN ENGLISH

Statistical Process Control (SPC)StatisticalStatistical ProcessProcess Control (SPC)Control (SPC)

IntroductionThe variability of a production process

Assignable causesNon-assignable causes

Control by variablesThe capability of a process. Capability indexControl by variables graphsInitial study of a process. Determining the capacityMonitorization process

Control by attributes

SPC. IntroductionSPC. SPC. IntroductionIntroduction

ProductionprocessRaw material

Finished product

Does it have sufficient quality? Is it adequate?Does it have sufficientquality in order to be

competitive in the market?

SPC. IntroductionSPC. SPC. IntroductionIntroduction

ProductionprocessRaw material

Finished product

OBJECTIVE

To test that the process is operating under optimum conditions

To obtain information that allows the process to be improved

HOW?

Measurement during the production process

Specified measurement intervals

Graphical representation of the evolution of a quality characteristic

SPC. IntroductionSPC. SPC. IntroductionIntroduction

SPC. IntroductionSPC. SPC. IntroductionIntroduction

Suppose X is the variable of interest (for example: length).Periodic samples of X are takenIn the control graphs the evolution of a statistic of X is plotted. (For example, the sample mean)

1x 2x 3x

Upper control limit

Lower control limit

If any point is outside the control limits, or if non-random behaviour is observed, this isevidence that the process is not operating under optimum conditions.

Statistical Process control. TYPES:

By variables (ranges): we analyze a measureable characteristic

of the production (for example, the diameter of an axis)

By attributes (defective / non-comforming fraction): we analyze

a qualitative characteristic of the production (for example: accept /

defect)

SPC. IntroductionSPC. SPC. IntroductionIntroduction

Statistical Process Control (SPC)StatisticalStatistical ProcessProcess Control (SPC)Control (SPC)

IntroductionThe variability of a production process

Assignable causesNon-assignable causes

Control by variablesThe capacity of a process. Capability indexControl by variables graphsInitial study of a process. Determining the capacityMonitorization process

Control by attributes

SPC. Variability of a processSPC. SPC. VariabilityVariability ofof a a processprocess

PROCESS CHARACTERISTICS:

VARIABILITY: two outputs never the same.

REPEATABILITY: the capability of production with a stable level ofquality

processinputs outputs

SPC. Variability of a processSPC. SPC. VariabilityVariability ofof a a processprocess

VARIABILITY: Even though an exhaustive relationship of the causes of the

variablity cannot be given, it is interesting to classify as either causes assignable

to specific factors (and therefore controllable) and unassignable causes.

ASSIGNABLE CAUSES

Produced by important factors that can be controlled in order to maintain

them at acceptable levels. The appear sporadically.

They produce unstable, significant and UNPREDICTABLE variability

NON-ASSIGNABLES CAUSES

Produced by secondary factors whose effect is not considered significant.

They are a permanent and inherent part of the production process.

They produce stable, non-significant, accepted and PREDICTABLE

variablity

PROCESS IN CONTROL:PROCESS IN CONTROL:PROCESS IN CONTROL:PROCESS IN CONTROL:We say a process is IN CONTROL when only

non-assignable causes are present

NON- ASIGNABLES CAUSES stable, non-significant, PREDICTABLE variability

SPC. Variability of a processSPC. SPC. VariabilityVariability ofof a a processprocess

The control graphs allow thepresence of assignable

causes in the process to be detected

Statistical Process Control (SPC)StatisticalStatistical ProcessProcess Control (SPC)Control (SPC)

IntroductionThe variability of a production process

Assignable causesNon-assignable causes

Control by variablesThe capability of a process. Capability indexControl by variables graphsInitial study of a process. Determining the capacityMonitorization process

Control by attributes

SPC. Capacity of a processSPC. SPC. CapacityCapacity ofof a a processprocess

PREMISE:

The characteristic of production that we control (for example: thediameter of an axis, the packaged weight ) follows a normal distribution WHEN the process is under control.

X N (, )

Almost all of the production (99.7%) will be in the interval:

( 3 ; + 3)

We call CAPACITY over the previous interval

CAPACITY = 6

SPC. Capacity of a processSPC. SPC. CapacityCapacity ofof a a processprocess

PROBLEMNot all of the products that we produce can be valid.

We only accept those that satisfy given specifications.

ToleranceToleranceToleranceTolerance intervalintervalintervalinterval:::: set of values of X that are considered to be

PRODUCT TOLERANCE INTERVAL (LT2 LT1)

LT1 LT2

This is defined by the client, or on occasions by the designer of the product or the personresponsable for the process (it often represents an objective value, , and an acceptable error, L):

L

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced in our process.

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3

LT1 LT2

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced in our process.

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3

LT1 LT2 DEFECTS!!

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced in our process.

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3

LT2LT1

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced in our process.

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3

LT2LT1 NO DEFECTS!!

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced in our process.

Mean,Std. dev.1,5

Normal Distribution

x

density

-24 -14 -4 6 16 26

0

0,02

0,04

0,06

0,08

3 + 3LT1 LT2

SPC. Capacity - Capability indexSPC. SPC. CapacityCapacity -- CapabilityCapability indexindex

The relation between the CAPACITY of a process and theTOLERANCE INTERVAL of a product gives us information aboutthe defects produced