Doctor Q’s Everything you need to about SPC( But your mother was unable to tell you)

Statistical Process Control Uses statistics and the distribution of data to

predict process results and improve on them Is THE MANAGEMENT OF VARIATION Normal distribution is heart and soul Things to know – average, std deviation and

normal distribution

Normal Distribution

Symmetrical distribution of data-BELL CURVE

Most data is normally distributed in operations or can be transformed to normal

Defined like the orbit of a planet

Normal Curve: Scores on a Standardized I.Q. Test

                                                      

    Carl Friedrich Gauss invented the normal distribution in 1809 as a way to rationalize the method of least

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The Normal Distributiontools Control Charts

tells manager when process is outside or tending outside normal distribution

Control Charts selection depends on the type of data– attribute or variable

Control Charts

Moving pictures of your normal distributions

Control limits represent the normal six standard spread

When outside the limits – you are out of control

There is an assignable cause if you are outside limits

The inside six sigma spread represents the “normal” distribution

The “normal variation”

Level of training and experience Equipment – capability Equipment – level of maintenance Standard Operating Standards

Level of ISO or TS implementation

                  

Ed edward Deming

Process capability

Compares how the process is actually performing ( normal distribution of sample) against what the customer wants (requirements or specifications)

Process Capability

Is the process capable of meeting the customer requirements

Cp – the potential of meeting requirements if the process is centered

Cp = ------------------------------

Cp should be greater ONE

Customer requirements or specifications

Six Sigma distribution of operating data

Cpk or actual process centering Is the process capable as centered

Cpk =

= difference of average and upper or lower spec/3 sigma

The lower is the reported Cpk (upper and lower spec

Greater than one

Cement example

You are selling 50 lb bags at spec of +/- 2 lbs

The sample average shows 50.9 with a std deviation of .4

First draw Spec 48____________________52 Actual 50.9 -1.2 (3 std Dev)______________ 50.9 + 1.2 (3 std

dev)

49.7_______________________________ 52.1

Note you have some out of spec 52.1 vs 52

Cp = 4lbs/ 6x.4= 4/2.4 = 1.666 Ideally capable BUT some out of spec thus

not centered Cpk upper = upper spec-average/3 sigm

52-50.9/3x.4= 1.1/1.2=.916 Cpk lower = average-lower spec /3 sigma 50.9-48/3x.4=2.9/1.2=2.9 Therefore Cpk is .916 and less than one –

not centered

Homework

You are manufacturing “12 inch” rods for an equipment manufacturer

The manufacturer has a specification of 12 inches +/- .1 inch

You sample your process and find an average of 12.06’’ with a standard deviation of .02”

Calculate Cp is you process ideally capable

Will all the material be in specification If the process was not capable (do

regardless of answer above) – name 5 options you have!

Name a sixth for extra credit Without calculating is Cpk less than one Extra credit calculate Cpk HINT: see example in course documents