Short Run SPC

ASQ Portland Section 0607

By Ed Landauer, C.Q.E., P.E. November 13, 2012

Short Run SPCWhat it isWhat it isn’t

SPC is not intended to be a substitute for inspection and testing

Programs such as JIT make Short Run SPC more common

What Constitutes a Short Run?A run of 25 parts?A run of 100 parts?A run of 1000 parts?A run of 50,000 parts?A single part?

General Categories of Short RunsNot enough parts in a single production run

to develop or maintain control limits on the process

Process cycles are so short that production runs are over before sufficient data can be gathered

Many different parts are made for many different customers

Examples

Example: CNC MachineExtremely complex machineHas dozens of different toolsThat produce hundreds of different

partsWith thousands of characteristics

Example: CNC Machine- Generic Operations

Select a tool

Position a tool

Rotate a tool

Move the part

Remove the metal

Example: CNC MachineNearly all problems encountered

after the initial set-up involve the ability of the machine to position the tool precisely

For example, the location of a single hole provides information on the location of the tool in both the x and y directions

Sample Set of DataTwelve subgroups- 3 observations per

subgroup 5.0037 5.0009 4.9973 5.9974 6.0021

5.9981 5.0015 5.0023 5.0029 5.9990 5.9983

6.0018 4.9989 4.9981 4.9985 6.0015 5.9977

6.0023

7.0014 6.9991 7.0003 6.0031 5.9987 5.9987

7.0009 6.9979 6.9975 5.9985 6.0011 6.0021

6.9979 6.9995 7.0021 6.0017 6.0021 6.0001

Method 1: Target Average and Range ChartsSubtract nominal (or target) value

from the actual measurements

x* = x - nominal

Can plot several parts from a given process on a single chart

Can plot several features from a single part on a single chart

Method 1: RestrictionsNominal (or target) values must be

close to each other with the same tolerance

Or the most restrictive tolerance must be used for all

Method 1: Control Limits• Calculate averages and ranges of the deviations from target

• Control limit formulas are the same as for traditional control charts

Method 2: Standardized Variables Control Charts

Good for recurring families of partsThey are independent of the unit of

measureThey are scaled so that different

characteristics can be plotted on the same chart

They are “true” process control

charts

Method 2: CalculationsUse the normalizing transformations

Need to use historical averages from previous data in the calculations

Method 2: Control LimitsX-bar Chart -A2 and A2

R chart D3 and D4

Short Run Attribute Chart- c-chartJob shop welding processProduces small quantities of single

order only partsOperation always involves joining

parts of similar material and size

Can plot chart of weld imperfections per 100 inches of weld

Short Run Attribute Chart- p-chartJob shop welding processSimilar operation to previous slidePlot chart of the number of weld

imperfections per total welds per 100 inches of weld

SummaryOnly minor change in mechanics to

move from traditional SPC to Short Run SPC

The biggest difference in the perception of Statistical Process Control rather than Statistical Product Control

Questions?

Thank You!

EDL22@comcast.net