Statistical Process Control
Case Study
3

The Quality Improvement Team at a manufacturer of wood components visited a customer and examined items in the scrap and rework bins. After looking at each item and talking with the customer, the team agreed on categories of nonconformities and developed precise definitions for each category. They created a table:

Reason Frequency Relative cost of rework
Size out o specs 194 4.92
Loose knots 18 2.34
Raised grains 4 1.76
Dents 3 1.75
Stain 31 1.00
Fuzzy grain 105 .72
Splits 11 .32
Machine tear-out 61 .09
Burn marks 44 .06
Oil grease marks 2 .03
Total 473

Question: Conduct Pareto Analysis to identify most important non-conformities with respect to frequency as well as relative cost to rework. Which is the primary non conformity?

The company now knows the primary nonconformities and therefore where to focus initial efforts of an SPC program. But it is not known, however, the specific processing steps that lead to a given nonconformity—that is, where and how the problem arises—and therefore XYZ does not yet know where or what to monitor.
To help discover the specific steps in the process that lead to a given nonconformity, it is helpful to develop a flowchart for the process.

Developing a flowchart: An example

XYZ Forest Products Company produces wooden handles for push brooms. Their customers produce finished brooms by adding a rubber grip to the top of the handle, inserting a threaded metal ferrule to the bottom of the handle, and attaching the broom head.
Last year, business began to fall off for XYZ; orders dropped 40 percent in just 6 months. Several customers stated that the competition’s quality was better. A few customers had begun asking XYZ to provide documentation for process performance—namely histograms, control charts, and process capability indices. Therefore, XYZ was inspired to use SPC.
Because customers reported several different quality problems, Pareto analysis