Case Example: Problem Solving In A Qc Circle Using Some Of The Seven Tools (Hamanako Denso)

Introduction:

The basic reason for dealing with stratification is that it enables an effective causal analysis to be carried out and so improves the design of effective prevention methods. The case was published in 1984 in Quality Control Circles at Work—cases from Japan’s manufacturing and service sectors.

(c) Reason for starting the project

The company had begun a zero-defect campaign. Since breaks were the most serious defect in the regulator coils for which we were responsible, we chose this as our project.

(d) Goal setting

We set a target of reducing the break rate from its current (June 1977) value of 0.2% to 0.02% by the end of December.

(e) Understanding the present situation

I looked over 500 defective coils manufactured during June to see where the breaks had occurred and showed 206 of them to the entire circle. Together, we summarized the general results of this study in a Pareto diagram.

(f) Study of causes

We decided to focus on breaks at the beginning of the winding and had each member suggest three reasons why breaks might occur there. We then arranged these suggestions in a cause-and-effect diagram.

1. Individual differences: to see how different workers performed, we had three people wind coils on the same machine (No. 1). These turned out to be no great difference in the rate of defects.

2. Material differences: three gauges of copper wire are used. All of the breaks occurred with the thinnest gauge (diameter 0.14 mm).

3.Machine differences: breaks occurred on all the machines but we found that the rate increased with the tension load.

4.Correlation between tension load and rate of breaks was determined from a study of

3000 coils produced at each workstation. I did this analysis myself and presented the results to the circle members.

Our QC circle then made another cause-and-effect diagram to try to find the reason for the variation in tension. We investigated and planned studies of the spring pressure, the felts and rollers A and B. A special instrument was needed to measure spring pressure, so we asked the production engineering section to make that measurement.