A Tool For Getting To The Root Of A Problem

We often focus on the symptoms of a problem without ever delving deep enough to find its true cause. Yet if we do not find and eliminate the cause then we are doomed to face the same problem again and again.

Columns From: 8/1/2007 Modern Machine Shop,

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cause-and-effect diagram

This cause-and-effect diagram shows why a machine is not making parts.

We often focus on the symptoms of a problem without ever delving deep enough to find its true cause. Yet if we do not find and eliminate the cause then we are doomed to face the same problem again and again.

Possible production problems include having trouble finding the right tools, fixtures, supplies or equipment required to run a job. Another common scenario is parts piling up at the inspection station, waiting for rework or other disposition instructions. These are symptoms or visible outcomes of problems within a company. A simple technique can help us get past the symptoms and focus on the root causes of the problems. A “cause-and-effect” diagram is one such technique.

A cause-and-effect diagram is an effective means of analyzing all possible causes of a problem so that all factors can be studied. In this type of diagram, a long horizontal arrow is used to describe the problem being analyzed. Expected causes of the problem are shown as radial lines stemming from the first horizontal line. Potential origins for these causes are then shown as horizontal lines extending from the radial lines. If the chart is developed further, then potential sources for other causes are shown as more radial lines originating from the last horizontal line. Although the diagram is a simple tool, it presents all of the necessary information in a clear, logical manner so everyone can evaluate what is causing the problem.

Let’s diagram a problem regarding machine availability. The machine is down, so that is the focal point of our diagram. In the figure on the preceding page, we show four primary causes as radial lines off the first horizontal line describing the problem “Machine not making parts.” These causes are 1) machine breakdown; 2) machine stopped/waiting for workpiece inspection; 3) machine being set up; and 4) no operator to run machine. As these “causes” are identified, sources of these causes must now be determined.

The first ultimate root cause shown on the diagram is associated with the “can’t find fixture” cause and identified as “poor fixture organization.” This root cause has now been identified and needs to be addressed by the company. Other root causes (not shown on the diagram) could surface upon further analysis. For example, on the “breakdown” line, we see that the company does not possess any spare parts. This means that the machine could sit idle for a significant period of time. The solution would be to review the parts that typically need replacement and maintain a small inventory to reduce repair delays. In addition, this company’s policy of stopping the machine while the first piece undergoes inspection could itself be a root cause of the machine not making parts. The solution would be to evaluate this current policy, comparing the lost machine time against the risk of a bad part being sent to first-piece inspection.

A company can use the cause-and-effect diagram to plan and manage permanent corrective action on machine downtime and other crucial problems we all face.

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