Manufacturing ERP Software
Manufacturing: Problem Solving in Production, Part 9

Manufacturing: Problem Solving in Production, Part 9

By Bob Sproull

Review of Problem Solving, Part 8

In my last post, I presented an indispensable problem solving device known as the cause and effect diagram (aka Ishikawa Diagram or fishbone diagram). I talked about how its structure makes it an efficient visual for understanding a problem and its possible contributing factors. Then I detailed the sequence of steps for creating a C&E diagram and demonstrated an actual example.

In today’s post, I will complete my discussion on problem solving tools by introducing the causal chain. The intelligent application of both the C&E diagram and the causal chain in a systematic problem solving process reliably leads manufacturing teams down an efficacious path to the ultimate root cause.

As I have stated in previous posts, much of what I am presenting in this series of posts is taken directly from my first book written in 2001, [1] Process Problem Solving – A Guide for Maintenance and Operation’s Teams.

Causal chains enable sequencing to lead to root causes

When problems are discovered and investigated, a chain of events usually leads to the source or root cause. One of the most effective techniques for uncovering the root cause is the causal chain (aka chain of causality). Causal chains are stepwise evolutions of problem causes. They are typically seen in one of two chain-like patterns, as depicted in the figure below.

ECi manufacturing blog: step diagram

Each step (or sawtooth) represents an object in a normal or abnormal state. The object is placed above the step-line and its state is placed below the line, directly underneath the object. Examples of objects and states include:

Object

State

Fuel Line

Plugged

Circuit Breaker

Tripped

Tire

Flat

Motor

Stopped

Each step down is the effect of the preceding step and the cause of the next step down. That is, the information on the step to the left is always the cause of the information on the step to the right. The diagram starts with the problem. In this case, a punch press has stopped. It then continues in a stepwise fashion, as the question “Why?” is asked for each effect.

ECi manufacturing blog: second step diagram

An investigation of the problem reveals that a pressure switch had tripped the circuit breaker to the electric motor powering the press. In this example, we started with the problem symptom—the stoppage of the punch press—and continued in a stepwise direction to the left by asking “Why?” until we arrived at the root cause of the problem.

In a list format, the sequence was as follows:

  1. The punch press stopped because the motor stopped.
  2. The motor stopped because the current stopped.
  3. The current stopped because the pressure switch was open.
  4. The pressure switch opened because the air pressure was too low.
  5. The air pressure was too low because the air compressor failed.
  6. The air compressor failed because the oil level was too low.

In six steps, we found that the root cause of the press stoppage was ultimately a low oil level supplying the compressor. This team could have continued on and asked why the oil level was too low, but that was clearly a personnel-created failure. So they simply refilled the oil supply, and the correction worked its way down the stairway to solving the press stoppage.

To take this example even further, let’s suppose we had two punch presses that used the same air compressor. If this had been the case, we would have had two identical causal chains that connected at the common source of the problem, the low oil level. It is important to remember that simultaneous symptoms usually have a common cause.

Coming in the next post

To continue guiding manufacturing teams, I will begin a new series with the next blog post, covering measurements for effective decision making. As always, if you have any questions or comments about any of my posts, leave me a message and I will respond.

Until next time.

Bob Sproull

References:

[1] Bob Sproull - Process Problem Solving – A Guide for Maintenance and Operation’s Teams, 2001, Productivity Press

[2] Ishikawa, Kaoru, Guide to Quality Control (Asia Productivity Organization, 1986)

Bob Sproull

About the author

Bob Sproull has helped businesses across the manufacturing spectrum improve their operations for more than 40 years.

facebook-icon facebook-icon linkedin-icon linkedin-icon twitter-icon twitter-icon blog-icon blog-icon youtube-icon youtube-icon instagram-icon instagram-icon Bookmark this page Google +