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Paths of Variation Part 1

Paths of Variation Part 1

By Bob Sproull


In my last post we completed our series of posts on variation by discussing the basics of a Queuing System, two very important “laws of variability” and finally, a ten point summary of primary points, principles, and conclusions relative to understanding variability.  These ten included:

  • Variability always degrades performance
  • Variability buffering is a fact of manufacturing life.
  • Flexible buffers are more effective than fixed buffers.
  • Material is conserved.
  • Releases are always less than capacity in the long run.
  • Variability early in a line is more disruptive than variability late in a line.
  • Cycle time increases nonlinearity in utilization and efficiency.
  • Process batch sizes affect capacity.
  • Cycle times increase proportionally with transfer batch size.
  • Matching can be an important source of delay in assembly systems.

In today’s post, I will present the first of three posts on a subject I refer to as Paths of Variation along with a real case study to demonstrate the teachings of paths of variation.

Paths of Variation

We’re all familiar with the positive effects of implementing Cellular Manufacturing (CM) in our workplaces such as the improved flow through the process, overall cycle time reduction, throughput gains as well as other benefits.  But there is one other positive effect that can result from implementing CM that isn’t discussed much. This potential positive impact is what CM can do to reduce variation. But before we reveal how this works, let’s first discuss the concept of paths of variation.

When multiple machines performing the same function are used to produce identical products, there are potentially multiple paths that parts can take from beginning to end as we progress through the entire process. There are, therefore, potential multiple paths of variation. These multiple paths of variation can significantly increase the overall variability of the process.

Even with focused reductions in variation, real improvement might not be achieved because of the number of paths of variation that exist within a process. Paths of variation, in this context, are simply the number of potential opportunities for variation to occur within a process because of potential multiple machines processing the parts. And the paths of variation of a process are increased by the number of individual process steps and/or the complexity of the steps (i.e. number of sub-processes within a process).

The answer to reducing the effects of paths of variation should lie in the process and product design stage of manufacturing processes. That is, processes should/must be designed with reduced complexity and products should/must be designed that are more robust. The payback for reducing the number of paths of variation is an overall reduction in the amount of process variation and ultimately more consistent and robust products.  Let’s look at a real case study.

Many years ago I had the opportunity to consult for a French pinion manufacturer located in Southern France.  For those of you who are not familiar with pinions (i.e. pignons in French), a pinion is a round gear used in several applications: usually the smaller gear in a gear drive train.  Here is a drawing of what a pinion might look like and as you might suspect, pinions require a complicated process to fabricate.

Pinion Gear

When our team arrived at this company, based on our initial observations, it was very clear that this plant was being run according to a mass production mindset.  I say this because there were many very large containers of various sized pinions stacked everywhere.

The actual process for making one particular size and shape pinion was a series of integrated steps from beginning to end as depicted in the figure below.  The company received metal blanks from an outside supplier which were fabricated in the general shape of the final product. The blanks were then passed through a series of turning, drilling, hobbing, etc. process steps to finally achieve the finished product.

The process for this particular pinion was highly automated with two basic process paths, one on each side of this piece of equipment.  There was an automated gating operation that directed each pinion to the next available process step as it traversed the entire process which consisted of fourteen (14) steps.  It was not unusual for a pinion to start its path on one side of the machine, move to the other side and then move back again which meant that the pinion being produced was free to move from side to side in random fashion.  Because of this configuration, the number of possible combinations of individual process steps, or paths of variation, used to make these pinions was very high.

Pinion Process

Next Time

In my next post, we’ll introduce you to the multiple paths of variation that these pinions could traverse and discuss how these paths can significantly increase the overall variability of processes. 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



Bob Sproull

About the author

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

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