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Paths of Variation Part 3

Paths of Variation Part 3

By Bob Sproull


In my last post I demonstrated the total potential paths of variation that pinions could travel through as they make their way through the total process.  As a reminder, the actual pinion production process flow and the total paths of variation were as follows:

At the end of my last post I asked you two important questions:

  • Do you think that the pinions produced through these multiple paths will be the same dimensions or will you have multiple distributions?
  • What if we were able to reduce the number of paths of variation from 32 down to 2, do you believe the overall variation would be less and how many distributions would you have now?  Or another way of saying it, do you believe the part to part consistency would be greater?

In today’s post, we will answer these questions and then demonstrate the impact of reducing the paths of variation in this case study.

Paths of Variation

In the figure below I have created what I refer to a “virtual cell” meaning that we limited the paths of travel that an individual pinion can take by removing the possibility for a part to traverse back and forth from side to side of this machine configuration.

Pinion Manufacturing Process Solution

In simple terms, the part either went down side 1 or side 2 rather than allowing the gating operation to select the path.  In this figure you can see that pinions passing through exterior turning machine A1 are only permitted to proceed to internal turning machine B1. Those that pass through internal turning machine A2 are only permitted to proceed to turning machine B2. In doing so, the number of paths of variation for the first two process steps was reduced from 4 to 2. Continuing, the parts that were turned on A1 and B1 can only pass through drilling machine C1, crack detection D1 and hobbing machine E1 while those produced on A2 and B2 can only be processed on drilling machine C2, drilling machine D2 and hobbing machine E2. To this point, the total paths of variation remain at 2 instead of the original number of paths of 16.

The part continues to hard hobbing where there are, once again, two machines available. The parts produced on A1B1C1D1E1 can only proceed to the G1 hard hobbing machine while those produced on A2B2C2D2E2 can only be processed on hard hobbing machine G2. We also instructed the heat-treater to maintain batch integrity and not mix the batches.  So at this point, because we specified and limited the pinion paths, the total paths of variation decreased from 32 to only 2! So what do you think happened to variation when we created our virtual cell?

The key response variables for this process were five, individual diameters measured along the surface of the pinion. As a result of limiting the number of potential paths of variation, the standard deviation for the various diameters was reduced by approximately 50 % on each of the diameters!

But even though we were very successful in reducing variation, there was a problem associated with making this change….sort of an unintended consequence, if you will. Remember in the original configuration pinions could move to the next available machine (i.e. either side) as they proceeded along the flow of the process. With the new configuration, they could no longer do this.  Prior to this change, when there was downtime on one side of the machine, the automation and/or operator simply diverted the pinion to the same machine on the other side so as to keep the parts moving. With the new configuration, when a machine in the cell went down unexpectedly, the parts now had to wait until the machine was repaired.

The immediate short term result of this change was a significant reduction in throughput of pinions because of unplanned downtime. However, in the longer term, it forced the company to develop and implement a preventive maintenance system which eventually reduced the unplanned downtime to nearly zero. When this happened, the new throughput surpassed the original throughput and the variation was reduced by 50 %! In addition, the scrap levels for this process were reduced by 40%!  And the great part was, not a single dollar—or should I say Euro—was spent in doing this, yet the payback was huge.   So as you are studying your process for variation reduction, keep the concept of paths of variation foremost in your mind because it can make a huge difference in some circumstances.

Next Time

In my next post, we’ll begin a new series on continuous improvement and present new tools and techniques.  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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