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Integrating Lean Six Sigma and TOC Part 9

Integrating Lean Six Sigma and TOC Part 9

By Bob Sproull

The Answer

At the end of my last post, I told you that having a perfectly balanced line (i.e. all processing times for all steps are equal) is not a good thing.  I asked you why you think this might be a true statement.  Here’s the bottom line:  If all steps had exactly the same processing times, then when one step goes down for whatever reason, the entire production line stops.  Consider this example; suppose every step in your process had a processing time of 15 minutes as follows:


Raw materials enter step 1, are processed for 15 minutes and passed on to step 2.  The semi-finished products continues through steps 3 and 4 until it exits as finished product.  If step 1 experiences downtime for any reason, then the rest of the steps must stop processing because there is no buffer of parts between each step.  Because each step has the same capacity, there is no opportunity to even produce a buffer of parts between the steps.  In an unbalanced line, this same problem does not exist because throughput is dictated by the constraint and the non-constraints have extra capacity to be able to keep running.  In this case, the only time the line stops is if the constraint experiences downtime.


Short Review

In my last post we began our discussion of Steps 3a, 3b and 3c, so today let’s continue with that discussion before we move onto Steps 4a, 4b and 4c.  As I’ve done in my last few posts, just to refresh your memory on the UIC steps, here are the two figures that represent the Ultimate Improvement Cycle.

The UIC Steps 3a, 3b and 3c, Continued

In this post we will finish our discussion on Steps 3b and 3c and then begin our discussion of Steps 4a, 4b and 4c.  In Step 3b we will focus our efforts on reducing processing time and improving flow by executing part of the plan we developed in Step 3a.

So far, we have analyzed our upstream and downstream non-constraint work-loads; decided what work, if any, can be safely moved away from the constraint operation; developed or proposed re-designed process steps to accommodate the unloading of constraint work;, and estimated the new processing times for both non-constraint and constraint operations, as well as the impact on flow and throughput. If we did our homework correctly, we should breeze through Step 3b and on to Step 3c. Remember the planning for this step had already been completed in Step 3a.

In Step 3c we will optimize the constraint, assembly and finished product buffers and refine our scheduling system based on any problems we may have encountered in Step 3b.  As a side note, the scheduling system we will use is based upon the teachings of the Theory of Constraints, which I will write about in a future post. We will analyze our new data on compliance to schedule and, if need be, make any corrections necessary to improve compliance.

We will insert a buffer in front of the constraint operation to protect the it in the event that one of our up-stream non-constraints has unexpected downtime. Keep in mind that our buffer isn’t necessarily product. In fact, usually our buffer will simply be a measure of time. That is, instead of having excess work-in-process inventory available, our production schedule will dictate how many parts to produce and when so that they will always be available in time for the constraint to process.

We will also insert an assembly buffer which consists of non-constraint parts in front of assembly, assuming the constraint part is required in assembly. And finally, we will establish a finished product buffer to protect our on-time delivery to customers. The important thing to keep in mind is that our non-constraints have “sprint capacity” or the capacity to produce product at faster rates than our constraint operation. So if an upstream non-constraint operation experiences downtime for some reason, when it begins production again, it should still have the ability to make product at a fast enough rate to re-supply the constraint buffer before the constraint buffer runs dry.  This completes our discussion of Steps 3a, 3b and 3c, so let’s move on to Step 4a, 4b and 4c.


The UIC Step 4a

In Step 4a we will develop a plan on how to elevate the current constraint (if this step is needed) and define appropriate protective controls. What do I mean by elevating the constraint?   Quite simply, the steps we have taken thus far will have increased the capacity of the constraint without spending much money at all.  We’ve been eliminating waste and variation which shouldn’t be in the process in the first place.  But what if, after all the improvements we have made, we still can’t supply enough parts to meet our market demand? 

Once again we turn our attention to planning and analysis. The premise here is, if in the previous nine steps we haven’t broken the constraint (i.e. increased the capacity of the constraint to satisfy the market demand), then we may have to acquire more resources   and spend some money to do so. Spending money, in this context, simply means that we will have to add additional resources, either by adding labor, time (i.e. overtime and/or shifts), equipment or a combination thereof to break the constraint.  This is the essence of the expression, breaking the constraint.

Based upon my experience, if there is a need to elevate the constraint only minimal labor increases are required and there is no need to spend large sums of money on new equipment. but in reality sometimes we do. Sometimes this labor increase can be achieved with overtime or additional shifts. However, in the event that we must spend money, we must do so under control and only after performing a detailed analysis of constraint processing times, and only add labor or equipment that is needed for the constraint to satisfy the needs of the market.


A Question to Ponder

In previous posts I have written about the negative aspects of manpower efficiency and equipment utilization.  Why do you think I have a negative opinion of these two performance metrics?


Next Time

In my next post we’ll complete our discussion of Steps 4a, 4b, and 4c. As always, if you have any questions or comments, leave 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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