How to use EPEI to track your progress towards flow

When working in machine shops, I’m always surprised to hardly see any flow-related indicators. You’ll typically see data on OEE-related indicators like reject rates, uptime, performance (mostly under the heading of efficiency), and maybe something on actual versus planned volume or schedule adherence. But flow? Yes, in a few cases, teams can show me the number of changeovers and the changeover time on the machine. But I’m mostly confronted with empty and questioning eyes, to be honest. So, how do you know whether you are improving flow in machine shops? In this post, I suggest you post the machine’s EPEI on or near the machine. That will indicate how well you are progressing towards one-piece flow and closing in on takt time, even in your machine-shop.

EPEI: Every Part, Every Interval

EPEI stands for “Every Part, Every Interval”. You can also come across this concept as EPEC (“Every Part, Every Cycle”) or EPE or EPEx. EPEI can be defined as the average interval between two subsequent batches of the same product. It is most often expressed in days, but depending upon your maturity, it could also be in hours. At one of my previous employers, we had quite a few cases where we changed over to the same item, several times a day. But I guess most of us are still in the range of measuring EPEI in days.

Now, why is EPEI important? Well, as you can see, EPEI is an interval; very much like cycle time and takt time. It, therefore, creates a language based upon the same underlying concept as takt and cycle time, namely the interval. You can see it as cycle time translated to the batch level.

So, put simply, you could state that an EPEI of an 8 hour day, and a lot size of 100 units, translates into an average cycle time of every single unit of 8 * 60 / 100 = 4.8 minutes per unit. And we try to balance our cycle time with our takt time. But whereas this cycle time could be close to our takt, the reality of batching means that we don’t really see these individual items coming off the machine of course… The effect of batching means we “lump” these cycle times together, which becomes the EPEI. EPEI can, therefore, be seen as the machine’s current position on the road towards reaching takt time for every single unit.

But where is this roadmap on the shop floor? I wish manufacturers would show me EPEI instead of OEE, to be honest. I’m not a big fan of a composite OEE anyway…

EPEI based upon changeover time

Now, why wouldn’t we changeover after every single product? Of course, this is a rhetorical question. We often can’t do this due to the time and effort it takes to change over from one item to another. So, we change over within the limits of our capacity, and the policies that manufacturers tend to have in this area. And when the team leader or shop supervisor decides…

A rule that I have highlighted in one of my previous posts was the 10%-rule. The rule implies that in order to absorb the changeover time with sufficient production time, a production lot should at least be 10x the changeover time. So as an example, when your changeover time is 30 minutes, you will define your production lot on the machine as 10x the changeover time, so a production lot of 300 minutes or 5 hours. Now if your processing time for a part type is 20 minutes, the production lot will be 15 parts. And finally, when the average demand is 5 parts/day, this part type will have an EPEI interval of 3 days. So, on average we will be changing over to this item, every third day.

Tracking actual changeover and EPEI

But are we? This where tracking you actual changeovers come into play. To see whether you are actually achieving 3 days, you need to log your actual changeovers. And to see whether your assumptions for your targeted EPEI still hold, you need to log the actual changeover times.

Typically, this is done using an A3 template for the month, where team members can log the changeovers on a daily basis.

Example of a C/O tracking log sheet (from our CO-EPEI application).

Based upon the recorded changeovers during the month, you can easily determine the EPEI for each product or product family running on the machine or the equipment, and for the machine, equipment, cell or line as a whole; and even much more.

It also enables you to check how many changeovers you have actually performed during the period, the average changeover time, overall and by item, and the overall time spent on changing over. This can then help you in verifying your assumptions on changeover time, that underly your EPEI objectives, and to focus your improvement efforts in further increasing flow in your plant.

After having worked this way myself for years, I have now made a small, but powerful Excel-based application out of it, incorporating my practical experience using this tool for years. If you are interested, then please check out our CO-EPEI application in our shop. You can download a free trial to try it out.

And have a look at the demo in the video below.

Translate the concept into practice

When you are serious about improving flow in your facility, why not track it in your machine shop? Flow is on all our lips, but without the evidence on the shop floor that demonstrates that you are serious, this is just empty talk to me. Use flow-related indicators like EPEI, integrate them in your daily shop-floor practice, ask for them in your gemba walks, and track your progress towards your “true north”.

There’s nothing holding you back. And maybe our CO-EPEI application can even help you in getting it up and running in no time!