Every part every interval (EPEI) is probably the least intuitive, least known, and yet most powerful metric for high mix, low volume (HMLV) value streams. That’s a big statement that we’ll begin to unpack in a series of posts.
For high mix operations, a value stream supports not just a single product but a product family, many products produced by common processes. With different products being produced in the same set of operations, changeovers between products can have a significant impact on total capacity especially for machine-oriented operations.
Here’s a descriptive definition of EPEI: EPEI is the time is takes to produce every member of the product family including the changeovers between products. And now here’s the math.
It takes a few steps to calculate EPEI. We start by calculating the total daily run time for each item in the product family. The demand to use is the same as for your takt time calculation but expressed per day. Use the effective cycle time in order to include the impact of downtime, yield loss, or rework.
Next, the time available for changeovers is the working time available per day minus the sum of the extended daily run times for every part in the product family. The time available per day is the same as used in your takt time calculation.
Finally, the EPEI is the sum of all changeover times needed across every member of the product family divided by the time available for changeovers. EPEI is most often expressed in days.
Here’s an example:
The Extended Daily Run Time is 440 minutes (170 + 150 + 120). With a working time of 500 minutes, we have 60 minutes per day of time available for changeovers (500-440). However, all of the changeovers add up to 180 minutes (45+60+75). Therefore, our EPEI for this process is 3 days (180/60).
EPEI is a length of time. Thinking about it another way, it’s how long it takes to accumulate enough available time to allow for all of the changeovers needed to produce every member of the product family.
There are two boundary conditions to consider that limit when EPEI is useful. First, if there are no changeovers then EPEI is meaningless and a comparison of effective cycle time to takt time is sufficient to understand capacity. Second, if the extended daily run time exceeds the time available then you do not have sufficient capacity no matter how quick your changeovers are. In this case, EPEI is also irrelevant until you solve the overall capacity issue. Now that we’ve defined it, how do we use EPEI? Here’s the short list which we’ll expand on in future posts:
- EPEI sets the lot size for each product for the process.
- EPEI sets the buffer size of work-in-process inventory
- EPEI contributes to lead time
- Driving EPEI lower reduces both inventory and lead time
- EPEI is an excellent indicator of capacity utilization for a value stream