Every Part Every Interval

Every Part Every Interval is one of the more technical lean metrics, and one of the more useful for designing a leveled production schedule. The acronym EPEI gets used in lean texts as if everyone knows what it means; most shop floor people do not. The idea is simple once unpacked: how often does the line cycle through every variant in its mix? Shorter EPEI means more flexibility, faster customer response, and a more challenging changeover regime. Longer EPEI means easier production but slower response. The right answer depends on the shop's mix and capability.

"EPEI is the question, how often can we cycle through every part on the list, before the line starts to feel it."

How every part every interval works

The calculation starts with the line's variant list and the run time each variant needs to cover demand in the planning window. If the line has eight variants and each takes 45 minutes of run time per cycle to cover demand, that is 360 minutes of run time per cycle. Add changeover time: if each changeover takes 10 minutes, eight changeovers add 80 minutes. The total cycle length is 440 minutes. With a 480-minute shift, the EPEI is one shift; the line can cover every variant once per shift.

Cut changeover time and the EPEI shortens. If changeovers drop to five minutes each, the cycle takes 400 minutes, and the line could fit a cycle in a partial shift, opening room for more cycles per day. Add variants and the EPEI lengthens; the line has more to fit into its cycle. The relationship between changeover, variant count, and EPEI is mechanical, and it makes the case for quick changeover work concrete: every minute shaved off changeover is a minute available for either shorter EPEI or more variants.

Why EPEI matters: it sets the customer response window for any variant in the cycle. If the EPEI is one week, a customer who orders a variant the day after it ran has to wait a week for the next run. If the EPEI is one shift, they wait at most a shift. Shorter EPEI means more responsive customer service for variants in the mix. Longer EPEI means customers either wait or the shop has to carry larger finished-goods buffers.

EPEI applies only to the variants the shop has decided belong in the cycle, usually runners and repeaters from runner-repeater-stranger classification. Strangers do not belong; they are scheduled separately as orders arrive. Including strangers in the EPEI cycle just lengthens it for no benefit, since strangers are not produced often enough to justify regular slot allocation.

Where every part every interval fits on the shop floor

Picture a small precision-parts shop running 15 variants of machined fittings for two HVAC OEMs. Without EPEI thinking, the shop ran each variant in a long batch, switching between them whenever the order book got loud about one. Customer response time on any specific variant could be up to three weeks, since the variant might not run in the next two-week window.

The shop classifies the 15 variants. Ten are runners or repeaters; five are strangers. For the runners and repeaters, the shop calculates EPEI. Each variant needs about 30 minutes of run time to cover steady demand. Ten variants, 300 minutes of run time. Current changeover is 20 minutes; ten changeovers add 200 minutes. The cycle is 500 minutes, just over a shift. With a SMED project, changeover drops to 8 minutes, ten changeovers add 80 minutes. The cycle drops to 380 minutes, easily fitting in a shift.

The shop sets an EPEI of one shift. The line now cycles through all ten included variants every shift, in a leveled pattern. Customer response time on any of the ten variants drops to one day. The five strangers run in reserved capacity slots when orders arrive. The shop's flexibility improves substantially, customer satisfaction goes up, and overtime drops because the line no longer scrambles to catch up on whatever variant just got hot.

Common mistakes with every part every interval

• Setting EPEI too long. A long cycle means slow customer response. Shorten changeovers and shrink the cycle.
• Setting EPEI too short without quick changeover. Frequent changeovers eat capacity. EPEI work depends on changeover work coming first.
• Including strangers in the cycle. Strangers do not warrant slot allocation. Keep the cycle for runners and repeaters only.
• Treating EPEI as fixed. As changeover times drop and the variant mix shifts, the EPEI should be recalculated. A target set last year may be lazy now.
• Confusing EPEI with takt. Takt is the demand pace. EPEI is the mix cycle interval. They interact but they answer different questions.

Every part every interval and related Lean tools

EPEI is the cadence parameter inside heijunka and production leveling. It works hand in hand with runner-repeater-stranger classification to decide which variants belong in the cycle and which should run separately. EPEI is the central metric for mixed-model production lines, where shorter EPEI delivers more responsive customer service, and it sets the pace context that the pacemaker process executes against.