TPS Relationships #11 - SMED & Heijunka

TPS Relationships

SMED & Heijunka

Making Production Leveling Possible

Imagine a production line manufacturing three different products: A, B, and C.

Demand exists for all three, every day. But changing from one product to another takes two hours.

What would you do?

The seemingly reasonable answer is to produce larger batches. Run A for several days, then B, then C. Fewer changeovers mean less time spent preparing equipment and more time producing.

From the perspective of machine utilization, the decision makes sense.

But what happens to the rest of the system?

Inventory grows. Lead times increase. Production becomes less responsive to changes in demand. A problem discovered in one batch may affect hundreds or thousands of units before the next opportunity to correct it.

The machine may be operating efficiently while the production system becomes less flexible.

This is where the relationship between SMED and Heijunka becomes particularly interesting.

Two Tools, Different Purposes

SMED (Single-Minute Exchange of Die) focuses on reducing changeover time.

By distinguishing activities that require equipment to be stopped from those that can be performed while it operates, simplifying adjustments, improving preparation, and eliminating unnecessary work, SMED makes changeovers faster and more predictable.

Its purpose is not simply to save minutes.

It is to reduce the operational penalty of changing what we produce.

Heijunka, or production leveling, addresses a different challenge.

Rather than allowing production to follow every fluctuation in orders, it seeks to level volume and, where appropriate, product mix over a defined period.

The objective is to create a more consistent production pattern while responding to customer requirements.

But there is a practical limitation.

If every changeover is expensive, frequent changes in product mix may become impractical.

And if we avoid those changes by producing large batches, our ability to level the mix becomes constrained.

Where the Relationship Begins

Consider our three products again.

Suppose each changeover initially requires 120 minutes.

Changing products several times a day would consume a substantial portion of available production time.

Now imagine that systematic SMED improvements reduce the changeover time to 15 minutes.

Suddenly, production sequences that previously seemed unrealistic become possible.

Instead of producing A for several days before switching to B, we may be able to alternate among A, B, and C in smaller batches.

The result is not automatically Heijunka.

We still need to understand customer demand, available capacity, product mix, process constraints, and the appropriate production sequence.

But SMED has removed an important obstacle.

Heijunka defines a more balanced production pattern. SMED helps make that pattern operationally feasible.

What Happens When We Separate Them?

If we apply SMED without considering the broader production system, we may celebrate shorter changeovers while continuing to produce the same large batches.

The improvement is real, but much of its potential remains unused.

We have gained flexibility without necessarily taking advantage of it.

Conversely, if we attempt to introduce a leveled, mixed-model production schedule without addressing long and unstable changeovers, we may create a plan that looks excellent on paper but is extremely difficult to execute.

Operators face constant pressure. Schedules become fragile. Expediting and workarounds begin to appear.

Eventually, the organization may conclude that Heijunka does not work.

But perhaps the problem was not the principle of leveling.

Perhaps we had not yet developed the capabilities required to sustain it.

The Relationship Is Not a Shortcut

Reducing changeover time does not eliminate every obstacle to production leveling.

Material availability, equipment reliability, process capability, demand patterns, and upstream constraints still matter.

Nor does every process need to alternate products continuously. The appropriate production pattern depends on the actual conditions of the system.

The important lesson is not that every company should pursue the smallest possible batch or the greatest possible number of changeovers.

It is that the economic and operational constraints influencing our production decisions are not always fixed.

Some of them can be improved.

And when those constraints change, decisions that once seemed reasonable deserve to be reconsidered.

The Bigger Picture

SMED and Heijunka illustrate something essential about TPS.

Tools are not independent solutions competing for attention.

They develop capabilities that allow other parts of the system to work more effectively.

SMED can create flexibility.

Heijunka can use that flexibility to establish a more balanced production pattern.

And that pattern can create new opportunities to improve flow, reduce inventory, and expose problems that large batches previously concealed.

Reducing changeover time is an improvement. Changing what the system can accomplish because of that reduction is where its greater value may lie.

That is the relationship worth understanding.

TPS Relationships | TPS by Katsu

Understanding not only what TPS tools do, but what they make possible together.