SMED explained

What SMED is

SMED stands for single-minute exchange of die. It is a method for changing a machine over from one product to the next in a single-digit number of minutes. Shigeo Shingo developed it in the 1950s and 1960s at Toyo Kogyo, now Mazda, and at Toyota. Japanese plants call the work changeover (段取り替え, dandorigae).

SMED sorts changeover work into two kinds. Internal setup can only happen with the machine stopped, such as removing a die. External setup can happen while the machine still runs, such as fetching the next die. Most slow changeovers do both kinds with the machine stopped. SMED moves the external work out of the stop, then shortens what is left.

Why changeover time matters

When a changeover takes hours, a plant changes over as rarely as it can. It runs a week of one part and stores it. That stock ties up cash and floor space, hides defects until someone uses the parts, and makes customers wait for the next run of the part they want.

In 1969 Toyota asked Shingo to cut the changeover on a 1,000-ton body press. It took four hours. Shingo reported that Volkswagen did a similar changeover in two. Six months of separating and converting the work brought Toyota's time to 90 minutes. Management then asked for three minutes. Three more months of work got it there.

If changeovers should take no more than a tenth of a machine's time, a four-hour changeover needs a 40-hour run after it. A three-minute changeover needs a 30-minute run. Short changeovers make small batches affordable, and flow, pull and level schedules all depend on small batches.

How to run a SMED event

Pick one machine: a bottleneck, or one that changes over often. Bring together the operators who do the changeover, a setter and someone from maintenance. Then work through these steps in order.

1. 1
   Film and time the current changeover

   Record one real changeover, from the last good part of the old run to the first good part of the new one. Record what the crew actually does, which often differs from the work instruction.
2. 2
   List every element

   Watch the film with the crew and write down each task with its time. Include the walking, searching, waiting and adjusting.
3. 3
   Separate internal from external

   Mark each element internal (the machine must be stopped) or external (it could happen while the machine runs). Mark it by what the work needs, even if the crew always does it with the machine stopped. Fetching, gathering tools and paperwork are almost always external.
4. 4
   Convert internal work to external

   Stage the next die, tools and material before the last part, and put the old tooling away after the restart. Then look again at the internal work. Preheating a mold on a bench or presetting a tool offline turns internal work into external work.
5. 5
   Streamline the internal work

   Replace bolts with clamps that lock in one turn or less. Give every die the same clamp height so nothing needs shims. Use locating pins and marked settings so the first part is good without trial runs. On a mold, a coupling plate connects all the water lines in one move. Split the work between two people with a written sequence, so they work in parallel without getting in each other's way.
6. 6
   Streamline the external work

   Put the tools and parts for each changeover on one cart, each in a marked place, so anything missing is obvious. Check the cart against a list before the stop. Store dies close to the machine they fit.
7. 7
   Standardize and retrain

   Write the new changeover as standard work: the sequence, who does what, and the target time. Post it at the machine and train every shift on it.
8. 8
   Repeat

   Time the new changeover and compare it with the first film. Then run the steps again on the same machine, or take them to the next one. Most machines need more than one round to get under ten minutes.

A worked example

A die change on a mid-size stamping press has six elements and takes 35 minutes, all of them with the press stopped. The numbers are made up for this example.

Moving three elements out of the stop saved 17 minutes. Clamps, locating pins and marked settings saved 10 more. The crew did the same work at the same pace. If the press changes over 4 times a day, that gives back 108 minutes of press time every day, which the plant can use to run smaller batches.

Try it on a packing line

A changeover on a carton packing line has nine elements, listed in the order the crew does them today. Every element starts as line stopped. Mark each one, then press Check my sort.

Common mistakes

• Buying clamps first. Without the film and the sort, you speed up the internal work while the larger external work stays inside the stop.
• Telling the crew to hurry. SMED removes waiting and searching. A rushed changeover leads to a wrong setup, and a wrong setup means a second changeover.
• Sorting on paper only. Marking an element external saves nothing until the cart is packed and the die is staged before the stop.
• Skipping the written standard. If only one person knows the new method, the time goes back up the first time that person is off shift.
• Starting on the wrong machine. Time saved on a machine with spare capacity only builds stock faster. Start where lost time means lost output.
• Stopping after one round. The first round gives the biggest drop. Time the changeover again and run the steps again.

How to start

Start with one changeover on the bottleneck. To total your times, enter each element in the free SMED changeover calculator and mark it internal or external. It shows the stopped time, the time you can move out of the stop, and the hours saved each year.

The EPEI calculator shows how often you can run each product at the new changeover time. The OEE guide shows where changeover time sits among a machine's other losses.