What is the difference between internal and external changeover work?

Internal work can only be done with the machine stopped, such as removing one die and fitting the next. External work can be done while the machine is still running the previous job, such as fetching tools, staging material or preheating the next die. SMED moves as much work as possible from internal to external.

How do you measure changeover time?

Changeover time runs from the last good part of one product to the first good part of the next, so it includes adjustments and trial runs. Only internal work adds to it; external work done while the machine runs does not.

How is the time saved worked out?

Every internal minute moved to external comes off the changeover. Multiply the minutes moved by the number of changeovers per week for the machine time freed each week, and divide that by the cycle time for the extra units it could make.

Why does a shorter changeover allow smaller batches?

Large batches are often run to spread a long changeover over more units. With a shorter changeover you can change over more often in the same weekly changeover time, so each batch can be smaller: today's batch size times the new changeover time divided by today's changeover time. Smaller batches mean less stock between runs and a faster response to orders.

Can I have the extra capacity and the smaller batches?

Not both in full. The freed time can go into making more units at today's batch size, or into more frequent changeovers with smaller batches, or be split between the two. The calculator shows both ends so you can choose.

Does SMED mean every changeover must be under ten minutes?

No. Single-minute exchange of die refers to the goal of a single-digit number of minutes. It is a direction to work toward; any internal minutes removed free up machine time.

Free tool

SMED changeover calculator

SMED (single-minute exchange of die) shortens changeovers by moving work the crew can do while the machine is still running, external work, out of the stop. The changeover time that remains is the internal work: the steps that can only happen with the machine stopped.

New changeover timeequalsInternal work − Minutes moved to external

Time freed per weekequalsMinutes moved × Changeovers per week

Capacity freedequalsTime freed (seconds) divided by Cycle time (seconds)

Smaller batch sizeequalsBatch size × New changeover time divided by Changeover time today

The smaller batch keeps the weekly time spent on changeovers where it is today: shorter changeovers, done more often. Capacity is rounded down and batch size up to whole units.

Internal work today

Done with the machine stopped: today's changeover time

External work today

Already done while the machine runs

Internal minutes you can make external

Fetching tools, preheating, presetting, paperwork

Changeovers per week

Cycle time

To turn freed time into units

Batch size today

Units made between changeovers

Pre-filled with the worked example. Changeover time runs from the last good part of one product to the first good part of the next.

New changeover time

60 min

Down from 90 min: 30 min (33.3%) saved on every changeover.

Machine time freed

3 h / week

180 min at 6 changeovers a week

Capacity freed

240 units

Per week, if batch sizes stay the same

Or a smaller batch

1,000 units

Up to 9 changeovers a week in today's 540 min

Internal share of the work

82% → 55%

Of 110 min of changeover work

Today90 min internal · 20 min external

After60 min internal · 50 min external

Yellow: internal work, the machine is stopped. Grey: external work, done while it runs.

How to calculate it

Split the work, then move it

Film or time one changeover and list every step. Mark each one internal (the machine must be stopped) or external (it could be done while the machine runs). Today's changeover time is the internal total.

Then find the internal steps that could be done before the stop or after the restart: fetching tools and material, preheating, presetting, paperwork. Those are the minutes you can make external. Steps you speed up without moving them, such as quick-release clamps, simply lower the internal time.

Changeovers count against availability in OEE, and shorter ones make smaller batches practical, the basis of just-in-time production. To see what the stop itself costs, use the downtime cost calculator.

Worked example

Illustrative numbers, not a benchmark

A press has 90 minutes of internal changeover work and 20 minutes of external work. The team finds 30 internal minutes it can move to external. It changes over 6 times a week, runs a 45-second cycle and makes batches of 1,500.

1New changeover time = 90 − 30 = 60 min, 33.3% shorter.
2Time freed = 30 × 6 = 180 min = 3 hours a week.
3Capacity freed = 180 × 60 ÷ 45 = 240 units a week.
4Or smaller batches: 1,500 × 60 ÷ 90 = 1,000 units, with 6 × 90 ÷ 60 = 9 changeovers fitting in today's 540 minutes a week.
5Internal share of the work: 90 of 110 minutes (82%) before, 60 of 110 (55%) after.

The changeover drops from 90 to 60 minutes. The press can use the 3 hours a week to make 240 more units, or run batches of 1,000 instead of 1,500 and hold a third less stock between runs.

Embed this calculator

Teaching this, or writing about it? Paste this code into your site, course page or intranet and the calculator works right there. It is free, with no sign-up.

<iframe src="https://www.theleansuite.com/tools/smed-changeover-calculator/embed" title="SMED changeover calculator by LeanSuite" width="100%" height="1310" style="border:0;max-width:1000px" loading="lazy" allow="clipboard-write"></iframe>
<p style="font:13px/1.4 sans-serif"><a href="https://www.theleansuite.com/tools/smed-changeover-calculator">SMED changeover calculator</a> by LeanSuite</p>

How LeanSuite helps

LeanSuite watches a video of your changeover, separates internal and external tasks and recommends a leaner sequence.

See SMED

FAQ

SMED changeover calculator: common questions

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