The page's own CNC worked example on one axis of minutes, with 0 the moment the machine stops. Before: all nine activities run inside the stop — 15 + 20 external at the front, 190 min genuinely internal, 15 external paperwork at the back = 240 min. Three of the nine (50 min, a fifth of the clock) were already external and were being done stopped out of habit. After: those three keep their full duration but move outside the window — 35 min before the stop, 15 min after the restart — and the six internal activities are converted and streamlined from 190 to 15 + 5 + 15 + 2 + 10 + 10 = 57 min. Total work still done falls from 240 to 107 min; machine-stopped time falls from 240 to 57, so the machine is back in cut 183 minutes sooner.
Why Changeover Reduction Changes Everything
Consider a CNC machine with a 4-hour changeover. With 8 available hours per shift, losing 4 hours to changeover means only 4 hours of production. To make this economical, you run large batches — maybe 3 days of output at once. This creates 3 days of excess inventory, hides defects for 3 days, and means if the customer needs a different part, they wait until the current batch finishes.
Now reduce that changeover to 15 minutes. You lose 15 minutes per switch. You can change over 8 times per shift and still have 6 hours of production. Now you can run every part number every day. Inventory drops from 3 days to 0.5 days. Defects are caught within hours, not days. The customer gets what they need within a shift, not within a week.
Same machine. Same operator. Same parts. The only change is setup time.
The 4 SMED Stages
Stage 0: Document the Current Changeover
Video record the entire changeover from last good part to first good part. List every activity, its duration, and whether the machine is running or stopped. Do not try to improve yet — just document reality. Typical finding: 30–50% of activities are already external but are being performed internally (machine stopped) out of habit.
Stage 1: Separate Internal and External
Classify every activity: Internal (machine MUST be stopped) or External (CAN be done while running). Then reorganize: all external activities happen before the machine stops or after it restarts. This stage alone typically reduces changeover by 30–50% with zero investment.
Stage 2: Convert Internal to External
Challenge every internal activity: “Can this be done while the machine is still running?” Pre-heat tooling while the current job runs. Pre-set fixtures on a duplicate table. Pre-stage material at the machine before stopping. Each conversion removes time from the critical path.
Stage 3: Streamline All Activities
Reduce the time of remaining internal activities: replace bolts with quick-clamps, standardize fixture heights to eliminate adjustment, use intermediate jigs for alignment, automate where economical. This is where investment may be required, but only after Stages 1–2 have been completed.
Current changeover: 240 minutes (4 hours).
| Activity | Current (min) | Type | After SMED (min) | Change |
|---|---|---|---|---|
| Find next program/tooling list | 15 | External | 0 (done before stop) | Moved external |
| Walk to tool crib for tools | 20 | External | 0 (pre-staged) | Moved external |
| Remove current tooling | 45 | Internal | 15 (quick-release) | Streamlined |
| Clean machine | 15 | Internal | 5 (air blast) | Streamlined |
| Install new tooling | 50 | Internal | 15 (pre-set on plate) | Converted + streamlined |
| Load program | 10 | Internal | 2 (pre-loaded) | Converted external |
| Adjust offsets/alignment | 40 | Internal | 10 (standardized heights) | Streamlined |
| First article run + inspect | 30 | Internal | 10 (reduced from confidence) | Streamlined |
| Paperwork/sign-off | 15 | External | 0 (done before stop) | Moved external |
| Total | 240 min | 57 min | 76% reduction |
Impact: Changeover from 4 hours to under 1 hour. Minimum economic batch drops from 3 days to half a day. EPEI improves from weekly to daily — enabling heijunka mix leveling.
Read the “After SMED” column carefully: the three zeros are not work that vanished. Finding the program, fetching the tooling and signing the paperwork still take their full 50 minutes — they simply no longer take them while the spindle is stopped. Only the six genuinely internal activities were actually made shorter, 190 minutes down to 57.
⚠️ Don’t Skip Stages 1–2 and Jump to Investment
The most common SMED mistake is buying quick-change fixtures (Stage 3) before separating internal/external activities (Stage 1). Stages 1 and 2 are free — they require only reorganization and preparation. They typically deliver 50–70% of the total reduction. Stage 3 investment should only happen after the free improvements are captured.
🎯 The Bottom Line
SMED is the enabler of everything: small batches, heijunka, one-piece flow, and daily EPEI. The 4-stage method (document, separate, convert, streamline) systematically reduces changeover from hours to minutes. Start with video documentation, then move external activities outside the machine stop window (30–50% free reduction), then convert and streamline the remaining internals. The goal is not zero changeover time — it is changeover time so short that batch size becomes a choice, not a constraint. Next: Total Productive Maintenance — ensuring the equipment is reliable enough to sustain the lean system you have designed.
Interactive Demo
Split the changeover into internal and external steps, then convert what you can. Watch the changeover time fall and the economic batch size fall with it.
Stop reading, start modeling
Model your process flow, run simulations, optimize staffing with TOC math, and test your knowledge with 107 interactive checks — all in one platform.