What Is SMED, Line Balancing, and TOC — and How Do They Work Together?

In lean manufacturing, three tools often appear together: SMED, line balancing, and TOC (Theory of Constraints). Each solves a different problem, but when combined, they reduce waste, shorten lead times, and increase throughput. This article explains what each tool is, how it works, and how they complement one another.

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What It Is

• SMED (Single-Minute Exchange of Die) — developed by Shigeo Shingo — is a method to reduce changeover time to under 10 minutes (the "single minute" refers to a single-digit number of minutes, not necessarily one minute). It separates changeover steps into internal (machine must stop) and external (can be done while machine runs) activities, then converts internal to external wherever possible.

• Line Balancing is the process of distributing work elements across workstations so that each station has roughly equal cycle time. The goal is to minimize idle time and maximize the efficiency of the production line.

• TOC (Theory of Constraints) — developed by Eliyahu Goldratt — focuses on identifying the single bottleneck that limits the entire system's output. The method is: identify the constraint, exploit it, subordinate everything else to it, elevate it, and repeat.

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How It Works / Steps

### SMED (Shingo's 4-step approach)

1. Observe and document the current changeover process.
2. Separate internal and external activities — internal means the machine must be stopped; external can be done beforehand.
3. Convert internal activities to external — e.g., pre-position tools, use quick fasteners, standardize dies.
4. Streamline all remaining activities — reduce wasted motion, use parallel work, improve transport.

### Line Balancing (basic steps)

1. List all tasks and their standard times.
2. Draw a precedence diagram (which task must precede another).
3. Assign tasks to workstations so that total time per station ≤ cycle time (takt time).
4. Calculate balance efficiency:

```
Balance Efficiency (%) = (Sum of task times) / (Number of workstations × Cycle time) × 100
```

Higher efficiency means less idle time.

### TOC (Goldratt's 5 focusing steps)

1. Identify the system's constraint (the bottleneck).
2. Exploit the constraint — get maximum output from it without major investment.
3. Subordinate all other processes to the constraint's pace.
4. Elevate the constraint — invest to increase its capacity.
5. Repeat — once the constraint moves, find the new one.

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A Worked Illustrative Example

Example data (illustrative only) — not from any real factory.

### SMED example

A press machine changeover takes 45 minutes. After observation:

• Internal activities: 30 minutes (die removal, setup, adjustment)
• External activities: 15 minutes (fetching tools, preheating, cleaning)

Step 1: Move external activities before the machine stops → changeover now 30 minutes.

Step 2: Convert internal to external — e.g., pre-set die height, use quick clamps → internal drops to 15 minutes.

Step 3: Streamline remaining internal steps (two operators working in parallel) → internal drops to 8 minutes.

Result: Changeover time reduced from 45 to 8 minutes — a 82% reduction, now in the "single-minute" range.

### Line balancing example

A line has 4 workstations with task times: 8, 10, 9, 7 minutes. Takt time is 10 minutes.

• Sum of task times = 34 minutes
• Number of stations = 4
• Cycle time = 10 minutes

```
Balance Efficiency = 34 / (4 × 10) × 100 = 85%
```

That's acceptable, but station 4 (7 min) has 3 minutes idle per cycle. Rebalancing could shift one task from station 2 to station 4, improving efficiency.

### TOC example

A production line has capacities: Station A = 100 units/hr, Station B = 80 units/hr, Station C = 90 units/hr.

• Constraint: Station B (80 units/hr) limits the whole line.
• Exploit: Run Station B without breaks, add quality checks before it to avoid rework.
• Subordinate: Stations A and C should not overproduce — they should match B's pace.
• Elevate: Add a second machine at Station B → capacity rises to 160 units/hr.
• Repeat: Now Station C (90 units/hr) becomes the new constraint.

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Common Pitfalls

• SMED: Trying to reduce changeover time without first separating internal/external — you end up speeding up the wrong steps.
• Line Balancing: Balancing to a cycle time that is not aligned with actual customer demand (takt time).
• TOC: Focusing on local efficiency instead of the global constraint — improving a non-bottleneck adds no throughput.
• General: Treating these tools as isolated; SMED reduces downtime, line balancing reduces idle time, and TOC tells you where to focus both efforts.

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Bringing It Together

SMED, line balancing, and TOC are not competing methods — they are complementary. TOC tells you where the real constraint is. SMED reduces the time lost when you change over that constraint. Line balancing ensures the rest of the line flows smoothly at the constraint's pace.

To apply these lean tools systematically, try the free SMED / Line Balancing / TOC tool at:

https://www.6sq.com/tools/smed_toc/