Twelve operators, six operations, and the supervisor puts two on each because it looks fair. The
line then makes fewer garments than twelve people should. Nobody is idle by choice and nobody
is slow. The line is simply running at the speed of its slowest step, and every other step is
waiting for it.
The idea in one line
Output is set by the slowest operation
Garments cannot leave the line faster than they leave the operation that makes the fewest an
hour. Anything faster before it piles up; anything faster after it waits.
For each operation, the garments an hour it can make is
operators × 60 ÷ the minutes the operation takes (its share of
the SAM, the standard minutes per garment). The smallest of those numbers is the line's output.
A worked example
These figures are made up to show the sum. They are not a benchmark and not a claim about any
line or any garment. The shirt has the 18-minute SAM used in the
line efficiency article, split over six
operations, and 12 operators. Everyone is assumed to work at the standard, so only the shape of
the answer matters, not the exact count.
- Collar: 3 minutes
- Sleeve: 4.5 minutes
- Side seam: 2 minutes
- Hem: 1.5 minutes
- Cuff: 3 minutes
- Buttons and buttonholes: 4 minutes
At the standard, 12 operators could make 12 × 60 ÷ 18 = 40 garments an
hour, 320 in an eight-hour day. That is the ceiling.
Two on every operation
With two operators on each of the six operations, the capacity of each is:
- Collar: 2 × 60 ÷ 3 = 40 an hour.
- Sleeve: 2 × 60 ÷ 4.5 = 26.7 an hour.
- Side seam: 60 an hour.
- Hem: 80 an hour.
- Cuff: 40 an hour.
- Buttons: 30 an hour.
The line makes 26.7 an hour, because the sleeve cannot do more. That is 213
garments in a day, 107 short of the 320 ceiling, or 66.7% of it. The hem
could make 80 and is working at a third of that, and the side seam at 44%.
Move people to where the garments are waiting
Take one operator off the hem and one off the side seam, and put them on the sleeve and the
buttons: collar 2, sleeve 3, side seam 1, hem 1, cuff 2, buttons 3.
- Collar: 40. Sleeve: 3 × 60 ÷ 4.5 = 40.
- Side seam: 60 ÷ 2 = 30.
- Hem: 60 ÷ 1.5 = 40. Cuff: 40.
- Buttons: 3 × 60 ÷ 4 = 45.
Same twelve people. The line now makes 30 an hour, 240 in a day —
27 garments more, a 12.5% rise, and 75% of the ceiling. Nothing was bought,
no one worked longer, and no one was asked to be faster.
The bottleneck moves
The slowest step is now the side seam, with one operator at 30 an hour. Everything else has a
little spare. That is how it always ends: fix the slowest step and the next one becomes the
slowest. The line gets better until the steps are nearly equal, and then the only way up is
more people or less time per garment.
The gap to 40 an hour is mostly the whole-person problem. The ideal for the sleeve is 3 people,
for the side seam 1.33. You cannot have a third of a person, so one operator who can cover two
neighbouring steps, a floater, is worth more than their own output.
Look for the pile, not the person
The bottleneck is where work waits. A pile of bundles in front of an operation is the line
telling you its slowest step. An empty table in front of one is a step that is waiting.
What to do with it
- Time every operation. The sums need the minutes per operation, not just
the whole-garment SAM. A stopwatch and ten garments is enough to start.
- Check where the bundles pile up at the start of the shift and again at the
end. If it is the same place, that is the bottleneck.
- Rebalance when the style changes. A new shirt has different minutes per
operation, so the allocation that was right yesterday is not right today.
- Before paying for overtime, check where the extra hours would go. Overtime
on a line whose bottleneck is not fixed makes more garments at the slowest step's speed and
pays everyone for it. The
overtime article counts that cost.
What the sums leave out
- Made-up minutes. The six operations and their times are an example. Use
your own, timed on your own machines.
- Operators are not equally fast. The sums put everyone at the standard. A
slower operator on the bottleneck can matter more than the arrangement.
- Bundles in between. A few bundles of buffer between steps hide small
differences for a while. They do not change the long-run answer.
- Breakdowns and rework. A machine that stops at the bottleneck stops the
line; the
rejects and rework article shows what a
reworked garment takes back from the day.
The efficiency calculator gives the minutes a line earns in a day;
the sums above explain why that number is usually below what the headcount suggests.