A buyer wants three thousand shirts in fifteen working days, and asks whether you can do it. The
answer is usually a feeling: yes if we push, no if the line is busy. The same question can be
answered with four numbers and a division, and the answer comes with a figure for how wrong it
can be.
The sum
Operators needed
(garments × SAM) ÷ (working days × minutes in a day × efficiency).
The top line is the work, in minutes: how many standard minutes the order contains. The bottom
is what one operator earns in the time you have. Divide, and round up, because you
cannot hire a part of an operator.
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 garment has the 18-minute SAM used in the
line efficiency article, and the efficiency is
that article's 81.25%.
- The order: 3,000 shirts.
- The time: 15 working days of 8 hours.
- The work: 3,000 × 18 = 54,000 minutes.
- One operator, one day: 8 × 60 × 0.8125 =
390 earned minutes.
The order is 54,000 ÷ 390 = 138.5 operator-days. Over 15 days that is
138.5 ÷ 15 = 9.2 operators, so 10. Ten operators have 58,500 minutes
in the fifteen days, about 4,500 more than the order needs — roughly 11.5 operator-days
of room.
The answer moves with efficiency
The same order, the same fifteen days, and only the efficiency changed:
- At 90%: 432 minutes a day, 125 operator-days, 8.3 operators. 9.
- At 81.25%: 138.5 operator-days, 9.2 operators. 10.
- At 70%: 336 minutes a day, 160.7 operator-days, 10.7 operators.
11.
Twenty points of efficiency is two operators. Which one you plan for is the decision; the
arithmetic does not make it for you.
Turning it round: what date can this line promise?
Say the line has 12 operators and no more. The days it takes are
54,000 ÷ (12 × 390) = 11.5, so 12 working days at 81.25%. At 70%
it is 54,000 ÷ (12 × 336) = 13.4, which is 14 days.
Round up to whole days, because a day not finished is a day not delivered. Notice what the
round-up does: 11.5 and 13.4 are different answers by two days.
The first days are not the average days
A new style takes a little time to run at the line's usual speed. Suppose, as an assumption,
that the first two days run at 60%. Those two days earn 12 × 480 × 0.6 × 2 =
6,912 minutes, not the 9,360 at 81.25%, a shortfall of 2,448 minutes. The remaining 47,088
minutes take 47,088 ÷ 4,680 = 10.06 days at full pace. The total is
12.06 days, which rounds up to 13.
That is one day later than the clean sum, from a start-up that cost two days at 60%. It is the
difference between promising twelve and promising thirteen.
Plan with a figure you have seen, not the best one
Use the efficiency this style, or one like it, has actually run at on your line. If you have
never run it, use a lower figure and say so in the quote.
What to do with it
- Check the machines. Ten operators need ten machines of the right kind free
for the whole fifteen days. If they are not, the sum is about a different line.
- Leave room for absence. One operator off can cost a line nothing or a whole
day; the absent operator article shows which.
- Check the slowest step. Enough total minutes does not mean enough minutes
at the slowest operation; see the
line balancing article.
- Keep overtime for emergencies. It is a way to make up a shortfall, and
the overtime article puts a price on it.
What the sums leave out
- Made-up order, SAM and efficiency. All are assumptions in the example. Use
your own.
- Cutting and finishing. The sums cover stitching only. Cutting, pressing and
packing need their own people and their own days, and the order cannot ship before they
finish.
- Fabric and trims arriving. A line cannot sew cloth that has not come. If
the fabric arrives on day five, the fifteen days are ten.
- Holidays and breakdowns. Count working days, not calendar days.
The efficiency calculator gives the efficiency to use, and the
cut-and-make rate article shows what the same
efficiency does to the price you quote for the order.