Sample frameDay 257·Operations · Batching

Morning. Nobody sat down and chose your batch sizes. Your setup times chose them.

Operations.

Batch Size and Changeover

5 min read·Apply by lunch

The question

Pick the thing your operation does in batches — the Thursday invoice run, the fortnightly release, the print run, the monthly board pack — and ask why it's that size rather than half of it: can you give a reason that isn't simply that switching over is a nuisance nobody wants to do twice? If not, you've been paying for that nuisance in flow time every week since, and never once priced it.

The idea

Ford Whitman Harris was a production engineer at Westinghouse when he published How Many Parts to Make at Once in Factory: The Magazine of Management in 1913 — two pages that gave the world the economic order quantity. His formula balances the cost of setting up against the cost of holding what you've made, and it carries a consequence people rarely follow through: the economic batch moves with the square root of the setup cost. Halve the changeover and the batch falls by about twenty-nine per cent. Quarter it and the batch halves. So batch size isn't a scheduling decision at all; it's a setup decision wearing a scheduling decision's clothes. Shigeo Shingo, the Japanese industrial engineer who began consulting to Toyota in the 1950s, is the one who took that seriously. Asked in 1969 to fix a thousand-ton press whose changeover ran to four hours, he got it — by his own account, and in under a year — down to three minutes. He published the method as SMED, single-minute exchange of die, in A Revolution in Manufacturing, which reached English in 1985. Which is why the schedule meeting is the wrong meeting. Teams spend years negotiating run lengths and release windows, defending a batch size that a few days spent on the changeover would have made indefensible.

The framework

01
Why is the batch that size — and who decided?
The batch you run is an answer your operation gave to a question nobody asked out loud: how much pain is a changeover worth? Before you change anything, take one repeated batch and write down how long the switch actually takes, what it costs you in lost output, and how many more times a week you'd have to do it to halve the run. Most teams have never timed a changeover with a stopwatch — they've only inherited a rule about it. The real number is usually smaller than the folklore and much larger than the schedule assumes.
02
Split the setup into what needs the work stopped and what doesn't.
Shingo's first move was a sorting exercise, done on paper before anyone touches the machine. Internal setup is anything that can only happen while the work is stopped. External setup is everything else — fetching the tools, staging the materials, warming the machine, briefing the person taking over, opening the file. In most operations a surprising share of what happens during the stoppage never needed the stoppage; it's done then out of habit, and habit is not a constraint. Sort your changeover into those two lists first, and do the external half while the previous batch is still running.
03
The conversion is where the hours go.
Sorting buys you something immediately; the real reduction comes from converting internal work into external. That means standardising fixtures so nothing needs adjusting on the day, pre-setting tools on a spare holder, replacing fasteners that need turning with clamps that need one motion, writing the sequence down so nobody has to remember it. Only then do you streamline what's left. The order matters: teams that skip straight to going faster end up rushing a changeover that still contains twenty minutes of work which never belonged inside it.
04
The gain belongs to frequency, not to output.
Here's where most changeover projects quietly fail. You halve the setup, the schedulers see free capacity, and they spend it running the same batches with more slack — so flow time doesn't move and the improvement vanishes into a utilisation figure. The point of a cheaper switch is to switch more often: shorter runs, a wider mix each week, less finished stock sitting about, and a customer order that waits days rather than a fortnight. Decide before the work starts that the gain is spent on frequency, and say so out loud to whoever owns the schedule.

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