Changeover Time: The Hours That Appear in No Costing

The symptom

Almost every manufacturer has a number for changeover time. It sits in the ERP, it feeds the costing, it determines batch sizes and delivery dates. And in almost every business, nobody can say precisely where it came from.

Usually it was estimated or measured once, years ago, under conditions that no longer exist: different tooling, different parts, different people. It has been carried forward since, because it looks plausible and because changing it starts an argument with sales.

Running alongside it is a second, unwritten number: what the people at the machine consider realistic. Everyone on the shop floor knows that figure, and it is recorded nowhere. The gap between the two numbers is the interesting part.

The mechanism

Changeover isn't a single operation, it's a chain. Fetch the tooling, strip the old fixture, clean, mount the new fixture, set the datum, load the program, run a first article, measure, correct, obtain release. The documented changeover time generally measures only the middle block — the actual physical conversion at the machine.

What is systematically missing are the edges. Hunting for tooling that wasn't returned to its place after last use. Waiting for the setter, who is currently standing at another machine. Waiting for measurement release, because the inspection room is occupied. The second and third first article, because the first came in outside tolerance.

These edge pieces share a property: they aren't the setter's fault, and so they don't get recorded as changeover time in the confirmation. They vanish into "other" or into nothing. The machine is standing still regardless.

An uncomfortable chain follows. Understated changeover times make small batches look attractive on paper. Small batches mean more changeovers. More changeovers mean more unrecorded edge time. The business plans itself, with entirely correct arithmetic, into a utilisation that doesn't exist — then wonders why delivery dates slip while the capacity calculation balances.

The cost

An anonymised example, altered in detail, real in pattern. A contract machining business producing turned parts, just under 60 employees, was struggling with chronic delivery delays alongside apparently free capacity. The costing assumed 45 minutes of changeover.

Over three weeks we didn't analyse the confirmations — we measured machine standstill, from last good part of the old order to first good part of the new. The mean was 97 minutes. The median was 84; the upper quartile ran over two hours.

The largest individual items weren't spectacular: 14 minutes average waiting for the setter, 11 minutes hunting for tooling, 19 minutes for first-article inspection and release. Together, more than the entire costed changeover time.

Extrapolated across roughly 1,100 changeovers a year on four machines, the difference amounted to about 950 unplanned machine hours — at an internal rate of €78, some €74,000 of capacity that existed in the plan and not on the shop floor. The business had spent the previous year considering a fifth machine.

The fix

The first step was unglamorous: the costing was corrected to the measured values. Internally this was the hardest decision, because in the short term it makes prices and delivery dates look worse. It also makes them honest, and everything else builds on that.

Then the three largest edge items were addressed — not the machine, but everything around it. Tooling got fixed locations with shadow boards and a mandatory return before end of shift. First-article inspection for defined part families moved to a gauge at the machine rather than the inspection room. And changeovers were spread across the day so that one setter was never serving two machines simultaneously.

Measured changeover time fell to an average of 61 minutes. More important than the mean was the spread: the upper quartile fell from over 120 minutes to 78. Predictability comes from less variation, not from better averages.

That separation — measuring what actually happens instead of carrying forward what was once true — is the core of a process analysis. Fixed scope, fixed duration, fixed price.

The next step

Stand at a machine and time a single changeover: last good part to first good part. Compare the result with the number in your ERP. That one measurement costs you two hours and tells you whether a closer look is worth it.