Downtime

Downtime is the most actionable signal in a small shop, and it is also the most poorly tracked. Most owners can tell you the rough number of breakdowns last month but cannot break them down by cause, by machine, or by shift. That gap is the gap between knowing the shop has a problem and being able to do anything about it. Lean and TPM treat downtime as the primary input to availability and the most direct expression of equipment loss.

"The five minute stops cost more than the four hour breakdown. They just hide better."

How downtime works as a measurement

Every minute of scheduled production time falls into one of two states: the machine is running good parts, or it is not. The "not" bucket gets split. Planned downtime is anything the shop chose to do during production time. This includes preventive maintenance, changeovers between products, scheduled meetings, breaks, and inspection time. Unplanned downtime is everything else: breakdowns, parts shortages, missing operators, quality holds, waiting on QA. The split matters because the response to each is different. Planned downtime is reduced by making the planned event shorter, like running quick changeover on setups. Unplanned downtime is reduced by removing the underlying cause.

Inside unplanned downtime, the categories should be specific enough to act on. "Mechanical breakdown" is too coarse; "hydraulic leak on press two" is actionable. Most shops settle on six to ten categories per machine, refined over the first quarter of tracking. The categorization also feeds the six big losses framework, which is TPM's canonical way of mapping every downtime category back to availability, performance, or quality.

The unit of accounting is time, not count. Two short breakdowns of five minutes each are the same downtime as one ten minute breakdown. The shop floor sometimes obsesses about the number of stops, but the time is what hits the schedule. A shop that tracks counts without minutes is solving the wrong half of the problem.

Where downtime tracking fits on the shop floor

Picture a 25 person sheet metal shop running three press brakes and four laser stations. The owner believes the laser is the bottleneck. A two week downtime tracking exercise tells a different story. The laser is down 12 percent of scheduled time, mostly for nesting changes and material loading, both planned. The press brakes are down 21 percent of scheduled time, almost all unplanned, almost all from a single brake whose backgauge keeps slipping. The shop had been hand wringing about a new laser purchase. The real fix is a one day repair on the back of an existing brake.

That kind of clarity only happens when downtime is tracked at the machine level and broken into categories operators can actually mark on a sheet. The tracking does not need software; it needs a clipboard, a posted board, and a weekly five minute review with the maintenance lead. Once the categories settle in, the shop can start attacking the top two categories with a focused improvement effort.

Common mistakes with downtime

• Counting events instead of minutes. Two five minute stops equal one ten minute stop. The schedule cares about time, not how many.
• Categories too broad to act on. "Machine issue" is not a category. Five to eight specific causes per machine is the floor.
• Ignoring the small stops. Minor stops under two minutes get waved away and then collectively eat 15 percent of capacity. Track them.
• Tracking only the worst machine. The other machines may be hiding their own pattern. Track everything, even briefly, before deciding what to focus on.
• Using downtime data to assign blame. The instant operators believe the data will get them in trouble, the data quality collapses. Lead with the problem, not the person.

Downtime and related Lean tools

Downtime is the raw input to availability, which is one of the three factors in overall equipment effectiveness. It is the gap that mean time between failures and mean time to repair explain. Categorized properly, downtime maps directly onto the six big losses, which is the framework TPM uses to attack equipment losses systematically.