The preventive maintenance schedule stops most failures. This page covers the ones that happen anyway — because the difference between a breakdown and an outage is usually not skill and not luck; it is whether the part that failed was already on a shelf you own. An OEM with an eleven-day lead time does not care that your machine stopped on day one of your biggest contract. The shelf is the only supplier whose lead time you control.
The list is five columns, built from the maintenance log in one afternoon with the person who actually fixes things: the part (named as the floor names it, with the manufacturer part number written next to it — "the little green board in the cabinet" is how a 2 a.m. order does not get placed); what it gates (not "lathe" — "lathe → the aerospace lot due the 14th, $31k"); source and real lead time (the last time you bought it, how long did it actually take?); shelf quantity with a two-bin reorder point (one bin in use, one sealed; empty bin goes on the purchase bench, not back on the shelf); and last-used, which feeds the kill list.
The 48-hour rule decides what belongs on the shelf: if the part gates revenue and its real lead time is longer than you can tolerate the machine being down, it belongs on the shelf. A two-day lead on a machine you can live without for a week — order on demand. A three-week factory lead on the machine that produces your payroll — that is the shelf. Cost is the tiebreaker, not the gate: a $340 drive that saves a $180k contract is not a $340 part, it is cheap insurance with a part number.
A shelf with no reorder point is a battery that drains silently. The first spare you use feels free; the point of the list is that using a spare starts an order, not that using a spare was possible. The worst version of a spare is the one you used in March and remembered in September — empty shelf, long lead, breakdown twice.
The kill list runs once a year with the last-used column open. Spares for retired machines, superseded revisions, anything untouched for two years: sell it, return it, scrap it — get it off the shelf and its value back into the business. One shop's audit found sixty-two spares; eighteen belonged to machines retired before the current owner bought the company. Dead stock: $2,100.
Five traps: buying spares for everything (the list exists to rank; spares for every part of every machine is capital that sleeps); the spare for the machine that left (shelves drift — the last-used column and the annual walk are the fix); no reorder point (two-bin or min-max, reordered the day the bin empties, not the day someone remembers); untested spares (the drive never powered up, the seal that does not fit the revision you actually run — a part-shaped hope; fit-check on the maintenance cadence); and spares without the fiche (manual, torque spec, wiring diagram, config backup stored with the part — otherwise the shelf serves whoever remembers, and the person who remembers is on leave the week it fails).
The worked example: a twelve-person CNC machine shop ran production through one 1998 vertical machining centre — it held tolerance like nothing else they could afford to replace. The servo drive died mid-way through an aerospace lot worth $180k with a delivery penalty attached. OEM quote: eleven days. The owner had heard this exact story from a neighbouring shop eighteen months earlier — same machine class, same failure, same eleven-day quote, a lost batch and a lost customer — and had spent one afternoon building this list from that neighbour's expensive education.
The shelf had the drive: $340, bought eight months earlier, bin-and-seal. The fiche was in the same drawer — parameter sheet, torque specs, configuration backup on a labelled USB stick. Forty minutes from dark machine to spindle turning. The lot shipped on time and the penalty clause never woke. The same walk caught a $140 fuse kit wound for the old spindle motor (replaced, fit-checked) and eleven spares for a bandsaw scrapped in 2023, sold to the scrap dealer's parts counter for $410 — dead stock out, coffee money in.
The shelf is not a cost. You are not buying parts; you are buying back lead time you do not control — at shelf prices instead of penalty prices.
The full page — with the five columns in detail, the 48-hour rule, the traps, and the worked example — lives at Hive Ops Notes.
Free: The First 30 Minutes — the incident quick-start checklist. If your operation needs the full system: Ops Starter Kit ($14), Ops Starter Kit Vol. 2 ($27), or the Ops Mega Bundle (all 5 kits, $49).
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