Off the Shelf

Every design review ends with a set of parts that are exactly right. Correct material, correct tolerance, correct margin. Nobody in the room is asked how long any of them takes to buy.

The stoppage is a fastener. Not a casting or a housing — a screw, specified to a tolerance that took it off every catalogue and onto one supplier’s schedule. Two shifts of work stand in front of the line and the line is not moving, because that single item is weeks out. The bin for it is labelled and empty. Everything else for the build is in the building.

Nobody over-specified anything deliberately. An engineer is rewarded for a design that performs and carries no risk, and tightening a tolerance is the cheapest way to take a risk off a drawing. The lead time this creates belongs to another function, lives in another system, and surfaces a year later. There was no forum where a tolerance ever met a delivery week, so the drawing was optimised against everything except the thing that would stop it.

The bill is paid in stopped hours and in expediting that never traces back to the drawing that caused it. The remedy is rarely engineering; it is a standing conversation between the people who set tolerances and the people who buy against them. I ask which parts on a build sit on somebody else’s shelf, and which exist only on ours.

So there is one question worth carrying into a design review long before the first order: which of these tolerances is the reason this part has one supplier? Every tolerance decides who is able to make the part. Most of them are set by somebody who will never have to buy it.

Where this shows up: Prototype to serial — closing the gap between what is designed and what can actually be built.