Attrition is the stock you lose to the assembly process itself — parts a pick-and-place machine rejects, the leader of tape a feeder needs before it can place anything, components damaged in handling. Plan a build for the exact BOM quantity and you run short partway through. PartsBox accounts for it so you do not.
Attrition depends on the part and the run. A reel feeder wastes a fixed length of leader no matter how many you place; a percentage of small passives is lost across a long run; some components are simply more fragile than others. PartsBox models this per part, with two parameters:
Set both on a single part, or select many parts in the table and apply the same values to all of them at once.
When you build or price a project, PartsBox adds the attrition overhead to every line, so it draws — or buys — more than the bare BOM. The overhead is the larger of the two parameters, not their sum. A project needing 500 of a resistor set to 1% and a fixed 10: 1% of 500 is 5, the fixed minimum is 10, so PartsBox takes the larger and allocates 510.
The build table shows the arithmetic rather than hiding it: the required column reads "510 (500+10)", the BOM quantity plus the attrition overhead. The same overhead flows into BOM pricing and purchase lists, so the offer ranking and the order quantities reflect what production actually consumes.
A meta-part has no attrition of its own — its members do. When pricing, PartsBox uses the worst case, the highest attrition among the members, so the estimate never comes in under. When building, it applies the attrition of the member actually chosen for the run.
Attrition is for production planning. For a prototype, a partial build, or a pre-counted kit where the quantities are already exact, you can switch it off when starting the build — globally, with the "Disable attrition?" checkbox, or per entry, overriding the global setting for individual parts.
Part attrition is on the Production plan.