BOMs don't survive contact with the shop floor.

Every manufacturing ERP project I've been close to has the same gap: the bill of materials in the system, and the thing the shop floor actually makes. The two start out identical and drift apart from day one. That's not carelessness — it's structural.

Why the drift is inevitable

Substitutions. A material runs short mid-batch, an experienced operator swaps in the approved equivalent from the next shelf, production doesn't stop, the customer never notices. The paperwork rarely follows. From the floor's point of view nothing happened — the product got made.

Batch-size reality. The BOM is modelled at a clean theoretical quantity. The actual kettle, mixer or line has a minimum fill, startup scrap, and losses that don't scale linearly. Halve the batch and consumption per unit quietly changes; the modelled BOM has one number for all of it.

Undocumented steps. The rework loop after the forming step. The extra sieve pass "we always do in summer". The manual quality check that eats ten minutes and a consumable. Everyone on the floor knows these; nobody has written them down, so nobody costs them.

None of this feels like a "change" to the people doing it — it's just how the product gets made. So nothing ever triggers an update in the ERP.

How it quietly breaks the system

Costing. Standard cost is built on the modelled BOM. If the floor consumes six percent more of an ingredient than the recipe says, margin is overstated on every unit — and you find out at year-end through an inventory write-down, not through a report during the year.

Planning. MRP explodes the modelled BOM to plan purchases. It buys the wrong quantities of the wrong things: stockouts of what you actually consume, a slow-building mountain of what you don't.

Trust. Production variances get so noisy that people stop reading them. That's the quiet death: once variance reporting is noise, the ERP has lost the floor, and every number downstream of it is decoration.

The fix is honesty, not discipline

The instinct is to demand that the floor "follow the BOM". In my experience that fails, because the floor's deviations usually exist for good reasons. What works:

Walk the floor before you model. Watch a real batch end to end. Weigh what actually goes in. An operator narrating what they do is worth more than the spec sheet from the design office — the spec is the intention; the floor is the fact.

Model the recipe people actually cook. Capture the as-made BOM first, even if it's ugly: with the substitutions, the scrap, the extra steps. You can improve towards the ideal recipe later. You cannot plan or cost from fiction.

Build the feedback loop. Make deviations cheap to report — a scan at the station, one line on the batch record that flows into a weekly review. Give the BOM an owner. Review variances on a cadence, and when the floor turns out to be right — it often is — change the BOM, not the floor.

The whole loop in one picture:

A BOM is a living document. Treat it as a snapshot from the design office and it starts dying the day you post it. Keep it fed from the floor and it becomes the one thing in the ERP everyone actually believes.