6. Engineering: bills of materials and routings

This is where the item master becomes a product structure — where the parts table stops being flat.

Bills of materials

A BOM says what goes into one unit of a part, one level down: the bike lists a frame and two wheels; the wheel, in its BOM, lists a hub, a rim and thirty-two spokes. Open a part and click BOM.

Some rules of thumb from forty years of other people's mistakes:

• Quantities are per one unit of the parent, in the child's unit. 2.5 metres of tube per frame, 0.35 kg of powder per frame.
• Use reference designators on anything with positions: R1, R2, R3; left, right. Free text, but future you will want them.
• One level at a time. Do not list the wheel's spokes on the bike's BOM. The explosion at the bottom of the BOM page walks every level and multiplies through; that is its job, not yours.
• A part cannot contain itself, and the system will refuse it — directly or through any chain.
• Phantoms are rare. A component whose part is not stocked is built inside a work order and consumed by the next step without ever being shelved — a hub assembly pressed and laced in one flow. If it ever waits on a shelf, or you ever sell it, it is stocked, not a phantom.

The BOM page's banner answers the question MRP exists for, for today: you can build N, and here is the component that stops you making more.

Routings

A routing is the sequence of operations that make one unit: for a wheel, hub build → lacing → truing → tyre fitting. Each step names a workstation group — the capability, not the machine — and carries setup and run times, and a batch size when the process is batched ("the bed holds six").

Open a part and click Routing. A part with no routing can still be made; its work orders get a single Build step.

The link between them

On the BOM page, each line has a consumed at dropdown naming a routing step. The hub and bearings are consumed at hub build; the tyre at tyre fitting. Unallocated lines go to the first step.

This is what lets a work order be honest about readiness. Without it, a wheel job could not start until the tyres arrived; with it, Joe builds hubs while the tyres are on a boat, and only Sarah's step waits. It is also what backflushing consumes at each step. It is worth the two minutes per part.

Groups and stations

Before releasing anything, make the groups your routings name — Welding, Wheel building, Assembly, Print farm — and the stations in each. Three truing stands are three stations in one group and three times the throughput of that step.

Next: making.