Bill of Materials Structure and Explosion
Structure
A BOM states, for each parent, the components, quantity per, and the level each component occupies. Level 0 is the end item. A component appearing under several parents takes the deepest level as its low-level code, and MRP nets it only when that level is reached. Explosion is the top-down calculation of gross requirements; implosion (where-used) answers the reverse question - which parents consume this component.
BOM types
Single-level BOMs drive one explosion step. Indented BOMs show the full hierarchy; summarised BOMs total each component across the product. Modular and planning BOMs forecast options with percentage allocations. Phantom items are assemblies never stocked - they explode straight through to their children.
Accuracy details that matter
Quantity per must match the unit of measure of both parent and component (per 100, per metre, per kg). Scrap and yield must be applied at the level where they occur, or every downstream order is short. Effectivity dates keep engineering changes from corrupting history. Alternate components and reference designators belong in the record, not in tribal knowledge.
Worked example
Parent A (level 0) needs 2 B and 3 C (level 1). B needs 4 D; C needs 1 D. D appears under both parents at level 2, so its low-level code is 2 and it is netted once at that level. For one A: D from B = 2 × 4 = 8; D from C = 3 × 1 = 3; total 11. With a 5% scrap allowance on D, the gross requirement is 11 ÷ 0.95 ≈ 11.6, rounded up to 12.
Traps
Skipping a level, which double-counts or misses requirements. Treating phantoms as stocked items. Forgetting scrap. Ignoring where-used when a shortage hits - implosion shows which parents are affected.