Bill of materials

A bill of materials (BOM) says what a product is made of: every manufactured part, every purchased item, every screw, with quantity, material and part number. It connects engineering (drawings), purchasing (what to buy) and production (what to make and assemble).

How it works

A BOM is usually multi-level: the finished product at the top, subassemblies below and parts and purchased items at the leaves. There is the engineering BOM (as designed) and the manufacturing BOM (as built, including process items). When generated straight from CAD and imported into the ERP, it avoids the retyping behind the costliest errors: buying the wrong part or the wrong quantity.

Practical example

A filling machine has 312 items. The BOM exported from CAD separates 86 machined parts (with drawing and material), 140 commercial items (with supplier and code) and 86 fasteners. Purchasing creates orders straight from that list, and production uses the same list to pick the assembly kit.

Common mistakes

• Keeping the BOM in a separate spreadsheet that does not follow CAD revisions.
• Mixing internal and supplier part numbers without separate columns.
• Forgetting assembly consumables such as adhesive and sealant.