Bill of materials (BOM)

A bill of materials (BOM) is a structured list of all components, subassemblies, parts and materials, with their quantities, needed to manufacture or assemble a product. It is an integral part of technical documentation and links engineering output with production, purchasing and logistics.

The engineering BOM (EBOM) captures the product structure as the designer sees it. The manufacturing BOM (MBOM) reflects how the product will actually be made and assembled. It may have a different structure because it accounts for manufacturing operations, semi-finished stock and consumables.

The BOM is a key document for purchasing materials and components, for production planning and cost estimating, for inventory management and logistics, and for service documentation and spare parts.

In small projects, a BOM is often a simple spreadsheet. For large products with thousands to tens of thousands of line items, BOMs are managed in PDM and PLM (product lifecycle management) systems such as SolidWorks PDM, Siemens Teamcenter or PTC Windchill.

A BOM is created for every assembly as soon as it is headed for production or purchasing. It is used to order purchased parts (bearings, fasteners, electronics), to send manufactured parts out to suppliers, to estimate the product cost and to plan assembly.

It pays to maintain one from the prototype stage. It quickly shows you how much the materials and purchased parts cost, which items have long lead times and where standard or repeat parts can be used.

The BOM must match the current revision of the model and drawings. Spreadsheets that are retyped by hand quickly drift out of sync with the CAD data, so it is best to generate the BOM directly from the assembly. For purchased parts, list the manufacturer's exact part numbers, not just a generic description ("6204 bearing" without specifying the variant, seals and internal clearance is not enough).

Do not forget items that are often missing from the CAD model: adhesives, lubricants, threadlocker, labels, packaging or cabling. If you hand the BOM over to a supplier, agree on the format and on how revision changes will be marked.