GlossaryKind of term: Manufacturing termBill of materials
What is a bill of materials?
A bill of materials lists the parts, bought-in items and materials an assembly is made of, with the quantity per assembly for each item.
Each line carries an item number, a part or article number, a description, the quantity per assembly and often the material or standard. The item numbers refer to the item balloons on the assembly drawing. A bill can be flat or multi-level, with a subassembly referring to a list of its own. It appears as a table on the drawing, in a spreadsheet or in an ERP or PDM system.
How does the bill of materials work in practice?
The bill decides how many times each price counts, and which items get an estimate and which a purchase price. A missing item, an outdated quantity or a list at a different revision from the drawing gives a total that is right on paper and wrong on the shop floor. Watch the units too: besides pieces, a bill can hold metres, kilograms or consumables.
What is the bill of materials often confused with?
A bill of materials describes an assembly but is not the product itself. A routing is something else again: it records in which order and on which machines a made part is produced. And the designer's bill, the engineering BOM, is not automatically the production bill: the latter adds what making and fitting need, such as stock material, consumables and the grouping into subassemblies.
Frequently asked questions
Which items on the bill get an estimate of their own?
The made parts: every item that goes through your own shop. Bought-in parts, including those a supplier makes to drawing, get a line at their purchase price, and the assembly work its own item. The quantity per assembly decides how often each line counts.
What do I check on the customer's bill before I calculate?
That every item appears on the assembly drawing, that the quantities per assembly are right and that the revision of the list matches the revision of the drawing. Query any item that is missing or unclear.
What is the difference between a flat and a multi-level bill?
A flat bill lists every item on one level. A multi-level bill follows the build: the assembly holds subassemblies, each with a list of its own. For the estimate you multiply quantities through the levels.