Bill of materials

A bill of materials is the structured list of everything in a product: made parts, bought parts, sub-assemblies and raw material, with a quantity on each line. On an assembly the list has levels: a top list refers to sub-lists for the sub-assemblies.

What it means for the calculation

For a machined part the calculation starts at the material line: the grade, the stock size and the price per kilo or per bar, plus the share that ends up as chips. On an assembly the list also brings in the bought parts and the assembly time. A missing or outdated line is one of the most common reasons a quotation is right on paper and wrong in reality. And the lead time of the slowest line sets the lead time of the whole.

Where this comes up

In a shop's own system, tied to the product number, and usually on the assembly drawing too, as a table with item numbers.

Common mistakes

Setting the stock size equal to the finished size of the part, which leaves out the chip loss and the clamping allowance. Also carrying on with an old version of the list after a customer revision.

Does a single machined part have a bill of materials too?

Usually one line: the raw material with its stock size.

What is a multi-level bill of materials?

A list that refers to sub-lists for sub-assemblies instead of putting everything on one level.

Why does a bill drift out of step with the drawing?

Through revisions applied in one place and not in the other.