A bill of materials, or BOM, is the organized parts list that tells a team what is needed to build a product. It connects the design shown in a CAD assembly to real items such as screws, plates, motors, washers, and purchased components. A clear BOM matters because purchasing, manufacturing, inventory, costing, and quality control all depend on the same part information.
Without it, a correct drawing can still lead to the wrong parts, wrong quantities, or production delays.
An assembly is built from parts and often from smaller sub-assemblies that are prepared before final assembly. In an exploded view, each item is pulled apart along an assembly direction so students can see how the product fits together and how callout numbers match the BOM table. The BOM usually lists item number, part number, description, quantity, material, supplier, revision, and make-or-buy status.
Once released, the BOM drives purchase orders, work instructions, inventory picking, manufacturing routes, and final inspection.
Key Facts
- Total part count = quantity per assembly x number of assemblies.
- Extended cost = unit cost x quantity.
- Assembly cost = sum of all extended part costs plus labor and overhead.
- A part number identifies a unique item, while an item number identifies its location in one BOM.
- A multi-level BOM shows parent assemblies, sub-assemblies, and child parts in a hierarchy.
- Revision control ensures that drawings, part numbers, and BOM rows all describe the same design version.
Vocabulary
- Bill of Materials
- A bill of materials is a structured list of every part, material, and sub-assembly needed to build one finished product.
- Assembly
- An assembly is a group of parts joined together to perform a function in a larger product.
- Sub-assembly
- A sub-assembly is a smaller group of parts built first and then installed into a higher-level assembly.
- Part Number
- A part number is a unique code used to identify one specific component, material, or purchased item.
- Exploded View
- An exploded view is a technical drawing that separates parts visually to show their order, orientation, and relationships.
Common Mistakes to Avoid
- Confusing item numbers with part numbers is wrong because item numbers are only positions in one BOM, while part numbers identify actual unique parts across the company.
- Forgetting to multiply quantities by the number of finished products is wrong because a BOM usually gives the quantity needed for one assembly, not for the whole production run.
- Mixing old and new revisions is wrong because a revised drawing may require different dimensions, materials, suppliers, or matching parts.
- Listing a sub-assembly and all of its child parts at the same level without a clear structure is wrong because purchasing and manufacturing may double-count parts or miss build steps.
Practice Questions
- 1 A motor mount assembly uses 1 base plate, 2 brackets, 4 bolts, and 4 washers. How many of each item are needed to build 25 motor mount assemblies?
- 2 A BOM lists 2 bearings at 14.00, 4 screws at 18.00. What is the total material cost for one assembly?
- 3 A design team changes a bracket hole diameter and releases revision B of the drawing. Explain why the BOM, purchasing records, and manufacturing instructions must be checked before production continues.