Bills of Materials (BOMs) and Purchasing

What a BOM is

A Bill of Materials (BOM) is the structured list of everything in your assembly: every part, how many of each, and key properties like material, vendor, part number, and mass. It answers the two questions the build team always asks: what do we need to buy and what do we need to make?

Onshape generates the BOM for you

Onshape builds the Bill of Materials automatically from your assembly. As you insert and mate parts, the BOM updates live. You can choose how it is structured:

• Flattened — a single combined list with no hierarchy (good for total counts and quick purchasing).
• Structured, top-level — shows the top assembly and its immediate children only.
• Structured, multi-level — shows the full subassembly hierarchy, so you can see which parts belong to the gearbox vs. the intake.

You can add custom columns (vendor, part number, cost, mass) and insert the BOM directly into a drawing, where each row links to a balloon on the view.

Why the BOM is one of your most valuable outputs

• Purchasing: Export the COTS rows to order parts early. Long lead times can sink a build season, so generating an accurate BOM quickly is critical.
• Fabrication planning: The custom-part rows tell the build team exactly what to make and how many.
• Weight tracking: If your library and custom parts carry correct masses, the BOM gives a running weight estimate you can check against the 2026 limits (115 lb without bumpers per R103, 135 lb with bumpers per R408). Catching an overweight robot in CAD is far cheaper than at competition.
• Cost accounting: FRC has cost-related rules (the BOM/cost accounting requirements in the game manual); tracking part cost in the BOM helps you stay compliant and on budget. Teams competing internationally must also produce a formal BOM for customs.

Good BOM habits

• Name parts clearly in the Part Studios so the BOM is readable ('Intake Roller Shaft', not 'Part 12').
• Fill in metadata (material, vendor, part number) as you design, not the night before competition.
• Use library parts (MKCad, vendor CAD) so masses and names come in correct automatically.
• Keep one source of truth — the Onshape assembly — so the BOM never drifts from the real design.

From BOM to robot

A clean BOM turns into a purchase order, a cut list for the shop, and a weight budget all at once. Teams that maintain a live, accurate BOM throughout the season buy parts earlier, build with fewer surprises, and arrive at competition within weight. REV's FRC Kickoff Concepts BOM guidance is a helpful, current reference for how teams structure theirs.