/bom-builder, BOM plus weight and power budgets

Three linked tables that decide whether the robot is legal, survivable and affordable. They live in docs/BOM.md. Keep all three current, because a stale weight budget is how you get disqualified at inspection.

Process

1. Collect every part from the current design

Walk the whole robot, and don't miss the small stuff.

• Drivetrain: motors, gearboxes, wheels, hubs, couplers
• Electronics: Arduino UNO Q, motor driver, edge sensors (×4), opponent sensors (×3 to 5), battery, regulator, switch, connectors, wiring
• Mechanical: chassis or base plate, blade, ballast, standoffs, fasteners, brackets
• Consumables: heat-shrink, threadlock, tape

Fasteners and wiring are always heavier than people estimate. Give them a real line rather than a guess of zero.

2. Weight budget, the legality check

• Fill Est. g from CAD, and fill Actual g as parts arrive and get weighed.
• Total against the confirmed 3 kg limit.
• Target about 2 900 g competition-ready with batteries installed. Magnets are banned and the arena is wood, so push = μ × weight and every gram under the cap is push force you don't have.
• Flag immediately if the estimate goes over 3 000 g. Losing weight late is painful; catching it in CAD is free. If the estimate comes in well under, add low-mounted ballast rather than leaving the margin unused.

3. Power budget, the survival check

The peak column sizes everything. Then check:

• Driver continuous rating at 2× total motor stall current or better. A sumo match is the stall condition.
• Battery C-rating × capacity at total peak or better, with margin.
• Regulator dropout verified against worst-case sag. The UNO Q's Linux side resets more readily than a bare MCU.

If motor stall current is a guess rather than a datasheet figure, say so explicitly and recommend measuring it before ordering the driver.

4. Cost the BOM

• Date every price and stock check, since they drift.
• Note the spec that matters, not just the part name. "TB67H420FTG, 9 A continuous" is useful; "motor driver" is not.
• UAE sourcing: check shipping and customs. Lithium cells often can't ship by air, which means long lead times or local-only sourcing. Flag long-lead items so they get ordered first.
• Spares are line items, not afterthoughts. At minimum: spare wheels, spare battery, spare fasteners, and a spare blade if machining allows.

5. Total and sanity-check

• Sum line costs into the category summary in docs/BOM.md.
• Flag duplicates and missing parts.
• Check the total against the user's budget ceiling, and ask if it isn't recorded yet.
• Confirm machining costs are included. The blade usually needs an external shop.

Output

Update docs/BOM.md in place with all three tables plus the budget summary. Report:

• Weight total against the limit, as a percentage
• Whether the driver and battery are correctly sized, with the numbers
• Total cost against budget
• Long-lead items to order first
• Anything still unknown that blocks ordering

Rules

• The weight class is confirmed at 3 kg (2026-08-09). Don't reintroduce the 500 g option or tag the class (assumed).
• Don't invent prices. If a price wasn't checked, leave it blank and say so.
• A part with no Actual g has not been weighed. Don't present estimates as measurements.
• Cross-check against the estimated budget already in docs/BLUEPRINT.md §3.1 and §4.1 rather than starting from scratch.