Check the spreadsheet before the cart: 6 things I look for in a hobby robot BOM

Disclosure: I help run MicroDuck Robot Community, an unofficial, independent fan and maker site. It is not affiliated with Pollen Robotics or Hugging Face. Everything below comes from reading public maker files; I did not build this robot or place any orders for this post.

A bill of materials (BOM) looks like the most boring file in a hardware repo. It is also the file that decides how much money you spend before you find out whether your build works.

Last week a community maker published a web preview of an assembly workbook for a small bipedal "duck" robot replica (the HD1910 servo route). Reading it closely turned into a useful checklist, so here it is in a generic form you can apply to any hobby robot BOM.

1. Rows are categories, not part counts

The preview has 33 print-item rows and 23 purchased-item rows. That does not mean 56 parts. One row can be "servo × 15" or "M2 screws, assorted". Some rows are alternatives (for example, two ankle variants, or optional roller parts) where you pick one, not both.

Check: before you add things to a cart, mark each row as required, pick one of, or optional for your build.

2. Re-add the totals yourself

In this workbook the total cell is SUM(I3:I58), but there is a real line item at row 59 (a sole grip pad, CNY 16) that the formula skips. Nobody did anything wrong on purpose; a row got added below the range.

Check: select the whole amount column and compare your sum with the stated total. If they differ, find the row that falls outside the formula range.

3. Quantity × unit price should equal the line amount

The battery row (an NP-F550-type pack) lists quantity 1, unit price 17, and line amount 47. One of those three numbers is wrong, or the line is really a bundle (battery plus charger, say) that the label doesn't describe.

Check: recompute every qty × unit = amount line. Mismatches usually mean a hidden bundle or a stale price, and you want to know which before ordering.

4. The part label should match the route you are building

The servo label in the table reads HD1901M while the project route is HD1910. That might be a typo, or it might be a different SKU. With 15 servos in a build, guessing wrong is expensive.

Check: confirm the exact servo SKU and voltage with the supplier listing, not just the BOM text.

5. Keep CAD, BOM and wiring on the same revision

The same update split the print files into separate HD1910 and XL330 routes. A BOM from one route plus CAD from another, or wiring notes from an older board revision, is how people end up with parts that physically fit but electrically don't. Connector families (for example EH2.5 vs AMP2.0 harnesses) are not interchangeable just because both carry power.

Check: write down, in one place, the CAD route, ankle/roller variants, board revision and pinout you are following.

6. Treat author budgets as history, not a quote

A second maker published a budget the same week: a body subtotal of CNY 2,529.70 (including a gamepad and spare stock, 15 servos at CNY 119 each), and a version with an assembled HAT/IMU board at CNY 2,827.70. Making boards yourself was estimated at CNY 3,400-3,700 once CNY 800-1,000 of tools and consumables are included.

Those are useful reference points, but they are one person's prices on one date. They leave out filament and printed parts, a computer, unpriced items like the microSD card, and shipping or losses. And you should not merge two different makers' lists into one "cheapest possible robot" number; they assume different boards and wiring.

Check: list what you already own (printer, soldering tools, a spare computer) and what's missing from the budget, then price only the gap.

The short version

Before ordering parts for any hobby robot:

• Classify rows: required, pick-one, optional.
• Re-sum the totals and recheck qty × unit price.
• Confirm SKUs and voltages against the supplier, not the label.
• Pin CAD, BOM, board revision and wiring to one route.
• Treat budgets as dated examples and price your own gap.

Buying parts doesn't finish a build; assembly, per-servo setup, IMU calibration and recovery checks still come after.

I keep the full source-checked notes for this route, including the BOM preview, both budgets and the calibration steps, on one page: MicroDuck DIY: BOM checks, build budget and calibration.

If you've been burned by a BOM detail I didn't list, I'd like to hear it in the comments.