The final week of a machine project has a familiar rhythm: freeze the models, finish the drawings, line up the exports, then release. Everything is done, everyone wants to move on, and between you and the button sits one last document: the bill of materials. Precisely because the model is right, it is tempting to let that list ride along unchecked.
Yet this quarter of an hour decides lead time more often than the weeks of modelling before it. A wrong quantity only surfaces once the saw has already run; an empty field only when purchasing has nothing to order. That is why one fixed habit pays off: a BOM check before release, in a set five-minute sequence that covers the biggest risks. Which of those checks you can automate with a toolbox like Thundercad is covered further down.
To set the scope: this article is about the check itself, not about the drawing work and not about the familiar pitfalls. Checking drawings has a complete checklist of its own, and the mistakes that slip through when this moment is missing are described in five BOM errors that only show up on the shop floor.
Why a separate final check, even when you work carefully
A bill of materials is a derivative: it follows the model, but in practice it trails slightly behind. In the last week of a project, plenty still changes. The customer wants the platform on the other side after all, purchasing swaps a standard part over delivery time, a colleague removes a support that turned out to be redundant. Each change is fine on its own, but the sum of them does not land in the list that goes to work preparation by itself.
On top of that, manual interventions quietly pile up. A quantity that was once overridden by hand stays overridden, even when the model now holds twelve fastener sets instead of eight. A part that was temporarily excluded from the BOM for an intermediate state sometimes stays invisible for good. So the final check does not exist because you are sloppy; it exists because an assembly with hundreds of parts and dozens of late changes simply produces residual errors.
Step zero: update first, then look
Checking only makes sense on current data. Open the top assembly, update the model and refresh the BOM, so quantities and properties reflect the latest state. Skip this step and you are checking yesterday's state while signing it off today.
With Update BOM from Thundercad that is a single action: the tool updates the bill of materials of the complete assembly in one go. Right after that, use Export BOM to put the list into Excel, in your own template, so you can sort and filter without clicking through the model. Together that takes less than a minute, and it is the starting point for every check below.
The five-minute check, in a fixed order
Run the five checks in the same order every time, from the error with the biggest damage to the error that is quickest to find. A fixed order also stops you from quietly skipping a step under time pressure.
- Spot-check the quantities. Recount three kinds of rows in the model: a part that appears in a pattern or mirror, a row that was ever adjusted by hand, and a left-right pair. Four bearing blocks in the model and two on the list: exactly the kind of difference you catch here.
- Units on semi-finished materials. Profiles, hoses and strips belong in metres or millimetres, not in pieces. One row reading "tube 40x40: 1 piece" where 5.2 metres is meant, and the sawing station stands still. Walk through every row that is not a discrete purchased part.
- Empty fields in the columns the shop floor reads. Filter for empty cells in article number, description and material. An empty description becomes a question downstairs; an empty article number becomes a guess. In an exported list this is filtering, not reading.
- Duplicate and clashing article numbers. Sort by article number and look at the neighbours: the same number with two descriptions is a clash, two numbers for the same part is creeping stock pollution. Both belong back at the source before release.
- Item numbers versus balloons. Put the BOM next to the assembly drawing: does the highest item number match the row count, and do the balloons still point at the right items? Especially after late deletions or renumbering, something shifts here.
It does not need to be more than these five. If the first two steps already turn up deviations, stop checking and go back to the model: the list is not ready for release yet.
Updating, exporting and filtering are exactly the steps you can speed up: Update BOM and Export BOM do the legwork, you keep the judgement. That leaves most of the five minutes for actually looking.
Try 30 days freeWhich checks you can automate
Half of the five-minute check is mechanical work, and mechanical work belongs to software. Build your export template in Excel properly once and let it raise the flags itself: conditional formatting that turns empty cells red, a formula that counts duplicate article numbers, a column that compares unit against description. Because Export BOM always exports into that same template, those checks run again on every export without you lifting a finger.
What remains for the eye are the checks that need project knowledge: do the quantities match what is being built, is that one row really a semi-finished item, does that purchased part belong to this machine. Automate the finding, not the deciding.
From finding to source
Fix every finding in the model or in the part's iProperties, never in the exported file. A corrected Excel sheet is a plaster: on the next export the error is simply back, and by then nobody remembers. After fixing, run Update BOM and the export again and only walk through the changed rows.
Also keep a short list of what you find. Three findings of the same kind, say empty material fields on purchased parts every time, is not a checking problem but a working-method problem: then a template or a team agreement is due. That way the check step by step makes itself less necessary, which is exactly the point.
Frequently asked questions
How long should the final BOM check take?
For an average machine assembly, five minutes is realistic, provided the list has been updated and is sitting in Excel. For a new machine type or an assembly with many manual overrides, count on a quarter of an hour. If it structurally takes longer, the problem sits upstream, not in the check.
Do I check the BOM in Inventor or in Excel?
Fixing happens in Inventor, looking is faster in Excel. Sorting, filtering and highlighting empty cells are exactly what a spreadsheet is good at. So export first, and use the model only to trace and resolve findings.
Can I try this way of working first?
Yes. Update BOM and Export BOM are part of Thundercad's free trial month: download the toolbox and run the five-minute check alongside a few releases. You will quickly see which checks you can hand over to the template.
