Balloon 14 on the drawing points at a bearing housing. Item 14 in the parts list is a retaining ring. The fitter building the frame puts a finger on the sheet, calls engineering and asks which of the two he should believe. How it got this way no longer matters at that point; someone is standing idle.
Afterwards the answer is almost always the same: a part was added along the way, a position was dropped or a number was overridden by hand, and ever since, the list, the balloons and the ERP system have been looking at different states. In this article we zoom in on the numbering itself: how item numbers in an Inventor BOM are assigned, where things derail and which ground rules keep everything pointing at the same state. Tooling like Thundercad helps with refreshing the list, but the foundation is a handful of hard rules.
This is the deep dive on one specific error; the full list of classics is in Five BOM errors that only show up on the shop floor.
How Inventor assigns item numbers
The item number is a property of the BOM of an assembly, not of the part itself. The same bearing housing can be item 7 in one assembly and item 23 in another; the part number stays the same everywhere, the item number is only the position in this particular list. Mix those two up and sooner or later you will be renumbering parts because an assembly changed, which is exactly the wrong direction.
Inventor numbers in the order of the BOM view, and there is a catch right there: the structured view, with subassemblies as their own level, numbers differently from the flattened parts-only view. The parts list and the balloons on the drawing follow whichever view you attach them to. If the drawing uses the structured view while the export to the ERP system uses parts only, you have two number series for the same machine. That does not have to be a problem, as long as it is a conscious choice that everyone knows about.
Important to know: a manually typed item number becomes a static value. Inventor keeps numbering around it, but the overridden value stays put, even when the rest of the list ends up looking completely different.
Four ways the numbering derails
- Parts added along the way get the next free number and end up at the bottom of the list, even when they logically belong near the top. Whoever inserts them "neatly" by renumbering shifts every number behind them.
- Dropped positions leave a gap. The gap itself is harmless; the reflex to close it is not, because renumbering after release invalidates every balloon and every ERP line behind the gap.
- Manually overridden numbers stay put while the automatic numbering shifts around them. That produces duplicate numbers or inexplicable gaps, and the list itself does not show which value was overridden.
- Two views, two series: the drawing shows different numbers than the export to the ERP system, and work preparation spends its days translating between the two.
Each of these four is small on its own. Together they turn "grab item 14" into a gamble on the shop floor, and a warehouse that gambles picks up the phone.
Pick one policy and stick to it
All four have the same remedy: a numbering policy of a few lines that everyone knows and that nobody quietly works around. The exact choice matters less than the consistency with which you follow it. A policy that works well in practice:
- Until the first release, renumbering is free; sort and number as logically as possible for the shop floor.
- After release, an existing number never changes again. New parts get the next free number, even if that puts them at the bottom.
- Gaps stay. A gap documents that a position was dropped; closing it silently renumbers half the sheet.
- Manual overrides are the exception, are reported and recorded somewhere, and never happen "real quick" just before release.
Why so strict? Because every item number leads a life of its own outside the CAD environment: on printed sheets at the saw, in the ERP system, on pick lists in the warehouse. One renumbering drags a trail through all those places at once. How to push such a change through in a controlled way, when it really is needed, is described in Implementing changes without a chain reaction of errors.
Consistent numbering starts with a list that shows the current state, not yesterday's. With Update BOM in Thundercad you refresh the BOM in a few clicks, even one last time right before a check or release.
Try 30 days freeRefresh and check before every release
Numbering problems are born during changes but only become dangerous at release. Right before releasing you therefore want two things: a fresh list and a quick check. Refreshing is what Update BOM exists for: it updates the BOM so that late model changes actually land in the list and your check covers the real state.
The check itself does not need fifteen minutes. Sort the parts list by item number and walk it top to bottom: duplicate numbers sit right next to each other and an unexpected gap jumps out. Then look specifically at the bottom rows, because that is where mid-project additions collect, and judge whether they are still grouped sensibly for the shop floor or call for a deliberate, documented renumbering.
Balloons that move with the list
The parts list can be perfect while the balloons lie. After a model change a balloon can lose its attachment to the part, a balloon of a dropped position sometimes lingers on the sheet as an orphan, and a manually overridden balloon value deviates from the list without the sheet giving it away.
So after every round of changes, walk the sheets with three questions. Does every balloon still point at an existing part? Does it show the same number as the list? And have new positions appeared without a balloon? A spot check of three balloons per sheet, picked in the model, catches most shifts in practice. If balloon work structurally eats time, that is usually a sign of renumbering too often or too late; the win then lies in the rules from the previous section, not in faster clicking.
Frequently asked questions
Should the item number equal the part number?
No, and preferably do not couple them at all. The part number identifies the component everywhere in your company; the item number is only the position in this one list. As soon as you try to keep them equal, every assembly change forces you to renumber parts, which is exactly what you want to avoid.
What do I do with gaps in the numbering?
Before the first release, simply close them by renumbering. After that, leave them alone: a gap is information, it tells you a position was dropped. Closed gaps mean shifted numbers, and shifted numbers mean balloons and ERP lines that no longer match.
How do I track down manually overridden numbers?
Sort by item number and look for duplicate values and gaps, and compare the highest number with the row count. Refresh the list first with Update BOM, so you are looking at the current state instead of an old one. You can try that, along with the rest of the toolbox, free for a month via the Thundercad download.