The call from the shop floor rarely comes at a quiet moment. The fitter stands at a half-built frame and is four mounting brackets short: the BOM said eight, the model turns out to contain twelve. Look it up, reorder, wait. The sour part is that the error was already in there when the list was released; it just was not visible on paper.
BOM errors are stubborn because they stay invisible until someone is holding the material. If you want to prevent BOM errors, it helps enormously to know where they are born, because it is nearly always one of the same five places. In this article we walk through them, covering for each error the cause, the damage and the way to catch it at the source. Tooling such as Thundercad helps keep the list current and publish it, but the first win is knowing where to look.
Error 1: quantities thrown off by mirrors and patterns
How it happens: mirrored and patterned parts are the classic culprits. A mirrored bracket saved as its own file counts along nicely, but a mirror that only lives as an operation in the assembly sometimes counts differently than you expect. The same goes for patterns with suppressed occurrences, and for parts accidentally set to reference: they silently drop out of the count even though they still need to be ordered.
What it costs: the saw shop cuts too few, purchasing orders too few, and the gap only shows when the fitter reaches for the last bracket that is not there. Count on a rush order, a day of standstill at that build spot and a fitter who has to switch to other work in the meantime.
Catching it at the source: after every serious model change, verify the quantities of mirrored and patterned items against the model, and deliberately check which parts are set to reference or phantom. It is not fifteen minutes of work; it is three focused looks at the places where things go wrong.
Error 2: standard parts that never made it into the model
How it happens: bolts, nuts, washers and other fasteners "will be added later". They live in a weld note, in the engineer's head, or nowhere at all. Sometimes there is a mounting kit in a text line on the drawing that never reaches the BOM. The model is finished, the list looks complete, and still a whole category of parts is missing.
What it costs: assembly grinds to a halt on the cheapest component of the entire machine. Someone drives to the wholesaler for a box of M10 bolts, and that is the good scenario; it gets worse with a special locking nut on a delivery time of weeks.
Catching it at the source: apply one hard rule: everything that has to be ordered or picked is in the BOM. If modeling it is impractical, use a virtual component, but add it the moment you think of it, not "later".
Error 3: the same part under two item numbers
How it happens: a part gets copied with Save As and receives a new file number while staying physically identical. Or the same article sits in two subassemblies with a slightly different description, or with a space or capitalization difference in the part number that stops rows from merging. The list then shows two items that are one article in reality.
What it costs: purchasing orders twice in smaller quantities and pays more, the warehouse counts differences that do not exist, and the shop floor wonders whether item 12 and item 31 are the same part or not. Doubt on the shop floor means phone calls, and phone calls mean standstill.
Catching it at the source: reuse existing parts instead of copying them, and keep part numbers tight. The check itself is surprisingly simple, see the tip below.
Errors like these only show up in a current, complete list. With Thundercad you refresh and publish the BOM in a few clicks, so you can still do it right before release.
Try 30 days freeError 4: descriptions lagging behind the design
How it happens: the design changes, the text does not. The 5 millimeter strip becomes 6 millimeter, but the description still reads "Strip 5x50". Copied parts inherit the text of their original and nobody looks at it again. The geometry is the truth; the description is an outdated summary.
What it costs: the shop floor and suppliers work from that text, not from the model. The saw operator picks material by description, the machining supplier quotes on the text in the request. One stale description can produce a completely wrong material order, plus the argument afterwards about which truth counts.
Catching it at the source: build descriptions from parameters where you can, so they change along with the model. And refresh the list right before release: that is what Update BOM is for, it brings the bill of materials up to date so late model changes actually land in the list.
Error 5: pieces where meters should have been
How it happens: bulk material sits awkwardly in a BOM that thinks in pieces. A cable tray, a hydraulic hose, a length of chain profile: modeled as a single part it counts as "1 piece", while purchasing needs twelve meters. The reverse happens too: twelve separate segments in the model where one length was intended.
What it costs: too little gets ordered, or far too much. One meter of cable tray where twelve were needed means a back order; twelve lengths where one was enough means leftover material and a skewed job costing.
Catching it at the source: give bulk material a fixed base unit and, before release, walk through every item that says "pieces" but is really length material. There are usually only a handful; one sort by description finds them all.
The catch routine before every release
Five error types, one routine. Work through this list at every release and you catch the bulk of them before the shop floor ever notices:
- Refresh the list with Update BOM, so you are looking at the current state instead of yesterday's.
- Sort by part number and by description and hunt for duplicates and stale texts.
- Verify the quantities of mirrored and patterned items against the model.
- Walk through the fasteners: is everything that needs ordering actually in there?
- Check the units on bulk material: meters where meters belong.
- Only then export, with Export BOM into your own Excel template, so purchasing and work preparation always see the same columns in the same order.
That fixed export is not a detail. How to set up such a template is covered in BOMs from Inventor to Excel: always in your own template. And why this is about more than tidiness is covered in Clean, reliable data into production: every error you catch here never needs repairing further down the process.
Frequently asked questions
Who owns the BOM: engineering or work preparation?
The source sits with engineering, because the BOM comes out of the model. Work preparation may enrich it, but errors get fixed in the model, not in Excel. Correct them in the export and you will simply be repairing the same error again at the next revision.
Does an ERP link help against these errors?
A link removes retyping mistakes, and that is real progress. But the five errors in this article live upstream of the link, in the model and the list itself; a link only forwards them faster. Get the source right first, then automate the handover.
How much time does this check take per release?
With the routine above, usually about ten minutes for an average assembly; refreshing and exporting take a few clicks with Update BOM and Export BOM. Want to try the routine on your own bill of materials: try Thundercad free for 30 days.