How do I show a modified purchased part on the BOM?
Give the modified part its own part number and keep the stock part as its source. The BOM lists the new number; the new part's drawing names the stock part it's made from and shows only the changes. Purchasing buys the stock part, the shop knows what to do to it, and nobody mixes the two.
Why this goes wrong
A team buys a stock part, then cuts, drills or machines it to fit. If the BOM still shows the catalog number, purchasing orders the stock part, it arrives unmodified, and the build stalls. If the BOM shows only a new internal number, purchasing can't find it in any catalog.
The reason for the modification gets lost too. One engineer we spoke with described a bought-in part sourced wrong because nobody had written down why that specific one was chosen: it looked right, and it didn't work.
Drawing standards have a name for this case. ASME Y14.24 describes an altered item drawing, which shows the physical alteration of an existing item, such as a purchased part, and establishes a new identification for the altered item (NDIA overview of ASME Y14.24). The principle carries over to a small team with no formal standard: the modified part is a different part, so it gets a different number.
Worked example: 170-1031 hinge pin
The team buys a 200 mm ground stainless pin from a catalog supplier, part STK-0607. They cut it to 180 mm and drill a cross hole for a retaining clip. The modified pin becomes 170-1031.
/170-1031 Hinge pin/
/released/
170-1031_RevA.pdf altered item drawing: references STK-0607, shows the cut and the cross hole
source-part.txt STK-0607, supplier name and catalog number, why this pin
The drawing shows only what changes: the cut length with its tolerance, the cross hole and its location, and a note: "Make from STK-0607. All other dimensions per supplier catalog."
Step 1: Give it a new number. 170-1031 is never the same thing as STK-0607, even though one becomes the other.
Step 2: Link it to the stock part. On the drawing, in a "make from" note, and in the BOM, so anyone can see what to buy.
Step 3: Choose how the BOM shows both. Three common ways:
| BOM layout | What it looks like | What purchasing sees |
|---|---|---|
| New part only | One line: 170-1031, qty 2 | A part they can't buy; they have to open the drawing to find STK-0607 |
| New part with the stock part under it | 170-1031 qty 2, with STK-0607 under it, qty 1 per pin | Exactly what to buy (STK-0607) and what it becomes (170-1031) |
| Material column | 170-1031, with "made from STK-0607" in a column | Clear in a spreadsheet, but easy to lose when the BOM is exported elsewhere |
For 170-1031 the team uses the second layout, because it's the one where purchasing sees what to order without opening a drawing.
Step 4: Write down why this stock part. One line in the source file: why STK-0607 and not a cheaper pin. Ground finish for the bushing, 316 stainless for corrosion. Then a later substitution doesn't quietly undo the reason.
Step 5: Decide who modifies it. In-house or by a shop, it goes on the routing or the PO. If a shop does it, send the 170-1031 drawing with the PO, not the catalog page.
Ways to handle a modified purchased part
| Method | Good for | Main risk |
|---|---|---|
| Keep the catalog number and add a note | Nothing beyond a one-off prototype | Stock and modified parts get mixed in the bin |
| New part number with a "make from" link | Most teams; clear for purchasing and the shop | One more drawing to keep |
| New part number, stock part as a child on the BOM | Teams whose purchasing works from the BOM | Quantities need care if one stock part makes several |
| Buy it already modified | Volume parts a supplier can modify | Lead time, and the supplier's revision to track |
When this stops working
- Unmodified stock parts keep reaching the build.
- The same stock part is modified three different ways with no clear numbering.
- Substitutions happen without anyone checking why the original was chosen.
- The supplier changes the stock part and nobody notices.
At that point the stock part, the modification and the reason need to stay together with each revision. Tracking what each supplier has helps here too: How do I keep track of which revision each supplier has?
FAQ
Does a modified purchased part need a new part number?
Yes. Once it's modified it's a different part, and drawing practice under ASME Y14.24 gives an altered item its own identification. Without it, stock and modified parts end up in the same bin.
What is an altered item drawing?
A drawing that shows the changes made to an existing item, such as a purchased part, and gives the result a new part number. It references the original item rather than redrawing it.
Should the stock part appear on the BOM?
If purchasing orders from the BOM, yes, as the source of the modified part. Otherwise, at least name it on the modified part's drawing.
How do I record why a specific stock part was chosen?
One line next to the part: what about it matters, such as material, finish or rating. That stops a later substitution from undoing a deliberate choice.
Internal links
- How do I keep track of which revision each supplier has?
- How do I document why a part was designed a certain way?
- How do I check a drawing before it's released to the shop?
Sources
- NDIA, overview of ASME Y14.24 types of engineering drawings (2008): https://ndia.dtic.mil/wp-content/uploads/2008/technical/GastonEngineeringDrawingsY14_24a.pdf
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