Why the bill of materials keeps changing mid-build

A machine is designed while it is being bought. The drawing office releases the frame early so that fabrication can begin. The pneumatics and the sensor layout are still on the screen.

Then the customer moves a conveyor, or a commissioning engineer finds a guard that blocks an operator. A bracket changes, a cylinder gets a longer stroke and a proximity sensor moves to the other side.

None of this is careless. It is how bespoke machines get built. The damage comes from what happens next to the parts already ordered or cut. The drawing office sees a one-line edit, while purchasing and the fabrication shop see a returned order and a scrapped plate.

The designer exports a parts list from the CAD tool. Purchasing pastes it into a sheet and adds supplier and rate. The store keeps a printout. The shop supervisor marks his own copy with a highlighter.

One edit now has four places to land, and it usually lands in one. A cylinder changes in the CAD model but the purchase order still names the old bore size. The wrong part arrives and sits on a shelf.

Short sentences describe the result. Parts are late. Parts are wrong. Parts are surplus. The store ends up holding stock that belongs to a design nobody is building any more.

A purchased part can often be cancelled or exchanged, though a custom shaft, a milled plate or a laser-cut bracket is already scrap once the drawing changes. Fabricated parts need the revision checked before the cutting program starts, not after.

Purchased parts have lead times that make the problem worse. A late change to a servo motor size can push the whole delivery by weeks, because the drive, the cable and the coupling must change with it.

Track the two groups apart. Fabricated items need a revision check at the job card. Purchased items need a check at the purchase order and again at goods receipt.

Release the BOM as a numbered document. Revision A is the first release, B is the next, and each carries a date, the author and the reason. The CAD export becomes a draft until released.

Each line stores the revision in which it appeared or changed. Purchase orders and job cards copy that revision. When revision C changes a line, the system flags every open order and job card that still holds revision B.

A change note links to the lines it touches. The cost and delivery effect is visible before anyone approves it. The design manager can then decline a cosmetic edit that would delay a bought-out gearbox.

Fixing every machine at once is unnecessary. Choose the machine currently in build. Release its BOM as revision A today, even if it is imperfect.

Then set one rule. Purchasing and the shop floor work only from the released revision, and a change is accepted only through a change note. After a month, count how many orders were placed against an old revision. That count tells you how far the habit has spread. Review the change notes at the weekly build meeting, and ask which edits could have been caught at the design review.