BOM Revision Register

What this tool does

CM8-346 is a working register of bill of materials revisions. You record what changed on which assembly, what the part cost before and after, how many you build a year and what happens to the stock the change makes obsolete. It works out what each change does to unit cost and to annual spend, totals the write-off exposure sitting in your stores, shows which changes are stuck in review or approved but never implemented, and prints a report you can take to a change board.

Everything runs inside this single file. There is no account, no upload and no network request of any kind, so part numbers, costs and supplier names never leave the computer you are using. Bills of materials and the prices inside them are among the most commercially sensitive documents a manufacturer holds.

What one row is

One row is one part affected by one change. A change that swaps a seal and deletes a bracket is two rows, both carrying the same change reference. That is deliberate: cost impact is a property of a part, not of a change note, and splitting it is the only way the arithmetic can be checked line by line.

Give both rows the same value in Change reference, the same revision from and to, and the same assembly. The register groups by assembly for the charts and by reference in the report, so nothing is double-counted as long as the build volume you enter is the volume of the assembly.

How the cost change is worked out

Each row carries a quantity per assembly and a unit cost, before and after. The line cost is the quantity multiplied by the unit cost, and the change is the difference:

A negative figure is money coming out of the product. The tool colours savings green and additions red throughout, and the bridge table shows every intermediate figure so somebody who disagrees with the answer can see exactly which number they disagree with.

Four change types have a shape worth knowing. A part added has a before quantity of zero, so the whole line cost after is new cost. A part removed has an after quantity of zero, so the whole line cost before is saved. A quantity change keeps the unit cost the same on both sides. A source change keeps the quantity the same and moves the unit cost. The tool checks the first two against the quantities you enter and tells you if they disagree.

Annualising it

A saving of thirteen pence means nothing until you know how many you build:

Enter the build volume of the assembly, not of the part. If an assembly is built nine thousand times a year and uses two seals, the volume is nine thousand; the two is already in the quantity per assembly. Getting this the wrong way round is the most common error in change costing and it doubles or halves the answer.

Leave the volume blank if you genuinely do not know it. The tool will show the per-unit figure and a dash for the annual one rather than quietly treating the volume as zero, because a change worth thirteen pence a unit is not a change worth nothing.

The annual figure is a run-rate, not a forecast. It says what the change is worth over a year at today's volume. It does not know that the product is being discontinued in eight months, that volume is seasonal, or that the price you have been quoted holds only to a minimum order quantity.

Superseded stock, and what it costs you

The cost of a change is not only the new part price. It is also whatever you are left holding. Record the units of the old part sitting in stores, in work in progress and on order, then choose what happens to them:

• Use up — the old stock is consumed before the change takes effect. Nothing is written off, but the saving does not start on the effective date either.
• Rework — existing items are brought up to the new revision. The exposure is the rework cost per item from Settings, multiplied by the units on hand.
• Return — the supplier takes the stock back. Treated as no exposure; if you are taking a handling charge, record it in the notes.
• Scrap — the stock is a loss. The exposure is what it cost you, less whatever percentage you recover, set once in Settings.
• Hold — no decision yet. Treated as no exposure, which is exactly why a change should not sit on hold for long.

The tool deliberately does not net this against the annual saving. One is a single write-off, the other is a rate per year, and adding them together produces a number that means nothing. Read them side by side: a change saving forty thousand a year is worth an eight thousand write-off, and a change saving four hundred a year is not.

Status, and the state the tool works out for you

You set the status — draft, in review, approved, implemented or rejected. From that, the dates and your review target, the tool derives a position that is more use than the status alone:

• Overdue in review — raised more than your target number of days ago and still not approved.
• Awaiting effectivity — approved, with an effective date still in the future. This is the healthy state for an approved change.
• Effective, not implemented — approved, the effective date has passed, and the status has not been moved on. This is the expensive one. The engineering work is paid for and none of the benefit is being collected, and there is a real chance the shop floor is building to a revision nobody has confirmed.
• Approved, no date — approved with no effective date at all, which means nobody knows when it starts.

Time in review

Age counts from the date raised to the date approved, or to today if it has not been approved:

The average shown on the tiles covers only changes still in draft or review, so it answers "how long is the queue right now" rather than "how long did last year's changes take". Set the target in Settings to whatever your change process actually promises.

Effectivity

The effective date is the date the new revision becomes what is built. It is not the approval date and it is usually not the same day: stock has to run out, a supplier has to deliver, a work instruction has to be reissued. Record both, and the register will tell you when the gap between them has stopped being a plan and started being a drift.

Where the disposition is "use up", the effective date should be the date the old stock runs out, not the date the change was approved. If you do not know it, leave it blank rather than guessing — the watchlist reports "not set" honestly, and a wrong date is worse than no date.

Recording why

The reason field is the most under-used field in most change registers and the most useful one in hindsight. Read as a group, the reasons say something about the product that no single change does. A register full of quality and BOM error says the problem is upstream, in design verification or in how bills of materials are created. A register full of supply says your component strategy is exposed. A register full of cost reduction on a mature product is exactly what you want to see.

The chart shows the count of each reason with its annual cost effect alongside, so you can see both how often something is happening and whether it is worth anything.

What this deliberately does not do

This is a register and a costing aid. It is not a configuration management system and it is not evidence of control. In particular:

• It does not release anything. Nothing here tells a shop floor what to build.
• It does not hold approvals. A name in the owner field is not a signature.
• It does not connect to an ERP or PLM system, so nothing reconciles automatically.
• It costs only the parts. Setup time, tooling, new fixtures, scrap during the transition, freight, re-qualification and the engineering hours to make the change are all real and all outside this register. A change that saves two pounds a unit and needs a nine thousand pound tool is not a saving until you have said so somewhere.
• It has no idea whether the change is safe. Nothing in a cost figure tells you a fluorocarbon seal is the right seal.

Printing and sharing

The Report tab prints the tiles, the charts and both tables with a title block you fill in yourself. Filter first if you want a report on one assembly or one status: the report follows the filter, so a change board pack is a filter and a print rather than a separate document.

Saving your work

The register is held in this browser, on this computer, and stays there between visits. Use the backup button to write a JSON file you control — that file is the only copy that survives clearing browsing data, a new machine or a different browser. The spreadsheet download gives you the same rows in a form your finance team can open.

Accuracy & disclaimer

Every figure here is your own figure multiplied out. The tool does no lookups and applies no assumptions of its own beyond the recovery percentage and rework cost you set. Check a change or two by hand against your costing system before circulating anything. Where a build volume is missing the tool shows a dash rather than a confident zero, and that is on purpose.