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Part numbers, BOM numbers, Assembly numbers and the revisions

Part numbers, BOM numbers, Assembly numbers and the revisions

Part numbers, BOM numbers, Assembly numbers and the revisions

I have seen this topic a few times in deferent companies and people always argue about this every time it comes up. Even here it is being discussed see this thread .

I would like to gather everyone’s experience and find out what is the best strategy to address the relationship between Part numbers (PN), BOM numbers (BN), Assembly number (AS), model number (MN) and linking their revision. To help me describe the situation I will use a scenario.

MN321A (computer mouse)
|_AN678A (computer mouse assembly)
|_BN456A (computer mouse BOM)
    |    |_PN123A
    |    |_PN124A
    |    |_PN125A

I will use the following convention; XXNNNR where
XX is the identifying prefix (not necessary but help to clarify the number)
NNN is the actual number
R is the revision

The questions I have are as follows:
  1. When should one use a separate MN to the AN? For example if we are building a computer mouse, then what is the advantage in having separate AN and MN.
  2. In what situations one would link the Revision of the model number and Assembly number (given that they are two separate numbers)
  3. To what level should you propagate the Rev changes? In the example above if revision of PN212A is changed to B should both corresponding BN and AN change only? Or do you propagate all the way up to AN678A and BN456A?
  4. Has anyone any experience in regulated industries and what is the best way to handle this? Every time a part number or even the revision of the product is changed then the company is up for resubmission costs.
  5. To make mater more interesting, what is the best practice in handling the software revision? For example if the software for USA version is deferent to Chines version and each of them is tracked and revised independently.
Please don’t hesitate to describe how you or the company you work for handles this issue (with out violating confidentiality). It is a common issue and everyone as an engineer had to struggle with it at one point or another. I plan to summarise the outcome of this thread and post a white paper on this forum. Everyone involved will be given the appropriate credit on the white paper.

Thank you

RE: Part numbers, BOM numbers, Assembly numbers and the revisions


I've mentioned the following reference a number of times in this forum but I think it is good enough to mention again.

"Engineering Documentation Control Handbook, Configuration Management in Industry."  Frank B. Watts

I've attended two seminars by Frank and I think he has some good ideas and a lot of experience in industry.  His book covers change orders and revisions concerning the philosophy of interchangeable parts versus new parts better than anything else I have seen.  Overall his book covers a wide range of topics and has many useful hints.  It might be a good reference to have on hand.  Other references that would be handy would cover bills of materials and product configuration.

A few comments on your part numbering:

The current theory is that part numbers should not be intelligent, in other words should contain no coding or other clues as to what the item is.  Your XX prefix is an example of this.  Part of the reason is because overly complicated schemes will often run out of numbers and something else will then need to be invented, and I've seen this happen.  My opinion is that a compromise or something in between is the best answer.  Remember that these systems are used my people too, not just computers.

Your part numbers also contain the rev number which is definitely not the way to go.  A part number should be interchangeable through the revision process so there is no need to put this into the part number.  Doing so will create more work processing change orders.  If a new part is not interchangeable it gets a new number.  You will however need a way to save old drawing files that have been revised and that's when I've seen the A,B,C tags be useful.

I don't have much to say about model numbers, they have been loosely controlled at the companies I have worked at and seem to be more of a sales tool than an engineering documentation function.

Where I've worked the assembly number and the BOM number have actually been the same.  The parts list on the drawing and the corresponding item list in the bill of materials system represented the same thing.  Some companies don't put parts lists on drawings and instead tie the drawing to the BOM.  This issue was recently debated here about one or two months ago.

These are my personal opinions and I admit there are other ways to do these things, but there is a lot of bad advise out there too.  Another factor to keep in mind is that a good system for mass production is not necessarily the best system for custom engineered products built one or two at a time.


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