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Precise Bill of Materials Don’t Exist

There is no such thing as precise bill of materials, they do not exist! So there I said it…

Well this does require a little bit of explanation and nuance. In one of my previous articles I wrote about Parts don’t have Revisions, where I explained the different data models in PLM, ERP and CM2. Here is a quick refresher of the different setups (for more details check the article):

All above models are known as Imprecise Bill of Materials and I on purpose did not include a Precise Bill of Material concept to not overcomplicate the article. However in the mean time I have been involved in various discussions on the topic of Precise vs Imprecise BoMs. So what is the difference? 

Imprecise vs Precise Bill of Materials

Imprecise BoMs are bill of materials where child parts are linked by Part Number

That essentially means that it does not matter which revision of the child part is used. However revisions rules can be applied to identify the valid or preferred revision at a specific moment in time e.g. Latest Released. If you do care about which revision is used, you should have re-identified the part (which you can read all about in my book: The Essential Guide to Part Re-Identification).

Precise BoMs are bill of materials where child parts are linked by Part Revision. 

That means the Parent Part Revision links to the Child Part Revision, and in doing so creating a very precise relation. Every change means you revise all the way up to the final product. Changing the toilet seat on a submarine, would require you to revise all parent parts up to the submarine itself. As pictures speak louder than words, see below the difference between Imprecise and Precise Bill of Materials.

While you could technically switch from Imprecise to Precise BoMs during the part’s lifecycle, that requires a very mature organization and good understanding of the logic involved. If Precise BoMs are applied throughout the part’s lifecycle, you will face challenges in the procurement, production and maintenance phases. With Precise BoMs the standard rule, that Parts/Materials are put on stock based on their Part/Material Number, not based on their revision, is violated. You will have to put revisions of parts on stock, so basically the revision becomes part of the Part Number. Every change forces you to issue a new Part Number from the perspective of the operational world (Procurement, Production, and Maintenance).Imprecise vs Precise BOM

That is also why you often see that a need arises to do small changes that are captured in some form of iteration or version to prevent from having to revise till the product level. Which will result in making the Precise Bom into an Imprecise BoM. Hence the title of this article: Precise Bill of Materials don’t exist!

Conclusion

If it comes down to a choice, I think in most cases, the Imprecise Bill of Material is highly preferred as it supports all downstream processes. And within this concept I would still recommend a combination of the Common ERP solution and CM2 Solution as a foundation, as I earlier explained in Parts Don’t Have Revisions

I have yet to see a Precise BoM implementation where the benefits outweigh their disadvantages (so feel free to surprise me). Often Precise BoM implementations are part of legacy implementations and this history is still being dragged along. It is better to invest in a good implementation of Imprecise Bill of Materials with concepts like Manufacturer/Vendor Parts Lists and a good to help guide part re-Identifications, than to create overly complicated solutions to make a Precise BoM work.

 

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Header Photo created by Martijn Dullaart based on a template from Creative Chaos 77 on Canva

2 thoughts on “Precise Bill of Materials Don’t Exist”

  1. Another great, thought provoking article. I commented recently on another article of yours related to “parts not having revisions”.

    The counter argument is that without precision you have no “digital thread” or “digital twin”. When in modern PLMs you have separate but connected domains/departments (Engineering to manufacturing, for example) how would this work without precision? Presumable something has changed (new revision) that may affect downstream work, for example I add a small chamfer to a part for a new revision. Manufacturing needs to now modify something in the manufacturing domain to add the chamfer manufacturing process. How does this work in an imprecise way?

    I think you need precision to accomplish this and this then implies a way to practically make these multidomain changes without going broke in the process. This leads directly to configured (150%) methods in each domain to get the best of both worlds: relatively easy modifications while maintaining continuity. I would really like your thoughts on this.

    1. Thanks Jeff for your comment. I do not see the need for precise bom concept to support a digital thread or digital twin. With Imprecise BoM you can achieve this as well.
      There are some key ingredients you need though:

      If you do these things well, you should not have a problem. I think doing it precise will result in problems with operations as it is unmaintainable.
      I do not yet see how 150% will make this work in both worlds. By adding precise you just make 150% more complex. But perhaps I’m missing the point you were trying to make.

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