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Understanding Precise and Imprecise BOMs

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If you work with BOMs in Teamcenter you have probably seen the terms Precise and Imprecise BOM structure. Even if you only use one type of BOM you need to understand the difference between them.

BOM View Revisions

In Teamcenter an assembly structure is represented by a BOM View. BOM Views are associated with Items, and each Item Revision will have it’s Revision of the BOM View — a BOM View Revision, or BVR.

The descriptions precise and imprecise apply to BOM view revisions. It is determined by an attribute of the BOM view revision business object.

What does Precise and Imprecise BOM Mean?

A BOM View Revision defines which components are used by a particular Item Revision. precise and imprecise describe what is meant by component. If a BVR is precise the components are item revisions; the BVR defines precisely which revision is used by the structure. If the assembly is imprecise the components are items; the BVR is not precise about the revision to which the assembly refers.

Why Precise Structures are Important

When you release an assembly, releasing the precise structure preserves the record of exactly what it was that was developed, approved and released. It is an important historical artifact, particularly of the CAD data.

Revision Rules

This post will be too long if I discuss revision rules in detail. The main thing to know though is that even if an assembly is precise, you can still configure it to load latest working revisions or latest released or whatever by picking the appropriate revision rule. Basically, you can choose to look as the structure precisely as it was last updated, or imprecisely using which ever revision fulfills your selection criteria.

What Use is an Imprecise Structure?

An imprecise structure represents how someone assembling a product might look at the components. Often it does not matter in the factory which revision of a component is used. The rules say that every revision of a component must have the same form, fit, and function. Therefore the individual revisions are interchangeable. The person building the assembly can select any revision that is available of the components.

(yes, there are exceptions to this. Aerospace being the prime example.)

But the factory worker is probably working from a report generated from Teamcenter and not the actual BVR. The report could be generated from a precise BVR and simply remove the revision information.

So, why have the option of Imprecise structures?

Honestly, there are many who would say that you should never use imprecise assemblies. Personally, I have not seen a situation where imprecise assemblies were required. I would never release an imprecise assembly. Your released data should be the record of exactly what was reviewed and released.

That said, I do have an idea for a use for Imprecise assemblies — but that is a topic for another post…

  • PB

    Hi PLMDOJO,
    Interesting topic to discuss.Consider a huge assembly, may be aircraft engine or rotor assembly.  Assume that the assembly is a precise structure.  After engineering design/ changes etc.. the assembly is released for manufacturing.Now, due to some new requirement or a late change, a low level sub assembly or a component needs to be changed (i.e. new revision needs to be created). In this scenario of precise structure, just to incorporate a component change (new revision of component), the structure needs to be up-rev from bottom to top. This will be very time consuming and unnecessary exercise. Would like to know what are options to handle such situations.Thanks,PB

    • http://plmdojo.com Scott Pigman

      Hi, PB, thank you for reading and participating.

      I question the need to revise the upper level assemblies to incorporate the changes. If you want to view the aircraft engine using the latest revision of all feeder parts at all levels you can do that by selecting a latest released revision rule, even though it’s a precise assembly. By using revision rules that have no Precise entry you can basically view the assemblies as imprecise structures. However you can’t apply a Precise revision rule to an Imprecise structure and retrieve its as-released structure.

      If you have access to the PLM World website I suggest you look for Mark Hoover’s 2010 presentation, Precise Structures are Da BOM, (or something like that — I’m going by memory). He gives lots of examples of configuring precise assemblies in different ways using revision rules.

      • XX

        Hi, first thanks for an interesting site.
        I have not found the presentation you mention above. I would very much like to go through it to understand this topic better.
        From my current understanding: In the scenario above the users would have to use the imprecise rule to find the current valid structure, something like “latest released”. In that case the value of the original released precise structure would be to trace historical data. If the structure was originally released as imprecise (bottom-up) the same historical tracebility could be achieved by revision rule “date released”. So my conclusion is that imprecise revision rule will always be needed to avoid “revision explosion” while saving a precise structure is not really needed. 
        Feels like I have missed something here..?

        • http://plmdojo.com Scott Pigman

          a few points:

          * There is no such thing “imprecise” revision rule. However, revision rules may be designed to select revisions other that what is recorded in a precise BOM (latest released, latest working, etc.). Perhaps that is what you meant?

          * Structures (i.e. BOM view revisions) may be precise or imprecise. Rev rules define which revisions to show to the user.

          * I’m not sure what you mean by revision explosion, could you describe a scenario?

          * I found the presentation, (plmworld account required): http://www.plmworld.org > events > conference presentations > Presentations from Siemens PLM Connection Americas 2010 > Teamcenter Connection > filter: Teamcenter Engineering Process Management > Why Precise BVRs are da BOM!

          You could also try requesting it from the author at his site: http://www.hoovernebrig.com/contact.shtml

          I need to do a follow up post with some pictures to illustrate the behavior of imprecise and precise assemblies under different revision rules.

  • L B

    Nice explanation,  This has been a big topic of discussion at my current and previous job and to be honest it has always been kind of confusing. Although I understand it, I still opt of both by controlling the rev structure through NX and using the “As Saved” load option. Maybe one day if we ever get configuration management group to load teamcenter I will go with precise load options in NX

  • Hector

    Hi Plm Dojo

    Imprecise Assemblies can be used in Industries where there is no concept of Revisions.
    For Example in Retail Industry there will not be any concept of Revising the Assembly (ex: Shirt Assembly). They will save as and create a new Shirt for the next season.
    In those cases i think Imprecise Structure concept is apt.

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