Jan 272014

Continuing , the series on the Bill of Material we began with the article Dr. Strangepart: How I Learned to Stop Worrying and Love the BOM, here is a full re-print of the next in the series.  You can find the original at ENGINEERING.com here:


In my post Dr. Strangepart: How I Learned to Stop Worrying and Love the BOM, we began talking about the extremely complex process of managing the bill of material (BOM). That discussion focused on intelligent part numbers, but today I want to move to the question of whether the BOM is primarily keyed off of the functional use of a part or the specific part number itself. This is a tension that has existed since bills of material were created, when man married an axle to a wheel.

Why do people care whether the BOM is driven by part numbers or the part’s function? The answer is that each has certain advantages. Driving the BOM by actual part numbers is very useful for purchasing and manufacturing, and when an engineer is focusing on an individual piece of hardware. However, when the engineer or a product manager moves from one product to another, a function driven BOM allows him to compare components more easily.

The ability to compare BOMs has huge implications for the ability of engineering to successfully re-use parts. Re-use lowers both the cost of the hardware itself (by re-using tooling and increasing production volumes) while also reducing the engineering time needed to design a new part. Re-use has been the great white hope of the engineering community from time immemorial, but it is very challenging.

Although Product Lifecycle Management software vendors talk about re-use, most companies do a very poor job. Trying to manage re-use without intelligent part numbers and a functional way to look at the BOM is like searching for a single item in a hoarder’s basement that’s jammed full of boxes with no labels. Eventually you give up, go to the store, and buy a new one! That’s one of the big reasons why re-use has struggled in the engineering community.

So, how do we balance the tension between these two things? Here is a simple framework for thinking about the BOM that I call Form, Fit, and Function.



By function, I mean what the part is actually doing, what its purpose in life is. By form I mean the specific piece of hardware or part that we’re dealing with. In Figure 1 there is a simple example of how function and form relate to each other. Typically, the superset will be function. The function in this case is to provide rear vision to a driver of a vehicle. The function is the result of the part operating correctly. The part itself is the form that “delivers” that function. In this case, we might provide rear vision by designing a mirror, a camera, or some sort of sensor.

The third part of the framework is the fit of the part. However, by fit I don’t mean the actual geometric tolerances or real estate that the part occupies. What I mean is, “What are the unique attributes of the part that make it “fit” the function, i.e. accomplish the function?”

Figure 2 shows another example of a functionally controlled BOM. In this case, we’re using an example of the propulsion of a vehicle, perhaps a jet aircraft. In this case we show propulsion as a top level Function, along with sub-levels of the powerplant, how the power is transferred, and the cooling system.

Hiller Associates Form & Fit Functional BOM

Figure 2 Hiller Associates Form & Fit Functional BOM Example


The coolant is the Form that satisfies the Function of the cooling system. We assign an intelligent part number for the specific coolant. One might ask, when do I move from a Function to a part number (Form)? The answer to that is, whenever it is convenient. It will take a little bit of time for your team to develop a functional BOM that has a manageable level of hierarchy in it.

TIP: Any more than three or four levels of hierarchy can be very difficult to manage.

There is a many-to-many relationship between the Function and the Form (part). Depending on how far down the Function tree we go, we may need to attach more than one Form (part) to satisfy a function. On the other hand, if our functional tree is deeper, on certain products there may be one assembly (Form) that satisfies more than one Function on the functional tree.

Moving to Fit, we see that each Form may have multiple attributes (ways of fitting the function). The coolant “fits” our functional need by the attributes that it has. In general there will be a one-to-many relationship between Form (part) and Fit (attributes) respectively.

Form, Fit, and Function is a simple way to look at how we structure our products. It lends itself well to re-use of parts, but also for the re-use of work break down structures that are used in aerospace and defense.

Some companies are now working on constructing skeleton “Starter” or “Universal” BOMs that they re-use for each new product. The idea is to start with a generic BOM, and then add and delete to match the needs of a new product. The goal is for the company to have one universal BOM, or one for each unique product group.

This is a great idea in theory, but it’s not trivial to execute. To do this in your own company will likely take your engineering team, product management, and a consultant a year or more to find a structure that suits your needs. However, once this is done, the speed and re-use advantages should be significant. Hopefully, the Form, Fit, and Function BOM Framework will give you a simple way to think about it!

  12 Responses to “Form, Fit, and Function – A Framework for your Bill of Material”

  1. Moderator re-posting from linked in:

    George Spiller, owner at Design for Profit Inc, SAYs:

    Many companies that are trying to recover, but are saddled with mediocre engineering find that assigning implementation of smart part numbers, piece count reduction, new part tooling reduction etc to the purchasing group or production control group helps with implementation. Many of the known improvement techniques are less welcome in the design engineering group because they highlight a lack of innovation which is needed to thrive in the current season

    • @ George,

      That’s a very interesting observation that, although it’s a BFO (blinding flash of the obvious), I had never pondered before, i.e. engineering is very innovative with their design of products, but can been very Luddite with respect to their own internal processes.

  2. Moderator re-posting from linked in:

    Michael Contino, President, On Target Performance Group SAYS:

    Great article! The real question is, how many companies really even understand the value of a BOM, let alone how many have them.

    • @Michael — huh, that’s interesting. I guess as a recovering engineer myself, I never really questioned that most product development teams do value the BOM, regardless of whether they are sophisticated in how they manage it or not. Has your experience been different?

      • Michael Contino, President, On Target Performance Group SAYS:

        My BOM point really focuses on the lack of visibility of many companies (e.g. retailers, manufacturers) into the cost of materials, labor, and profit of purchased finished products and/or components. This level of BOM knowledge/lack of visibility can have a dramatic impact on the balance sheet (e.g. asset losses, write-offs, scrap recovery, reserves). This results in an environment where it becomes very challenging for the financial folks to communicate concise explanations when balance sheet swings occur.

  3. Moderator re-posting from linked in:

    Maher Rai, CCDM, Lead PLM Process Architect at ITC Infotech

    Hi interesting discussion.
    Most people shy away from the Fit, Form, and Function (FFF) subject. However, the word interchangeability is missing here.
    There is no value of the (FFF) designation without interchangeability as it is pertain to the part purpose of an assembly and/or end item.
    Also, intelligent numbering system is weak and create havoc across the enterprise.

    • @Maher – Why do you think that a simple intelligent part system would create havoc? I see no reason it would. It would PREVENT confusion. Interchangeability can still be managed by the PLM system. The power of the PLM Interchangeability, or even an attribute (fit) for interchangeability is independent of having an intelligent part number.

    • Maher Rai, CCDM, Lead PLM Process Architect at ITC Infotech SAYS:

      I understand the FFF and the numbering scheme are independent but wanted to comment on both of them.
      There is NO simple intelligent part numbering (IN) system. It may start simple and clean but ends up complex and not lean… The IN system logic would break within years so most places would add additional prefix to continue using the IN scheme, then you would have several intelligent numbering schemes
      Furthermore, imagine, you have a middle company that uses IN system acquires another company that uses different numbering scheme, so to share parts and/or reuse design and, the parties need to decide what system to use, then if the business is doing well (profitable), purchases other companies, so to keep the original IN system, will be costly and the level of survival is limited.
      I do not understand your point of view about the “PLM will manage interchangeability” Could you elaborate?….

      • @ Maher,

        Sure, happy to elaborate regarding the PLM interchangeability comment. My point is simply that one of the focuses of a PLM system is to provide attributes that note things such as:
        1. What part the new part replaces
        2. Which parts can replace (be interchangable) with the new part
        3. etc.

        In my specific FFF framework, these could be part of the “Fit” (attributes) of the part that are stored in the PLM system. But, my point is that a PLM system typically would include such interchangability markers by default, right?

        Certainly, the Ford release system WERS (Worldwide Engineering Release System) included all of these sorts of interchangability markers. WERS was essentially a Ford specific release system and it worked extremely well.

        To your point about the IPN (intelligent part number) getting too complex, I completely agree. I am guessing that there were violent arguments in Ford about what to include in the IPN, but at the end of the day they chose Program–Funtion–Revision. All other attributes went into the release system. However, it’s not right to say “There is NO simple intelligent part numbering (IN) system… would break within years.” That’s tautologically incorrect as discussing Ford as a case study, who do have an IPN system that works well for them and has been in place for years, right?


  4. Moderator reposting Linked-in comment:

    Mike Cheles, Supply Chain Integration and Optimization | Consulting | Acquisition Integration | CIO – IT Leadership | Lean SAYS:

    It is a great goal to try to marry up the engineering BOM with the manufacturing BOM. Maintaining both is something that many companies fall behind on and probably more on the MFG side than the engineering side. Especially if they’re implementing Lean. Changes in how the items in the BOM are controlled such as flattening the BOM levels, collapsing part numbers by adding more operations in a cell or building through parts that used to carry inventory transactions generate the need to update not just the BOMs but the control parameters in the item masters. It is not the sexy part of Lean but I have seem BOM quality fall substantially behind in such efforts. Factor in what Engineering is doing to improve things like function or cost and the BOM becomes a fairly dynamic record. I like the theory in the article but my concern is will the enterprise buck up the resources to implement and maintain it. If not, the problems could be compounded by creating such an all inclusive record with that much intelligence in the key. I truly admire those companies that get it and maintain their BOM’s meticulously. So many do not.

    • @Mike Cheles,

      DISCLAIMER: I am actually not trying to discuss the E-BOM vs. M-BOM questions in the article.

      That’s another very important aspect, but somewhat orthogonal to what I am trying to discuss. Maybe I’ll delve into that rabbit hole in a later article and incurr the passionate wrath of every obsessed BOM worshiper on the internet. 🙂

      Your right, though, that a minimum level of discipline is required to use an intelligent part system (IPN), but the rewards far outweigh the effort.


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