Part numbering systems design for PLM software

PDXpert PLM software has been designed to support a wide variety of part numbering systems, including manually-assigned intelligent part numbers, category-based semi-significant part numbers, and easy-to-manage sequential part numbers. This topic explains why most industries have adopted sequential identifiers as part numbering system best practice.
We'd be happy to explain how PDXpert PLM software's item identification capabilities, coupled with its flexible sourcing relationships, can help companies transition an existing part numbering system to a simpler design.

Part numbering system origins

Traditional part numbering systems and document identification schemes originated over 50 years ago. At the time, a basic consideration was that unstructured information was very difficult to find, and it was therefore necessary to overload document identifiers and part numbers with search-related "helper" data. In the absence of better alternatives, this approach and the resulting practices represented reasonable, though costly and cumbersome, compromises.

Fortunately, times and technology have changed.

Goals for part numbering system design

Let's step back a moment and consider our essential requirements for a part numbering system.

The goal of any part numbering system is to uniquely identify the component approved for a specific application. Accurate, consistent, unambiguous part identification is essential for correct product assembly, testing and maintenance.

We must ensure that a new identifier is assigned whenever a variation in attributes has a material effect on the item's form, fit, or function in the application.

A part number allows us to clearly distinguish one part from other parts when the difference is meaningful, and to ignore irrelevant differences. This is what distinguishes non-interchangeable changes from interchangeable changes. We also need to ensure that parts with non-meaningful variations can be used for the same application.

Part numbering system constraints

Any part numbering system design must also consider some purely human elements.

A practical part numbering system design should account for the limits of short-term memory. The "magic limit" is typically considered to be 7¹. Many years of academic study, verified by real-world experience, proves that data entry errors increase as the number of characters increase. After a certain length, errors increase at an increasing rate: at 15 characters, the error probability is close to 100%². Anything longer than seven (a) requires most users to write down, rather than simply remember, the part number for even short-term usage; and (b) increases the likelihood of data-entry errors³.

Another part numbering system design consideration is eye-hand coordination, i.e., "clerical speed and accuracy". Many of our users will prefer using only the characters commonly available on a computer's numeric keypad. This allows single-hand operation, and the layout is easily memorized and generally independent of local language variations (e.g., if you build or service your product in another country, how easy is it to enter your non-numeric characters on their keyboards?). Restricting our part numbers to numeric characters will provide the fastest data entry for heavy users in purchasing, manufacturing, receiving, service and other places where employees constantly work with a wide variety of part numbers.

Five to seven numeric characters permit a universe of up to 10 million unique parts, far more than most companies will see over their entire lifespan.

Human-factors constraints prove that a part numbering system using 7 or fewer numeric characters is the easiest to manage for the majority of our users.

Intelligent part numbering systems: problems and costs

To encode or not to encode?

There's an almost magical fascination, even an obsession, with designing the perfect part numbering system. Everyone starts by envisioning how convenient it would be to tell, at a glance, the important characteristics of a part, or the document number that describes the part. Yet, a century after the development of mass production, the goal remains elusive because each person has a different idea of what part attributes are important - and that person's opinion will change over time. This fluid set of usefully encoded properties inherently conflicts with the part numbering system's essential role in establishing an unchanging reference to an item.

Recall that our basic part numbering system requirement is to distinguish parts, based on their meaningful attributes for the application.

Good part numbering practice: Our first clue

Let's first examine the two possible extremes: encode all meaningful attributes in the part numbering system, or encode no attributes. It's immediately obvious that a part numbering system that encodes all meaningful attributes of each part would be impossible to manage. The super-intelligent part number could potentially include: all physical dimensions; environmental requirements; contract & regulatory conformances; purchasing and inventory management characteristics; accounting treatment rules; maintenance & servicing intervals; field interchangeability properties; inspection and test procedures; and a host of other attributes.

So, if we are forced to the extremes, only a non-significant part number is viable - particularly if we're also aiming to stay at "only" 7 characters.

More practical intelligent numbering schemes benefit fewer users

However, suppose we take a middle path, encoding only certain attributes in the part numbering system, and leaving other attributes as "look ups". Which of these attributes shall be favored?

Whoever designs the part numbering system will decide the "most important" attributes: engineers will focus on technical properties (size, weight, power, tolerance, material, etc.); purchasing agents may prefer procurement attributes (make/buy, vendor, and commodity codes); production may desire handling codes, bin locations, tooling and fixture dependencies, and shelf-life limits; and finance may want to encode general ledger accounts and purchase authority. Quality, marketing, field service, and customer support will have their own competing views.

To enforce any of these will require some additional human and/or automation resources for the initial design as well as on-going monitoring.

Moreover, the part numbering system designers typically are not high-volume users such as document control analysts, warehouse clerks, purchasing agents, production supervisors, and field technicians. These people need to accurately and concisely convey part numbers, often over the phone in noisy environments.

At the outset, one group will have relatively useful part numbers while everyone else will be using an error-prone and costly system. The high-volume users' only consolation may be in knowing that even the favored group's part numbering system won't last forever. Very "intelligent" part numbering schemes are also very fragile: initial design assumptions, practical day-to-day decisions, and the march of technology inevitably make the original categorizations obsolete. Repeated part numbering system tweaks and, finally, redesign are inevitable because:

• Mistakes are often made in part number assignments, some of which can be quite costly. The engineers who "know the code" use the part number, everyone else reads the part description. If a "10 amp fuse" number is assigned to a "1.0 amp fuse" description, a lot of product can get shipped or serviced with the wrong fuse.
• You can never simply ignore an incorrect part assignment. Otherwise, the category number becomes an unreliable indicator of its content. An aluminum casting that's accidentally assigned to the steel castings category requires an engineering change and full dispositioning. And the error becomes more painful if your number has been cast into the part.
• Overlapping or ambiguous categories are functionally equivalent to having no categories. For example, is it immediately obvious that an LED is a "display" and not a "diode" ... or an "indicator"? Who's to say, and how do they protect the original definitions over the next few decades?
• Categories become over-used ("Miscellaneous"), ambiguous ("Hardware"), or irrelevant ("Vacuum Tube").
• Encoded attributes may force unnecessary new part numbers despite on-going interchangeability. For instance, a stainless steel part is revised to use brass; if the base material is reflected in the part number, this revision forces a new number even though the new part is completely interchangeable in the application.

Avoiding these errors requires additional processes, which adds cost, time and management. The cost of processing a single engineering change to re-issue an incorrectly-categorized part may wipe out any hoped-for advantage of the intelligent part numbering system.

Using a part category ("commodity code") prefix

Where the business rules are fundamentally coupled to the physical part, or nearly so, they'll sometimes be usefully embedded in the part number. The leading example is when the physical part is handled differently within the inventory or production management process. For example, if you manage a wide variety of part types, say structural steel and electronic components, then these may be managed in different parts of the warehouse. It may be convenient to prefix the part number with a "hint" as to how the part is handled.

So, in a "semi-significant" numbering system, an electrical component may start with "1" and the structural steel may begin with "2", but each part within the category is assigned a sequential, non-significant identifier (e.g., 10001, 10002,... and 20001, 20002,...).

Since these part categories represent clear and unchanging attributes, you'll want to keep the numbering groups very large. There's no benefit to creating subgroups that have no distinguishing business rules, or to requiring any thought about whether a part belongs to one category or another. For instance, in the vast majority of cases an "electronic component" number will be quite sufficient, and there's no need to create separate Resistor, Capacitor and Diode commodity code prefixes unless the physical parts are — and always will be — treated quite differently from one another.

If the business rule (such as part handling) could change over the life of the part, then the business rule shouldn't be reflected in the part number.

What about part revisions?

Part revisions are not included in the identification of the physical part because, by definition and best practice, all revisions of a single part number are interchangeable. Consequently, all revisions of a part number can be intermixed in a single inventory location.

If this isn't true, then you don't have interchangeable parts, and they require distinct part numbers and inventory bins. For a more detailed discussion, see Do parts have revisions?

Since a part revision merely distinguishes one interchangeable part (and its PLM data record) from another, the revision identifier format will not influence our part numbering system design.

Authorities for using short, numeric, non-significant part numbers

Many experts strongly recommend the use of short, numeric-only, non-significant item numbers. We have not found any modern authority that supports using either alpha characters or any type of significance in part numbering. Here are some relevant comments from our PLM book list:

Clement, et al.: Manufacturing Data Structures, page 20:

Another important point about item numbers is that they should be as short as possible. Part numbers are keyed, copied and used as verbal identifiers. The shorter the numbers, the more accurate people can be. Obviously, the greater the number of digits in a part number, the greater chance of error. We also recommend that only numeric digits be used.

Garwood: Bills of Materials: Structured for Excellence, page 73 (author's emphasis):

The solution...is to use shorter non-significant part numbers. We have found that part numbers of 5 or 6 digits are the most effective.

Mather: Bills of Materials, page 100:

All tests point to numbers alone as being more easily identifiable with far less chance of error... Tests have also shown that smaller numbers are easier to write and remember accurately. Hence, the ideal part number is all numeric with as few characters as possible.

Francis Criqui, General Motors Corp.:

As Director of Engineering Standards at General Motors, I had responsibility for issuing blocks of part numbers for all GM engineering units. I was involved [in] multiple efforts to define a "smart" part number scheme that works. We never found one. .... The easiest and simplest solution is a purely sequential numbering system where the number has no significance other than [as] a tracking ID.

Part numbering system best practices for PLM

Product lifecycle management software provides a significant level of automation, and will influence how your part numbering system is defined.

In a computer-based item numbering system, use a short, non-significant number for all document numbers and part numbers. Most attributes that people typically want to encode in their part number system can be either explicitly defined within the PLM system attributes (such as document and part type, make or buy sources, commodity codes, component versus assembly, etc.) or in the item description (part name or document title). Use the product structure to manage the relationships of documents-to-documents (i.e., a "data list"), parts-to-parts ("bill of materials"), and parts-to-documents ("part control data"). Other item identifiers defined within the PLM database can be used to satisfy almost any user's information or query needs.

If you've decided to adopt category prefixes, use a very limited set - preferably fewer than 10 - with categories sufficiently broad that the correct choice is immediately obvious to the most casual user.

If we record all meaningful part attributes within a database, and use the part number simply as an index to that data, then our part numbering system goals can easily be met using only 5 to 7 digits, which can represent 100,000 to 10 million unique part numbers⁴. Relying on a Google-like free-form text search to find part data shortens the part length, speeds data entry, reduces clerical errors, and eliminates the need to periodically redesign an attribute-based part numbering system as usage and technology alter the initial categorization.

Part numbering systems supported by PDXpert PLM software

A good part list tool will automatically assign an appropriate part number, prompt users with how the part should be described, provide information on the part supplier sources, permit user-attached specification and process files, and track the part release history. The best parts list software is part of a complete PLM system that includes bill of materials management, document & file control, and change control.

Like most part numbering software, PDXpert PLM software can follow industry part number best practices by sequentially assigning document and part numbers. If you help your users by separating your part numbers into short groups (e.g., 123-456), PDXpert PLM software will ignore the delimiting characters when incrementing: 1234-999 will become 1235-000.

If your part numbering system uses a "semi-significant" scheme, PDXpert PLM software can assign document and part numbers based on your defined item types. Each part and document template can "subscribe" to a unique or shared item numbering format that permits category prefix (e.g., "HW-"), sequentially-assigned base number, and a fixed suffix (such as "-01"); the next part number from HW-12345-01 will be HW-12346-01. Users can further modify these assigned values as needed.

And, if your company must use an "intelligent part numbering system" that requires human interpretation and assignment, PDXpert PLM software will accept manually-entered document and part numbers for all of your items.

In addition to accommodating your specific part numbering system, PDXpert PLM software supports all organizational stakeholders:

• Duplicate part numbers are automatically prevented based on rules that can optionally include part or document class or subclassed type.
• When creating a new part record, users are prompted for consistent item descriptions using "text templates" based on the part type.
• A specifying designer can provide unit costs for part list roll-ups.
• Each part can have its own supplier source parts list, which includes the ability to rank preferred usage.
• Users can add a virtually unlimited number of file attachments that are either "locked down" on item release, or can be modified throughout the entire part life cycle.
• Even non-technical users can easily find parts by relying on familiar "Google-like" free-form text searches that not only return the most relevant parts, but also can return similar items that may be substituted, with the resulting part list ranked by relevance.
• Complete part histories are tracked by releasing and canceling parts revision records using change forms.

Notes

1. G. A. Miller, Psychological Review (1956) "The magical number seven, plus or minus two: some limits on our capacity for processing information."
2. Dave Garwood, Bills of Materials: Structured for Excellence (1995), page 70.
3. Ehap H. Sabri, et al: Purchase Order Management Best Practices (2006), page 114: "Typical error rates for manual data entry are about 1 error for every 300 keystrokes."
4. There is one other good practice you should consider: never start a part number with a zero. A number with a leading 0 has two unfortunate effects: some people may write 123 for your part 000123, and some computer applications (like Microsoft Excel) discard all leading zeros during import. Although this rule reduces your possible part universe by 10%, it's well worth the cost.

Contact us if you'd like to discuss how the general concepts summarized in this note may be applied to your situtaion. We'd be happy to address other PLM software good practices - ask us!