Part numbering system design

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. But which approach is best practice?

• Part 1 (this note) describes how to design a short, sequential, non-significant part numbering system, and why this has become preferred engineering practice.

• Part 2 explains why "smart" part number schemes are complex, fragile and expensive — yet offers some useful rules anyway.

Topic contents

• Part number design goal
• Rules for a good part number system
  • Part number length
  • Part number character set
• Best part number design solution: short, numeric, non-significant
• Authorities for using short, numeric, non-significant part numbers
• Part numbers versus source numbers

Part number design goal§

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

Any part numbering system uniquely identifies an item approved for a specific application. Accurate, consistent, unambiguous identification over the entire part lifecycle is essential for correct product assembly, testing and maintenance.

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

A good part number design allows us to:

• Meet design requirements by clearly distinguishing one part from other parts when the difference is meaningful.
• Ensure procurement and production efficiency by ignoring differences when these are irrelevant in the application.

Rules for a good part number system§

Our design goal hasn't imposed any constraints so far. But as with most designs, the part numbering scheme must consider human factors and how the design will be used.

The part numbering system must accommodate high-volume users — people who work with hundreds or even thousands of part numbers each week. Document control analysts, warehouse clerks, purchasing agents, production supervisors, and field technicians need to accurately and concisely convey part numbers, often over the phone in noisy and distracting environments.

Part number length§

For most companies, a consistent length of up to 7 characters works quite well.

Consistent length§

All part numbers should be the same length. Varying lengths make it difficult to know whether any characters have been omitted or truncated. Different computer applications may sort varying text lengths unpredictably.

Maximum length§

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 approaches 100%².

Any scheme longer than 7 characters:

• requires most users to write down, rather than simply remember, the part number for even brief use; and
• increases the likelihood of data-entry errors³.

Part number character set§

Best part number design considers data entry eye-hand coordination, i.e., "clerical speed and accuracy", as well as compatibility with common office applications and practices.

Numbers, not letters§

Computer keypad characters are most efficient for high-volume users. The keypad allows single-hand operation, and the layout is easily memorized.

Numeric characters are universal, visually distinguishable, and independent of local language variations. In many fonts, alpha characters appear quite similar to numbers. If your product is built or serviced in another country, can users easily enter your part number on their keyboards?

If you must use non-numeric characters, avoid characters outside of ANSI Basic alphanumeric.

No leading zeroes§

Avoid starting a part number with a "0" (zero) character.

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, making a mess of your bills of materials. Although this rule reduces your possible part universe by 10%, it's well worth the cost.

Hyphens as delimiters§

Separate a part number into chunks when it exceeds about 6 characters. For efficient data entry, we'll limit our delimiter choices to symbols on the numeric keypad.

Part numbers are often used as the filename for importing bills of materials, exporting design packages and viewing file attachments. Therefore, do not use:

• Reserved computer file characters such as < > \ / : " . | ? *
• Characters near the font baseline (period and underscore) that can be obscured within a hyperlink. For instance, 276543-01 is more obvious than 276543.01 or 276543_01.
• Characters commonly used by search engines to indicated operations and grouping. For example, characters like & | ! ^ " ~ * ? : \ may indicate commands: !resistor, wildcards: t?re or th*, or escape sequences. Paired symbols like (x) "x" {x} 'x' [x] may be used to group search tokens.

And never use a space character.

The safest delimiter character is a hyphen: 276543-00. This is the hyphen (minus sign) on the numeric keypad, not the Unicode en dash ( – ) or em dash ( — ) characters.

For consistency, if your number scheme includes a delimiter, all part numbers should share the same format. For example, don't use -00 for some parts, -000 for others, and omit the dash entirely on yet others.

Best part number design solution§

Taking into account our rules, this very simple part number design has been successfully applied in a huge variety of industries, organization sizes and technical environments:

Conclusion: A part numbering system using 7 or fewer numeric characters is the easiest to manage for the majority of our users.

Five to seven numeric characters permit a universe of up to 10 million (without leading zero, 9 million) unique parts, far more than most companies will see over their entire lifespan.

Recommendation: 6 numeric characters, no delimiter. Start at 100001 and increment.

Our short, numeric, non-significant part number provides the fastest data entry, with the fewest possible errors. It's the most efficient solution for heavy users in purchasing, manufacturing, receiving, service and other places where employees constantly work with a wide variety of part numbers.

PLM software can easily generate the next part number in sequence, prevent conflicts with previously-issued part numbers, and find parts based on the number (if you happen to know it) and description and other attributes when you don't.

If you're now wondering how part characteristics can be included in your part number, you're thinking of a "significant" ("smart", "intelligent") part number scheme. Although significant numbering schemes are no longer considered good practice, we identify the risks and offer suggestions for avoiding some of the problems.

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

Experts strongly recommend the use of short, numeric-only, non-significant item numbers. We have not found any modern authority that recommends using significance in part numbering.

Here are some relevant comments from our PLM book list:

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.

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

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.

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

Identification numbers are preferably short, not long. The characters that make up the number are preferably numeric, not alpha. Any symbols to be used with the numeric characters are preferably limited to dashes.

Guess: CMII for Business Process Infrastructure, page 81

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.

Mather: Bills of Materials, page 100

I prefer a non-significant number because there is a longer life and less error... Typically, companies run out of numbers in certain categories of a significant number. Also, a non-significant part number is more cost-effective to use than a significant part number.

Monahan: Engineering Documentation Control Practices and Procedures, page 33

The most critical of these issues is that, over time, the significant numbering systems tend to break down. ... As time passes, variations arise which were not foreseen. One digit was set aside where two are now needed. Significant numbers thus tend to lose their significance. They no longer do the classification coding function intended by their inventors.

Watts: Engineering Documentation Control Handbook, page 49

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. [note: GM part catalog numbers use eight numeric characters without delimiter, e.g., 23291488]

Francis Criqui, Director of Engineering Standards, General Motors Corp.

Part numbers versus source numbers§

Source numbers identify saleable items to the outside world.

First, it's important to distinguish part numbers from source numbers (also called product, model, catalog, marketing, vendor or sales numbers).

In customer communications, a source number can provide more flexibility than your engineering part number. This separation allows you to substitute non-interchangeable engineering items without affecting brochures, datasheets, catalogs and similar customer-facing materials.

Stable source numbers suggest feature continuity despite non-interchangeable product changes. A BMW 320i car built in 2021 is quite different from the version sold 45 years earlier, yet long-time customers understand the evolution. Customers, sales and marketing personnel, distribution channel partners and customer purchasing agents all refer to the "same" general product while engineering makes radical product changes over time.

All changes to the source part are, by definition, "interchangeable for the application" — that is, for marketing or support purposes. This interchangeability is maintained for as long as marketing or product support considers the evolving item sufficiently similar to its earlier versions — there's no objective technical criteria. Non-interchangeable engineering changes simply bump the marketing number's revision (which is never exposed outside the company).

For this discussion, the terms smart number, intelligent number, significant number are all used to indicate the same idea.

Some companies encode high-level marketing characteristics into the source number: a "smart" catalog number.

Product model numbers, like that BMW 325i, usually represent very general characteristics. Or, source numbers — such as the purchase number of a resistor, screw or engineered plastic — might summarize published datasheet specifications. However, source numbers can't possibly encode all specifications for a complex part, but only identify the most useful characteristics. It's unlikely that most customers will bother to learn your specific numbering details, so the "smart" number (if any) should be as short and simple as possible.

In any case, the "significance" risk remains: errors in significant source number are easy to make and expensive to correct. An extensive product catalog or very complex part attributes drive towards non-significant product numbers.

Part numbering supported by PDXpert PLM software§

PDXpert PLM software makes it very easy to adopt a non-significant numbering scheme. PDXpert software can be used to auto-generate part numbers, and its free-form (Google™-like) text searches make part look-up quick and simple.

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., 1234-5678), PDXpert PLM software will ignore the delimiting characters when incrementing: 1234-9999 is followed by 1235-0000.

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. The software easily accepts mixed schemes, such as importing legacy significant numbers while auto-assigning shorter non-significant numbers.

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 and type subclasses.
• 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."

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