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Life after MIL-STD-100: Decoupling part numbers and their documents

PDXpert product data management software allows you to easily establish relationships between your parts and the documents that define them. Although PDXpert PLM software lets you create part numbers in sync with your document numbers, we'll explain how to simplify your engineering change process with (a) independently-assigned document and part numbers, and (b) explicitly-defined relationships between a part and its defining document.

Back in the old days, good practice linked part numbers with their defining documents. MIL-STD-100G, STANDARD PRACTICE FOR ENGINEERING DRAWINGS, declared that:

404. Part or identifying number. The Part or Identifying Number (PIN) shall consist of letters,
numbers or combinations of letters and numbers.... The PIN shall be or shall include the
design activity drawing number, and may include a suffix identifier (if applicable).

The modern replacement for MIL-STD-100, ASME Y14.100, says essentially the same thing:

The Part or Identifying Number (PIN) is an identification assigned ... for the purpose of uniquely identifying a specific item. A PIN is the same as, or is based on, the controlling drawing number.

The original rationale for tying document and part numbers

At one time, searching for a part's supporting documentation was slow, labor-intensive and expensive. The quick solution was to explicitly tie the document and part numbers together. There were two versions of this technique:

A document number defined one or more parts. If multiple parts were defined, such as o-rings with varying diameters, the document provided the "base" or "root" number (e.g., 13579), and each part had a related "dash" number (13579-01, 13579-02, etc.). Problems occurred if, say, you needed more part variations than permitted by the dash number (122 different o-rings?).
A part defined one or more documents, each of which defines one aspect of the part. For example, circuit board assembly 97531 has a schematic (97531-01), board artwork (97531-02), silkscreen (97531-03), and an assembly drawing for each different "pick & place" machine (97531-04...97531-06). Alternatively, all the documents could have the same document number, but are of unique document types (schematic, artwork, silkscreen, assembly). But here the problem is restrictions on re-use, such as when you want schematic 97531-01 to describe assembly 97544.

A modern twist for tying document and part numbers

In certain narrow instances, a fabricated part may be fully described by a CAD model, which of course has both a "document" number and revision.

It may be argued that this one-to-one relationship between the CAD model and part is sufficient to say that the part number is the document number, but that "part revision" is always identical to the CAD model revision. However, there's no additional reason to treat a CAD file as anything other than a particular document representation. And since we've already established that parts don't have revisions, saying that the CAD model revision is the part revision just can't be true.

Making document and part numbers independent

Our principal goal in item numbering is to make identifiers our permanent "handles" throughout their lifecycles.

Documents have different life cycles from the parts they describe, and document identifying numbers may change less frequently than part numbers. A document is assigned a new revision identifier when its content changes. This happens in two instances:

The document describes an existing part where (a) the document and part numbers are related, and (b) the revision does not cause an non-interchangeable change. In other words, the part remains interchangeable before and after the document's contents change.
The document adds a new part to an existing series of similar parts. For example, your specification lists all available 0.1 watt, 2% axial lead resistors from 1Ω to 10MΩ; when the supplier adds a new value, say 12MΩ, the document can be revised since the addition doesn't affect any existing product.

A document revision may introduce a part change that requires a new part number assignment based on the rules of interchangeability. It's a fair bit of extra work to change the document number simply to shadow the part number, and the task is difficult if the document describes several parts, only one of which requires a new number. This is particularly true when the drawing shows mating parts, or part families - one ends up with documents showing partial families or, in the end, decoupling the drawing number entirely.

As stated by Dave Garwood in Bills of Materials, page 74-75:

If the drawing number and the part number are the same, you run into many new problems. ... Try to keep the part numbers and drawing numbers separate...

Using short, numeric, non-significant drawing and document numbers

For reasons similar to those described in our Part Numbering System Design topic, a drawing or document number should be:

as short as possible, commonly no more than 7 characters
purely numeric, without any letters or symbols (if symbols are necessary, then limit to the characters available on the computer keyboard's number keypad, typically "-")
without any significance or value attached to any of the characters, or to a part number

Benefit: It's all about simplifying your change process

Nowadays, best practice in identifying both parts and documents is to use short numeric sequential identifiers. As Michael Grieves advocates in Product Lifecycle Management (page 166), "... the part number has no meaning itself but simply acts as a pointer to the always accessible product information".

Similarly, the clear trend, based principally upon computerization, has always been away from establishing any identity relationship between parts and their supporting documents. This is a change from older practice, especially popular in defense contracting, of tying the part number to the defining document number.

If your document numbers are independent of the parts they describe, you won't necessarily need to change document numbers when your revisions affect part interchangeability. You can minimize the document references that must be manually updated, and change forms that must be processed, simply by assigning document numbers without regard to how the related parts are identified.

As always, let the computer do the tedious work so you can focus on the interesting stuff!

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