Item revision rules

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Parts are not stocked by revision

The debate, for some reason, continues to rage: do parts have revisions?

The obvious answer is, yes, of course, parts get revised all the time. And there is certainly no harm in marking the revision on the part if you want to trace a particular iteration to a specific drawing revision.

But the real part revision question goes to interchangeability, which drags in the old "form, fit and function" discussion.

A part number is, for all practical purposes, both

  • an item identifier, and

  • a location in the warehouse for that item.

From the perspective of those people who work with the real-life physical part, the part number is used to consolidate all interchangeable parts at one warehouse location. A part received into inventory will be stocked in a location identified only the part number. If you need to stock a new iteration of a part in a different bin because it is not compatible with previous iterations, then you issue a new part number, not a revision.

Many ERP systems can only track, manage and purchase parts by number. Sometimes, in order to sidestep this "limitation", users will add the revision as a suffix to the ERP part number, thereby forcing the system to order, receive and stock by revision. This approach merely creates a new part number where every revision, interchangeable or not, has its own inventory bin.

Revisions are not used to reflect design confidence

In the old days, a design that wasn't yet approved for production had a numeric revision (e.g., "01"), and when it was production-ready it was assigned an alphabetic revision (typically "A" or "AA"). The revision format communicated a business rule whether it was safe to buy an item in production quantities.

These "intelligent" revision formats could distinguish 2 lifecycle phases, but there are typically not just two sets of business rules. At various phases, you may want to say "don't buy any, we're still working" (design), "buy only enough to do a field test" (beta), "stock only enough to support repairs" (service-only), or "this is no longer stocked" (obsolete).

You probably would not propose having different revision formats for each lifecycle phase, even though a revision production-oriented revision "C" may represent "build whatever you need" (production) or "don't build any" (obsolete). Instead, most PLM systems support an unlimited number of lifecycle phases to reflect your organization's inventory business rules.

So in a modern PLM system, the simple "preproduction" versus "production" revision formats cannot indicate the true business rules currently applicable to the item.

Furthermore, embedding the lifecycle phase in the revision forces the item owner to open the file, edit the revision value, and ensure that nothing else will be modified, by intention or accident.

Consider instead:

  • Document revision refers to the technical content of the document. You change the revision only to change its contents. 

  • Lifecycle phase refers to a set of business rules that are applied to the document.

An item's lifecycle phase should be a simple database attribute that represents a company's commitment to the item, a reflection of the item's "maturity". One should not revise a document if the document itself has not changed, but its usage has.

Item revisions use letters or numbers, but not both

An alphanumeric revision is not necessary, and can be confusing. Is the revision after "A9" supposed to be "A10" or "B1" (or "B0")? Even if you and your colleagues know, have your suppliers trained each of their staff?

The argument in favor of using alpha characters for a revision — beyond the historical fact that DOD/MIL-STD-100 required it — is that "it's different from an item number" and therefore easily distinguished, assuming that your item numbers are numeric. You may be able to set your computer system to accept only numbers for identifiers and only letters for revisions.

On the other hand, numeric characters offer a more visually distinct set; most people can easily identify the digits 0 through 9 regardless of handwriting style or computer typeface. It's often more difficult to distinguish C/G, D/O/Q, L/I, U/V/Y, or W from VV.

Revisions are a fixed length

Ideally, your items should always display their entire "potential" revision sequence, such as "AA" or "001"; users will then be able to tell if a character has been omitted or accidentally truncated. If some of your revisions are "A" and others are "AC", how is a reader to know whether the first revision is correct, or is missing the second character?

Dates are not appropriate for item revisions 

Because it's possible that an item could be revised more than once on a single day or are effective across more than one time zone, and dates are notoriously prone to misinterpretation based on the reader's geographic location (is "03.06.05" in March or June — or maybe May?), you shouldn't consider a date for a revision.