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Do parts have revisions? What the "form fit function" rule tells us

PDXpert product data management software easily lets you identify your parts and the documents that define them. As designs evolve, the original documents get revised, and new parts are created from these revisions. This has led some to conclude that parts must also have revisions, and that these revisions must be reflected in the manufacturing management system (MRP/ERP). We'll show that, even though they may be revised, parts aren't identified by revision.

Part interchangeability and the "no revision" rule

Proper part identification is the foundation of inventory management. Part identification and interchangeability must be correctly applied because a part number is functionally equivalent to an inventory bin location.

One part number = one bin location.

A part is interchangeable with another part when their relevant functional and physical properties are equivalent. Part interchangeability - that is: affecting form, fit, or function - applies to design iterations of the same part, as well as to substitute or alternate parts. (For a complete discussion, see Applying principles of interchangeability to your bill of materials.)

Practically speaking, if a revised part can be intermingled in production inventory with previous iterations of that part without any negative effect, then the part iterations are interchangeable and the part number remains unchanged. Similarly, if one supplier's parts can be intermingled with another's, then these are also interchangeable and can be stocked in the same warehouse location.

However, perhaps after revising the documentation (or qualifying another supplier's part), the new part iteration is not compatible with previous iterations (or you simply want to track its usage because you're not sure of its effect). In this case, it's not interchangeable. You must issue a new part number, and the new part must be stocked in a different inventory bin.

Therefore, design revisions that do not affect a part's interchangeability are, for inventory and production management purposes, irrelevant. Of course, any part may go through an evolution which is directly traced to revisions of the part's documentation. But the well-accepted rule is that parts are not identified by revision.

Parts binned by revision simply have weird part numbers

At some companies, assigning new part numbers can be overly bureaucratic, and this may lead to using revision identifiers to distinguish non-interchangeable iterations.¹

Of course, the whole point of using the form-fit-function rule is to decide when a part iteration requires a new number and inventory location. If your warehouse is locating non-interchangeable parts by revision, then the revision has become a de facto part number suffix. Although this sidesteps assigning a new part number, it merely shifts the burden to a more complicated decision about whether a particular revision justifies the creation of a new inventory bin. The frequent result is that all parts get marked with revision identifiers, and all changes are treated as non-interchangeable, creating many more inventory locations than would otherwise be necessary. The inventory management cost of maintaining superfluous number+revision bins overwhelms any perceived savings in avoiding a new part number.

Authoritative references reflect the "no revision" rule

Many authorities have confirmed that parts don't have revisions, and we have not found any authority who supports part revisions. Some useful comments (click here for PLM books):

Clement, et al.: Manufacturing Data Structures, page 265:

Revision level - A number or letter representing the number of times a part drawing or specification has been changed.

Guess: CMII for Business Process Infrastructure, page 88:

Revision levels are preferably excluded from the [part] identification number. ... Lack of trust is why the revision level of each physical item is often included in the bills of materials.

Samaras: Configuration Management Deskbook, definitions on pages 495 & 499:

Revision letter, a letter added to a drawing or other document number to indicate that a change was made to the original document.

Part number, a number used to uniquely identify a part or item. ... The revision letter is not included with the part number.

Watts: Engineering Documentation Control Handbook, page 52 (author's emphasis):

The revision letter or number is the change status or level of the document....

Rule: The Revision is not part of the part number.

Rule: Revision is never marked on the parts.

Rule: Never stock by revision level.

DOD-STD-100, page 700-7:

The term revision refers to any change after that drawing has been released for use.

And in DOD-STD-100 section 402.6f:

Part numbers shall not include the drawing revision.

MIL-STD-100G, section 406.6:

d. PINs [Part or Identifying Numbers] shall not include the drawing revision.

Part database records have revisions

As an important side note, certain records that are about a part (called "part metadata" in a product data management system) can and should be tracked by revision, similar to other documentation that defines the part.

In PDM software, the part database record includes, for example, the default unit of measure and approved supplier sources. Obviously, these part data elements aren't the actual physical part, but define certain characteristics of the part that may not be in the part CAD file, specification, or other documentation. Since these attributes can change over time, and you may wish to track these changes, the attributes are associated with a specific database record revision.

However, a part's identification remains distinct from both its documents' revisions and its database records' revisions contained in the PDM software.

Using interchangeability rules to auto-update bills of materials

PDXpert PLM software relies on part interchangeability rules to simplify bill of materials management. If you retain the part number at each interchangeable change, PDXpert will update every bill of materials immediately upon release of the new part record revision. Similarly, a document that's listed on a part or document structure will always display the most current released revision.

Similarly, when a supplier part record revision changes while retaining the previous part number, the Source list on the consuming part will be updated without requiring an engineering change for the using part.

Notes

1. One reason why part number assignment can become cumbersome is that the part number is meaningful, and must be "protected" from incorrect categorization. If every new part number must be created or reviewed by a part number czar, it's time to abandon significant part numbering in favor of sequential assignment.

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