Drawing Revision & Part Revision 2

[CajunCAD91]

The company I work for is in the process of implementing a MRP system which needs part and assembly revision levels. We have not had a method to document the revision levels for parts & assemblies in the past however we have been able to maintain a list of revision levels for documents.

MFG wants the part or assembly revision level to match the revision level on the respective drawing. This way the PO will list the part number & revision being ordered and the drawing number will match.

We engineers contend that the part or assembly revision level is independent of the drawing (drawings may be revised to add views or sheets to make them easier to read which has no impact to the revision level of the part).

Are there any references to determine which method is correct?
Any experiences that can be used to show why one method is better than the other?

Thanks for any assistance,
Louis

 

[plmguy]

A few points for consideration...

First and most important, a part is not identified by its revision. If it's interchangeable, meaning that receiving inspection can throw one revision or the other into the same inventory bin, then MRP need not track the revision. OTOH, if one revision can't be put into the same bin as the preceding revision, then they're not interchangeable, and they should have distinct part numbers. (This is not to say that part records do not have revisions, they do, but only the parts themselves do not.)

Parts are built to a specific document revision. While it's sometimes the case that a part number and document number share the same identifier (not a particularly useful practice any more), the match is only a matter of small convenience and there's no compelling reason to keep them in lockstep. Since (1) the number match is rather arbitrary, (2) documents get revised for reasons having nothing to do with a part's definition, and (3) parts do not have revisions, then we can safely say that trying to keep the revisions matched will be an exercise in frustration.

References:

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." (Emphasis mine.)

Garwood: 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..." (Obviously, this implies there's no relationships between revisions.)

Guess: CMII for Business Process Infrastucture, 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."

Watts: Engineering Documentation Control Handbook, page 52: "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." (Author's emphasis.)

I think most of these authorities are supportive:

http://www.buyplm.com/pdxpert-plm-books.htm.aspx

- Ed

 

[ewh]

A star for that info, Ed. It is especially helpful to have all of the references you mentioned.
Until recently, we maintained separate revision levels for parts and drawings. While the parts were interchangable, there might be manufacturing improvements, for example.
Recently, an important customer informed us that due to their implementation of an MRP system (stop all production), we now need to supply both parts and drawings reflecting the same revision level.
I will look into the info you have provided and try to present a case to upper management to drop the revision of the part altogether. Thanks!

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[plmguy]

Thanks, glad you found it useful. Went to my wayback machine for a few more to add to the collection, starting with the then-definitive authority from 1988...

Samaras: Configuration Management Deskbook, page 499: "Revision letter, a letter added to a drawing or other document number to indicate that a change was made to the original document."
Ibid, page 495: "Part number, a number used to uniquely identify a part or item. ... The revision letter is not included with the part number."

The granddaddy could be DOD-STD-100. We have on page 700-7: "The term revision refers to any change after that drawing has been released for use." And in section 402.6f: "Part numbers shall not include the drawing revision." And later, MIL-STD-100G says in 406.6: "d. PINs [Part or Identifying Number] shall not include the drawing revision". These and MIL-HDBK-61 can be downloaded here:

http://www.product-lifecycle-management.com/legacy-military-standards.htm

I'm guessing a similar statement may be in ASME Y14.100 or a related standard.

Now, there are many aspects of CM that made perfect sense in 1988 that no longer work, but the clear trend -- based principally upon computerization -- has always been away from establishing any significance between part and document numbers or their revisions. As Grieves advocates in Product Lifecycle Management, "... the part number has no meaning itself but simply acts as a pointer to the always accessible product information" (page 166).

It's a pity that your customer is tracking part revisions, but must be even more frustrating that you're forced to match document identifiers and revisions to the part. It's tough to imagine a more brittle engineering business rule, unless they also have significant part numbering.

Kind regards.

- Ed

 

[KENAT]

Nice infor PLMGuy.

Simplistically, and not thinking about it too much. What your purchasing are proposing is what I've usually seen and seems to make sense.

As others point out, usually revisions should only apply to the doc, they should not have any significance to interoperability of the part.

That said purchasing will normally want to only purchase parts to the latest rev of the drawing, the drawing was revved for a good reason right!

With our CAD system the revision property of the model actually drives the revision level of the drawing so ours always match. Even if there's not a physical change to the model it gets revved in order to rev the drawing.

KENAT, probably the least qualified checker you'll ever meet...

 

[CajunCAD91]

We had a meeting this morning to come to an agreement on how to proceed. Using the info provided by plmguy, my boss and I were able to convince the MFG folks that parts & assemblies would not have a revision level. MFG will also get what it wants by placing the drawing revision in the field MRP uses for part/assembly revision. MFG will get all the documents MRP produces (PO's, Sales Orders, Work Orders, etc...) customized to place the text Drawing Revision in front of the revision letter to make it clear to all that this is not the part revision.

Thanks plmguy, the information & references you provided really helped us.

Louis

 

[plmguy]

Louis:
Great, glad to be of some assistance. Seems like win-win all around, since it appears that MFG has better appreciation for distinction between buying a part number and buying to a document revision.


KENAT:

Ah, but...

Your method works only where the drawing number and the part number are one and the same. I propose that we should be reluctant to maintain part & document numbers in sync.

Exact matches between document numbers and their resulting parts is often not sustainable. Part interchangeability rules may force a new part number where only a document revision is required; likewise, for clarity a new document may be created to describe one or more existing parts. And a specific set of documents (for fabrication, plating, assembly, testing, adjustment, inspection, etc.) may be applied to a variety of different parts.

Another document identification approach assigns related design document types a common root, and extensions ("tabulated drawings") indicate a particular document type. For example, a circuit schematic might be 45678-01, printed circuit layout 45678-02, and printed circuit board (PCB) drill pattern 45678-03. All these describe part 45678. But you may later need to "upsize" the schematic with a new document number to cover an entire product family with other PCB layouts and drill patterns, or an alternative PCB layout based on the same schematic, and the original relationships would no longer exist. (PLM systems are built on relational databases to ensure the relationships are explicit under all conditions.)

I suggest that explicitly tying business rules (like "match this part to this document" or "match revisions") to item identification is bound to fail. Our principal goal in part & document identification is to make identifiers our permanent "handles" to the underlying data or object. In contrast, business rules are, by their nature, flexible and constantly evolving.

These conflicting goals may sometimes mesh by chance, and give us some beneficial side-effects, but this should not be taken as more than passing good fortune. The fact that our CAD tools create models that can be immediately transformed in parts may be convenient in a smaller manufacturing environment, but won't scale.

And, if we grant that document numbers and part numbers may have (at best) a loose relationship, then there is little rationale for enforcing a link between their revisions.

- Ed

 

[KENAT]

I think this is where my having started out in the UK (at least my little defense sector) and now in the US causes issues. We rarely had tabulated drawings or variations there on.

You're correct that with tabulated drawings or similar things get a lot more complex.

While I may have worded it badly what I meant was, use the document revision for the part revision.

However, I'm tired and not sure I grasped the full implication of a couple of points you made. Perhaps the light bulb will illuminate at a later point.

KENAT, probably the least qualified checker you'll ever meet...