question on changing part numbers 1

[greg2835]

In my experience all employers have employed the "every change gets a revision." Meaning if I have Part1 at Rev A, get it manufactured and sent back to me, then find out it doesn't fit, I then revise Part1 to Rev B (no new part number).

I am aware of the "parts get no revisions" rule and it makes sense to me, but am curious how it plays out. Lets say I make part 12345, and at some point after release a change needs to be made. By now part numbers have been generated by others in the company, so I create the next sequential number, let's say 12406. This p/n goes through the approval process and is eventually released, and no further changes need to be made.

What should I then do with initial part number 12345 in the system? I guess I could do nothing and leave it as is, but would you recommend an "obsolete" or "superseded" note? I also fear tracability will be an issue, so perhaps those notes are necessary with identifying new part numbers. To assist with this issue, I thought about having an indexing system for creating part numbers such as initial part being XXXXX-01, then index the "-" number for each change (XXXXX-02, then XXXXX-03, etc.). This IMO at least implies those lower "-" numbers to be obsolete/superseded and also makes the history of that part much easier to trace.

That's it for now, I might add some more detail if a discussion gets started. Let me know your thoughts!

Greg2835
Mechanical Engineer

 

[IRstuff]

As a general rule, that's what configuration management is all about. If you didn't have unique part numbers, how could you possibly be sure that everything around it works as intended with that particular configuration? Our typical numbering system utilizes dash numbers for essentially cosmetic changes, while changes to form, fit, or function are part number changed.

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[greg2835]

Thanks IR, I have not considered that approach but I do like that these cosmetic changes have a place in your CM. Let me go a bit further then, and ask if you do anything special with a previous part number that required a FFF change? For example, part 12345 did not fit, so part 12406 was created and now in use. Do you specify somehow the was/is relationship between these numbers?

Greg2835
Mechanical Engineer

 

[IRstuff]

If it's something that's in production, then there would have been an engineering change order (ECO) requesting the change and the reason. If new part has dependencies, then the next higher assembly (NHA) or higher would require some sort of configuration notation, particularly if there's some incompatibility between the versions.

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[greg2835]

I see, ECO is placed and new part dwg may list something like "REV A - REPLACEMENT FOR P/N 12345" in the rev block, I assume.

Thanks IR, I actually was going to expand on things a bit but after thinking it through I at least have a decent starting point. I'll be joining a start-up company in a few weeks and was told I'll be tasked with the CM portion, so doing what I can to have something ready.

Greg2835
Mechanical Engineer

 

[cowski]

I recommend picking up a book on documentation control. I have a copy of the one linked below and can recommend it. I'm sure there are other good ones out there as well.

https://www.amazon.com/Engineering-...4&sprefix=engineering+document,aps,155&sr=8-1

 

[CWB1]

Revisions shouldn't be released until you have thoroughly tested a part, which should involved a test fit so your first scenario shouldn't happen.

Your second scenario is a bit confusing to me. If your parts dept simply needs to support an "old style" and "new style" with two distinct parts then I would simply have two revs of a part. OTOH, if the "old" and "new" are each going to see new design in the future then they need two separate p/ns. As for part marking, if you have inventory of one that can be reworked into the other, there shouldn't be any issues simply reworking the part marking to match.

 

[greg2835]

Thank you for the recommendation cowski, I will certainly give it a look!

To CWB1, we might have different working environments - so I'll provide a bit more detail. My company does all the design work and outsources manufacturing. Small changes come up time to time after drawings/models have been given to suppliers, so new numbers/revisions are made to distinguish these changes on the shop's floor.

My second scenario is the times where the old versions of the design are found incorrect and a new part number is created, either in the prototype or production phase. I have heard of different approaches such as notes left on these older version that say "CANCELLED AND REPLACED BY P/N XXXXX", just leaving the file as is and releasing new part number with "REV A - INITIAL RELEASE", or (what my previous employers have done) keep the part number and create a new revision.

The approach you use also adds to the list (use revs for different versions that will see new designs in the future), so I appreciate it as I think through these things.

Greg2835
Mechanical Engineer

 

[IRstuff]

If the old part is unusable or unsuitable, then it should be designated as such; I think in those instances, the part number should labeled as "void" so that no one attempts to use one or procure one. Again, some NHA documentation should clearly indicate that the part is not ususable for that purpose.

In electronics, it gets particularly tricky, because form and fit might be identical, but function might result in bricking a customer's equipment, so configuration management and validation is mandatory.

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[truckandbus]

the strict definition of meeting form fit and function for part revision versus new part number sometimes needs to be tempered with common sense and practical use of resources

One of the better takes on this discussion can be found using Jorg Eisentrager's interchangeability rules (if you google his name you should easily find this document)

 

[greg2835]

Thanks truckandbus, that is the heart of the manner that I am trying to research, how much "tempering" I will need in my situation. The resource you suggested looks great! I will certainly consider its methodologies.

In my experience, I have seen instances where parts are revised for every change, and then down the line get manufactured to the incorrect revision in one way or another. However, I was told that manufacturers don't like part number changes in BOMs and that they prefer part revisions (which I can see some sense in that).

As mentioned before, I will be joining a start-up company in a few weeks, and their current methods are no revisions, no part number changes (barely a title block either), just make a change and send the files. They are giving me free reign to put these processes in place, and I am quickly learning the pros/cons of different scenarios. What I am noticing, no system is perfect. I will do my best to set something in place with reasonable logic/defense.

Greg2835
Mechanical Engineer

 

[IRstuff]

probably, "free rein" as with a horse.

I think that what you are doing is all the more important because it is a startup, so entrenched bad habits are ostensibly easier to break. Much depends on what the ultimate business model looks like; onesy-twosy prototypes, or mass production. The former might not care than much about this, but the latter can be crippled by rampant mis-numbering, particularly in the post-delivery service and maintenance, and especially if there are electronic parts. Even when we do everything possible, sh!te happens; we once CM'd everything to do with an FPGA and still managed to not be able to repeat the build a few months later.

There are companies where multiple software builds and deployments can occur in a single day; not maintaining configurations, or even knowing the configurations could crash someone's site for a long duration.

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[CWB1]

My second scenario is the times where the old versions of the design are found incorrect and a new part number is created, either in the prototype or production phase. I have heard of different approaches such as notes left on these older version that say "CANCELLED AND REPLACED BY P/N XXXXX"...

That's simply terrible practice. Product/assembly structure flows top-down, not bottom-up. Your individual part print should never have information relating to the assembly much less its production status on it, otherwise you've created more work/cost for others who want to use that part in their own product/assembly in the future. If another engineer wants to use the first rev of a part, 25 years and 125 revs later, they shouldn't need to spend the time/money to create a new print. If a product/assembly needs a new rev of an individual part, then the part gets rev'ed and the product/assembly print updated to callout the new rev.

 

[greg2835]

Aha, yes IR stuff you are correct - I have been misusing "free rein" for quite some time now I have been walking through best methods for electronics part numbering, and that task seems MUCH more cumbersome (and I have never worked at a place that has done it well). Luckily, we do not have a lot of custom electronics - so I will have to work with others to find best practices for this.

To CWB1, I must not be clarifying enough, because I agree with your top-down structure. The cancelled note you have in your quote would be placed on the old part drawing, with the new drawing number listed (reasons for p/n change are mentioned in truckandbus' comment). As I mentioned, I have not employed this method but have heard of others doing so, so I included it in my list of examples. Again, this is an instance when there was something incorrect in the design, which deemed it ultimately unsuitable for the final release. This is not when a design was initially correct and then later changed for one reason or another. It doesn't happen often, but it does happen.

Greg2835
Mechanical Engineer

 

[CWB1]

Again, this is an instance when there was something incorrect in the design, which deemed it ultimately unsuitable for the final release.

The cancelled note you have in your quote would be placed on the old part drawing

It sounds like you're creating revs and releasing prints during development, which shouldn't happen. A "release" means the print has received final approval from engineering for form/fit/function after prototype/testing, and through the usual manufacturing, purchasing, and other depts' reviews for the current project. Releases get a new rev assigned to them. While you're working through prototyping, testing, etc (DMEDI) drafts of each print should be open to change at any time by the responsible engineer. For assembly updates over a product's lifetime (usually years), parts need updated occasionally, hence we rev the lower level part and update the assembly print to reflect this. Once a part is released there should be no changing or additions made to a released print. Literally, as part of the releasing process the print is "signed" (either via PLM or in rare instances today on the print itself), and becomes a legal document. The only way to add a note is to create a new rev of the print, and as mentioned in my last post is counterproductive and needless cost/time spent.

Not to beat the horse, but there is no such thing as a "final release" nor should you ever think of one rev as "replacement" for another bc in reality, they only "replace" the existing in specific assemblies. As you release/rev parts, you're essentially adding to an ongoing "catalog" that future engineers will draw from. For p/n 123 they might want to use rev A, C, & D, so notes stating "D replaces C" or otherwise are simply nonsense.

 

[greg2835]

Thanks for the input CWB1. When I said release I meant prototype release (release to outside manufacturing to begin prototype parts). Final release meaning the prototype has been production approved (you are correct in your definition for "release," this is just the vernacular I am used to from latest employer).

Greg2835
Mechanical Engineer

 

[truckandbus]

Another element to consider is how well the purchasing group manages notification to suppliers of required revision level of parts (my current company POs do not specify revision and there is no formal process for notifying them of a rev change - insanity)as well as how material handling processes the parts on the shelf (do they purge or rework old revisions, use first in/first out to establish a cut-in date or serial number change over of revision, or do they pig-pile revisions of parts on top of previous revisions of parts)