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Guidance on interchangeability guidelines for A&D

Guidance on interchangeability guidelines for A&D

Guidance on interchangeability guidelines for A&D


Please forgive me if this has been asked before.  I have searched the archives and drawn a blank.

I'm in charge of a CM group for a manufacturer of communications products that are used (primarily) in Space and Airborne applications.  Our company President recently suggested that the rules that we are following regarding when to make a revision change vs. a part number change are not consistent with his understanding of how this should work.  In discussing this with him and my CM manager, our disconnect relates to a detail that I have not seen described in MIL-STD-973, EIA-649, or any of the other standards that we reference in our CM process.  

Our company standard for interchangeability is that if an item is changed in such a way that full backwards/forwards interchangeability is broken, it must be assigned a new part number, and all superior assemblies must receive a new part number, up to the level of assembly where interchangeability is re-established.  Simple enough.

Consider an assembly (ABC-1) that is in production at Rev A, and a design defect is discovered that requires rework.  There is no change to the form/fit/function/reliability of the assembly, and when rework is complete it will function within the original specification.  We have never shipped any of the end items that use this assembly, but there are one or more assemblies in WIP that will need to receive the rework.  

In most cases, we would write a change order that would take this assembly from Rev A to Rev B, and we would disposition all of the WIP as rework. The Rev A build documentation would be recalled from the shop floor and replaced with rev B documentation.  All units in WIP would be built and inspected to the Rev B documentation, and would ultimately be marked ss Rev B's.  We don't do a part number change because the entire wild population has been captured and reworked, and our interchangeability test is applied ot the assembly AFTER it has been reworked.

Our company president believes that because the Rev A design did not work as intended, it is not interchangeable with the Rev B version version that does work. He is in effect applying the interchangeability test to the assembly BEFORE it has been reworked to its new configuration, and  is suggesting that we need to assign a new  part number when we implement the change rather than rolling the revision.  So instead of part number ABC-1 Rev A going to ABC-1 Rev B, it would go to ABC-2 Rev A.  His reason for advocating this appraoch is primarily that he doesn't trust that the dispostion specified on the ECO is always executed with 100% accuracy.  This creates the possibility that the revision of the product with the defect could escape rework and (since we don't stock or kit by revision) pass to the next higher assembly or ship to a customer.

It would be very helpful for me to hear your thoughts on how you think this should work, or how you handle this within your CM system.  If you have references to definitive standards, texts, or articles, I'd appreciate those as well.

Thanks in advance!

Kit Cheves
San Diego, CA

RE: Guidance on interchangeability guidelines for A&D

Your new Presidents approach is more robust and to my understanding more closely follows the 'letter of the law' as laid down in ASME Y14.100 and the like.

I don't remember ASME Y14.100 making a distinction between new products in development and fully released product when it comes to rev change V new part number.  I also don't remember it saying anything about new rev being OK as long as you can guarantee reworking all the old parts to the new rev or similar.

That said, what you currently do is fairly common practice.

I'm just surprised it's your senior management pushing this and not the other way round - or maybe that's a reflection on what I've seen the last few years.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Guidance on interchangeability guidelines for A&D

The rules you describe regarding inter-changeability, part number v. rev level change, are correct in my opinion as well as Frank Watts "Engineering Documentation Control Handbook". If all existing material in the world is reworked, making it inter-changeable with the upcoming rev, then a new part number is not required. If all existing material in the world is Scrapped, a new part is not required.

Over the years I have dealt with issues regarding completion of tasks that ensue upon ECO approval. For ECO systems I devise, I always like to see a set of approvals, typically from operations, that basically indicate "I've done what I was supposed to do" before Doc Control closes the ECO. This is not foolproof but there are far fewer issues.

Good luck

RE: Guidance on interchangeability guidelines for A&D

I'm way behind you guys on change management practices, but as a designer I'll give my perspective on this. In short I agree with your president.

If revision A has been deemed defective it should be declared obsolete and superceded by part number... and as part of the obsoletion process, there is a disposition such as use up inventory or scrap inventory.

MOO is revisions should be used for "little" things, cosmetic changes to drawings, maybe a unit of measure change, or a spec change on a serialized part.

My thoughts, love to hear other opinions.

Just a pert.

RE: Guidance on interchangeability guidelines for A&D

I have to agree with the OP and FineThread. As an Aerospace Manufacturing Engineer, if you can capture all parts in WIP and scrap or rework as applicable no part number roll is required or necessary. I have never 'witnessed' it done any other way (at The Lazy-B, LM or AB). Part number rolls cost money and are typically the bane of the design group, new programs especially. I'll see if I can search my files for some documentation regarding this (Lunch is just about over).

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