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Form, Fit & Function

Form, Fit & Function

Form, Fit & Function

Good Morning All,

I have been having an in depth discussion with the owner of my company as to when a dash number needs to be rolled. It is my contention that any time the fit, form or function of a part is changed than the dash number must be rolled. He has countered that what happens in the case of a material change? Everything about the part is the same but it is now made out of Ti or some kind of composite rather than 7075 T6. Is changing the material considered a form change and therefore the dash number must be rolled or because all the dimensions are the same the dash number does not need to be changed.

I'm hoping someone on this site can give me a definitive answer but I would like to hear from anyone with an informed opinion.

Thanks guys.

RE: Form, Fit & Function

What if there's a contractual prohibition against a particular material that's only present in one of your material choices? How will the customer be certain that you haven't slipped them a different material? Or, more simply, Ti is much stiffer than Al; will your customer be using the part "off-book," i.e., using it based on the fact that the Ti version is stiffer? Or, perhaps Ti is more resistant to a particular chemical, say, phosphoric acid; Al would be badly eroded with phosphoric acid.

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RE: Form, Fit & Function

dasmann, not the best forum for this question - configuration management might be better and similar questions have been posed there before forum781: -Engineering configuration management.

"Form Fit Function" is not a very good way of fully describing what changes need a Part Number change rather than revision (at least to ASME Y14.100 series, see section 6.8 of 2004 version).

The best summarized test is interchangeability - not just on "Form, Fit & Function" but in every conceivable way including any requirement for trace-ability etc.

i.e. could you have a bucket with parts to both version in and pick out either part and use it without caring?

Generally completely changing the material would probably be a new PN but there are situations where it might not. Functionally it will have different material properties as IRstuff alludes to which make it not fully equivalent in most applications.

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RE: Form, Fit & Function

It requires engineering evaluation, not evaluation by buzzword.

Same size bolt steel --> titanium? Function has changed because material properties are very different.

Same size bolt AISI 304 --> SUS 304 (Japanese equivalent)? Same part because the material properties are identical. Revise the drawing to indicate that both materials are acceptable.

RE: Form, Fit & Function


The first company that I worked for had a philosophy for part numbering that eliminated this quandary.

Part number changes were only mandatory when a configuration or material change materially affected the part such that it could not be installed universally in all existing installations [retro, mod or repair].

This allowed minor changes, such as...
Added holes for pass thru or fastening
Revised flange shape by TRIMMING [take material off, NO adding material]
Minor temper changes [EX: -T4 to -T42]
Minor finish changes [EX: zinc chromate primer to epoxy primer]
These minor changes were designated by a 'letter' following the basic PN thus: 12345-6 > 12345-6A > -6B, etc... NOTE: I am sure that the original -# was always considered as 'no letter code'. IF the code was simply "C" then only the change designated by C would be accomplished to the base part. IF multiple changes were required then all appropriate letter codes would be defined by production planning [EX: 12345-6B,C,E,J]

Fastener holes were always shown referenced on the basic P/N, so even adding a referenced fastener location, made no difference to the part. Fastener holes were then drilled on installation [assembly/install drawings]. I think that the only fastener holes shown on a drawing that were required were for details fastened to that part which then made a small sub assy; Mated to another sub Assy; or had unique functions such as lug and pin-holes.

The face of the drawing and the drawing effectivity block would then be annotated with aircraft serial number and the change letter(s) applicable... although this was not firmly reinforced.

In real terms,[RE P/N example above] the basic 12345-6 could be altered to include any/all of the lettered changes without materially changing the fit, form, function, strength, stiffness, etc of the base part.

IF however the change was irreversible, such as a significant alloy or temper change... or had changes to the configuration that were impossible to implement on the basic PN [12345-6], then a part number change ['roll-over'] would have to be implemented... and the letter code recycles to [new] for that new -#. Example of such changes...
Added flange area
Added Joggle
Added integral stiffening bead
Holes with edge stiffening [flange or bead]
Increased flange-width
Angular changes
Alloy/temper changes [increased strength, etc].

Regards, Wil Taylor

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RE: Form, Fit & Function


I figure that any change that may matter to you is a functional change. Titanium is stronger, more chemically stable and heat resistant to aluminium 7075-T6. Aluminium 7075-T6 is lighter. Note how I typed "may", as opposed to "will".

If there is any possibility that the parts are not interchangeable, the new part needs a new number.


RE: Form, Fit & Function

You can create a new part/assy dash number for any reason you like. On the other hand, you can continue to use the same part/assy dash number after a change provided you maintain a sufficient level of effectivity/traceability. There can be several parts in service with the same dash number, but they might be different due to repair/rework. The way these changes are tracked is through configuration management. This usually involves detailed documentation like ECNs.

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