CAD part numbering across parts and assemblies

[adamlunar]

I scoured the forum extensively for discussion of this topic but found nothing, so please point to prior topics if this has been covered (or a better location for the discussion as it seems to cross a couple areas).

I work at a design consulting firm and our CAD standards team is attempting to codify some of our best practices.
I'll paint the scene then get into the problem.

I'm designing a sheet metal part, and in its pre-production incarnation it's got some inserts -- PEM nuts, pins, etc -- so in CAD the "part" is actually a CAD assembly though if I were to order one it would arrive as a single piece with one number stamped on it. As the product makes its way into production, we'll cost reduce by stripping out all or most of the inserts and replace with formed features. In CAD, the raw sheet metal part is assigned a number, and one level up, the assembly is also assigned a number.

The question is: should the CAD part and CAD assembly be given the SAME number? (For example "xx-2-0001.prt" and "xx-2-0001.asm") Or to ask it a different way, why should this NOT be the case?

I think I've heard most arguments on both sides of this, and i realize neither is risk-free. My goal is to identify a system that's a balance between idiot-proof-ness and flexibility. I use the sheet metal example but the same issue applies to injection molded parts with inserts and other manufacturing methods that include multiple components that define a fabbed and purchased part.

The real issue what happens when the part+inserts becomes just a part. Do we stop using the .asm? What happens to the BOM? Should the PN reported there also change? There are CAD implicaitons as well, since I may end up having to recreate the drawing as the views, dimensions, notes, etc will not transfer along with the switch from .asm to .prt. How will the vendor handle this change? Will it show up as a completely new part? The new part is functionally identical to the old one, but a new PN may mean scrapping older, yet perfectly serviceable parts. There are more downstream implications, I'm sure.

Adam

 

[ctopher]

I would make it an assembly with BOM. Call it an assembly. How it's ordered should not affect how it's modeled. It's not a 'part', but one of your 'products'.
Changing it from an assembly to a part would mean that you would have to maintain both. Not efficient.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
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[MadMango]

In your case, I would model it as an assembly, and give it the same number since you are only concerned with the purchased "component" that just happens to be an assembly.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[fcsuper]

At my company, we use the same part number for all components of inseparable assemblies and use that part number as the file name for both component and assembly level; with the components having added descriptors after the part number when necessary.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ewh]

Since this is is in development, I would give the naked sheet metal piece a dash number, and use the base number for the assembly. This will allow people to know just what part is being refered to (purchasing, for example).
When the assy is developed further, you can still use the base number without dash numbers, provided a documentation trail is left and form, fit and function are not changed. Some may argue that the form has changed if previously assembled parts are now incorporated into the final part, but if the final version is completely interchangable with the development version, I think your covered.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

adamlunar - we experience similar situations with inseperable assemblies like yours. Ours is compounded by the fact that our doc system can't handle dash numbers.

The decision we made is that as we only ever buy in the completed assembly and don't do it our selves only the final assy would be given a part number.

Other components are refered to by the manufacturers PN and for the custom sheet metal part it has no PN as such, just the item number on the drawing & entry in the parts list - this isn't quite ASME compliant but close enough for us. As for file naming, the components if not standard parts are made 'assembly reference' they have the same file name as the assy but with -AR1, -AR2 etc added on the end.

As to your issue of going from assy to part in CAD we do have some assembly files which now only have a single part, we normally leave them as such since the effort to recreate the drawing to be fed just from the part can't normally be justified.

On whether to change PN, as mentioned by ewh the important thing is are the new and old part fully interchangeable, this is the important thing more so than 'form, fit, function' which are just areas to assess when considering interchangeability.

Also I dont' see why the new PN would neccesarily mean scrapping existing good parts. We have the option on ECO's to disposition 'use up stock'. So they don't cut in the new part till they've use up the old, can you do similar?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[pierdesign]

in my years as a company for hire I have worked with numerous companies and found not one implements part number schema the same.

I have seen so many different ways to name and number parts and assemblies it never ceases to amaze me.

The conundrum is that the every uniqe numbering scheme is always representative of the same thing: parts in development, production, bom parts, purchased, customized, multi part assemblies, revisions, etc.

So, when someone says, we use xxxxxx-001, -002 so people will understand..... No one will understand without you explaining your company's interpretation of -001, etc. as I guarantee you the next company that uses -001 interprets it differently or they use -ar2, -bom, to mean the same as you.

I thought that the introduction of linked PDM systems would address this parochial practice. Turns out every companies implementation of their chosen PDM system is customized and no one company uses the same PDM part, assembly, product numbering nomenclature.

Second job out of college I worked briefly with an old semi retired engineer. We carpooled and one night driving home he expounded on this subject as I had been drafting a D-size sheet part table with part numbers and one changing dimension. He was beating the steering wheel saying "The only part numbering system that will work over time with fewest errors is a sequential numbering system, no prefixes, no suffixes, etc. Every other scheme will fail or come up against situations requiring extreme work around." I tend to think he was right, not that it may be an ideal solution.

From my experience there is no best practice in the area of part numbering and configuration management.

 

[KENAT]

Pierdesign, the general issue of part numbers has come up before and current opinion is generally that a dumb system, as your old colleague says, is preferable.

However, that isn't really the OP issue. Also even if dumb part numbering is used, how that translates into CAD may still vary.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[drawoh]

adamlunar,

What does your company plan to do with the information?

At my place, we subcontract all fabrication. Our fabricators order PEM fasteners, not us. Any time I design a fabricated part that may turn out to be more than one piece, I define the model as an assembly. Any aluminium part may require thread inserts, for example.

At your place, you may have to order and stock PEM nuts, or helical thread inserts or whatever. In that case, your sheet metal drawing describes an assembly, with a procurement process attached to it.

If the sheet metal thing is an assembly, you will have to decide whether or not to do a separate fabrication drawing for the piece of sheet metal.

JHG

 

[adamlunar]

Thanks for all the responses so far. As KENAT points out, the issue isn't whether or not I model as a part vs assembly or how the numbering scheme works, but rather downstream issues if a CAD part and assembly file have the same number.

Since we're a design consultancy, we have to take into account any number of inventory management schemes and basically support whatever scheme our various clients have or would like to implement. Hence our desire for general idiot-proof-ness Our direct involvement in the process is usually limited to prototype vendor management and any confusion is easily resolved with a quick phone call.

I raise the possibility of scrapping parts as that's one outcome I can imaging if the scheme is not well thought out, and it's almost certainly a corner case.

There are two decent options we hadn't considered strongly before: add a dash number or descriptor to signify a sub-component, or leave the PN off the naked part completely. We'll hash these through at the next meeting. Thanks again all!

Adam

 

[MadMango]

You haven't mentioned if you are working with a PDM system, as that may be the deciding factor on how to assign part numbers in the long run.

I worked in a sheet metal fabricator for many years. We always assigned the same part number to the CAD files for parts or assemblies. The major difference being that we did everything in-house, so for us (on the floor) it was easier to track if everything was under the same identifier.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[adamlunar]

@MadMango:

Again, since we're on the services end, we only use PDM internally on some programs, another reason the solution should be agnostic in that respect as well.

The good news is that any client with an existing PDM system in place already has a naming / numbering scheme that we're required to respect, so this is less of an issue. In that case we usually stick with descriptive names until the design is feature complete, then change all the file names in one go according to the PNs the client has issued.

Adam

 

[PeterStock]

One company I worked for in the past had a policy to create sheet metal and plastic parts as assemblies in case inserts were needed. This was to inclued those that did not need them at creation as it might be needed later. This enabled the addition of inserts to be done without requiring the recreation of the drawing.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services