Engineering Procedures Manual

[49again]

We have been making some of our parts for our airplane mods (we have a GV that we use for atmospheric research) with such swoopy-curvy shapes that it is difficult/impossible to define with a paper drawing - we go directly from my SolidWorks model to the CAM software to the machining center. About the only thing I put on the drawing is "refer to 3D CAD model." I have a feeling that that's not going to be sufficient for the FAA DAR to be able to conform the parts. I have an FAA powerpoint presentation about "Acceptance of Digital Product Definition Data for Type Design" that talks about the FAA's ability to accept 3D models, but it make reference to setting up policies and procedures to insure integrity, etc., and establishing an "Engineering Procedures Manual."

So, I'm wondering, has anyone gone through such a process recently - set up such a Manual - that might be able to give me some pointers and/or guidelines I could follow to start setting this up.

I hope my request is clear enough. Any help appreciated....

Steve

 

[rb1957]

why not show an isometric view of the shape on paper, just to put something on paper. then i guess you have a controlled CAD file, maybe locked with a time/date stamp; this is the definition you want. then i guess you're going to send that definition to someone to make it, so you've got configuration control. then i guess the makers are going to verify that they've made your shape with a CCM inspection of the final part, proving that it conforms to your requirements.

i think there's a sufficient trail.

also, how critical is this shape ? if it doesn't conform to your profile, is there a safety issue ? is there a manufacturability issue (are you making lots; probably not) ?

good luck

 

[kontiki99]

I used to work with CNC systems as a student. I believe there are post machining CNC measuring systems.

After cutting the part, a different CNC system would use some type of probe (now maybe lasers) to dimensionally compare the finished product to the data file and produce a QA report.

 

[der8110]

Your request is clear and the posts touch on some of the elements, let's see if I can relate it to an endeavor that DIDN'T work too well (we never learn from things that go great) and wrap it up for you.

The problem at hand (complex shape) used to be handled with a "loft drawing", basically the contours at various stations, like topographical maps showing lines of constant elevation. These were used for cowlings, canopies, fairings, etc. and the parts were made on a tool. Of course the drawing says "make on tool xxx," but what to do in case the tool wears out. Or how to make the tool in the first place. The loft drawing was also used to make templates which can be used as go/no-go gages. Typically fine for the tolerances involved.

Nowadays, coordinate measuring machines like a Faro arm can be used.

The program mentioned was more ambitious than just 3D CAD, one of those "change the world" efforts. One early shortcoming was excluding certification personnel in the (thought) process. The powers that be had the same PowerPoint presentation, so they thought there was nothing else necessary to satisfy the FAA. Never mind one thing that has NOT changed is the definition of Type Design data, information "necessary to define the configuration and the design features of the product" and "information on dimensions, materials, and processes necessary to define the structural strength of the product". So tell me what a purple blob on a screen is supposed to signify.

How this all panned out was that everyone was going to get viewers, FAA included, but like your feeling is telling you, that was not enough. Basically, nothing conformable, no STC. An early suggestion (not adopted) was to include a few key dimensions, i.e., mating interfaces, so the DAR had SOMETHING to write down, and the stress analyst could at least recognize what needed to be done (not much anyway - primarily passenger convenience items). There's nothing that says you can't have a set of data for certification, and another set for manufacturing. Most companies do anyway, in the form of process routing sheets, travelers, work cards. Only trick is that these cannot be different than the certification data (thus the Engineering Procedures Manual!).

So, a few inspectors got trained in 3D CAD and running CMM's. Imagine having to teach SolidWorks to your DAR.

Advice would be to think of your downstream users. Your machinist only needs a toolpath, so he does not need a "drawing". Your DAR and DER do, so spend half an hour and throw a few key dimensions on whatever you eject out of the printer. Even better yet, talk to the stress analyst and see what he needs. You might be shocked on how little it actually is. Many dimensions and specifications are for planning purposes, not getting it certified.

As far as your first question regarding the Engineering Procedures Manual, yes, it's useful for things like how you ensure data integrity (meaning revision control). But every time you expect some new type of document to solve a problem, all you may have done is avoid the original issue. Do it only if it makes sense for the organization as a whole.

 

[wes616]

I know that a number of OEMs (boeing, airbus, dassault) are moving from drawing configuration mgmt to model based definition; I'm curious how this will downstream into us in the aftermarket world. AND - what does the FAA requre for MBD? Is there some published requirements that I've missed?

Wes C.
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[49again]

That's what I'm trying to pin down....published reqiurements. I've picked up bits and pieces of info from AC21-31, -35 & -36, but none really get down to the "meat and potatoes." What I was kind of hoping for was someone to step forward and say, "Our company just got done going through this. What WE did was X, then Y, then Z." Sort of a template I could use.

Maybe I'm trying to make this more complicated than it needs to be. For now, I'll do like der8110 and rb1957 suggested, just put a couple of views of the part on paper with some overall size dimensions. Hopefully, if the DAR sees a picture on paper that looks like the part he's holding in his hands, that'll be enough.

Thanks guys.

Steve

 

[ewh]

That's pretty much how we handle it. Overall dimensions, a few critical dimensions, some GD&T (such as profile and positional), datum point callouts, and a note stating that the included model file is the master definition.

 

[49again]

ewh -

Do you do anything to control that model file IAW guidelines in AC21-35?

 

[ewh]

I wish I knew! I am unfamiliar with the standard. We do have document control procedures in place to insure the integrity of released parts.

 

[rb1957]

like ewh, it's not my "ballywick"; but i expect your designers have a drawing release process so that the models aren't sitting on someone's drive, available for change. the key is to be able to point to a file and say that's the one (file name, time/date stamp).

 

[Sparweb]

Have you considered plotting a few section views at 1:1 scale, pasting them onto a board, and cutting a contour template? You can hold it up to the finished product and by eye see the gaps/bumps. The template must be clearly identified with the name, date, and revision of the CAD file that generated it.

I would avoid a full-blown EPM if you can include the templates as part of the approved STC data. Call out the inspection procedure and template identity on the fabrication / assembly drawing.

Also on the topic of templates, we obtained a damaged helicopter floor for one project, and still have it as the "template" for producing small parts that match all the existing holes.



Steven Fahey, CET

 

[FORD150HD]

maybe this is a stupid idea, but have you tried changing the background color to one which contrasts with the drawing color and then printed it?

 

[aerodog]

I can't speak to what the FAA might agree to, but aircraft parts are typically accepted by inspection departments for all the major companies simply by comparing the 3-D part model to the actual part. The X-Y-Z coordinate data can come from a CMM, Faro Arm or Faro Laser Tracker.

Since you have NC milling capability, you can also machine check templates for various stations, validate the templates back to the 3-D model, stamp reference data on them and declare them to be check tools.

If you want to take it one step further, connect the templates with all thread rod to the proper spacing and make a check fixture.

But in advance, I would tell the FAA what your plans are and get them to buy off on the approach.