Straightening Formed Aluminum Parts 3

[Neubaten]

Hi,

So I'm reviewing our workshop procedures, and I've found a problem that may seem trivial, but I still haven't found no easy solution.

Normally when assembling sheet metal for aerostructures, some of the parts need a subtle straightening for a correct mating, or to correct a small concavity, etc...

We do it by hot or cold forming, depending on the alloy, size of defect, etc...

We have stablished fixed parameters for defining the repairs & acceptability limits BUT they are based on other customer's standards and our previous knowledge on shop practices & repairs, and I haven't found the way to justify by analysis or literature those values for my customer (which does not have any specific standard/procedure for this purpose).

Do you guys know of any guide or standard (FAA, MIL, AIAA, whatever...) where I can check acceptable shop procedures for hot & cold straightening of aluminum sheets for aerostructures, or any basis for the justification of acceptable procedures in previously formed sheet metal?

Any help would be greatly appreciated.

 

[AviatorJim]

If the part needs anything more the hand pressure and draw clecos to pull it into position for I confirm that the part is in error, and not the structure it is going into, and then I send it back. And rework that is needed beyond trim to fit is treated as a repair, even if the part is new. Stamped parts rarely come as drawn, and the more complicated the forming needed to make the part, the more twist and flex they seem to have prior to being secured.
..
I also don't do production. I am a repair shop.

 

[berkshire]

Most of the Major Manufacturers have process specifications detailing the methods used to hand straighten parts to acceptable limits following heat treatment or manufacturing operations. These spec's are usually part of the type certificate process specifications and are proprietary.
Although they do follow standard metal working practice.
B.E.

 

[Neubaten]

AviatorJ, yes those parts are treated like repairs (under our MRB authority). We have the problem mainly with extrusions and sheets, though.

Berkshire, yes that is my exact problem. But one of our customer does not have an specific process spec... As I told we know exactly the repair practices and acceptability limits, but we need to reference or justify those practices using non-propietary standards. And it is a pain in the lower back because it is not so easy as it seems!

 

[berkshire]

Neubaten
Can you write your own process specification and incorporate it as part of your repair station manual?
B.E.

 

[plasgears]

'Metal finishing' is what we called routine straightening of prototype parts. In production, this is usually not encountered. However, some of my aerodynamic vane castings were 'rounded' prior to welding to sheet metal sections. this was followed by HT and finish machining of mating surfaces.

 

[WKTaylor]

NeuBaten...

1. Parts should be designed for optimum "fit" +/-manufacturing tolerances. The tolerances should allow for dimensional and angularity discrepancies inherent in sheet metal so that minor hand-pressure brings adequate alignment with only low press-stress.

Typical formed metal tolerances for aluminum or magnesium are +/- 0.032-inch [+/-0.8-mm] and +/-2-degrees .

Typical formed metal tolerances for steel, Ti & CRES are +/- 0.016-inch [+/-0.4-mm] and +/-1-degree (due to significantly higher stiffness/strength)

Typical machined metal tolerances are +/- 0.010-inch [+/-0.7-mm] and +/-0.5-degrees due to stiffness and machining/inspection capability.

NOTE. Whatever Your design tolerances, the shop MUST make all parts within the specified tolerances, or at least as close as conventional inspection will allow. Having widely variable parts is a nightmare and leads to great inconsistencies in Assy... and lots of cussing.

CAUTION [Lesson Learned]. Detail parts for multi-aircraft modifications MAY require careful on-aircraft measurement to determine exactly how the original acft structure was assembled (including production-blocks and/or year-models].. relative to the "precision" drawings. Serious variations have been noted and MUST be accounted for!!!! In some cases reality is NOT-EVEN remotely close to the drawing configuration... and-or other factors are present [repairs, other modifications, poor original Assy or detail parts dimensional controls for replacement parts/Assys, wear-and-tear, etc].

CAUTION [LL]: The same (as above) may be said for "simple" or complex modifications/repairs, for a single aircraft depending on how critical details need to be made. In some cases details should be made from splash/contour molds instead of drawings... and sized accordingly. I have used cartoons and mandatory splash molds of the real structure for years ILO precision drawings.

NOTE. Primer/paint finishes may change the fit of tightly fitted parts... that were sized bare.

NOTE. Raw sheet-metal thickness should be verified before fabrication. Nothing worse than forming a part from the WRONG thickness sheet... and/or a sheet that is on the thin or thick side of tolerances.

NOTE. In older aircraft, sheet metal tolerances could be substantially different than today's standard thicknesses/tolerances. For instance, 0.056" +/-0.003 was a standard thickness... hard or impossible to find today. The 0.050 or 0.063 [+/-0.002”] substitute may prove challenging to fit, if this thickness variable is NOT properly considered when fabricating off of a drawing or an old/damaged sample part.

2. It is then incumbent on the assembly technician to ensure tight fit of the details in the fixture and jigs... and/or the partially assembled aircraft structure [in-the case of modifications and repair installations]. If a good tight fit occurs with gentle hand pressure, then most sheet metal will "adjust" with low pre-stress. If excessive pressure to distort for fit [push/pull/twist/bend] is required, then You have limited options.

(a) Modify the detail for a better fit in the exact place [aircraft] where it will be installed: IE: trim, sand, grind, re-shape, etc. [this option may be combined with option (c), if/as needed]

(b) Make a new detail for a precision fit in the exact place [aircraft] where it will be installed. [this option may be combined with option (c), if/as needed]

(c) Shim as needed and as allowed by engineering drawing/data/policy (may be tricky, if NOT defined by data or a spec or one-off engineering analysis for fastener bending). Shims may be made from solid metal, laminated foil sheets [peel-ply shims], laminated composite materials [pre-cured or uncured pre-preg] or fiber/metal-particle reinforced paste adhesives. The solid materials will have to be hand-trimmed and tapered for an exact fit in gaps. The paste-adhesive shim material must be applied to fill smaller gaps on Assy.

NOTE. Shims only work for GAPS (duhhhh) and the mating details cannot obviously fit in interference. In some instances, details may have to be deliberately trimmed-back for "best-fit"... and then shimmed in the resulting gaps for a tight structural fit.

CAUTION. Thin "floating shims" are OK if used carefully. Thicker shims impose significant bending loads on fasteners and transfer shear-loads thru the laminate so they MUST be analyzed by stress engineers. In some cases structural shims may be used that actually fasten other places away from the joint to assure proper load transfer (and hands-free installation).

NOTE. Moldable [pre-preg and/or paste adhesive] shims allow some freedom to get a precision fit and gap-fill with little effort. However, these shims are typically softer/less stiff than metal shims and may squeeze-out or crush-out in high interference or high-load situations. In some cases these materials can be directly bonded to a member [flange/web/etc] before-hand and… molded/trimmed to fit… then cured outside of the joint against one of the mating parts.


Regards, Wil Taylor

 

[Neubaten]

WK, what a great advice. It goes beyond what I expected, many many thanks.


But what about sheet metal formed parts or extrusions that undergo an unexpected deformation (hit, fall to the ground, tool mark, etc...) before final assembly?

I think it is acceptable (if the damaged zone has small radii, isn't too deep, etc...) and a spread practice, to repair them by heating & straightening the part (under careful time&temperature control) or cold forming the part, what is your opininon on this repairs?

 

[berkshire]

Neubaten

"" But what about sheet metal formed parts or extrusions that undergo an unexpected deformation (hit, fall to the ground, tool mark, etc...) before final assembly? ""

You now have a non conforming part which "should" follow the inspection trail for MRB. The potential problems arise with what happens next. Is the part reworked and reinspected to a conformity standard. Or does the AMT pick it up off the floor. Beat on it with a hammer and stick it on the airplane?
B.E.

 

[WKTaylor]

Neubaten...

Berkshire hit the nail on the head. Sheet metal fabrication, handling, installation, etc discrepancies (however they occur) should be dealt with by MRB disposition [typically after QA, shop and liaison engineering review].

Boeing, Lockheed, Piper, Cessna, etc parts fabrication specs (or manuals/handbooks) provide clear detail as to what is acceptable practice: however, anything outside of these guidelines becomes an engineering disposition.

These large companies have spent a tremendous effort to provide guidelines for fabricating, handling and installing airworthy "sheet metal parts". If discrepancies occur, MRB resolves the problem.

NOTE: The FAA has learned the hard way there are hundreds of obvious and subtle ways to screw-up a single part... and there are fairly narrow ways to do a job in an "airworthy" manner.

AC43-18 SUBJECT: FABRICATION OF AIRCRAFT PARTS BY MAINTENANCE PERSONNEL [

http://www.airweb.faa.gov/Regulator...c5ce163ff0f862571470072833f/$FILE/AC43-18.pdf

] provides stark explanation as to how serious the FAA takes even 'trivial detail parts'.

NOTE. I discovered for myself how easy it is to screw up even simple sheet metal parts, while helping my dad build his Thorp T-18 (as a kid in the late '60's and early '70s). Since the penalty for poor workmanship could be death, my Dad [licensed pilot and mechanic... and the one ultimately responsible for the aircraft] was my tough QA/conscience: he would NOT settle for sloppy workmanship on anyone's part.

Further FAA Regs place legal responsibility squarely on the shoulders of the last individual to handle/install/sign-off-on installed parts. The mechanic must be convinced the part is legally airworthy and is installed with every "good [airworthy] maintenance practice" established for the aeronautical equipment" [IE: by aircraft or component maintenance manuals, etc].


Regards, Wil Taylor

 

[Neubaten]

Yep, I understand that. As I said that before, we treat those situations under MRB dispositions, what I wanted is exactly to justify acceptability limits for defects to be repaired and acceptable repair procedures under those circumstances.

Anyway, a little bit of safety remindering is never wrong, specially in this field...

 

[GlennE928]

You should be directing your questions to the aircraft manufacturer - the holder of the airworthiness certificate - either by reference to the aircraft's approved structural repair manual or directly.

As wktylor has warned, the FAA does not consider structural repair issues trivial.