Grain direction in bending sheet metal 1

[JQCF1]

Has anybody heard of bending sheet stock (.063 5052-H32 Aluminum)with grain direction on the diagonal? Local sheet metal shop (not an aircraft shop) experienced cracking on folding up a simple bracket. The bracket is like an upside down 6"long x 4"wide x 2"high box but has integral attaching flanges on the two 4" ends. I say with a zero bend radius (not 2-3t) and no concern for grain direction they were asking for cracks and failure even with this material.
I have never ever heard of a spec calling for diagonal grain direction.

What say you??

 

[desertfox]

Hi JQCF1

I agree with a very sharp bend radius you would probably experience cracking.
Have a look at this site:-

http://archive.metalformingmagazine.com/1999/11/Science.pdf

regards

desertfox

 

[MechanicalChef]

If the bends are occuring along the 0/90 directions, and the material experiences cracking along the grains (created through rolling), it could be worth rotating the parent sheet 45 degrees. Hence, bending across the grain-diagonals. If it were just a single bend, rotating it 90 degrees, like shown in the .pdf would work also.

 

[737eng]

A few years back I attended a short sheet metal course to actually experience how to bend up sheet metal, install rivets, hi-loks, trim out damage, etc.... During that class the instructor emphasized to bend the sheet 45 deg to the grain. When I asked him why he emphasized this 45 deg bend, he told me that you get the best of both worlds, higher strength in the axial direction and less likely to crack across the radii. He told me that when he was taught years and years ago, they were taught to bend perpendicular to the grain or 90 deg to the grain and this was always shop practice, but an Engineer later critized him for this because the Engineer was basing his design on the "L" direction and the load was now going to be in the "LT" direction. This got me thinking that I did base all my calculations on the allowables for the "L" direction in the MMPDS (back then Mil-Hdbk-5) and I did agree that good shop practice would be to bend it at a 90 deg. I did a check and found for most aluminums that the L and LT direction allowables were very close, however, some materials (301 1/2H comes to mind) is very dependent on grain direction, especially for Fcy due to the rolling process effect on the material. Anyway, I have since left that company and I think that instructor has retired, so I don't know what they are teaching in that particular class now. For your issue, I feel that the bend radii is your issue, in the H32 condition, your min bend radii should be around 0.12", you could probably get away with a 1t bend radii if you bend it in the "O" condition.

 

[rb1957]

I'm a bit suprised about using 5052 ... in lieu of 2024, 7075 ... and the previous poster is right about the bend rad, I'd specify 3t for a shop not used to aircraft parts

 

[berkshire]

5052 is a pretty soft alloy and if the part is air bent there should be no cracking problems either with the grain or across it.
You will experience cracking problems if the bends are coined with a zero inside radius.
B.E.

 

[WKTaylor]

JQCF1...

My ref handbook calls for a minimum 0.12-BR for 0.063 thick 5052-H34 (close to H32). This may be tweeked downward (maybe 0.09-BR) if bent 90+/-10 to the sheet grain... and the following is observed...

NOTE: grain is the rolling direction which elongates grains in the same direction. When bent transverse to the grain, long edges of grain boundaries are exposed to high strain with high potential for crack initiation.

Scratches and sharp/burred edges contribute to crack initiation when bending to severe radii. Polishing and radiusing the sheet metal edges and blending-out scratches or surface roughness will virtually eliminate cracking when bent over a SMOOTH radius. CAUTION: burrs, gouges, nicks, chips, scratches, roughness, corrosion, etc on the sheet-metal brake edge radius also need to be eliminated.

OLD sheet-metal guy trick...

Form a 0.032 thick sheet of "O" aluminum sheet [or annealed CRES 3XXX sheet] TIGHLY around the "sharp" edge of the brake (usually sharp edges on brakes are actually a very small polished radius [R~0.016]... or the "point" is squared-off [truncated]).

Lightly oil the outer surface of the bent sheet... and form ANOTHER layer of 0.032 OVER the base sheet. Repeat this "stack forming process", until the total thickness of the stack is "equal-to, or slightly higher than" the required Bend Radius. Form the production part around the sheet metal stack. This "back pocket trick" allows use of a single "sharp-edge" brake blade...for precision bending of "tin sheet metal [~0.080+/+ depending on sheet metal yield strength BR rqrmnts.

CAUTION: There are limitations to this "trick"... and every time the radius stack thickness is increased, then small adjustments to the fixed-blade-edge position, relative to the forming flange of the brake (the rotating surface that forces the sheet metal to bend around the "fixed-blade-edge" of the brake) need to be accomplished by the metal tech, to avoid binding.

NOTE: an acft manufacturer I used-to work for had a special v-knife-brake die for forming the trailing edges of the RH/LH elevator and rudder skin trailing edges [same part all (3) places]. Somehow it got slightly bent... and all the parts coming off the tool had a barely perceptable bowed trailing edge which was determined to be aero- & structure- wise acceptable "as-is"... but when You knew what to look for it was really annoyingly apparent.

Regards, Wil Taylor

 

[JQCF1]

To Wil, BE, Desertfox et al,
Great comments and I still stand my ground that the old school is best. Use 2-3t BR and bends that you want to survive to be transverse to rolling/grain direction. I had to teach these guys that the world uses inside BR (not outside) in callouts on dwgs. For example set-back calcs would be little confused if one used outside BR.
Wil, as you and I both know the T-18 is a great sheet-metal learning exercise. Thorp knew the rules and used them to the nth degree!! Thanks to all.
Regards,John Cragin