Aluminum Plate Engineering/Forming Question

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Victor Bravo, Sep 16, 2019.

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1. Sep 16, 2019

Victor Bravo

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I am hoping that someone will be kind enough to share a spoonful of formal education and/or technical knowledge with me.

I need to form a pice of 3/4" or 7/8" thick 7075-T6 aluminum plate. The piece will be aout 3.5 to 4 inches wide. Yes, it's for a landing gear. Yes I already know that Grove Aircraft makes landing gears.

I already know that 7075 aluminum, particularly in the tempered (T6) condition, really prefers not to be bent.

But I want to bend it anyway, and I want to bend it in the T6 condition (time, cost).

I have seen some "minimum bend radius" charts that (apparently) show 7075-T6 CAN be bent, but you have to increase the bend radius quite a bit. Those bend radius charts all seem to stop at .190 and .250 sheet thickness, and something like 7-10T min. bend radius.

My question is: If I want to bend a .750 or .875 thick plate that is already in the T6 condition, just how large does the minimum bend radius be, in order to not have structural difficulties with the metal ?

12T, 14T, 20T???

2. Sep 16, 2019

FritzW

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You know you can just buy the gear from Grove's because 7075 T6 is hard to bend

>>>

Not that it'd necessarily give you the right answer but... can you project what the bend radius would be from the charts you have? ie. plot a non-linear curve
I suspect it would look like half a smiley face and the radius would be bigger than your gear before you got to 3/4" thickness.

3. Sep 16, 2019

Geraldc

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4. Sep 16, 2019

Victor Bravo

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If it makes the whole thing possible to do, I am willing to have it follow one large radius from the side of the fuselage all the way down to the bolt-on axle, making it a "hoop" gear. The airplane this is going onto cannot be "cosmetically rehabilitated" under any circumstances anyway

5. Sep 16, 2019

TFF

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For the poor, that’s why you make gear out of 4130. It can be done in shop.

Forget bend radius, do you have a 500 ton press? You are not bending it that thick without one. If you do or have access to that kind of beast, someone will know how tight it can be bent. Did you know Grove can make gear for you?

And as an aside, I would not want to be bending anything that thick personality. I worked next to a shop that had a 200 ton press. It killed two people when the part shattered and went through their chest. Your not stopping that beast. If the part is in there wrong, someone is getting hurt.

6. Sep 16, 2019

Geraldc

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At radius of 10 times thickness bend pressure should be approx 52 tons per foot for steel so 53 * 83ksi(7075t6) /60ksi(steel)
so about 73 tons per foot width = in the region of 20 ton for your width.

7. Sep 16, 2019

wsimpso1

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Plastic strain can be computed for the outer and inner fibers for any given bend radius. The worst strain has to be significantly less than the max plastic strain at failure of the metal.

In three point bending, you can form the metal cold in little tiny steps and work your way around the long curve.

Having bought my 5/8" thick 4340 gear leg blanks bent by a drag racer shop, and then had them heat treated, I can tell you that it is all doable. See my build log on this.

Billski

8. Sep 16, 2019

wsimpso1

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OK, went upstairs, set up the math, looked up the data. 7075-T6 has 73 ksi yield and 11% max strain at failure.

A 30 inch bend radii on 7/8" stock requires 1.5% deformation, so you are well within the 11% max.

4" wide, 7/8" thick will require a bending moment of 37,000 in-lb. In three-point bending with 4" between the supports, that is 37,000 lb. A 20 ton press can just barely do it. That will take a lot of little strokes to do the whole thing. Similar forces are required to run in four-point bending with 2" of overhang of the outer rollers from the inner rollers.

My biggest concern in all of this is not in "can you bend it?" - you can. My concerns have to do with it lasting:
• These legs will have a bunch of residual stress in them that is likely to find a way to relax under load and operating cycles - you can anticipate that and put a somewhat smaller radii on the legs than you can live with, and when they settle the stresses a little and sag, you will still be ok. Heat treat after forming prevents this;
• The residual stresses and deformation strains may be locally much higher than the gross strain in the immediate area of tool contact (bending marks), and thus your parts could be closer to failure on a rough landing. Minimize these effects by making your tooling with four-point bending and with large radii;
• You will have local marks from bending that you either leave in and can serve as crack starters, or polish out and maybe can still serve as crack starters. Polish out bending marks, heat treat, and blast after forming prevents these issues.
I do not know about SoCal, but in metro Detroit, we have shops that do bending (rings, arches, etc) of big aluminum and steel structural shapes for architectural stuff for decent money, including annealing services. As for heat treat, I only paid a couple hundred dollars for the quench and temper work on my steel legs. I gotta believe that someone offers such services on a fill-in basis somewhere around SoCal.

https://www.homebuiltairplanes.com/forums/threads/billskis-fiberglass-bird.8344/ post 17 for the gear.

Whatever route you take on this, good luck and be sure to report back on how it all worked out.

Billski

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9. Sep 16, 2019

Winginitt

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Why not contact Grove and see if they will custom bend something to your specification. Then you get the benefit of their experience and one stop shopping.

10. Sep 16, 2019

Victor Bravo

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I appreciate the responses. The question I was asking is how large of a bend radius do I need in order to NOT have any stress and longevity issues.

I am pleasantly surprised that only 74.3 percent of the respondents cheerfully suggested something that I had already addressed in the OP (Grove).

I am thinking of having it done in a plate roller, not a 3 point press brake.

My question is with this thickness, what radius will work without putting cracks, stress risers, or other structural problems in COLD BEND metal.I know it will be a large radius, but how large is enough?

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11. Sep 16, 2019

FritzW

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You can't count mine, I was obviously joking.

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12. Sep 16, 2019

lakeracer69

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Getting ready to do this as soon as I finish welding up my jig. 2024-T3 .750 thick. Inside bend radius in the neighborhood of 4.5 times the part thickness. Using a 20 ton press. Doing it pretty much like post #3 link.

For everyone who parrots "Grove, Grove, Grove", I don't think are not interested in making a one off part for you. I just recently called them, and asked the gum chewing girl on the phone if they could make me some radius plates one inch longer than they sell.
Her reply was no, I asked again if she could ask someone and her reply was no they aren't interested in doing it.

Ended up making my own. On a 3-axis CNC I would only have to change one line of code to accommodate that. Unfortunately, I don't have one and had to do it the old fashioned way. Poor customer service in this case, absolutely! Maybe to bend up a $2000.00 set of gear they might move on it, I don't know. I do know that the material I bought was around$375.00 shipped and worth it to give bending it up a shot.

YMMV

Last edited: Sep 16, 2019
13. Sep 16, 2019

Victor Bravo

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Robbie Grove is a good friend and does top quality work. Absolutely first class. The budget for this project just cannot support that at this moment. My fault, not theirs. So I am trying to figure out a work-around that is acceptable on this cosmetically challenged aircraft.

14. Sep 16, 2019

wsimpso1

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A plate roller does put the plate in three-point bending and then rolls that along the part. That is perhaps the ideal way to do a continuous arch.

If you do go for a continuous arch, direct clamping probably won't work. Take a good look at the Vari-EZ/Long EZ/Defiant/Cozy arrangement. The arched gear flattens out somewhat during landing, which can put big spreading loads on the fuselage. The successful airplanes using an arch gear use a saddle clamped or bonded to the leg, with a pivot pin axle above the leg. Pick the spot that does not move during gear deflection for the pivot points...

Billski

Last edited: Sep 16, 2019
15. Sep 16, 2019

wsimpso1

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When I was considering aluminum gear, Robbie told me to send a print and that he does custom gear legs and associated hardware. For a gear leg job, yeah, i imagine he would make rocker plates too. But just the plates?

Billski

16. Sep 16, 2019

flyboy2160

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Their quote to me for a custom set with their customary 'small' radii was 3X to 4X the cost of a similar OTS set. This seems like a very reasonable hike for custom work. But With the closest OTS set at ~\$2000+, I went with flat, water jet cut Ti main legs.

17. Sep 16, 2019

flyboy2160

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Why not make the legs from composite?

18. Sep 16, 2019

Victor Bravo

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Would certainly consider it, who do you recommend as a fabricator ? I even have a roll of carbon tow.

19. Sep 16, 2019

flyboy2160

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I recommend......................................YOU! If I can't warm form my Ti nose leg, I'm going to RTM it with glass, NOT carbon. I want the security of through the thickness fibers [edit:for a gear leg.] I haven't yet found anybody with an OTS woven, tufted, or stitched preform, so I'm planning on hand stitching the dry preform myself.

Last edited: Sep 16, 2019
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20. Sep 16, 2019

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