Size of forming block; how big?

[Martyfeehan]

I'm building fuel tanks for my Wag 2+2. There aren't any drawings so I'm on my own. I built a wood mock-up that fits exactly how I want it when finished. I'll be welding the tank from 5052 .050 aluminum. I'm going to use a form block to shape the ends and baffles; basic rib construction. Problem is, how much undersized do I make the form block so it will produce the correct size tank? I know I need to subtract .100 for the thickness of the top and for the flange thickness I will be bending, how much more? Is the .100 enough or should I be closer to 3/16" all the way around?
Thanks,
Marty

[Aaron Novak]

I'm building fuel tanks for my Wag 2+2. There aren't any drawings so I'm on my own. I built a wood mock-up that fits exactly how I want it when finished. I'll be welding the tank from 5052 .050 aluminum. I'm going to use a form block to shape the ends and baffles; basic rib construction. Problem is, how much undersized do I make the form block so it will produce the correct size tank? I know I need to subtract .100 for the thickness of the top and for the flange thickness I will be bending, how much more? Is the .100 enough or should I be closer to 3/16" all the way around?
Thanks,
Marty

Hi Marty!
Experience has shown to add about .005" to the metal thickness for the form block dimensions. The metal naturally gets thicker there, plus you want it snug. Couple questions though and some hints that might help.

1-Why .050 5052? Thats way overkill for that size tank.
2-how are you going to retain the baffles?
3-Tig or OA for the welding?

Most of the Cub, T-craft etc wing tanks are in the .025" range of thickness and are made of 3003. 5052 does not form as easily, work hardens faster and cannot be brazed for sealing rivet heads etc. I know its popular now to tig over the rivet heads, but I have seen guys chase leaks doing that, plus you lose the clamp load on the rivet if you weld it, brazing does not. I know 5052 was the craze for some aerobatic guys years ago due to the increased strength, but when you weld it the material gets annealed anyway so its a moot point. Just a thought.

[Martyfeehan]

Hi Aaron,
As to material and thickness, I posted an EAA link below that details .050 and 5052. I will be welding with OA unless I can find someone to weld the tank for me. I have done a lot of practice and will be doing a bunch more before I decide to weld it my self. I did the short demo class at Airventure on O/A aluminum and bought the correct rod and flux. So far, my seams are ok. I haven't attempted to weld the thick flanges needed for outlets, sumps, etc so I'll have to see ho that goes. As to the baffles, couple of ideas. If I go with a 4 piece (sides, top and "U" shaped bottom, separate top than I could weld the baffles inside before welding on the top. Another idea is to cleco together the baffles and rosette weld the holes; I need to practice that first. I'm going to try forming the sides first; I have some .050 5052 on hand for that. I made my forms about 3/16" smaller all around for the forming block. I need the flange on the side, the top plate, and what ever slop comes in the bending. If necessary, I can make another form and make it bigger if necessary. Smaller to start is better I think. I'm going to try to bend a 3/16" flange around the form. I know I will need some cuts. I have attached a picture of the forming blocks so you can see the shape of the side of the tank. Having never done this before I have no idea how it will form but willing to try. Take a look at the form shape and let me know what you think.
Thanks,
Marty

EAA link: http://www.eaa.org/en/eaa/aviation-c...inum-fuel-tank

[Aaron Novak]

Hi,
Yeah Tony always liked thick tanks, I think he figured they were easier to weld. Production tanks were always much thinner and never have been a problem. His material choices are for Av fuel only though. Iv you ever want to run auto fuel you need to stick to 5052 or 3003/1100 as the others will be subject to corrosion. Your form blocks look nice, to help them live you can soak up the edges in thinned down epoxy. I think this is where the heavy 5052 is going to be a pain though, you really do not want to make any pie slices out of your flange, and you shouldnt have to. For sure you will need to anneal the material before making the flange, maybe more than once in the process. For the tank bung hole flares you will want to anneal as well before you flare it. When it comes time to weld them preheat the bung then move the flame over and make the weld around the flange. When you make the flares, make the bungs a snug fit. Good choice on the OA, you wont regret it!

[Martyfeehan]

Good progress made. I made an oak form and made my first side piece. It came out very nice but was a bit undersized. I spoke to a welder after making the form and got a better idea of correct size for the end piece. So, I made another oak form (easy, no big deal) and formed another side piece. The forming went off without a hitch, no cracking and very smooth. I also did some more mock up work with fittings and such for final size and fit. I'll be taking these to the welder next week and go from there. So far, everything going great. Here's some pictures.

Marty

[Aaron Novak]

[QUOTE=Martyfeehan;58167]Good progress made. I made an oak form and made my first side piece. It came out very nice but was a bit undersized. I spoke to a welder after making the form and got a better idea of correct size for the end piece. So, I made another oak form (easy, no big deal) and formed another side piece. The forming went off without a hitch, no cracking and very smooth. I also did some more mock up work with fittings and such for final size and fit. I'll be taking these to the welder next week and go from there. So far, everything going great. Here's some pictures.

Marty


]/QUOTE]

Looking Nice, The radius you used helps quite a bit in forming. What bung fittings are you using? The standard AN867 weld flange?
-Aaron

[Martyfeehan]

Yeah, standard bung fittings from Spruce and from Wag Aero. I have a welder who I'm going to talk to next week. He is old school fabricator who has been at this for 40+ years; gas welds the tanks and heilarc's on the bungs after tank is together. Made contact from a Bonneville racer neighbor(world record holder, fly's gyro's now) who knows this guy. He works a lot on aircraft tanks and gave me hints on design that I used to change the pattern some. He builds from .060 3003 vs .050 5052 but can do .050 if I get the material for him. The weight would be a couple pounds more for .060 but tank sure would be strong. I'll see what he has to say next week. Mock-up now is exactly as needed and the side piece I formed came out exactly to size and shape. I really liked the forming process.
Marty

[Aaron Novak]

Yeah, standard bung fittings from Spruce and from Wag Aero. I have a welder who I'm going to talk to next week. He is old school fabricator who has been at this for 40+ years; gas welds the tanks and heilarc's on the bungs after tank is together. Made contact from a Bonneville racer neighbor(world record holder, fly's gyro's now) who knows this guy. He works a lot on aircraft tanks and gave me hints on design that I used to change the pattern some. He builds from .060 3003 vs .050 5052 but can do .050 if I get the material for him. The weight would be a couple pounds more for .060 but tank sure would be strong. I'll see what he has to say next week. Mock-up now is exactly as needed and the side piece I formed came out exactly to size and shape. I really liked the forming process.
Marty

Hi Marty,
Im curious if he flanges them like they were designed to be used or not. .060 3003 is WAY overkill for that size. I personally would not go over .040 considering most originals are in the .025 range. But to each their own, it will just be heavy. Keep us posted.

[martymayes]

Like your version of SOLIDWORKS Marty!

[martymayes]

Question: Will your tank support the wing fabric on the cambered (top) side of the wing? Or do you plan on installing a false capstrip over the fuel tank area? Just curious - it's been my experience that otherwise the fabric will be flat in that area once tautened. Or maybe the picture is throwing my scale off and the tank was designed to fit between to normally spaced ribs?