Plywood form blocks were made for the end plates. I bought a piece of 1/2" hardwood plywood for the actual form blocks and used some scrap 1/2" plywood for the backing blocks. The form blocks have a 1/16" radius on the bend corner. I used the belt sander to put a 5 degree spring back relief angle on the edge.
The blocks are made undersized to allow for the end plate thickness, in this case 0.040" thick.
I did all of this twice. I started with a plan to use 0.040" 5052-H32 aluminum but could never get good at welding it. I still think I can learn to weld it. I finally gave up and remade everything with 0.040" 3003-H14 aluminum. The guys making cool aluminum bodies use 3003 because it's easy to work with.
I put a 1/2" lip on the plates even though I don't want that wide of a lip for welding, you'll see later. To lay it out I just used a pencil and a buffing pad from my Dremel Tool. It was quick and it worked. I found I only wanted a 1/4" lip at the 2 tips so I used a smaller pad there when I made the parts the second time.
The aluminum gets clamped between the 2 blocks so it's centered well. The aluminum is soft enough it doesn't take a lot of clamps. The backing block allows you to form a nice tight square bend.
As you start forming the aluminum with a rubber mallet it gets quite wrinkled. Just form the wrinkles into fairly distinct wrinkles and then hammer them down with a plastic or wood mallet. You're shrinking the aluminum to form a smooth flange.
The rubber mallet works great on the inside bends at the 2 bottom corners.
I probably could have used a slightly flatter spring back angle since the aluminum had a fairly tight grip on the block. It did come off without bending it. At the same time, I would rather have it over bent slightly then under bent so the skin fits tightly when trimmed for welding.
The skin pattern was used to layout the skin. I made the seam offset to one side to allow an access panel at the filler neck. I reused the panel I made for the old tank. It worked well to allow access if you needed to repair a leak, etc.
Because the top is sloped to fit the cowling there is some curve to the edges of the skin.
I try to avoid using tools for their intended purpose. I'm bending the end curves around the tubes on the ShopSmith. It worked perfect.
The skin is clamped to the end plates with straps so that holes can be punched to Cleco clamp the parts together.
I don't plan to rivet the thank together. The Clecos are just to hold it together while I tack weld it. I plan to nibble away the aluminum between the Clecos. Then I'll tack weld between the tabs with the Clecos. Once it's all tacked I'll snip off the tabs and put a tack weld where each tab was. I don't know how else to hold it all aligned while tacking it without a jig. The straps held because there was a wide flange. If the flange ended at the corner radius I don't think the straps would hold it as you tack weld. There are a lot of odd forces as you heat each spot to tack it. It really wants to bend and twist.
It's looking like a tank.
Even though the bottom is curved I want a sump to help assure water find its way to the drain.
I made a form block and hammered a shallow sump.
The next problem was how to make a fanged hole for the drain fitting. The flange will get welded to the flange on the fitting.
I discovered one of my ball peen hammers was the exact same diameter as the fitting. I sawed a hole in a piece of particle board. Amazingly the hole saw was the exact size to allow clearance for the 0.040" aluminum flange.
I sawed a hole in the aluminum to allow about a 1/8" flange and just pounded the hammer into the form block with a mallet, awesome.
The fitting fits perfect with just enough flange to easily weld the fitting to the tank.
The next problem was how to locate the hole in the tank bottom so the flange would be the same height all the way around.
I drew a circle on the aluminum to use for aligning the hammer.
For the form block I cut out a piece of foam rubber to fit the hole and drew a circle on it concentric with the form hole.
With the foam in the block you just position the aluminum so the circles are aligned.
Align the hammer with the circle on the aluminum and give it one quick wack with a sledge hammer. I probably could have used the mallet but I wanted to be sure I did this in one blow.
It worked perfect.
The fitting is in tightly and ready to weld.
The last thing was to shape the old access panel so the filler was close to vertical and so the curve matches this tank. There is a ring on the bottom of the panel with nut plates riveted on and a ring that sets on top of the tank to clamp everything for a tight seal.
I think it's time to weld the tank.
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