Aluminum hood build questions

[Chris_Hamilton]

Hello all,
Question for Peter and anyone else who feels they can point me in the right direction
Getting ready to make several aluminum hoods for Nissan S30's (240Z, FairladyZ's). Thought I'd lay out my plan and have you tell me what you think. Not looking to make scrap as Aluminum sheet is fairly expensive.

Material will be 3003 .063. Plan is to make the outer skin in one piece and the inner structure in several pieces. This post is more about the outer than the inner as I plan on making a hammerform for it. We'll see.

Start with a paper pattern off a good steel hood. Make profiles off that hood and mark/label them on it as reference for the new part.

The outer skin is not really a compound curve. It has a lot of shape front to back but side to side is flat with the exception of the blister. There is also a peak in the center of the hood that runs from front to back. It is flat from the side edge to that peak. Being that it is this way my thinking is to simply bend it to the curve (or close to it) it needs (front to back) and then in order to hold that shape do the initial turn of the side flanges over as they will be when finished. (flanges will require a couple of bends in order to finish)

Then block out the blister and the body line in the center. Gather the material with the hammer and dolly and make the center line. Or would it be faster to simply use a chaser after I've formed the blister and make the line?

As for blocking the blister I feel that's pretty straightforward. Block it, move the material around, use my profiles to check progress.

Here is where I'm a little unsure. After I get the blister shaped, if I need more curve (front to back) can I shrink the flanges to accomplish this? Shrink from the very top, same place on both sides? (if shape is needed in the same spot on both sides) Or should I make the curve/bend in the panel, block and make the blister, then turn the flanges? Doing it that way won't the shape/curve be pretty much set after doing the blister and be much harder to manipulate than if I turned the flanges over before blocking the blister?

[skintkarter]

How many do you plan on making Chris?

[neilb]

err... personally if it was me i would take a really good GRP mold (probably have to block sand the gel coat to remove ripples) of the steel bonnet (without the edge flanges) supported really well, and supported as solid as possible in the centre. you can use the paper pattern to show where you can block the centre hump, then clamp in the mold and finish as a hammer form. for repeatably i would have a go at that method. probably all crap but i would learn a way of how not to do it

[skintkarter]

That was where I was going too Neil - if the plan was to make more than 2-3, a big thick GRP mould and then flowform with a rivet gun. Dunno how the flowforming would be on the flatter bits, but even if the centre was made in a mould. The sides could be wheeled, then the centre welded in. But I'm theorising here.

[Peter Tommasini]

Chris making that bonnet in one piece it's a big ask...... BUT it can be done ! The way I would do that in one piece would be to wheel the main shape first and follow all your profiles ,then start to block the blister , you would need to have quite a few profiles for the blister as well ... you would need to keep your eye out when blocking the blister simply because it will take some of the shape out on the rest of the bonnet ...meaning that you would need to re wheel the main shape while blocking the blister, ...... meaning... a bit of work on the bonnet and a bit of blocking on the blister and so on .
Another way to do that would be in two pieces, so.... go ahead and shape the main shape fully, mark where the blister is going to be, then cut a hole on the bonnet making sure that the hole is smaller than the blister in width and length, turn the side of the blister upwards and stretch the sides of the blister while turning them up with the hammer and dolly, you would need to stretch the sides to reach the same contour of the already wheeled bonnet , then simply wheel the top of the blister turn the sides down, and put the weld half way down the two sides and front ,the line down the middle can be done with a wooden chaser on a sand bag (just like the lines on the Monaro quarter) the 12/ 14 MM flanges that ere to be turned down over the frame should be done at last once the bonnet skin is finished and the blister welded to the rest of the bonnet
Peter T.

[Chris_Hamilton]

Thank you Peter and Neil for the responses. I'm assuming Peter that you would do it the second way you described? Being that the hood only has shape in one direction (front to back) and it's not a compound curve, with the exception of the blister (sorta), how would you approach the wheeling? Flat anvil and almost no pressure? Pumping it? Wheel it from side to side with the go kart tire and anvil with radius that will give it the curve it needs?
If I choose the first method you described above and blocked out the blister, how would you go about controlling/correcting the shape of the flat area while blocking? Meaning keeping the shape/curve front to back and keeping it flat from the side to the middle?

Sorry for all the questions? I'll take some pics of one of the hoods I have to better show the flat area I'm talking about if you'd like. Thanks so much for the help.

[Peter Tommasini]

Chris
to be sure ....post some good pics of the bonnet in question, from the top view and side view so I can see the height of the blister
Peter