how to shape a small circle to the OD of a tank end?

I find I have no idea how to shape e.g. a 4" soft aluminum

I've had fair luck with a shot bag and small mallet.

If you have access to the inside of the tank and you have not cut the hole yet, you should be able to beat it into a slightly smaller radius by tapping with a ball peen hammer round and round from the outside edge in, using the inside of the tank as a form. If the radius is too tight then move to the outside and and beat to fit again.

Carl Boyd

I hammer domes in a depression carved into the end of a piece of oak firewood, or for heavier steel I use a short ring cut from 4" pipe. The center hole in an old car wheel ought to work. The shape of the depression doesn't matter too much since the metal is mostly stretched between the hammer head and the rim. You can fine tune the shape on a sand bag.

Jim Wilkins

Hey, Grant. Did you ever get one of the canvas lead shot bags that Boeing Surplus had years ago? My son and I found them and each got one. He cleaned and washed his really well and used it to hold fossils while he cleaned the stone matrix away. I still have mine in original condition and don't think I have ever used it.

Paul

You can do this on a flat surface.

-Cut the piece to shape and file or smooth the edge.

-Find a hard flat surface. An anvil or a half inch thick or thicker steel plate would be good.

-Using as large a radius domed hammer as you have, simply begin hammering at the center of the piece and make a spiral of hammer marks out from the center. Be careful not to hit anywhere close to the edges of the piece. Stretching will occur anywhere you hit with the hammer, but, since you are NOT hammering the edges, the stretched (hammered) material will have to go someplace and it will dome out as you need.

-Don't try to get all the doming you need in one pass. Just work from the center to the outside, then go back and make another complete spiral pass, hammering between the previous hammer marks if possible.

-You do need to be a little cautious that you don't overly work harden the piece to the point where it cracks.

Note that the larger the radius of hammer you use, the nicer the finished part will look.

Pete Stanaitis

----------------- Grant Erw> Picture the little aluminum disk riveted to the top of a Weber kettle. It

Ah. That would be "raising". I read about that once in a book about how they used to make copper pans. Thanks, Pete.

GWE

spaco wrote:

Spin it using your lathe.

Dan

I've done both hand raising and metal spinning but these days I do metal spinning as it's so much quicker for doing round objects. If you can turn up a blank to match the inside of your piece then you should be good to go. For a one off very simple tools can be used depending on your quality requirements. I would be inclined to join and post here about details to start with

. You can use traditional spinning tools or for things like spinning a tank end I have used a cam follower roller bearing mounted on what amounts to a radius turning attachment. See

I have recently done this on the same lathe, Harrison M300, upto 12" diameter and may try at max 18" using the face plate as a former.

Cast the conjugate shape against the original with Bondo. Or epoxy plus quartz sand instead of Bondo. Use that as a one-time spinning mold. Or make a 2nd cast (positive this time) and use the pair in a press to form the sheet.

We can combine modern technology with the classic sculpter's wisdom:

1.Chuck a piece large enough to create it in your CNC turning center. 2.Remove everything that does not look like a domed disk.

Carve the shape you want in a wooden block. Beat aluminum into it. try "dapping" for a generally smaller scale application of the concept.

You can also work it over a suitable sized ball from the other direction.

Or - work the aluminum on a shotbag, more or less freehand.

In either case you can use soot or chalk to mark the wooden form where you need to cut it, or the aluminum where you need to beat it, unless you really want the mess of using prussian blue, which is overkill - but the mark and cut process is very similar in concept to the same process in machine scraping.

Also look up "raising sheet metal" and "raising sheetmetal".

If your aluminum is not dead soft, you'll have a much happier time if you get it to dead soft before you start trying to shape it.

this is a 4" disc, with not much concavity, does it have to be aluminum, does it matter how thick it is? oftentimes (if the project doesn't have a heavy time restriction on it) i'll just keep my eye open for objects at the local recycling center/dump. how about *getting* a tank end and cutting (plasma, jigsaw, etc.) a 4" disc out of it already pre-formed? sometimes i get lucky and find exactly what it is i need at the dump.

sorry grant if this doesn't help you at all.

b.w.

(at first i thought you were talking about making a whole weber style dome. i tried to find a link i used to know about a guy making a expansion pipe for a two stroke motorcycle by hydroforming it, wondered how big of a dome you could blow up before it started wrinkling

(see links at the bottom) there are other better links on the web about this. it surprised me how smooth the blown up shapes can be.)

We don't seem to have dumps like that where I live. We have "transfer stations" and people do drop metal off there but no one is allowed to pick through it. That is a vanishing way of life in the Greater Seattle metropolitan area, sad to say.

I have decided to make a different kind of vent - a tube with a top-mounted vent. I know how to hold a tube vertical and scribe it to a curved surface. Then I'm going to cut out the tube and grind it until it fits the tank end closely, then hold it in place and scribe around the tube and cut a bit inside the scribe line, then smooth the cut a bit and then weld the tube in place.

The surface isn't regular is the problem - it will have to be off to the edge, and the tank end isn't spherical. Unless the surface is regular a rotating disk Weber-style vent won't work.

Grant

On Jan 23, 1:04=A0am, Grant Erwin wrote: =2E..

Then put a Diesel rain cap on it.

Jim Wilkins

A friend of mine has been making domes with hydraulics.

He forms 321 stainless into headlight reflectors,and similar domes, using his grease gun, and a quick-n-dirty jig. Basicaly a flat plate with a grease zerk, a layer of stainless, then a ring of appropriate size. There is a ring of 1/4" bolts around the rim if the ring.

With a 4 inch diameter ring, he says he can get a little less than half that in height, before the grease ends up all over his shop.

Me, I'd cheat, just like some manufacturers do, and build the vent as a flat assembly, and weld it into a cut out hole.

Cheers Trevor Jones