Sheet Metal Working Advice

Can't beat that [pun intended].

You can buy these legally in a lot of places. Light one, set the inverted bowl over it, step back.

Boom. Done.

I'm a bit confused: Are you looking to 'fill-out' the hemisphere by attaching a suitably-shaped end cap to the flat? Or are you speaking of pressing-out the flat using the 'end cap' in some manner to impart the curvature to the flat?

The application will of course determine the particular choice of material.

Please clarify

No, just to hammer it into the right shape, neatly. It seems I will need a male and female former to do that so I might as well go for a globe as suggested below. Thanks

This beeing a home project I guess you cannot well the bowl together and apply a little water pressure from inside?

Did you measure the bowls properly so that they do not miss material around the "equator" to become fully spherical?

How spherical do you need it?

I am serving salad in a stainless steel bowl now. Will have a look when we hit the bottom of it.

Andrew Westman hit it on the nose. Will go with the globe idea. Thanks for the idea though, maybe useful for a next program.

The credit for the globe idea actually goes to Rixter, but thanks all the same!

Have you thought of buying a world globe and using that? (You can use paint remover to remove the countries and oceans ).

<...paint remover...>

Waiting a couple of billion years for the sun to do it would also work.

No, I hadn't thought of that but it is a good idea and I will adopt it. Thanks.

I found a cheap one ready made 12" diameter $50....probably get a used one cheaper...or you can go big....

....but not so cheap....

http://www.houzz.com/photos/42156156/Stainless-Steel-Gazing-Globe-Silver-12-contemporary-garden-statues-and-yard-art

To answer your questions:

You can probably press that out without tearing, not sure what buckling is.

Aluminum or brass would make a better mold form. I would make the form a little larger than 7 cm to help press out the ripple where the flat began.

If you can make the form, can you also make a form to press into? That would let you know when to stop and even out the formed area.

There will be a line/ripple where the original flat was. It would take huge pressure and probably some heat to completely remove the ripple

Good luck!

Bowls are either spun or pressed in a hydraulic press, or hydro-formed.

As Andrew Westman said more information please.

If your only goal is a full hemisphere, I'd simply form the shape out of wood and bond/glue it in place.

Re-forming the parent material of the bowl could conceivably be done by making a matched metal forming tool (One convex/one concave) and using a hydraulic jack to form it, after much heating with a torch.

Metal spinners use a torch to aid in forming SS bowls.

Your turn:

Building one of these? A Van de Graaff machine?

My brother built one as a science fair project in school, using a world globe, hence my suggestion in #4. I was amazed at how well it worked.

Van de Graaff machines are a lot of fun, but large sheet-metal spheres are bloody expensive. I quickly found that out when building a Tesla coil years ago. Ended up using a torus for the HV terminal instead, one I made by covering an inflated inner-tube with aluminum foil.

Your idea of using a globe is a good one. Of course here it depends on the OP's application. He mentioned using 'hardwood' along with other materials for the 'end cap,' so maybe using a globe would work instead of fiddling with two metal bowls, I dunno.

It sounds like the OP may be using the 'lipless' stainless bowls at IKEA. I have thought about using those for a VdG generator and brazing two bowls together to make the top load. I wasn't going to worry too much about the flat spot on the top because the discontinuity is nicely rounded.

I haven't seen Ikea's bowls but as long as the discontinuity's radius is large enough it should work fine for that app, and the flat on other half would be cut-out anyway for the column and belt.

thanks

The flat had to go for mostly artistic reasons. I have seen a VDGg that had two domed ends on a cylinder and it worked fine. I would prefer no sudden edges though.

Again, yes, definitely better than using 2 bowls.

yep, you got it. Have wanted to build one for over 40 years and finally can. I will go with the globe idea for sure now, as I said above, my original plan. The pictured one is a beauty. Thanks for the lead.

Ah! So that is your project; a VdG machine. I'd wondered.

Actually, prof_peanut, I'd leave the flats if the discontinuity radius isn't too sharp. Try it with the flats and see how it goes. You can always cap the top later. I wouldn't hammer it out because, unless you're very good at sheet-metal work, you could conceivably introduce even sharper edges than the discontinuity radius that you're trying to remove. Really, the greater issue with these machines is quite often adsorbed humidity and/or contamination of the belt, making it harder for the belt to transport charge. The belt and rollers must be very dry and clean.

Thank you for this feedback. I have decided to continue with the bowls even if it is flat on top. I was thinking of using a bit of plastic sheeting for the belt but if you have a better idea for that I would appreciate your advice.

I think you may need to know the ductile temperature of the stainless steel for pressing out the flat part to same radius. I cannot envision doing this cold.

Someone will obviously correct me.

As long as your press can apply sufficient force, and you have form fitting upper and lower mold parts that can also take the pressure, go for it. Let us know how things roll.

It would be better to do it cold. SS forms quite nicely without being heated. One problem with heat is getting it applied evenly. Too much heat in a small spot and there is your tear.

However, it would be almost impossible to completely remove the line between sphere and flat without heat as that area has already been cold formed and therefore work hardened.

These Guys can probably help you out. It looks like they have a lot experience: https://www.youtube.com/watch?v=oMOBqRVDOYQ You will have to copy and paste the link in a browser, it is worth it!

If the original bowl was pressed then it will have work hardened and your chances of stretching the flat into a dome without splitting it are low. You will not be able to heat anneal stainless steel without distorting it. You would be better starting with a flat sheet and hammering out a dome from scratch. If you can gain access to an English Wheel which can loosely be described as a three dimensional roll former the job would be easier. If the original bowl was spun then you can put it back onto a lathe and continue spinning the base into a dome without problems.