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Sheet Metal Working Advice

12/12/2016 4:39 AM

I have a stainless steel bowl with a diameter of roughly 28 cm. The sheet is about 0.5 mm thick. The bowl is in the shape of a hemisphere except for an approximately 7 cm diameter flat base at the south pole First, will the base be able to be pressed out to form a full hemisphere without tearing or buckling and second what would be the best way to go about it. I thought I would make a cap with the right shape, that is with the end radiused off to 14 cm and with a diameter of 7 cm. Would brass, hardwood or aluminum do the job Any advice would be appreciated. This is not a homework question though I am at home and need a hollow stainless steel ball for a project that I have been designing for over 40 years and finally am able to proceed with it. Thanks.

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Anonymous Poster #1
#1

Re: sheet metal working advice

12/12/2016 5:01 AM

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#6
In reply to #1

Re: sheet metal working advice

12/12/2016 8:10 AM

Can't beat that [pun intended].

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#7
In reply to #1

Re: sheet metal working advice

12/12/2016 9:26 AM

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

Boom. Done.

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#2

Re: sheet metal working advice

12/12/2016 5:11 AM

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

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#16
In reply to #2

Re: sheet metal working advice

12/12/2016 11:09 PM

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

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#3

Re: sheet metal working advice

12/12/2016 6:17 AM

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.

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#17
In reply to #3

Re: sheet metal working advice

12/12/2016 11:12 PM

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

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#23
In reply to #17

Re: sheet metal working advice

12/13/2016 6:22 AM

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

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#4

Re: sheet metal working advice

12/12/2016 7:56 AM

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

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#5
In reply to #4

Re: sheet metal working advice

12/12/2016 7:59 AM

<...paint remover...>

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

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#18
In reply to #4

Re: sheet metal working advice

12/12/2016 11:15 PM

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

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#8

Re: Sheet Metal Working Advice

12/12/2016 10:44 AM

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

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#9

Re: Sheet Metal Working Advice

12/12/2016 11:13 AM

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!

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#10

Re: Sheet Metal Working Advice

12/12/2016 11:38 AM

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:

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#11

Re: Sheet Metal Working Advice

12/12/2016 12:14 PM

Building one of these? A Van de Graaff machine?

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#12
In reply to #11

Re: Sheet Metal Working Advice

12/12/2016 12:20 PM

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.

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#13
In reply to #12

Re: Sheet Metal Working Advice

12/12/2016 12:55 PM

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.

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#14
In reply to #13

Re: Sheet Metal Working Advice

12/12/2016 1:04 PM

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.

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#15
In reply to #14

Re: Sheet Metal Working Advice

12/12/2016 1:17 PM

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.

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#22
In reply to #15

Re: Sheet Metal Working Advice

12/12/2016 11:38 PM

thanks

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#21
In reply to #14

Re: Sheet Metal Working Advice

12/12/2016 11:37 PM

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.

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#20
In reply to #13

Re: Sheet Metal Working Advice

12/12/2016 11:30 PM

Again, yes, definitely better than using 2 bowls.

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#19
In reply to #11

Re: Sheet Metal Working Advice

12/12/2016 11:29 PM

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.

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#24
In reply to #19

Re: Sheet Metal Working Advice

12/13/2016 7:39 AM

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.

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#29
In reply to #24

Re: Sheet Metal Working Advice

12/16/2016 8:33 PM

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.

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#25

Re: Sheet Metal Working Advice

12/13/2016 10:34 AM

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.

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#27
In reply to #25

Re: Sheet Metal Working Advice

12/13/2016 10:52 AM

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.

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#26

Re: Sheet Metal Working Advice

12/13/2016 10:46 AM

Word o' warning.

If the stainless steel was cold formed, it can spontaneously split due to residual stress if you heat it up. I tried resoldering a spot welded handle back on to a Revereware stainless steel saucepan and the thing spontaneously split down and around one side once the torch warmed it to silver soldering temperature. Stainless is really hot-short, that is, much weaker when heated to high temperatures.

As far as forming out the base to a spherical contour, I'd use a sand bag and an auto body domed hammer to rough it and then a planishing hammer and domed stake to finish the shaping. the planishing hammer is just a flat face hardened light hammer and the stake head is domed to the contour you want on the inside.

If you're going to heat with a torch, try to heat he whole thing up evenly to try to reduce residual stresses.

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#28

Re: Sheet Metal Working Advice

12/13/2016 12:14 PM

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!

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#30
In reply to #28

Re: Sheet Metal Working Advice

12/16/2016 8:34 PM

Frightening, the risks don't even seem to have been assessed.

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#31

Re: Sheet Metal Working Advice

12/17/2016 10:40 PM

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.

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