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Forming a bowl

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  • Forming a bowl

    So I've got a problem. I "need" to form a steel bowl the approximate dimensions of a wok, though with a material thickness of somewhere between 1/32" and 1/16 and I just don't have much of an idea how to go about it.

    The end result is going to be used in a musical instrument, so I want it to be relatively smooth and consistent (i.e. hammering it in to form is just out.)

    I thought about starting with a positive form and using something like a shop press. But I'm just not sure how that would work. I imagine I'd have to over-bend it and I'd have to deal with material crimping.

    The other thing that caused a couple grey cells to fire off was a process I've seen a couple videos of where a brass positive is used on a lathe, and a burnishing tool is used to form the material to the positive. Neat stuff. But I don't see it working with something like 4130.

    Any ideas? Or should I just get a hammer?
    Proud machining permanoob since September 2010

  • #2
    I believe you're talking about metal spinning -- but I don't know enough about it to tell you if 4130 is out
    of the question.

    I don't think a press would work -- at least not an HSM press -- I think we're talking serious tonnage to get
    a finished part out.

    I'm afraid hammer and maybe an english wheel might be your best bet but I'm eager to hear what the experts

    Can you buy anything and adapt to suit? like you a say, wok, small sat dish, (metal) parabolic mirror, etc?


    • #3
      Sadly, no. this is the initial "R&D" stage. I'm going to want to repeat this process and make metric buttloads of these. Plus, most of the things I've found in the wild are far far too thick to be useful, though perhaps that suggests a solution a
      Proud machining permanoob since September 2010


      • #4
        Fwiw, the discs from plows are often used for wok like cooking devices. Apparently they work very well for that, and probably musical instruments.

        One Saturday I watched a couple of guys form five silver goblets on a big lathe by spinning sheets of silver, one heating with the torch, the other forming. Each came off flawlessly and quite quickly. Gorgeous. Stems were to be turned and soldered on later.

        They were formed by the owner of the shop, whom I'd never actually seen fab anything but who held a reputation as a fab anything. It was well deserved, and I probably should have paid them for the show.


        • #5
          Check the dollar store for stainless steel bowls.
          Free software for calculating bolt circles and similar: Click Here


          • #6
            Do a google search for metal spinning. I do quite a bit of design work for a local company that makes spinforming lathes, very similar to the old manual spinning lathes, only with the added twist of CNC. I'm not suggesting that you buy one of these lathes, because they go for about a quarter million dollars each. However there are many spinforming companies who will build a "die" to your specifications, and spin the bowls for you. This is a good approach if you are talking large quantities. The largest spinforming lathe I have designed tooling and automation for was spinning 8" diameter 4140 steel pipe into oxygen bottles. The pipe was preheated to a white heat, and had two oxy/propane torches playing on it as it was spun.--Brian
            Brian Rupnow
            Design engineer
            Barrie, Ontario, Canada


            • #7
              Hydroform it.

              Didn't really understand the process until I saw Mythbusters do it to form a torpedo thing they were making.

              They basically clamped the workpiece between 2 pieces of 1/2" or so steel of which had a hole cut in it to the shape of what they wanted. The used a pressure washer (if I recall correctly) to apply pressure to a fitting in the lower plate that deformed the workpiece up through the hole in the top plate.

              Make a round hole in the top plate and this should work fairly well.

              Here's a thread on another board with a video of the Mythbusters clip.



              • #8
                I've used that last method to form lens shapes from plexiglass and lexan. Clamp the material to a flat bottom with a ring, then heat to forming temperature and pressurize the gap. To do that in steel, you would probably do it cold, as heating the entire jig would require a pretty big oven, and the pressurizing medium would be subject to the heat.

                Not a big deal though- a good solid steel plate could be the bottom, and you make up a ring to clamp the disc down all around it. It would take a lot of bolts and drilled and tapped holes, or you could use a couple dozen strong C clamps. You would likely make a groove in the base for a large o-ring to seal it. Pump it with water- I wonder if a pressure washer generates enough pressure to do the job- quite possible. You are looking at a lot of overall pressure when you multiply the 2000 psi or whatever by the sq inches of the disc inside the clamping ring.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                • #9
                  Would the end of a 20lb propane bottle fit your need? Maybe not as deep as a wok but it has roughly the right shape and thickness. There is a thread on one of the knife making forums by a guy who makes drums from them.


                  • #10
                    Another approach would be to form them on a heavy duty planisher. The bowl in the photos is 1/16" galvanized sheetmetal and was formed with a Japanese Vibro Shear in about 15 minutes. My Vibro Shear takes 5/8" square shank tooling and will planish, shear, bead, joggle, form louvers, stretch and shrink and probably much more with the proper tooling.


                    • #11
                      A/C refrigerant tanks might work too. They're a little smaller than a propane tank. The salvage yard down here is full of them. The problem is that most have dimple feet so they stand up.

                      I press a lot of parts and a shop press should work if you don't have a punch/press. The problem there is that you need to hold the blank on the perimeter while pressing then shear off the excess. I don't use bolts or c-clamps. I use a spring loaded clamp as part of the moving die assembly. Or you could make it separate. And, you'll need male and female dies but these can be made of cast zinc. They wouldn't necessarily have to be machined steel. And 4130 will be hard to form/draw unless it's dead soft. Is 4130 a requirement? Does it have to be heat treated? Something like 1008 CR would form easily.

                      Spinning may be easier if you don't have a punch press. The 4130 will still be a problem there too.


                      • #12
                        Hey thanks for the responses guys.

                        - Most of the tanks mentioned are just too thick walled.
                        - No, 4130 isn't a requirement. It's just what I have a bunch of test sheets of, in different thicknesses. My metallurgy is for shyte. I don't have any real knowledge about tonal qualitites of different grades of steel at a particular thickness, so whatever I can work the easiest I'll go with I suppose.
                        - I'm really encouraged by a couple of the ideas, an English Wheel seems like the right idea (read: the easiest for me to wrap my head around without pictures) if I could rig up something that would hold the center and eek it in a bit as I cranked (with a die/anvil wheel set I'm sure "even I" could turn out on the lathe.)

                        Spinning was the first thing I thought of, but the size is just too big for a normal lathe (the target diameter at the widest point is pushing 22".)
                        Proud machining permanoob since September 2010


                        • #13
                          I haven't heard anything "home shop" to make "buttloads" of units. After you make your prototype, however you do it, You might look for a local outfit with a punch press to form these things. After initial die cost, it isn't very expensive to hammer out a hundred or so at a time (if that is what buttload means).


                          • #14
                            I did a search for "tank ends" and found this;


                            There are a bunch of shops who make nothing but what you are looking for.

                            Also, 20lb propane tanks have a wall thickness less than .090". And then there is the question, if you need a wok shape, why not just use a wok?


                            • #15
                              Maybe you can adapt a tank head. Here's one supplier: There are many more.