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I want to make a relativelly small diaphragm spring for a project of mine. I'm thinking somewhere around 50-70 mm diameter, I can size the other parts to match the spring.

picture of a diaphragm spring

It's basically just a cone with some cutouts.

I have a sheet of material that I've been told is Phosphor bronze. I've measured it and it appears to be 0.2mm thick. According to Wikipedia

Phosphor bronze is used for springs, bolts...

Which makes me think this could work. The problem is I don't really know how to work this material. In particular, I need to turn it from a flat piece into a cone and most likely add some reinforcement pieces to the 'fingers', though I could probably make do with bolting those on.

So the main question is: How do I go about turning a flat sheet into a cone? Do I stretch it somehow? Or should I make a circular cutout and join the edges somehow? Do note that I don't a lot of force from the spring. Worst case I'll make several and put them together.

In terms of tools I'm decently capable with a soldering iron (soft soldering) and have access to a drill that has an attachment that is meant to make it usable as a primitive lathe. Those are all that seem relevant right now.

Now, to avoid the XY problem, I shall describe the X as well.

I want to make a small model of a manual automotive clutch for myself. The key piece that I've been unable to get is the spring. Thing is, I want one that is way smaller than the ones in cars and those that are sold at that size are mostly gonna be way too strong for my application, as they are made for strength, not for looks.

I don't mind if what I make is weak, it's not meant to transfer power, it's meant to look realistic and that is my end goal.

Attempts

Attempt number 1: https://imgur.com/a/zKjpghs

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    Are you familiar with metal turning practice which results in bowl shapes? Do you think you could spin your bronze disk and form a cone on your current equipment?
    – fred_dot_u
    Sep 6 '19 at 9:29
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    What dimensions are you after? I'd be surprised if some sort of spring washer couldn't do it, perhaps a Belleville washer. Here are some specs from the spring supplier I use in work. Wave-spring washers may be more suitable if a Belleville washer is too stiff, though Belleville washers can be stacked back-to-back to reduce the force for a given deflection
    – Chris H
    Sep 6 '19 at 9:42
  • @ChrisH Yes I considered washers, but all I've had in hand were way way way too stiff. And give the application, it needs to be one piece, so stacking won't work. Wave-spring is a completely different thing that wouldn't work for the application. I've added the expected size.
    – Mr Redstoner
    Sep 6 '19 at 9:53
  • @fred_dot_u Not really, though I won't know until I try, which I certainly could if I knew that's what I want to do. I should have plenty enough material for a few failures.
    – Mr Redstoner
    Sep 6 '19 at 9:54
  • That is big. The ID:OD ratio wouldn't work for washers for a shaft in the middle, even apart from the stiffness.
    – Chris H
    Sep 6 '19 at 10:30
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This is a little speculative as I haven't tried it with phosphor bronze, or anything as thin as you got.

First cut a circular piece, and clamp it between 2 rings. A holesaw and scrap plywood (to support it and to make the rings) may be helpful here, but you don't yet want a big hole in the middle of the workpiece. Mark the centre accurately, a set of equally-spaced radial lines (I suggest 12), and a circle a little smaller than the inner diameter of your clamp rings.

Then beat or even press it into a conical shape, using a round-ended tool in the middle (maybe even the blunt end of an old drill bit). You should have a cone with a slight brim round the edge.

Now drill out the centre hole (using a step drill or cone drill would be best). This is already a spring, but perhaps rather too stiff - this is a good time to test. Cut along 3 of the 12 lines (equally spaced) and test again. If it's too stiff, cut along another 3 lines so every other line is cut, test again, and if necessary cut the last 6 lines. For the cutting, I'd use a hacksaw with something supporting the workpiece, as you want to remove a little material. You might be able to use tin snips and make two parallel cuts (my tin snips are heavy duty and not fine enough for this).

If you want it to be flat around the centre hole, instead of using a round-ended tool, drill a small hole and use that to clamp discs front an back, then apply pressure to the discs.

The important bit is to keep everything concentric - you'll probably spent more time jigging than actually making the part.

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    I hope this is clear enough. It would be easy to illustrate if I hadn't just upgraded my machine - I don't have the right software at the moment.
    – Chris H
    Sep 6 '19 at 10:47
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    Seems to be somewhat working, though it looks like I might need to add some heat to get it done in a reasonable amount of time.
    – Mr Redstoner
    Sep 6 '19 at 16:00
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    Enough heat will probably anneal it. It should be easier to bend, maybe even after cooling, but may be less springy as a result. This behaviour is very dependent on the exact alloy
    – Chris H
    Sep 6 '19 at 16:23
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    I've looked it up and found a source claiming 475 – 675 °C as annealing temp. I want to try the soldering iron, so I should be below that no problem.
    – Mr Redstoner
    Sep 6 '19 at 17:57
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    Alright the first results are in. With a bit more patience this might work! I think I might try to modify the jig a bit, so that I'll have a big bolt pulling the center down as I'll thighten it over the course of several days rather than hammering it. Feel free to check out the gallery. I'll leave the question open for a while longer to see if anyone has other good suggestions, if not I'll accept yours.
    – Mr Redstoner
    Sep 13 '19 at 11:48

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