Material selection for weeble wobble principle 2

[Toxotoxo]

I am working on a project using the weeble wobble principle

http://en.wikipedia.org/wiki/Weeble#Design_principle

.

At the moment I am using a stainless steel 4mm diameter hemisphere but this is not heavy enough.

I've been looking a more dense materials like copper and was wondering what you guys recommend?

The material can not be toxic or harmful to the user.

 

[dgallup]

Depleted uranium

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[btrueblood]

Bismuth, Indium, Tungsten...there are lots of metals with densities higher than nickel/copper/iron. Lead would work, and be low-toxicity if encapsulated.

I think that the bottom of the weeble in your link is not a perfect hemisphere, but more like a section of an oblate spheroid. Would want to do a little math (like calculus) to convince myself...

 

[IRstuff]

As the article points out, the surface must be CONVEX

I like tungsten; it can look a bit like stainless steel, until you try to pick it up.

TTFN
faq731-376

 

[Toxotoxo]

btrueblood: thanks for the suggestions. The first three seem like quite expensive materials and hard to machine (I'm using a lathe to make the hemisphere) but I will make one out of copper and see what results I get.



IRstuff: "a bottom which is a more or less smooth (unfaceted) hemisphere" I does not seem to say convex anywhere.


Also as I'm new to this post is there any way I can reply to individual posts?

 

[btrueblood]

Bismuth and Indium, if alloyed with tin or similar heavy metals, can be quite easy to machine. Tungsten too can be machined easily (well, compared to the pure stuff) if you buy the alloy that is basically sintered tungsten powder with several % of cobalt or nickel binder.

Never mind the latter part of my post - a hemisphere, or more better a spherical segment, will work.

Copper is not a lot heavier than stainless steel, but I'm still not clear how much heavier you need to get. Do realize that the weebles are a plastic shell, with a weight (likely a chunk of zinc) inside it. I.e. try hollowing out the hemisphere (or spherical segment) and loading a small amount of heavy alloy (like lead that you then coat with epoxy adhesive/sealant) near its base. Bismuth/tin solder is readily available and quite non-toxic, and would be a good choice for this kind of idea, as you can easily melt the stuff and pour it into the hollow.

 

[Toxotoxo]

thanks for the advice

 

[EdStainless]

Or when you machine out the inside do it with an offset center point so that the wall in the bottom ends up thicker than the sides. This would place extra weight there that is an integral part of the unit.

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[mrfailure]

What is the light material you plan to use? Could you stick with the stainless steel but instead use a lower-density material for the light side?

Aaron Tanzer

http://www.lehightesting.com

 

[KENAT]

You don't say anything about cost, so how about using something like Gold? Nice and inert and dense.

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[EdStainless]

or Pt, it would be little less expensive than Au

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[btrueblood]

I think Iridium is denser yet...

 

[Toxotoxo]

Thanks for the replies.

The light material I am using is hollow plastic how ever it is quite high, almost 30 cm tall.

Costing is also an issue as it is meant to mass produced (10000+ units) so the price of the end product is around 20 British pounds.

EdStainless: Im not quite sure what you mean, I was a bit unclear in my original post. The thing I'm making is not egg shaped, it is more like a plastic stick with a solid bottom hemisphere. Sort of like this but not as tall

http://dornob.com/self-balancing-staff-weighted-walking-stick-stays-upright/

I will looking in to all the other suggested materials.

Just like to add I'm new to this forum but I can already tell is great!

 

[EdStainless]

You will need to add a lot of weight in the bottom to make this work. after all you need to get the center of gravity near the bottom.
One low cost way to do this is to use shot (lead) and pot it in a resin inside your base. It would be inclosed, fastened, and easy to place (no dimensional tolerances).

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