Building a lead bulb

[wheels]

Originally Posted by messabout

Anthony; lead shot and resin is a workable solution but to achieve a specified weight the volume must be larger. That is to say that shot and resin are not as heavy as solid Pb. For least drag, the pod should be as small as can be, so extra volume is undesireable.

Plaster of paris will make a good quality mold. But there are hidden dangers when using plaster or concrete. Either material must be allowed to dry very thoroughly or all hell will break loose when the hot lead hits it. Dougs remark about not letting water get anywhere near hot lead is to be taken seriously.

back when I was a welder/machinist we had to occasionally cast our own pieces. one of the more important steps that most laypersons didnt understand is that you need to slowly heat the mold first before pouring in the molten metal. easy to do if you have an industrial oven for heat treating. If I were to do it at home I would dig a small pit and use some scrap angle iron to hold the mold. fill the pit with charcoal for BBQs or scrap wood. heat it slowly to 300-400 degrees before you pour and you will avoid alot of the shock to the mold from pouring 620 degree molten metal into it.

[DianneB]

I have worked with molten lead quite a bit and feel the need to point out the dangers and drawbacks.

#1 - NO WATER anywhere near molten lead!!!!!!!! Even a drop of perspiration into molten lead will cause an 'explosion' like you have never seen! (The water turns to steam instantly and splashes molten lead everywhere!)

#2 - Wear full-body protection - face shield, gloves, NO exposed skin. Lead is extremely dense and even a small splash will give a serious burn.

Drawback to casting lead is its high coefficient of expansion - as it cools, it shrinks back and you need to make sure your casting remains full. If you pour more lead on top of cooling lead, they will not bond together.

Lead is neat stuff to work with because of its low melting point but it needs to be treated with great caution!

[idkfa]

How important it is to use antimony, shouldn't there be about 3% for strength?

They used a squashed bulb but did not fan the tail out?

[Tim B]

Concrete is a pretty good mould material. It can handle heat, and is cheap. Pre-warming the mould is quite cunning, but the guys in the video failed to put enough breather holes in. When you've cast the bulb you should only have minimal finishing to do, which should be possible with hand-tools. You can then fair and paint as appropriate.

Any type of resin next to hot lead is likely to lose significant structural integrity, so you would need to consider this if you were going to use a GRP mould.

Personally, I'd use concrete, as I think the level of finish you would get from a GRP mould is probably not worth the extra trouble.

Any form of glue/resin/wax used in addition to lead is pretty inefficient. Any designer will tell you that denser material is better. The drag goes up pretty quickly as the surface area increases to accomodate the extra volume required, and lead is the densist material that's readily available. Gold tungsten and uranium, while denser are not usually considered readily available.

Tim B.

[fastwave]

An easy way to build small bulbs is to make them a body of revolution and use a lathe.

Assuming you use a length/diameter of 5 then you need a metal pipe 18cm diam and 90cm long. You cast the lead in the pipe and then head to your local machine shop with a lathe. Give them a plywood template of the section and presto.
With a bit of sanding on the lathe the finish can be impressive (assuming you cast the lead hot enough)

Then all you need is a pocket to attach it to the fin.

[DianneB]

Don't forget that if you are going to use concrete, the mold will have to be heated (slowly) to the casting temperature. If you pour molten lead at 800 degrees into a room-temperature concrete form, the inside of the form will try to expand rapidly while the outside is still cold. I would bet money on a mold failure in the same mode as tempered glass (i.e. explosive!)