Rotational casting of EFP platters

[nbk2000]

A question came to my mind, after reading the copper furnace thread, of what to do with it once you had it melted.

First thing that came to my mind was EFP platter!

But, in order to deform into a proper EFP slug, the platter must be thicker in the center than the edges, and taper evenly from the center outwards, otherwise the slug doesn't form symetrically, thus being unaerodynamic and lopsided, resulting in poor performance.

http://nbk2000.freeyellow.com/Furnace_Rotational.jpg

Rotational casting seemed like a simple solution for the home experimenter to use. The mold is spun in two axis's at once, both parallel to the ground, and vertically. The two speeds would need to be adjusted so that there is only a slight difference, with the fastest being the parallel, so as to cause it to be thrown "out", creating the thickest part towards the center of the platter, while the vertical spin causes the platter to be tapered towards the edges.

The mold is attached to a small electric motor which spins the mold, and the the motor is itself mounted on a board that is spun upon a central pivot, with a lead counterweight on the end of the board opposite the motor to provide balance.

Natually, you must spin the whole assembly at sufficient speed to keep the copper from pouring out of the mold, while NOT spinning the mold so fast that the copper creeps out up the sides of the mold. Molten copper splatter would likely hurt...:eek:

Since I got this idea from an SciAm Am Sci article, I thought it only appropriate to create an illustration worthy of such an illustrious publication. :)

Ph3Ar |\/|i p|-|0+O$h@P! $k!L|_Z! :p :D

[Jhonbus]

I don't think I understand this properly. Why can't you just make the necessary shape using a dish-shaped greensand mould?

[DBSP]

It does seem a bit to complicated to fully function in a home workshop, although I belive it would work really good if you could get it all together.
http://w1.478.telia.com/~u47804009/EFP_platter_mould.JPG
This would be a bit simpler.

I was just wondering about what materiel to use for the mold. Clay would probably work.
I wonder if regular (white) modelling plaster would work? It would be great scince its so easy to work with. The problem might be that it perhaps wouldn't stand the heat and crack. I've been thinking about making one of these moulds for SC cones for a while now and I might just try it soon.

[Jhonbus]

Greensand (http://backyardmetalcasting.com/greensand.html)

[Observer]

Tell me, what kind of shape are you striving for?
http://w1.478.telia.com/~u47804009/EFPs.jpg

I'm simply interested in why you choose that specific shape of the liner.

[Mr Cool]

A light gas gun would require WAY more work and expense to beat the 2 km/s of an EFP slug, and you can't really shoot projectiles weighing a pound out of one. Light gas guns are good for top speeds, but not so good for KE.
But with a good mould and design you could churn out potentially tank-busting EFPs for a few £ each.
I imagine a backward fold design would be the most likely to work for us. I don't know why, I just think it'd be easier to fold something than to make it flow, especially if using ANNM or other similar explosive that doesn't quite have the power of HMX etc. For this you would want highest power directed at the centre of the disc, with slightly less at the edges (but not too much difference or it'll stretch out and break mid-flight), and a normal "hemispherical" wave front, such that the front hits the centre of the disc first. I think.
I've been looking for ages to try to find a good description of the dimensions of an actual platter, but have not been able to find one :(.

[nbk2000]

When you mold cast a metal, it contracts towards the center, from all directions. This introduces stresses in the metal that could be detrimental to the formation of the EFP platter.

Rotational casting counteracts this by using G forces to force the metal to cool in one direction only, from the edges inward. This isn't my idea, I just cribbed it from some patents that mentioned its use for such. :)

Also, one spin mold can create many different platters since you can vary the speed ratios between the two axis to create platters that are thin centered/thick edged, or vice versa, or even all the way across. This allows you to tune the platter to the job, assuming you've performed enough empirical tests to know the variables.

You could simplify this by setting the motor-mounted crucible vertically, which still allows you spin taper the platters edge thickness.

Practice using wax, instead of copper, would allow you to see the difference spinning makes.

Now, the Big Boys use metal spinning of solid metal plates to do the job, but this may be a bit harder than casting since it requires applying significant angular forces on metal plates at high rotational speeds. Bending a metal plate isn't easy, and gets downright dangerous when it's spinning, since the stress may cause the plate to frag.

[Anthony]

Would a cast platter, rather than one formed from a sheet, be brittle?

I imagine the platter must be fairly flexible to form properly in flight, and casting seems to produce more brittle materials.

That said, I haven't much experience with cast copper, maybe it is like lead and doesn't go brittle?

[nbk2000]

Any time you work copper, you have to anneal it, to restore it's softness, because it does get brittle.

Stick the platter in an oven set at max, heat for an hour, then turning down the oven by 50°F/hour, will remove the hardening.

A patent mentions the use of a buffer plate made of aluminum to transmit the explosive shock to the copper platter without shattering it. If you can melt copper, you can certainly melt aluminum.

Another patent uses inner-tube rubber as the buffer. So there's all kinds of ways of doing it. :)

The simplist platter to make is the backward fold, which is also the most penetrating. Forward fold is better for smashing through masonry, but we'll get to that later ;), right now the goal is to punch through metal plate.

[kinetic]

I think Anthony's right. According to my demo manual and the way I was always taught, you can't use cast metal for a platter. However, nbk2000's method may hold up- i've never tried it so i won't say that it won't. If you actually try this, please post results!!

[nbk2000]

I've found a huge copper EFP platter, ready-made, at K-mart! :)

It's 30" in diameter, 13" in depth, and costs $70. It's solid copper spun from a plate, and has nice curvature.

Given it's size, it'd have a penetration capability of a foot of steel plate and a meter of reinforced concrete at 100 meters. Not too shabby for a K-Mart product. ;)

Though it's normal price is $100, so stock up now. :D

[mongo blongo]

What is the actual product/use? I doubt they would be selling these for use as EFPs :D
On a side note I was thinking it could be possible use the bottom of those small gas cylinders you push into camping stoves. Maybe electro-plate them as you mentioned.

[nbk2000]

Decorative lawn fireplace/beer cooler. :rolleyes: What a useless use of a useful product. :D

You mean the little propane canisters?

[mongo blongo]

Yea the ones which have no regulator and get pierced with a spike when fitted into the stove. I can't get a pic of the one I have but here is another one "similar" to mine.
http://www.preparedness.com/coldualstov.html
See that it has the metal casing for the explosive already attached and the underside is curved inwards, perfectly centered in the casing which will be the template for the EFP to be electro-plated onto.
I had a quick go at electroplating one of these with copper but I encountered a problem. There seems to be some kind of material coating the bottom of the template (only the bottom). It looks like metal but it will not conduct electricity. I thought maybe some kind of plastic but no solvent I have can remove it.
I have no idea what this is or why it is only on the part I don't want it to be on :mad: but maybe it could be polished off.
Anyways I just sanded it off and did a very bad job of plating it. It was extremely uneven and it didn't stick very well. :( Well it really needs to be done properly with electroplating chemicals, low voltages etc.

[nbk2000]

It might be laquer or enamel, requiring mechanical removal.

It might not be copper, but even steel will work, as long as it's got the proper curvature and isn't hardened.

If you want to try electroplating, get a book on the subject, as there's many variables to the process that need to be just so for it to work well.

[Dave Angel]

I realize that this has got a little off topic, but as we are on the subject of electroplating, I've had very little success with my few attempts to make EFP platters (and SC liners) via electroplating.

I've been sniffing around google a bit to see what I could find on the subject. One forum I came across is full of arrogant bastards, only my opinion ;) who work with it at an industrial level and tell anyone who says "I want to try a bit of electroplating at home" that it is very dangerous and you can get acid splashed in your eyes and that cyanide is bad for you etc etc. :rolleyes:

Fortunately the above mentioned forum had a link to a yahoo group which actually dares to give information to would-be home electroplaters:

http://groups.yahoo.com/group/Electroplating/

I've had a look at a few of their files, including some giving details of plating solution compositions, surface cleaning and preparation, power supply construction and current pulsing. It's all rather informative and worth looking into if anyone wants to take this further without having to fork out for books/find them in the library.

Something of note is that copper sulphate plating isn't very effective unless something called a 'strike' is made with a copper cyanide solution first, though I think this can be over come with professional (expensive) formulations. Copper pyrophosphate is possibly a better option as the strike can be made using a dilute solution of the plating solution, thus one stock solution is all you need.

I'm still not sure whether to invest in furnace construction or the electroplating method but I'm leaning towards a furnace as I will probably have more uses for that. It depends how much it'll all cost and how much I can scrounge. Still, electroplating looks good because under the right conditions you can plate a good quality layer of ductile copper.