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Saz43

Fri Mar 02 2012, 07:26PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

The cylindrical projectile would probably have slightly better magnetic coupeling with the coil, but I think the losses that come with sharpening the last few centimeters are going to be miniscule. The sharpened tip gives you greater advantages, such as less drag and more importantly, more penetrating power. I like to build my coilguns to do as much damage as possible, and the sharp tip is necessary to get the killing power.

klugesmith

Fri Mar 02 2012, 08:29PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Here are three other ideas for making laminated projectiles to reduce eddy currents.

1. Glue together a stack of "I" laminations from a transformer core, to make bar with a square cross section. Could wrap with fiberglass for more strength.

2. If you can get a sufficiently thin sheet of steel, roll it into a very thick-walled tube. OK to leave a steel mandrel in the center (like a sardine-can key?) The spiral cross-section can't conduct circumferential eddy current, if the turns are insulated from each other. For example, use painted metal from a "tin" cookie container.

3. Use a dense bundle of straight steel wires, bonded with epoxy. Could use glass tubing or a test-tube as sacrificial mold -- make barrel to fit projectile, instead of the other way around. Vacuum methods might help to get good resin penetration.

Have you got your hands dirty yet? How about making and characterizing a coilgun with traditional solid-steel projectiles? Then you would have a reference system, against which to compare the alleged benefits of eddy-current suppression or "external iron".

Turkey9

Sat Mar 03 2012, 06:54PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Yeah I may vote for a no sharpened tip projectile, but all of mine are sharpened!

Have you thought of using JB weld? As far as I know it's not conductive but is impregnated with steel filings so you would already be part of the way there. Plus, JB weld is some of the strongest epoxy I know of. It is actually used to fix cracks in the pistons in engines where my step dad works. You might not have a problem with chipping.

Forty

Sat Mar 03 2012, 09:31PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I already suggested J-B weld :P Their website said it was insulating, so that's good. I'll try it out myself sometime soon.
I've tried the I laminations before. If you use pieces from several transformer sizes then you can form more of a rounded shape by putting the widest sections in the middle and decreasing the width as you stack them up (might provide slightly better fill factor that way if you choose to surround the laminations with epoxy to make it round.) You could also just leave it as a square and use square holed transformer bobbin to make your coil on. A square barrel could be tricky though...
I've considered the wire segment idea before because i saw the same sort of core used in a classroom demo device. Can't think of a source material to use though.
the sheet metal spiral is a new one. I wonder if that ultraperm shielding they're always trying to sell on goldmine would work well. Just a thought. Would those super permeability materials even be good to use?
Oh and directly comparing two different projectiles using the same coil setup wouldn't be the best comparison unless you could somehow get their size and mass to be equal. I think what you'd have to do is find the optimum coil/initial position for each projectile and then compare the efficiency (using the same bank and switching.)
There's gotta be a better way than that to compare the materials though. Something with a permanent magnet maybe? or use it in the core of an electromagnet and do something with it? any ideas?

I agree that the projectile should be pointed if you intend to shoot it at anything hard. If the front is flat, then a ding or hard impact could misshape the edge and cause it to catch on the inside of the barrel.

Anyways, Pink have you tried anything yet or are you still just theorizing?

ZakWolf

Sat Mar 03 2012, 09:47PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

I wanted to give this a "shot" (sorry) and im going to use steal filings and shape lock.

Im going to put it to a section cut from the barrel for the mold, and as it cools shape some stabilizing fins on the back.

just my two cents

Pinkamena

Sun Mar 04 2012, 06:10PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Forty wrote ...

SNIP
Anyways, Pink have you tried anything yet or are you still just theorizing?

Just theorizing :)
I'll be building it the summer, and I am taking my time planning it.

Yandersen

Fri Oct 05 2012, 04:25PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>

Yandersen

Wed Dec 05 2012, 02:49AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Guys, I want to buy a machine for making projectiles out of iron rod. Any suggestions? Who has it already - where the hell did you get one?! =(

Yanom

Wed Dec 05 2012, 03:06AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

Yandersen wrote ...

Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>

Thought about this. It seems that you'd want your projectile mass closer to the wall of the coil, so it could be hollow....
and shorter projectiles are better right? Is that true up to a point or would it be good to have a coin-shaped projectile shorter than it is wide?

Yandersen

Wed Dec 05 2012, 11:54AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

The hole through projectile will increase efficiency only for coils with few layers of wire. For most efficient shape (3x outer to inner diameter) it will not add any significant benefit as field inside such coils is almost constant all over the hole.
I am not completely sure about overshortening the projectile. The shorter the coil the better, yes, but overshortening projectile by making it's length close to diameter or even shorter may be not efficient even for such short coils. And the shorter the projectile the less stable it in the air.

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