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Permanent Magnet Rail Gun (PMRG)

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Marko

Tue Oct 30 2012, 10:56PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

Sorry to hear that the french's alloy contains cadmium and lead... I just looked it up myself. If the system works, there would be little or none of it vaporized per shot (probably the most at the moment projectile leaves the barrel). i'm not particularly afraid of metallic lead either, I already have few hundred kg of it laying around the garage! But I'm unfamiliar with cadmium, and I wouldn't want it to get onto everywhere and into environment.

I've also found field's metal on ebay, only a bit more expensive but alsoa higher melting point, would need more heating to get going.

I also hoped that some of you guys would be able to tell me more about chemical and surface properties of these alloys, like ability to wet copper, and also the possibilities of troublesome amalgamation that may occur over time (gallium spectacularly attacks aluminum, for example).

I want to know if I wet my rails with liquid metal, that it won't form an alloy with copper over time and lose it's ability to melt (then I'm screwed!)

Cheers,

Marko

Marko

Mon Nov 05 2012, 09:15PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

Update with some pictures and future prospectives!

Marko

...

Mon Nov 05 2012, 11:40PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I know that indium (and most alloys of it) will wet copper, I have used it for soldering to copper in the past. That said, it requires flux, any oxidation on the copper prevents it from sticking. I wonder if it would be worth it to pre-wet the rails with the metal? I am envisioning a pre-firing procedure where you stand the gun on end, put a plug in the breech, pour in some flux and your solder of choice, and then push/pull the plug down the rails to give them a nice even, fresh, coat of solder. Then load in the projectile, maybe with a little plug of solder in front of it for good measure. Free added benefit of having the barrel lubricated

Marko

Tue Nov 06 2012, 01:56AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Peter

Yes, I originally intended to have entire rails pre-wetted; I wouldn't expect the projectile to do this well during firing! And yes, I also thought some flux would be necessary for this to go well. I'm not sure if normal high temp soldering flux would do - is there any alternative you would recommend for lower temperatures?

By the way - I also know a possible alternative to liquid metal, and that is those electrically conductive greases used by linemen for power line connections. I don't know what properties of this stuff are nor where could I get some. I'm afraid it could actually be more expensive than low melting point alloys!

However, I'm currently concentrating on power supply for this cannon, as well as measurements. It's crucial to get some idea of the contact resistance and how it changes with various types of brushes. It'll take a while before I reach the liquid metal wetting phase!

Marko

Shrad

Tue Nov 06 2012, 01:01PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

why not try liquid tin like the one you'd use for printed circuit board chemical tinning?

Marko

Tue Nov 06 2012, 01:44PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Shrad

Well, pure tin solder has a melting point at over 200C which would destroy my insulation, glue, and possibly the magnets. I need stuff with melting point reasonably close to room temperature in order for this idea to be worth of hassle.

Marko

Patrick

Tue Nov 06 2012, 05:22PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

tin is one of those scary metals, tin whiskers have killed other peoples work on many occasions. At high temp and high velocity i wouldnt want to tempt fate either.

Shrad

Wed Nov 07 2012, 07:51AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I was talking about chemical tinning agent which could be applied to the rails for wetting, but if you say it's not suitable.... ;)

I think you're stuck to indium or even lower melting point metal, but price is not the same

Patrick

Wed Nov 07 2012, 05:49PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

cheap, low melting point, non-poisoinus, seems to be his most immeadiate requirements.

Josh Campbell

Thu Nov 08 2012, 01:09AM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Difficult task... it seems gallium is the best choice given the cost and it's ability to wet metals even with a bit of oxidation in the way. This is primarily the case be cause gallium is corrosive to all metals except tungsten and tantalum. Which presents another issue. In fact liquid metals can dissolve most metals. That might make their use as a room temp wetting agent tricky. However the level of corrosiveness on say, oxygen-free hard tempered copper may be low enough that the effects of corrosion are tolerable.

Interesting table that shows how high melting points are grouped around tungsten (W) which has the highest melting point.

1352336848 5258 FT145747 Metalmeltingppoints

Will be interesting to see what you come up with in testing.

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