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Coilgun Barrel Composition, Armature Composition

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Dave Marshall

Tue Feb 21 2006, 01:54AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I would say that for a small single stage coilgun, the teflon ought to cut it. I'm not really familiar with just how ridgit it is, but the bit with the coil is sure to be plenty ridgid, so long as the coil is constructed properly. The rest of the tube could be supported if needed to be.

I worked with .25" inside diameter Delrin tubing for my 6 stage 4.7Kj design back in high school. The walls were too thick, but it worked pretty well overall. I wouldn't really be too concerned with friction losses for a single stage system. If you had a 6' barrel for multiple stages (the trouble I'm having now) you might want to invest more time and money into a low friction barrel. In a low power system, wall thickness is the most important aspect. So long as it isn't like sandpaper inside, I wouldn't be overly concerned. On a side note, metal barrels should be avoided if at all possible. They can be made to work just fine, but they generally are more headaches than they're worth.

As for a projectile, I would encourage you to get one thats as close to the same length as the coil as possible. Short projectiles make for short pulses, and minimal coupling between the coil and projectile. A projectile thats atleast half as long as the coil is more desirable. I always used projectiles as long as the coil, and was never dissappointed. Cold rolled steel makes a fine projectile for low to mid power systems, and you can't beat the price. If you're worried about pressure build up, port the barrel.

For more powerful systems, the cores from big solenoids are usually made of silicon steel or some other material with a very high saturation point, and they make for great projectiles.

Dave

Barry

Tue Feb 21 2006, 02:02AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Quantum Singularity wrote ...

As for the projectile material, is there a way to tell how good a projectile will be without having a working gun? For example, I know you should see the inductance rise as the projectile is placed in the center. If I wanted to try 10 different materials before I built a working gun could I pick out the best just by which causes the inductance to raise the most? Maybe there are some other tests too? There are so many variables that I think it would be good to start designing with certain variables already optomized if you get what I am saying.

Yes, the amount of work done on the projectile is indeed related to the change in inductance. You can run some simple tests to compare one material to another. Just keep in mind a few points...

For a fair comparison, you need to use identical size and shape of projectiles. That's because a longer projectile will generally cause a greater change than a shorter one. But it won't necessarily perform better in the barrel due to the additional mass.

The inductance change is a complicated function, not a direct proportionality. If one material has twice the inductance change, then you can be sure it will not have double the velocity. I've done some FEM modelling to examine inductance change, and the results were difficult to interpret. I wasn't able to extract specific numerical results or guidelines that would relate inductance to work done.

The ratio of minimum to maximum inductance is called saliency and it's a useful figure in rotary motors. Read more about it here.

To measure inductance, use your coilgun firing circuit under normal conditions. Since a good coilgun has the projectile near (or past!) saturation, a small-signal inductance meter will not tell you anything about actual performance.

Hope this helps. And I hope others have some suggestions about this, too.
Barry
It's the 99% of the lawyers that give the rest a bad name.

aahz

Tue Feb 21 2006, 02:05AM

Registered Member #186 Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42

Where can you get a smooth rod of cold-rolled? I'm about an hour's drive from the nearest big hardware store, so I wouldn't want to go just to be dissapointed.

Anyway, I was planning on a 1/2" by 1" piece of iron or steel, slotted every 45 degrees with a dremel, with the coil being 1" as well.

Dave Marshall

Tue Feb 21 2006, 03:36AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

From my experience just about any average hardware store has atleast a limited selection of cold rolled steel. If not, its pretty universal stuff. Anyone with a machine shop, garage, or even an old storage shed probably has *something* laying in a corner somewhere. My local Western Auto has small lengths up to 3/8" diameter.

Dave

Quantum Singularity

Thu Feb 23 2006, 02:39AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

You could always call to see if they have it before you go. But if they dont maybe you could get a long carraige bolt and cut pieces out of it? Not sure how its composition compares to cold rolled steel. How would 'high carbon' steel work? And how ideal would a projectile be the was approximately the same size/shape as a .30-06 round that weighs ~100 grains? I'll have to find how to convert that to grams...

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