Hi,
my coil was 4 cm long with one layer
the diameter was about 1 mm
is there any thing wrong with that ?
Not entirely sure what you mean. With that cap charged, as IanP siad, thats a fair bit of energy. What determines how powerful the magnetic field is is current and number of turns. Although, even a few turns around a plastic tube should make a nail near it twitch.
The more turns you have, the more resistance the coil has, the less current can flow (I=V/R). Its a careful balance, so thick wire would be a good idea, to lower the resistance, and allow for lots and lots of turns. Maybe gauge #28?
1mm diameter? of the wire? or of the entire coil? Thats VERY small for a coil in this sort of application. I would put together a simple 'test' coil to check that your circuit is working correctly......THEN you can design a coil for your app.
An obvious thing to check would be, is your projectile magnetic? Most nails are iron, so that shouldn't be a problem. Also, connect your coil to a 12V supply, keep the current low, say, 1A. Now you have a solenoid, move some iron things around it to check if the field is there/how strong it is, you also might want to check that the coil is actually conducting
If you've tried the above, and it makes a good solenoid, a decent electromagnet, then the problem is with your circuit. The cap may be disacharging via some other path.
Is the coil wrapped around a non-magnetic former? I can only assume you want a hollow coil, with a nail just outside of it, so that when you discharge your cap through the coil, the nail is pulled into the coil and thrown out the other side. I once used a spool of gauge 28, I bought from an electronics shop. Its the spool the wire came on, I just soldered the wires to my circuit and let her rip.
Some examples of what your coil should look-like/do:
http://www.global-defence.com/1997/High-speed/CoilgunLG.jpg
**broken link removed**
http://www.sampson-jeff.com/rsoh/010420/rsoh05.jpg
So, just to recap, some simple checks you can do:
Check continuity of coil.
Check power of magnetic field with the coil carrying
DC
Check if projectile is magnetic (although, an aluminium ring WILL move)
Check diodes around the coil, one might have blown and be shunting the power, bypassing the coil.
Once you have a working circuit, that can create a magnetic pulse, then you can worry about your 'ejecter seat' mechanism.
I hope this helps in some way. I'm sorry if I'm asking questions that are obvious, or have been asked before, but once I get a good idea of what you want, and what your coil looks like, then I can try and help you solve your problems.