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Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Hello Folks, New to the forum first post. I have built a very rudimentary coil gun using 7500 mmf caps at 400 VDC and a number 14 wire coil. The barrel of this thing is a piece of K&S 7/16" .4375" OD brass tubing. The projectile is a solenoid plunger that is .392" in dia. The ID of the tubing is .407"
Since I am new to this I am using a variac to limit the voltage to 200 VDC until I get the bugs worked out of it and finalize the design.
I have tried finding a non conductive barrel material that is close in size to the projectile I am using but no luck. The brass tubing is as close as I can get.
Does anyone have an idea as to how much loss I might be seeing because of eddy currents in the tube? Should I persue getting a few projectiles turned so I can use another non metallic tube that has a different inside dia.?
Plans are to make this a multistage at some point but for now I want to get the basics down. What a blast this stuff is!!!!
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
With a low power system its relatively negligible. With permanently magnetized projectiles, or with higher power systems, it can actually be a substantial source of losses, as the eddy currents will actually oppose the motion of the projectile.
I'd suggest looking into this stuff as it is only minimally conductive, and very easy to work with. In my experience it isn't very smooth inside, which can cause some problems. A chunk of cold rolled steel wrapped in 120 grit sand paper, and a half hour of shaking the bejesus out of it fixes that right up though.
You might also look into a plastic called Delrin. McMaster-Carr sells it, as does Amazon's new raw materials site. Delrin tubes are a bit thick walled to be ideal, but its pretty rigid, and highly machineable. When I used it, I just stuck it on a lathe and thinned down the walls where I wanted to wind coils. Very low friction stuff as well.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
In my very low-power coilgun (12,000uF at 60vdc ~= 20J) the brass tubing caused a 24% decrease in velocity compared to a plastic firing tube. My brass tube was not slotted.
I expect the effect would be larger at higher energies, but this is already significant.
Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Hey Guys, Thanks for the help on the barrel material. The next evoloution of this project will either use a non metallic barrel or my friend will mill slots in the brass currently being used. This is quite a fun experience ....
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Coil gun barrel material
