Coilgun Barrel Composition, Armature Composition
aahz, Thu Feb 16 2006, 07:58AMIn order to reduce friction, I was thinking of ordering a couple of feet of Teflon Flouropolymer pipe (Look up 8547K61 on McMaster-Carr), though I'm a little concerned that it might not be rigid enough for the gun. It seems to be somewhere between 1/2 and 1/4 the rigidity of polycarbonate of the same thickness. Since I'm only planning a single stage at the moment, the length could be less than a foot, with a thickness of 1/16in. Has anyone had any experience with the stuff?
Also, I was thinking of using a ferrite bead as the armature for two reasons. One; to reduce eddy currents, and two; to help a small bit with aerodynamics. I figured with only a .02in total gap between the armature and the barrel, (.01 evenly around) a small amount of pressure would build up at the front of the armature, increasing drag a little. The small hole in the middle of the bead would help reduce that pressure. Basically.... Would a cylindrical ferrite bead, with an opening in the middle, be more efficient than a traditional iron, bundled/slotted iron, or a solid ferrite armature of the same mass?
Re: Coilgun Barrel Composition, Armature Composition
WaveRider, Thu Feb 16 2006, 02:39PM
Go with a slotted or bundled iron (or mild steel) bead. Ferrite has a low saturation field (of the order 0.25-0.5T), hence does not experience as much force as steel (Bsat: 1.8-2.2T). Bundling of thin steel rods (insulated from each other) reduces eddy currents considerably and maintains the desired high-field force...
WaveRider, Thu Feb 16 2006, 02:39PM
Go with a slotted or bundled iron (or mild steel) bead. Ferrite has a low saturation field (of the order 0.25-0.5T), hence does not experience as much force as steel (Bsat: 1.8-2.2T). Bundling of thin steel rods (insulated from each other) reduces eddy currents considerably and maintains the desired high-field force...
Re: Coilgun Barrel Composition, Armature Composition
TheMerovingian, Thu Feb 16 2006, 02:57PM
Remember that Bsat is the most important factor for choosing the projectile since the force depends mainly on it. Initial permeability isn't so important and can range between various values. No sense to reduce the eddy currents to zero if force will drop a lot. Use laminated projectile or slotted one. The skin effect of iron will confine the currents into the external shell of the projectile,so slotting will reduce them somewhat without much reduction in force (only a slight , because cross-section will reduce)
TheMerovingian, Thu Feb 16 2006, 02:57PM
Remember that Bsat is the most important factor for choosing the projectile since the force depends mainly on it. Initial permeability isn't so important and can range between various values. No sense to reduce the eddy currents to zero if force will drop a lot. Use laminated projectile or slotted one. The skin effect of iron will confine the currents into the external shell of the projectile,so slotting will reduce them somewhat without much reduction in force (only a slight , because cross-section will reduce)
Re: Coilgun Barrel Composition, Armature Composition
Quantum Singularity, Sun Feb 19 2006, 09:27PM
Pertaining to barrel and armature composition... is there any kind of rule of thumb or way to calculate the crushing affect on the barrel? Like say plastic is good up to a 100J pulse but higher needs a metal barrel (example)? I know polycarbonate is a very strong plastic is there a cheap source for this stuff in tubes? Any do clear tubes cause problems with optical triggering?
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.
Quantum Singularity, Sun Feb 19 2006, 09:27PM
Pertaining to barrel and armature composition... is there any kind of rule of thumb or way to calculate the crushing affect on the barrel? Like say plastic is good up to a 100J pulse but higher needs a metal barrel (example)? I know polycarbonate is a very strong plastic is there a cheap source for this stuff in tubes? Any do clear tubes cause problems with optical triggering?
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.
Re: Coilgun Barrel Composition, Armature Composition
..., Sun Feb 19 2006, 10:40PM
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Me thinks...
..., Sun Feb 19 2006, 10:40PM
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Me thinks...
Re: Coilgun Barrel Composition, Armature Composition
aahz, Sun Feb 19 2006, 10:52PM
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
But like it's been said so far, ferrite would be too quick to saturate, so I'm probably going to go with a hacked up and slotted iron bolt instead. As for aerodynamics, the slots in the bolt would serve the same purpose as the hole in the middle of the ferrite, so that problem is solved too.
aahz, Sun Feb 19 2006, 10:52PM
... wrote ...
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Me thinks...
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Me thinks...
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
But like it's been said so far, ferrite would be too quick to saturate, so I'm probably going to go with a hacked up and slotted iron bolt instead. As for aerodynamics, the slots in the bolt would serve the same purpose as the hole in the middle of the ferrite, so that problem is solved too.
Re: Coilgun Barrel Composition, Armature Composition
..., Sun Feb 19 2006, 11:37PM
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
But it is not really worth arguing over, go ahead and give your bolt a try and see how the coil handles it.
..., Sun Feb 19 2006, 11:37PM
aahz wrote ...
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
But it is not really worth arguing over, go ahead and give your bolt a try and see how the coil handles it.
Re: Coilgun Barrel Composition, Armature Composition
Electroholic, Mon Feb 20 2006, 07:16AM
The crush problem is already solved
search for "another coil gun site"
I used the same method, no crushing at all.
And for projectile material, i would say cold roll steel rods.
but i guess you can always test different materials with a NiB magnet?
Electroholic, Mon Feb 20 2006, 07:16AM
The crush problem is already solved
search for "another coil gun site"
I used the same method, no crushing at all.
And for projectile material, i would say cold roll steel rods.
but i guess you can always test different materials with a NiB magnet?
Re: Coilgun Barrel Composition, Armature Composition
aahz, Mon Feb 20 2006, 07:46AM
My original issues with rigidity weren't related to the crushing force of the coil, rather the flexibility of the barrel. I didn't want something that would bend under it's own weight, or something excessively flexible....
As far as projectile material.... if saturation density is the only real factor, iron or steel would be the best for its availability.. so that's what I'm going to try out first....
http://www.oz.net/~coilgun/theory/materials.htm
aahz, Mon Feb 20 2006, 07:46AM
My original issues with rigidity weren't related to the crushing force of the coil, rather the flexibility of the barrel. I didn't want something that would bend under it's own weight, or something excessively flexible....
As far as projectile material.... if saturation density is the only real factor, iron or steel would be the best for its availability.. so that's what I'm going to try out first....
http://www.oz.net/~coilgun/theory/materials.htm
Re: Coilgun Barrel Composition, Armature Composition
Quantum Singularity, Tue Feb 21 2006, 01:23AM
I assumed attractive forces were between the coil and a ferrous object nearby, not at some angle, but between them. One attracts the other right? The greatest 'barrel curshing force' would be when the projectile is directly in the center, the coil would try to crush in around the projectile correct? Before the projectile enters the coil the force is more horizontal with the plane of travel. In a misfire or when some problem causes the projectile to not exit and there is still much charge left then all the force would be on the barrel, and I have heard of barrels get crushed this way. In a good working design obviously the discharge should be over by the time the projectile gets dead center.
I saw that website before but thought the barrel-less design had some inherent problems (not to say that a design w/ barrel doesnt have problems too). I wasnt sure just how much force adding superglue to the layers would hold up to. And if you only have 1 or two layers then the super glue coating wouldnt be as strong as opposed to having 8 layers coated with the stuff. I also saw a movie clip on that site before where one coil goes flying off when fired...
Quantum Singularity, Tue Feb 21 2006, 01:23AM
... wrote ...
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
I assumed attractive forces were between the coil and a ferrous object nearby, not at some angle, but between them. One attracts the other right? The greatest 'barrel curshing force' would be when the projectile is directly in the center, the coil would try to crush in around the projectile correct? Before the projectile enters the coil the force is more horizontal with the plane of travel. In a misfire or when some problem causes the projectile to not exit and there is still much charge left then all the force would be on the barrel, and I have heard of barrels get crushed this way. In a good working design obviously the discharge should be over by the time the projectile gets dead center.
I saw that website before but thought the barrel-less design had some inherent problems (not to say that a design w/ barrel doesnt have problems too). I wasnt sure just how much force adding superglue to the layers would hold up to. And if you only have 1 or two layers then the super glue coating wouldnt be as strong as opposed to having 8 layers coated with the stuff. I also saw a movie clip on that site before where one coil goes flying off when fired...
Re: Coilgun Barrel Composition, Armature Composition
Dave Marshall, Tue Feb 21 2006, 01:54AM
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
Dave Marshall, Tue Feb 21 2006, 01:54AM
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
Re: Coilgun Barrel Composition, Armature Composition
Barry, Tue Feb 21 2006, 02:02AM
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.
Barry, Tue Feb 21 2006, 02:02AM
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.
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.
Re: Coilgun Barrel Composition, Armature Composition
aahz, Tue Feb 21 2006, 02:05AM
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.
aahz, Tue Feb 21 2006, 02:05AM
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.
Re: Coilgun Barrel Composition, Armature Composition
Dave Marshall, Tue Feb 21 2006, 03:36AM
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
Dave Marshall, Tue Feb 21 2006, 03:36AM
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
Re: Coilgun Barrel Composition, Armature Composition
Quantum Singularity, Thu Feb 23 2006, 02:39AM
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...
Quantum Singularity, Thu Feb 23 2006, 02:39AM
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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