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Muzzle Knife: inner or outer bevel?

A place to ask general spud cannon related questions.
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Muzzle Knife: inner or outer bevel?

Unread postAuthor: elstevedore » Wed Aug 08, 2007 9:22 pm

Hi I have built a lovely ABS combustion cannon. The muzzle knife is beveled along the inner edge of the barrel. I did it that way because I figured it would be less likely to split the potato and make a good seal, but I recently read a post by a guy who said his cannon blew up because he made it the same way. I've probably put about 50 pounds of spuds through this cannon with no problems other than the fact that it's sometimes tough to cram the spuds down the barrel because it's such a tight fit.

Should I be worried?

Do most of you bevel your muzzle knives on the inner our outer edge of the barrel?

Thanks!
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Unread postAuthor: hi » Wed Aug 08, 2007 9:27 pm

you need to sharpen the outer side, that way the barrel stays the same diameter.

if you sharpen the inside then the potato is cut to large for the rest of the barrel.

just hack it off and do it over, it will take 5 minutes and will save the cannon.
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Unread postAuthor: Pilgrimman » Wed Aug 08, 2007 9:28 pm

I would highly advise against beveling on the inside. It is highly likely that your cannon will fail, especially with better fuels such as propane. If your potato makes more friction with the barrel, it won't fly as far anyway. The outer beveling method will still produce a near perfect seal, and without the risk to life and limb. Just be safe.
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Unread postAuthor: Scotty » Wed Aug 08, 2007 9:31 pm

Bevel the inside just a little (bout 2mm) if you are worried about a tight fit.

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Unread postAuthor: hi » Wed Aug 08, 2007 9:34 pm

thats to much, you need to start on the outside and work your way in.

2mm is accually alot.

just hack it off and start over, it will ake 5 minutes and will save your cannonn.
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Unread postAuthor: SpudBlaster15 » Wed Aug 08, 2007 9:41 pm

I suggest both an outer and an inner bevel. If you have a 2" barrel, and the potato requires 30 pounds of force to be shoved down the barrel, it will begin to move at only ~10psi. Once it starts moving, frictional forces are much less, meaning a tight fitting spud will not negatively effect performance. Considering that combustion guns produce a maximum of around 50psi in the chamber, no amount of static friction created by a spud is going to cause a launcher to explode.

Combustion guns need the projectile to have some static friction in order to perform well. If the spud has no static friction, it will begin to move very early, and leave the barrel long before combustion is complete.
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Unread postAuthor: iPaintball » Wed Aug 08, 2007 9:49 pm

I know that this is off topic, but why is it that combustions can be so powerful? I mean, if the combustion only produces around 50 psi, wouldn't the launch be equivalent to a pneumatic firing at the same pressure?
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Unread postAuthor: Fnord » Wed Aug 08, 2007 9:57 pm

Ipaintball: A pneumatic of equal size would be more powerful.
The peak pressure is about 50-70 psi, but the pressure drops off quickly as the potato is traveling.
The only advantage a combustion has is there is no valve involved=no flow restriction/opening time.
At least, that's what I've heard.


I always use a small amount of inner bevel on my guns, but not that much.
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Unread postAuthor: hi » Wed Aug 08, 2007 10:09 pm

iPaintball wrote:I know that this is off topic, but why is it that combustions can be so powerful? I mean, if the combustion only produces around 50 psi, wouldn't the launch be equivalent to a pneumatic firing at the same pressure?


because there is no valve.

valves take time to open. if there isnt a valve there then there is nothing to open.
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Unread postAuthor: SpudBlaster15 » Wed Aug 08, 2007 11:00 pm

Combustion launchers do have valves. It's called combustion, which takes between 20 - 50 milliseconds to complete for an average sized launcher.

A combustion launcher would outperform a pneumatic launcher of equal size that pressurized to 50psi, because the gas within a combustion gun is hot, on the order of 4,000*F.
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Unread postAuthor: MrCrowley » Wed Aug 08, 2007 11:09 pm

Pilgrimman wrote:I would highly advise against beveling on the inside. It is highly likely that your cannon will fail, especially with better fuels such as propane. If your potato makes more friction with the barrel, it won't fly as far anyway. The outer beveling method will still produce a near perfect seal, and without the risk to life and limb. Just be safe.


Care to tell me why it would fail? If your sharpening the inside of your barrel it's just going to be harder to get the potato slug in and down the rest of the barrel, it's not going to make it much harder for the potato slug to come back out as it will already be the size of the barrel.
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Unread postAuthor: boilingleadbath » Thu Aug 09, 2007 9:16 am

Wow, it's actually possible to make a spud cutter that has a "forcing cone" all the way from the outside diameter?

Amazing.

That said, tight fitting projectiles <i>do</i> increase the maximum chamber pressure... although the data hasn't come in on how much they do so.
Regardless, a properly constructed launcher can survive the pressures created by a stuck projectile.
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Unread postAuthor: jimmy101 » Thu Aug 09, 2007 1:22 pm

I agree with the more experienced spuders (boiling and spudblaster), the barrel should have an inner bevel.

An inner bevel will not increase the risk of the gun failing.

An all outside bevel gives too low of static friction, the spud starts to move early.

An inner bevel increases the static friction and delays movement of the spud until more chamber pressure has built up. Basically the same as a burst disk(though the "burst pressure" is pretty low).

The more inner bevel there is the harder it is to ram the spud down the barrel. An all inner bevel takes a heck of a lot of force to ram the spud home. For a muzzle loader you have the problem of the ram rod breaking the spud. Not a problem with a breech loader.

It is generally assumed that a double bevel, inside and outside is a good compromise. Boosts static friction but not to the point where it is impossible to ram the spud down the barrel.

Even with a massive amount of static friction of say 60 pounds force, in a 2" barrel that means you need 60pounds/~3in<sup>2</sup>=20PSIG before the spud will start moving. Still well within the safety limits of a properly glued gun.

As the static friction is increased (by increasing the inner bevel) you will also increase the dynamic friction. Since dynamic friction is usually a lot less than static it has a fairly small affect on the guns performance. Indeed, I've calculated that at 15 pounds dynamic friction for a 77ci chamber and 2"x30" barrel dynamic friction takes just ~1% of the energy in the chamber.

So, add an inner bevel. How much is up to you.
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Unread postAuthor: MrCrowley » Thu Aug 09, 2007 2:50 pm

boilingleadbath wrote:Wow, it's actually possible to make a spud cutter that has a "forcing cone" all the way from the outside diameter?

Amazing.

That said, tight fitting projectiles <i>do</i> increase the maximum chamber pressure... although the data hasn't come in on how much they do so.
Regardless, a properly constructed launcher can survive the pressures created by a stuck projectile.


jimmy101 wrote:I agree with the more experienced spuders (boiling and spudblaster), the barrel should have an inner bevel.


I was arguing against sharpening the inside of the barrel so the I.D would be changing and not the O.D, im not sure if BLB understood what I was saying there but I was wondering how sharpening the I.D of the barrel would make the cannon explode.
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Unread postAuthor: paaiyan » Thu Aug 09, 2007 3:05 pm

I'd say bevel the outside, I thought about it a lot when I did mine. My thinking was, you bevel the inside and the spud is going to be compacted. now a raw spud has some elasticity to it, so when the inner bevel pushes it in, it's not going to just stay that way, it's going to continue to put more pressure on the inside of the barrel. This could prevent the spud from exiting. Even if it doesn't, it is going to increase the friction of the spud as it travels down the barrel, slowing it down.

I would say bevel the outside, then take some fine grit sandpaper and just barely bevel the inside, this will ensure the spud is sealed, while not causing undue friction.
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