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auto-piston prototype

Built a BB Machine Gun? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 2:04 am

The tube you casted in needs to be consistent as well, if it sin't a perfect circle then it's going to jam. This is why I love syringe tubes so much :D
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Unread postAuthor: rna_duelers » Wed Jan 30, 2008 2:07 am

I used brass tubing.I thought it might of been the problem but the ID seems to be pretty consistent in my mould.I'll figure it out,have you seen the new musket since it's been finished?
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 2:16 am

I thought I had commented, clearly I didn't - but I wanted to say it's looking *really* good, especially with the stock! If i were you I would remove the plastic red bit from the blowgun trigger and paint the blowgun black, then polish up the other metal bits and give them a couple of coats of clear laquer.
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Unread postAuthor: rna_duelers » Wed Jan 30, 2008 2:21 am

That is the plan :wink: . Although I do have one serious problem,my dad. . .He wants me to pull it apart,because he says if the cops catch me with that there is no getting out of saying it's just used to launch spitballs etc,because it looks and shoots like a real gun.But he hasn't won yet!Although he does have a very valid point.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 2:27 am

Isn't there a law about curios and relics in Oz, making them an exception compared to other air weapons? You should check a few garage sales and look for an antique blowgun, then you could tell the cops it was made by your great great grandfather....
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Unread postAuthor: rna_duelers » Wed Jan 30, 2008 3:33 am

You could be onto something here.But a cop around here isn't going to give a rats ass about some Relic or Antique they will take me and it together and lock me up,you don't know what it can be like down here with these things.Especially with the crooked cops.
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Unread postAuthor: SPG » Wed Jan 30, 2008 6:36 am

Quick question/thought:

It looks like we're (well ok you lot are) getting into territory where the materials available will begin to restrict the ideas floating around. Particularly in this case the springs. We're beginning to see set ups that need to be finely balanced by these springs, and that means a lot of shopping around for just the right thing, or a lot of calculations using Hook's etc, to create piston set ups which work with commercially available springs.

So a thought, a lot of people are masters at pistons, and at what I'm going to call "gas management", tailoring chamber sizes to pistons, to whatever. So why not use air as the "spring" material? Looking at Jack's latest.

Image

You can see there's the portion above the piston where the spring is housed. If the piston itself is providing an air tight seal for the chamber then this portion can also be made air tight. Rather than use a spring to reseat the piston, we could equally well put a schraeder in this portion and use pressurised air to reseat.

This should give you an infinitely adjustable "spring" and it uses technology and physics which we're already confident with.

Just my two-penny-worth.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 7:36 am

Good point, but it would also create the need for an air-tight seal, that means friction which is not a good idea. Also, it jars a bit with this incarnation of the design that combines the piston with the breech and makes more sense in my view:

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Unread postAuthor: SPG » Wed Jan 30, 2008 9:50 am

True an air-tight seal does cause friction, but surely in this instance that would be overcome by the air pressure? So the firction wouldn't prevent the action from cycling, all it would do is slow it down.

But yeah it doesn't make for successful integration into a combined valve and breach set up, well not very easily anyway.

(notice how I ignored mister arsehole)
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Unread postAuthor: Antonio » Wed Jan 30, 2008 1:56 pm

jackssmirkingrevenge wrote:
Image


Now looking at the airchamber. Why is it there? I think u could do with a valve and the breech all in the same diameter tube. This makes it more simple. Is the chamber supposed to control the rof? As altering the tension of the springs would bring the whole system out of balance (automatic fireing mode). Btw, as we are using a blow gun. Why not use it with a piston parallel to this system that acts as an actuator. With a spring on one side it wont stick. So its ganna look more like a spyderpaintball gun but then instead of a spring pushing the hammer forwards, it pushes it back after the cylinder (with blowgun or solenoid) has pushed it back. I am not sure which paintball gun uses this action but I have seen an animation before.
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Unread postAuthor: Hawkeye » Wed Jan 30, 2008 4:10 pm

I don't see how this system is going to be started. Pressurizing from behind the green ball will just shut it off, push the rod slightly forward and lock the whole system up. I think you still need a way of manually cocking the barrel/ bolt/ hammer to get it started.
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Unread postAuthor: Skywalker » Wed Jan 30, 2008 11:16 pm

The spring strength will be relatively strong. The spring will hold the piston back and keep the valve open until the chamber reaches firing pressure, and then it will start to move forward. At that point, the area on the face of the piston that the air pressure is applied to increases (from the area of the small connection port -> the area of the piston tube). This pushes the piston forward, overcoming the spring.
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Unread postAuthor: Hawkeye » Thu Jan 31, 2008 8:37 am

Then I can see the spring strength being a huge issue. One strong enough to hold the chamber open and yet go through such a dramatic shortening will be tough to locate.
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Unread postAuthor: jackssmirkingrevenge » Fri Feb 01, 2008 5:59 am

Hawkeye wrote:One strong enough to hold the chamber open


That's not really a problem, the entry port in the chamber doesn't need to be very big, even a schrader valve will do.
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Unread postAuthor: Skywalker » Fri Feb 01, 2008 2:17 pm

But you also have to hold against the small diameter seal to the firing chamber, which is larger than the little entry port valve. I still think it'll work. Just keep your bolt stroke pretty short.
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