SpudFiles

So, for about a month now, Ive been working on an automatic bb-firing thompson submachine gun replica.

<img src="http://de.academic.ru/pictures/dewiki/1 ... ompson.jpg">

Initially, I had planned on a blow back system with a wooden frame. I've since come to realise that there is no point risking failure when I could just use a tried and tested auto valve that is easy to make.

I have all the materials I need to make the valve. It will be a direct copy of JSR's rattlesnake V3, with a co2 inflater as the air source (16 gram cartridges) It will pop off at 300 PSI with about 3.5 cm cubed of chamber volume to give a nice bit of energy to an 0.25 gram bb.

The feeding system will be a spring loaded mag that seals with the barrel so that no annoying blow-forward or blowback bolts will have to be dealt with- the bbs will fit in the mag tube quite nicely so there won't be too much power loss with that system.

Making the valve and feeding system will be the easy part, as I have all the stuff and tools I need to get that done.

The problem is the frame: I have tried to make it out of wood and got pretty far, but the more I work on the wood and chisel it out and stuff the more fragile it gets until it just becomes a splintered mess.

So the plan is to make the internals, and then cast a mould of the gun and cure an epoxy frame around the internals to create a really solid gun. The feeding system and holes in the barrel for hopup and feeding would be drilled after curing because obviously the inner workings have to be sealed off from the epoxy.

My dilemma is, how to make a mould for the epoxy, so that I can cure a mould of the thompson around the internals?

EDIT 2 July 2010- Today I've started making the popoff valve. I'm using a 1.6cm inner diameter syringe as the piston, and a 0.75cm inner diameter steel pipe for the chamber. The chamber will be 6 cm long with the air inlet on the right.

<img src="http://img194.imageshack.us/img194/149/95296582.gif">

PVC tape is wrapped around tubes to make them fit into others and allow them to be glued and epoxied.

I forgot to notate that the red is 1/2 inch pvc pipe.

So this should be done tomorrow and then I've gotta figure out the mould for the thompson body cast.

EDIT 5 JULY 2010- So I went ahead and made myself what I hoped to be an autovalve- what I got was not even a farter- All it did was piss out gas in a very provocative way- almost like it was saying "Fuck you I'm just not going to work now go home you lose."

Well, If at first you don't succeed...

Seriously, I don't get why the gas just freaking pissed out, but I'm not going to remake it with the same parts. I'm going to make an epoxy piston in 1/2 inch pvc with bath plug rubber on the end to seal. It will hopefully be similar to JSR's V2.

Good luck, the theory of the pop-off valve is pretty simple and well explained in my various threads so if you follow the specified valve seat:outlet and valve seat:piston dimension ratios you should have no trouble making it work.

I usually make moulds out of foam/plastic sheet/cardboard, anything thin, workable and non-porous, herewith some examples:

I like the diagram, I wonder if an air cushion could be used instead of a spring? It would be eaiser to adjust the fired rate by adjusting the air spring pressure.

If built exactly as in your diagram, the valve will "hang" and you will have nothing but an amusing farting machine. The piston seat area should look something like the diagram below, with the outlet exposed the instand the piston begins to move. Since more air is coming in than can come out (hence piston seat area>outlet area) there will still be an overpressure that causes the piston to pop.

jackssmirkingrevenge wrote:If built exactly as in your diagram, the valve will "hang" and you will have nothing but an amusing farting machine.

... of which you have about twenty-seven and a half.

You also need to restrict the flow at the end inlet of the chamber. As small as possible, or the valve won't cycle properly.

Ok, I will change it so that the steel pipe goes a bit further in and makes that gap you're talking about, thanks for the advise.

Ranger wrote:I like the diagram, I wonder if an air cushion could be used instead of a spring? It would be eaiser to adjust the fired rate by adjusting the air spring pressure.

Precisely what I did here, but I think a coil spring is less of a headache and stil ladjustable with a screw or shim system.

... of which you have about twenty-seven and a half

Sad but true, though now that the principles are understood the collection has stopped growing

You also need to restrict the flow at the end inlet of the chamber. As small as possible, or the valve won't cycle properly.

Using a pump like the one below inlet flow can be adjusted appropriately, and important feature if you want to be able to control rate of fire.

Ok, I will change it so that the steel pipe goes a bit further in and makes that gap you're talking about, thanks for the advise.

Try not to leave too much dead space though as this will reduce power. Remember:

Mu CO2 pump is a genuine innovations air chuck, and I can open it as little or as much as I like.

I am trying to reduce the dead volume as much as possible by placing the gas outlet as far to the left as possible to reduce the amount of pipe travel I will need to get the gas to the barrel.