SpudFiles

Hola chicos,

following my brief stint at home, I returned to Spain with a few hundred mL of epoxy in my suitcase. Add that to some PVC piping from the kind guys in the Maintenance department, a syringe scrounged from the lab and an old inner tube from the garage and there you go, the biggest coaxial I've ever made. It feels a little primitive compared to my other efforts, given limited facilities I had to build it, far far away from my workshop.

How does it shoot, you may ask? Well, I don't know if it does yet as the last pouring of epoxy still has to cure. My concern is that the piston diameter is very close to the barrel diameter, so sealing might be an issue if the alignment isn't close to perfect. As always with epoxy, it's a matter of wait and see! I'll let you know in a few hours.

Meanwhile, have a photo and a diagram:

ei carumba ! thats pretty big lol. The biggest ive made was like 3" long and it didnt work . lol

Haha typical JSR keeping to your minimal pilot volume, let's just hope it doesn't come to bite you in the rear if the piston doesn't work.

What do you plan on shooting?


I may as well get it out of the way for anyone who may happen to ask...

Q:What's your piston made out of?
A: Epoxy. Deal with it.

ajdevi92 wrote:ei carumba !

It's not huge in terms of what's been featured on this forum, but on a personal level, relatively speaking, it's massive!

Haha typical JSR keeping to your minimal pilot volume, let's just hope it doesn't come to bite you in the rear if the piston doesn't work.

Don't curse it, I already have a bad feeling... should have gone for a smaller barrel!

What do you plan on shooting?

Ammunition or targets?

jackssmirkingrevenge wrote:

Haha typical JSR keeping to your minimal pilot volume, let's just hope it doesn't come to bite you in the rear if the piston doesn't work.

Don't curse it, I already have a bad feeling... should have gone for a smaller barrel!

I always have the same feeling, with most of my cannons. Even with the V.A.L where I knew nothing could go wrong, it probably would. But it didn't

jackssmirkingrevenge wrote:

What do you plan on shooting?

Ammunition or targets?

Hmm, both

MrCrowley wrote:I always have the same feeling, with most of my cannons. Even with the V.A.L where I knew nothing could go wrong, it probably would.

The joy of epoxy though is that that feeling is prolonged over hours of nail biting

Ammunition, anything that fits in the barrel really, though I do have a stock of airsoft BBs so (again, if it works) shotgun! As to targets, we'll see

edit: aaaargh! Well it seals perfectly well, but as to firing, just exhausts everything through the schrader. Diagnosis: too little difference between piston and barrel diameter, it should have been an obvious design flaw from the start but I was perhaps a little too enthusiastic to get building

Ah well, back to the drawing board.

I hope for you that this thing works. The piston is unserviceable (unless you want to waste epoxy) and has almost no frontal area...
We already had the discussion about such low frontal areas some time ago where you expected a thin layer of air to push the piston, and you'd better be right. If not, you are screwed

Its indeed massive for your personal standards. How did you came up with the idea of making something that "huge"?


Wait.... is that a load of wrapped duct tape?! *coughghettocough* You should definitely take a better look at this great epoxy casting tutorial over here. Or PM the creator of it, he is known to be the master of epoxy casting.


EDIT:
Oh just saw your edit

edit: aaaargh! Well it seals perfectly well, but as to firing, just exhausts everything through the schrader. Diagnosis: too little difference between piston and barrel diameter, it should have been an obvious design flaw from the start but I was perhaps a little too enthusiastic to get building Confused

Seems your theory FAILED.

Sorry bout that, are you going to try and rip the back of the gun off?

Wait is the porting about the same size as the piston?
To me, it looks like the barrel is smaller but if you trying to seal something with the same diameter as the piston, you know that wont work

O, ic that im a little late

But its ok, just salvage the piston, have a smaller porting, and there you go!

cant you just put in a bigger schrader valve?

nice try, but it is missing some more volume, or gap in between the piston and compression tank. how bout shaving down the epoxy?

Hold-up, so you made this in Spain without any of your tools? Pretty impressive.

psycix wrote:I hope for you that this thing works. The piston is unserviceable (unless you want to waste epoxy) and has almost no frontal area...
We already had the discussion about such low frontal areas some time ago where you expected a thin layer of air to push the piston, and you'd better be right.

I'm still right ;p The issue here is the difference between the piston diameter and the diameter of the barrel porting. The area over which that thin layer of air has to push on makes a big difference, especially with a poor pilot like a schrader.

Its indeed massive for your personal standards. How did you came up with the idea of making something that "huge"?

I don't want it loud, so I figured I could get power with less pressure and noise

Sorry bout that, are you going to try and rip the back of the gun off?

Thanks for the support - and probably ;p

jackssmirkingrevenge wrote: I'm still right ;p The issue here is the difference between the piston diameter and the diameter of the barrel porting. The area over which that thin layer of air has to push on makes a big difference, especially with a poor pilot like a schrader.

I think it's the schrader. Chuck on a ball valve and you'll be fine IMO.
I've got quite a tight tolerance on my piston where you can only just see the gasket around the barrel when it's sealed.

If you can get that initial pressure difference to happen a lot faster, the piston should come unseated enough for the air to get in the gaps between it and the seat and push it back.

Well Jack you could have failed harder...
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