SpudFiles

I've been on winter break and there's no better way to spend time and Christmas money than building a hybrid!

Here's some specs for anyone interested.
Chamber: 2" galvanized steel, ~1135 mL
Barrel: 1.5" Intermediate Metal Conduit
Ignition: stungun (thanks again Mark!)
Fueling: Inject propane via syringe through 1/4" ball valve

Well I have a nearly unlimited amount of golf balls, and I needed a new golf ball gun due to the first being confiscated by authorities. The first was just a small PVC sprinkler valve gun that could lob the balls a couple hundred yards despite staying around 90 psi. I decided to upgrade.

Putting the chamber together was pretty easy. I had to make a new spark plug but that's easy too, just a wire, hollow schrader, and a lot of epoxy. My fueling system is 100% recycled parts, which gives it a "fun" factor of being imprecise! I thought finding a barrel would be the most difficult part of the build but that fell quite nicely into place as well. Everything is steel and the gun ends up weighing around 22 lbs.

I never knew how awesome wire brushes were until I tried cleaning some label goo off one of my fittings and ended up stripping the dull grey finish off the fitting itself. I then proceeded to wire brush every part of the cannon except the barrel, giving the whole thing a nice shiny steel look.

My ignition is a little ghetto but functional and feels fine, so I'll keep it. To fire I just connect a battery wire to the chamber. I bought a 10 foot pipe for the barrel, meaning to cut it to 4 feet and 6 feet, but I failed at eyeballing and now I have 4.5 foot and 5.5 foot barrels. I think I should be able to get somewhat near supersonic at 6X or 7X, but I might need a bigger chamber/longer barrel. Oh well!

I dry fired it last night for the first time and it actually worked! It surprised me with a loud bang and a sweet jet of fire out the empty barrel

If I forgot to mention anything feel free to ask or demand!

I might be able to post damage once I get to shoot the gun, but that might not be for a week or so til I go up to his ranch. Video could also happen presumably.

Noice! And syringe fuelled too JSR approved!

This should put golfballs through pretty much anything, even below 10x, looking forward to seeing it shoot!

Nice. Looking forward to video and damage pics.

Nice high tech ignition system!

Thanks guys!
I've dry fired it a few times at 2x and 3x in my back yard and it's really loud. I've been thinking all day about where I'll be able to shoot it without getting in some kind of legal trouble...

So far my plan is if I see police roll up I'm gonna stash the propane tank and say it's a project for a material science class to see what pressures it takes for certain materials to fail. Sounds fine right? With a pressure gauge and a bike pump out it seems like a reasonable answer. The barrel is there to direct the sound away from my ears of course.

Nice work.

Just keep your barrel unthreaded and chuck on a piece of pipe with an endcap, screw it in with no Teflon tape. Works perfectly for setting up burst discs without a tonne of noise.

yo yo!! The gun looks sweet! Listen i have to deal with the cops too man and i came up with great solution. Hopefully u own a smartphone. Go into the app store and get the police scanner app. It works like a dream. I actually have to drive from park to park testing my guns. Whenever someone complains i hear the call come over the air and im out. U always got to be one step ahead of them. In order to be the man u have to beat the man!!!!! They would have to pry my spud gun from my kung fu grip!!!!

I took this hybrid up to 6x today and finally shot something out of it! Turns out that launching a projectile (a pair of socks) dampens the noise a lot, I'm much less worried about police crashing my pew pew party now.

I do have a problem though. I can't get an aluminum can burst disc to work for the life of me. I used them all the time with my old hybrid (1/2" union) but for some reason in this cannon the burst disc edges crimp a little at around 30 psi and leak the mix out. I don't think it's a matter of having the discs perfectly centered but that's all I can think of. I'd like to use cans as discs since they can take way more pressure than aluminum foil layers and I'd like to have burst discs that can take 300+ psi...

I have had similar issues.

How is the fit when inserting the disk in the union? I think that if the diameter is too tight it makes them crimp more. On the other hand soda cans just crimp easier than foil. Also soda cans are slicker and creep more under pressure. Can you tell if the foil kinks due to creeping (slipping) or immediately upon tightening the union. If it is creeping, tightening the union tighter may help. I found that really tightening helped, but it is a hassle to carry big wrenches or big channel lock pliers.

Another option is to use craft foil sold for tooling (embossing) designs in. Craft stores like AC Moore or Michaels stock it. It is much heavier than kitchen foil. I think it is a bit thicker than soda cans but softer and less slick. It comes in sheets and rolls. It is similar to disposable roasting pan foil.

I have not yet tried it but some folks apparently grind flat surfaces and/or use rubber gaskets in their unions.

Please report back on your progress. I am sure your results will be good info for the rest of us.

I have been meaning to do some testing myself but got more tied up in some piston hybrid and pneumatic projects.

Nice job noname! Glad that circuit still works good.

As PeteS said, grinding the union flat and adding a gasket could work better. IIRC DYI did that to his burst disk cannon and I was set to follow suit on the steel hybrid that I never finished. My smaller steel hybrid I used foil in, though, and it sealed fine.

It is a fair bit of work to grind the sealing face flat, as well. An angle grinder would help a lot before you got to the point of using sandpaper.

Good luck!

mark.f wrote:It is a fair bit of work to grind the sealing face flat

It should be easy if you have a lathe. Other cast iron fittings seem to machine fairly well, so I guess I have no excuse for not having done it. Maybe I will head down to the shop, chuck up the parts in the lathe, and give it a go now.

UPDATE:
OK so I just did a 1" union on the lathe. It took maybe 15 minutes total. I will have to do some testing, but today isn't a good day for the noise. I wonder if the machined cast iron will grip and seal the aluminum well enough without a gasket. I think the disk might be less likely to creep without a gasket if the seal is good enough.

I'll have to find the 1-1/2" union that I have around somewhere and do that one too. It will probably go back on my 1-1/2" burst disk hybrid replacing the camloc that seems prone to occasionally letting the disk slip and leak.

It would be nice if they would seal well enough if hand tightened, but I am not holding my breath on that.

The sealing face is really flat, I can't imagine it being anything other than a flush 90 degrees. Which is good because I don't have an angle grinder or lathe to use! I also forgot to add that I've been using a gasket the entire time. I'm using the exact same setup I used to with my old 1/2" hybrid, just scaled up: chamber sealing face, gasket, burst disc, barrel piece of union.
The aluminum can burst discs generally hold pressure until I get to around 30 psi, then the leaks form. I've tried sandwiching a soda can disc in between foil discs but that didn't work either. I guess I'll just use foil until I find a better method, it's not the end of the world.

PeteS wrote:I think that if the diameter is too tight it makes them crimp more

This has been my experience, making them slightly undersize and tightening properly should solve the problem. Is there a rubber gasket of some form involved in the holding of the burst disk?

noname wrote:The sealing face is really flat, I can't imagine it being anything other than a flush 90 degrees.

Are you sure? Every iron one I have seen had angled faces, and your crimped disks look like the ones that came out of my union with angled faces.

The ones from the local home depot or lowes have maybe a 45 degree angle and some bronze on the sealing faces. When I turn them down to make the faces flat the bronze gets machined off.

If they are already flat folks might be interested in the brand and source. Please post those if they indeed are flat. It may save folks some grinding.

EDIT:
Just speculation on my part but... I have to think that creeping or other slippage is more likely with a gasket than without. I have more problems with it in a rubber gasketed camloc than I did in iron non gasketed unions. The iron unions were really tightened though. I have yet to experiment with the flat face union I just modified, so will not speculate much on how different it might be.

EDIT again:
I machined both faces so the full surface from the inner diameter out to the threads is flat on both faces. This makes for much more contact surface area, but does cut the male threads shorter.

The unions look something like this before modification:

That's def a home depot union. I machined mine also and bought the gasket material and that bitch seals up nice and tight. I don't have to use the pvc anymore.. Thank God!!