SpudFiles

The maximum I'd be able to do as of now would be 10x mix, although even that would be pushing the limits of my pump. I might get a better pump (220ish PSI) later on. But for now let's say I'll be going for 9x, is there any cheaper solution than a 30 dollar ball valve (the cheapest I found so far including shipping and handling, although I'm still waiting for a response on a few others)?
Edit: I found a few on eBay for 20$ a piece, I'll probably just order one of those to be safe, unless I find another one cheaper elsewhere or a different solution

I was about to go out and buy parts for a hybrid but in searching for unions I came across this It's called a dielectric union or something like that (this is the first time I actually came across it XD)
Anyway, most stores boast that it will hold up to only 200 something PSI, I was wondering if this was true or whether it would hold up to 10x mix. This would be a lot easier than finding someone to face down the tapers on a regular union.

It should work, although you'll obviously need to adapt one side to copper tube. Copper tube makes for fairly consistent, safe barrels anyway, so on a low pressure design like this there isn't much against using it. The pressure rating on parts like these usually has more to do with their shoddy tolerances causing leaks than with actual rupture, although, as usual, you should either hydrotest to an appropriate multiple of the working pressure or only operate the gun remotely.

Also note that it's possible to do a pretty decent job of removing the lip from a union by yourself, using only a bench grinder and a belt sander. A lathe would be nicer, but several Spudfiles members have done beautiful jobs of it with only those basic tools.

I might just get one of them (dielectric), I believe Ace hardware had one they called a galvanized dielectric union with threaded pieces on both sides (I wanted to use a piece of conduit pipe for the barrel). If I do find someone to modify a regular one I'll get it (I may or may not be able to get to a bench grinder) since it seems a bit safer and looks to have a little wider flow.

An old comment that's been said many times before but one that bears repeating from time to time....

In hobbies such as this there are often only two ways to do something: Expensively or dangerously.

Choose wisely.

I scraped the dielectric union at the store, I didn't quite trust it. That and I figured I could make a regular union work. I actually found an o ring whilst walking around the plumbing section that fits into the area between the tapers on my 3/4'' union perfectly! I've only tried it up to 60 PSI (haven't sealed the threads on the chamber yet, so it's leaking a lot) and it seems to do the trick.

An unrelated question concerning piston hybrids though, has anyone ever tried using a burst disc for their pilot valve? I figure it's got to be simpler than a spool valve and have a higher amount of flow than a pop off valve, the downside being you have to replace the discs every shot. Just curious since of all the ones I've looked at on here I haven't seen such a thing. Or would it be an absolutely abysmal idea (I'm full of those)?

The point of a piston hybrid is to trade off some performance for increased ease of use. Using a burst disk pilot on a piston hybrid combines the difficulty and decreased performance of a piston hybrid with the inconvenience of a burst disk hybrid.

I just realized that later today. Sorry for the spamming of stupid questions XD

It's something I did consider and while DYI is right, I believe there's another reason too. The beauty of the spool valve is that it opens AS SOON AS peak combustion pressure is reached. You can dial it in very nicely so the valve is opening when you want it to. With a burst disk, you'd have to have the disk strong enough to safely survive the pilot pressure (in my cannon: 300PSI at a 10X mix) yet be weak enough to burst soon after the piston starts moving back and compressing the pilot air. Maybe it's actually easier than I imagine and you can actually have a burst disk dialled in to burst at a pressure significantly higher (e.g. 400PSI) but you might risk slowing the valve down or even having it rebound prematurely.

Additionally, you have flying shrapnel coming out the rear

A spool valve isn't so difficult to make, I was actually surprised how easy it was to get one to seal. I just drilled a hole through a dowel for the bolt, made a rough groove on the outside of the wooden dowel with a hacksaw blade and a random o-ring seemed to work. I eventually changed it and made a small o-ring groove from a series of washers of various sizes held by nuts.

Just uploaded this, by the way:

MrCrowley wrote:Additionally, you have flying shrapnel coming out the rear

Only if you fudge up the design. There's absolutely no reason for a burst disk to generate shrapnel if "no shrapnel" is a design goal.

Fair enough, though it goes back to what DYI said about trade-offs between performance and ease of use.

You could even skip making the burst disks fail in specific ways and just chuck on a shroud to catch any shrapnel.

MrCrowley wrote:Additionally, you have flying shrapnel coming out the rear

That's part of the fun! (Unless you're standing behind it)

MrCrowley wrote:Just uploaded this, by the way:

I actually watched that twice xD

I think I've got all the knowledge I need to start designing a piston valve hybrid, I will probably do it over the school year and then get a summer job to afford the parts and finally I get to build it (hopefully). I may need help with the piston (machining and perhaps design, although I will try to do it myself as much as I can ) and perhaps the housing (length and whatnot) but otherwise I think I can figure it out
Random question though, how large should my pilot area be to avoid piston bounce? (If I have 2'' piston housing and piston sealing against a 1.5'' barrel)

Well you only need the piston to move back about an inch or so but you also need to fit in a bumper.

Really, you want to keep the pilot valve as small as possible so you have less air to vent. It may be trial and error. Instead, focus on maximising flow through the pilot valve. A burst disk is an obvious solution if you want to go that route.

Thanks, I'll have to think about it all and start designing. Maybe I can get the different parts and pieces and build small sections over the school year. Then I'll be able to thoroughly test each component and make adjustments and things and by the time I get the whole thing built it'll be perfect (hopefully!). Sorry to bore you all, I just get excited talking about it

~NooooooB~ wrote:Sorry to bore you all, I just get excited talking about it

I was looking at notes I had taken in uni during my first semester the other day and I saw pages and pages devoted to designing Mjollnir or calculating o-ring tolerances and piston forces