Granta - my copper APH (design phase currently)

[cantab]

Yeah, a gallon reservoir would be seriously large. (Though a US gallon is only 6 2/3 imperial pints, not 8). At that point it's probably better to go with a backpack.

[aEx155]

Originally Posted by SilentGuy

devvo: I think aEx's point was that drink bottles do use standard thread sizes - just ones other than what we're used to. If those threads adhere to the MPT standard, then there shouldn't be a problem, as long as you can find an FPT adapter.

I wouldn't say that soda bottles have a standard thread (I guess you could call it SBT (SodaBottleThread) ). The point I was trying to make was that soda bottles could use standard threads, but with a bit of work involved.

Originally Posted by devvo

there are no fittings designed for plastic bottle threads, and if there was it would leak as they aint that watertight, the seal is made by that plastic thing on the inside of the cap,

I do have a seal on the inside that replaces the "plastic thing on the inside of the cap". If you've read my thread about my homemade, I used two rubber hose washers; the fact that there is two means that there is a rubber seal presses tightly against the plastic bottle; one you couldn't achieve with one. If you say my way isn't watertight, then tell me how my water gun isn't leaking...

Originally Posted by cantab

I don't like the idea of cutting new threads into the bottle. It seems it will make it rather weak.
If glue will hold then that's the simplest option.

The bottles I've used hold pretty well, since the neck is so thick.

...and just FYI: I've used "still" drink bottles under pressure and they take it just fine, apart from some bulging from the various parts (especially the bottom). I'm not sure whether or not that's bad, but I've never had anything bad happen because of it.

[Ben]

A bladder tank might be best for you. I have no experience with them but Biopyro mentioned where to get them in the UK. If you can find a 1 liter one for a reasonable price that'd be perfect.

[devvo]

I experimented today in our fab shop and found that 3/4 did kind of work with a lot of PTFE on the thread, while I was there I also did a little pressure test on the 3 bottles I had and they all went at about 190psi (coke, dr pepper and tesco lemonade if you want to know).

The bladder tank (an expansion vessel as known in the trade) is a great idea, ive used them before and they yield the far better results than a plastic bottle, scrap yards have them ten a penny ( we scrapped 60 of them a few weeks ago ourselves) they are lighter than they look. BSS or plumbcenter have them cheap too

[devvo]

just another idea to throw in there, why not use a tank adaptor? you could use a bottle with a flatish bottom (like a lucozade bottle) and have any bore size you want since they go from 15 to 54mm

[cantab]

OK, I went looking at parts today, and what I've seen has made me make some changes.
Notably, I'm doing away with 28mm pipe. I've looked at compression fittings for 22 mm and they're really bulky, so 28mm would be ridiculous (Homebase don't even sell them). Like I said, I'm not soldering Granta, learning to solder can wait. I'm not sure whether to do 22mm throughout (for possible economy) or 22 barrel 15 elsewhere (for lightness and higher pressure).
The basic design's going to remain the same though, including the idea of multiple smaller PC's.

I've also seen a range of plastic pipe. It's not PVC, but something else. Pressure rating is 12 bar (170 psi) at 20 celsius (room temperature) and they have push-fit connections. It's only in 15 and 22 mm.
Valves are however not available in plastic, so I'd have to still use metal for that. The connectors can take either, but that means plastic - connector - copper - valve - copper - connector - plastic, which is a bit of a pain.
Finally, this stuff isn't rigid. A metal valve on the end of the barrel would make the barrel droop, which is NOT wanted!

So Granta's going to stay as a copper gun, despite the weight. The lack of rigidity of the plastic means I'd probably have to do an internals and body type design with it. THR has shown one approach to that, getting a big cheap toy gun and gutting it would be another.

[Silence]

Hmm...exactly how long is your barrel? If you're talking about the distance between the chambers and the valve, that doesn't need to have any length. I'm guessing you're referring to the length including the chambers, but that could be supported by something between the pump and the barrel.

That said, I'd use metal since that's what the rest of the water gun uses.

In the US, most copper is sold in telescoping sizes. Diameters come in increments of 1/32" with wall thickness of 1/64". That allows for some neat sliding designs and light weight while offering enough strength for Nerf and water guns. It's too bad you only have heavy duty stuff in the UK.

[cantab]

With the plastic pipe, I'd want to make almost everything plastic. However, the few metal parts needed would cause the plastic to flex. To be honest, the wight of itself, and the PC's and reservoir, would also.
True, I could support it. Wood would probably work fine. But it's an added complication.

I might see if the push-fit connectors can be used to connect the PC's though. If I file off the screw threads, it might work. It depends how much tolerance the connector has, as the bottle is a bit wider than the pipe usually used.

Also, general question: does pipework from the trigger valve to the nozzle need to be pressure rated? It's not holding any static pressure, so it might not. If not, that gives me another interesting idea, using multiple trigger valves to increase flow (when larger valves are unavailable). The valves could then be opened together with an XXP275-style trigger.

In any case, a lot of this is ideas for guns after Granta.

[devvo]

I wouldnt worry about the pressure ratings, you couldnt pump it up to a high enough pressure, the plastic bottle would be the first thing to go.

regarding copper sizes here in the uk there pretty much the same, if you really want the best pump i would use 22mm and 18mm for the pump handle as it is a perfect fit

[cantab]

I've not come across 18mm pipe; it's either 15 or 22. I'm going with 15 for the pump tube, both for lightness and more importantly since it allows higher pressures - about 45% more with the same pumping force. For the rod I'll probably use smooth dowel with some sort of rubber seal; ideally o-rings but washers if that's all I can find.

[Ben]

Hmm... I've missed this thread.

Given that some people don't even use pressure rated pipe for the continuously pressurized parts, I don't think you have anything to worry about with the intermittent pressure at the nozzle. As devvo said too, the soda bottle will break first anyway.

Multiple valves sounds like an option when larger valves aren't available, but it seems fairly complicated and costly too. It'd be best to avoid it but if you can't then it should work.

[cantab]

It looks like the project's going to be a bit more expensive than I'd expected

Parts I need (and can get). Pipe is copper, fittings are compression.

2m 15mm pipe
2m 22mm pipe
22m check valves x2 (15mm are unavailable)
22mm lever arm valve
22mm equal tee x as many PC's as I have, so 2 or 3
22-15-22 mm tee
15mm elbow x 3 or more
22-15mm reducer
dowel

cost of all that lot will be about £67. ouch. of course i'll have lots of pipe and maybe a few fittings left.

I still have a few aspects of the design to work out:

The reservoir. I want it to be robustly fixed to the gun (don't want it swinging around when the gun is moved). It also ideally needs to have its base level with or above the base of the pump tube. I'm not confident a single pipe will hold it, so I may use some sort of wood frame at the rear. I think a 4 or 6 pint milk carton, mounted horizontally with the handle up, will be good. (remember british pints are 20 oz). The exact size will depend on what I use for the PC's - I want the PC and reservoir tops to be roughly level.

The pump seal. I'll use o-rings if I can find them. Otherwise I guess i'll have to make do with rubber washers.

I've just now figured out the nozzle. I'll take another drinks bottle, cut off the threaded bit from the neck, glue that around a stub of 22mm pipe, and use drinks caps for my nozzles! Also I can easily lengthen the pipe enough to have a laminator if I want.

EDIT: There are two main reasons this gun's going to be rather expensive. The first is the use of 22mm pipe and fittings. 15mm is rather cheaper but would of course reduce performance. And anyway I'm not sure where to get 15mm check valves
The other is the use of compression fittings. Solder fittings are much cheaper (less than half the price). They'd also be lighter and less likely to rub against the arm. However, as I said I'm not ready to learn soldering just yet, and also I'd prefer something more easily dismantleable. In particular it could be dangerous to try to desolder the PC connections when bottle plastic is still glued to them. If the gun works well I'll definitely consider a Mark II of the same design but using soldered fittings.

[coolocat]

If you're wondering where to get check valves I would suggest you try your local plumbing/hardware store and ask if they have check valve.One thing maybe you should ask for non return valves because in the store I went into I asked for check valves and the shopkeeper looked at me strangely but when I said non return valves he knew instantly what they were. Finally if they don't have the check valves ask where you might be able to get them. Anyways thats just my two cents lol .

[cantab]

Yeah, I've come across them called non return valves too. But I can get them, just only in 22mm, not 15. That makes it a bit heavier and possibly more expensive, but it's no big deal.

[Silence]

Good points, coolocat...I've also heard these valves called "one-way valve". In a way, "one-way valve" is best because it's the most intuitive; even somebody who normally calls them by a different name should know immediately what you're talking about.

WaterWolf wrote this guide for building homemade PVC check valves. I don't know how well it could be accomplished with copper fittings, but it's cheap and compact if you embed it in a tee.