[skunkfire]

Hello all,
After giving this some serious thought its too damn tempting not to make. Chamber will be 500mm of 110mm PVC, completely sleeved in couplers then plated in 2mm steel, with some foam in between for good measure. The steel sheet will not be a pressure holding part of the cannon, it’s just meant to get between myself and fast flying PVC shards in the event of a chamber explosion.
Barrel will be 2m of 75mm PVC, tennis ball calibre. Ignition will be using a flyback (working nicely now) and the mix will be limited to x2.
Here’s a picture of progress so far, possibly the most work done on a spudgun before even going near any pipe:



For scale the 2 long poles are old 6 foot long Bo staffs
This is just a framework for the cannon, I have made this for 4 reasons;

1) Provide structural support, removing loading from the chamber barrel joint and preventing unsightly ‘barrel sag’
2) Support the barrel whilst allowing it to slide forward for breach loading
3) Provide a framework around the chamber for mounting the steel sheet onto
4) It will look cool

[Cornishtiger]

Thats looking great. I dont get what the steel will add other than a load of weight and a load of work but I like the sound of everything else.

[Biopyro]

That is awesome. I don't think you run the risk of an explosion unless you are expecting DDT, so I shouldn't think the steel is necessary

[spanerman]

^epic fail, pvc is a bad material for hybrids with any mixture over 1.5x what pvc will you use? if you are plating in steel, why not use a steel chamber?

[skunkfire]

Cheers for the comments guys,
I can see your point on the 2mm steel being overkill, it would weigh a ton, however the PVC will be taking peak pressures close to its rating so I would like to have something between it and me, I'll probably derate to 0.8mm. It will also be useful for bolting things on to, propane cylinders and the like.
Spanerman, putting together a steel chamber this size using threaded fittings would cost a fortune, i might be able to buy a piece of unthreaded scrap pipe for less but i lack the equipment and skills to weld the ends, besides it would weigh a ton and i have a PVC fetish
I am using 16 bar metric PVC, and as mentioned above I will be glueing modified couplers over this to give a combined wall thickness of nearly 20mm, thats not going to give. I am slightly more worried about the end fittings as there is not a lot i can do to strengthen them.
On the subject of things weighing a lot i picked up this rather solid 2"x2"x4" lump of Ali today:

I'm going to turn this into a manifold of 3 solenoid valves so I can automate the fuelling, not quite sure how i'm going to do it yet but it will definately be a learning experience

[spanerman]

how are you housing the burst disk?

[MaxuS]

Where did you get that block of Aluminium from?

[einkleburt]

skunkfire, on Jun 17 2008, 10:57 PM, said:

this rather solid 2"x2"x4" lump of Ali today:

Mackays? in CAMBRIDGE

[MaxuS]

Cheers mate.

[skunkfire]

the burst disk will be held between a barrel section and a modified coupler with some fat O-rings. I will use a couple of homemade tension clamps to hold everything together.

Yes you guessed it, the ali block came from Mackays, the good thing about that place is you can go in and ask for a measly 10cm of something and they'll just go and cut it off instead of saying you need to buy 6m of the stuff. Although in this case just 10cm was quite expensive, i'm tempted to polish it up and mount it on the wall as a piece of minimalist art instead of drilling a load of holes in it

[skunkfire]

seeing how its been kind of quiet around here I thought I would double post and share the fun and joy which is making a solenoid valve(or three). I have sketched a diagram of my crude design below:



so far I have drilled the "T" into the body, threaded the input and output connections and cut the grooves into the armature for the O-rings. The hole is a 1/4" (6.3mm, use a hole saw pilot drill) and the armature is a piece of 6mm iron bar, giving about 0.15mm of clearance, topped off with 1/4" o-rings. Technobots sell some nice 6mm precision ground shafts but this is silver steel, being harder to machine and possibly less magnetic so not so good for a solenoid armature. Bearing this in mind I bought some crap 6mm black iron rod from B and Q, which seems to do the job.

I have tested the valve by holding the armature in and applying 10bar to the other end, the seal held nicely, I had a chuckle letting go and launching the armature across the garage at high speed. However my mirth was short lived as the O-ring soon got cut up on the sharp edge in the centre of the T and started to leak, hopefully the edge will wear down a bit with use.

next job is to wind a coil capable of generating a field strong enough to counter the 0.5kg - 1kg of force which will be exerted on the armature at its working pressure of several bar, and find a spring with the same compressive force.

[Cornishtiger]

This is fantastic work I cant wait to see how you get on.

[skunkfire]

why thanks CT, progress on the valves is a bit slow but i'm getting there, the first coil I wound was a bit wonky making it hard to align with the armature properly but it just about works, the next one should be much better.
I've just put together a plan for the fueling section, its a fairly conventional metering system with a few extra bits:


I noticed the flow from the propane cylinder via the blowtorch was very sluggish, i could try and modify or drill out the valve but this would bugger up my blowtorch, instead i plan to put a buffer chamber in between the blowtorch and regulator to allow for quicker filling of the metering chamber.
I have also included a reg. on the air side of things to facilitate quick filling of the chamber for multiple shots. As i will only be operating at 1 BAR (x2 hybrid) this is quite a small pressure differential so the chamber will be quite slow to fill, i might try setting the regulator to a higher pressure and timing the valve to inject just the amount of air necessary. All in all i hope to achieve about a 5 second refuel time with the 3 solenoids being controlled by a pic.
Anyway the main reason for posting, I was thinking of using a mini keg- i think they are 5 or 10L kegs of heineken or carling you can buy from tescos- as an air tank. I was wondering if anybody has had a look at one of these and if they reckon they would be any good as a pressure vessel, ie. strong enough and possible to connect up?
Cheers

[Pete]

For added chamber saftey, you should consider going to Viking direct and buying a few rolls of 2" fibreglass tape.

This stuff is the strongest stuff you can buy in roll form. We use it for pyro to stop things exploding, and to duct explosions upwards.

Just a quick story about how strong this stuff is:

We taped some live cake units (the whizz-pop type) to a small tree bracch, about 3inches indiameter, with one peice of fibreglass tape. 2inches wide with a one inch overlap. The cake unit went off, the tree brach was sliced in half and the ring of fiberglass tape landed, unharmed, on the grass.

Its very strong stuff, if you do do the steel thing, id wrap the chamber up in fibreglass tape before you put the steel on.....

Tape

[Hotwired]

Aha!

I'm also thinking of getting a bit of ally from mackays. Also 2" square bar.

I'm still tossing up whether it's even possible for me to make straight 1.25" holes through it or if I should pay mackays (a sack of gold) to do it...

Also floating around is whether a comparable valve could be made from fittings and a much cheaper welding job.

[skunkfire]

Cheers Pete, i mght try some of that, it wont look pretty but if it's going under some sheet steel, no matter

Hotwired, what are you going to use to attempt such a feat? i've never seen a twist drill that big and it would need a reduced shank to fit in your drill, and a small morgage to afford. you could use a bimetal holesaw but that would be tricky, not very accurate and limited in depth. my pillar drill hasn't got enough torque for big holesaws so i have to use my monster 1200W SDS drill, which isn't very good at drilling a straigh hole not to mention dangerous, some of the things i have done trying to drill holes include:

once when i didn't bring the holesaw down perfectly square wiith the surface, the teeth bit into the ali unevenly, gauging out big chunks of metal before half the teeth were ripped off and the pilot drill bent.

when the holesaw jammed, the torque from the drill stripped the thread off the holesaw, not to mention nearly pulling my arms off.

all in all, good fun, i should probably get a drill with a safety clutch. Anyway another option would be to go for a forstner type bit if you can find one big enough, i know they're meant for wood but i've used them to go through ali before and they cut a clean hole, never tried one that big before though.

[RTH]

I still don't know who's got a lathe here (despite the frequent discussion of lathes) but you would probably be able to lathe out a hole, depending of course on whether the hole is central or not. Might be a bit of a task delivering oil to the tool head if you were to do it like that.

[Hotwired]

I don't think any drill will be worthwhile for that kind of hole, If I can't think of something exceptionally imaginative it'll have to be a lathe/mill job. To make things interesting it probably won't even be a centred hole.

It just feels kinda cheating to get it done by professionals

As for the kegs they have to take the pressure of the carbonated lager anyway plus be thick enough to hold their shape when being knocked about unlike carbonated drink cans which crumple at a touch...

Just have to find £14 and some way of getting rid of it's 5 litres

[skunkfire]

here's a quick picture of my valve manifold so far, nearly finished now, just need to wind a few more coils, a job i dislike intensly.

It will be a bit bigger than originally intended but no matter, with the current springs it is set up to work at about 1-2bar, at this pressure the force on the piston from the gas is close to that exerted by the spring so only a amall amount of force is needed from the coil to open the valve.
The only problem i anticipate is the coils overheating if the valve needs to stay open for too long. I deliberately chose the high power coil strategy with fewer turns of a thicker guage wire(~500 turns of 0.54mm wire) to give low resistance therefore high current flow and heat dissipation, but i nearly went to far, it gets quite toasty after about 5-10 seconds

[Hotwired]

Fantastic looking setup coming alive there.

I think thinner, higher resistance wire and so a reduced current could be better. Overheating coils might cause problems if for any reason they're left on too long.

More of a bitch to wind though


Try adding some resistors to your existing coil to restrict current - if it can still operate then that could be the overheating issue fixed.