PVC + Delrin piston valve cannon (picture heavy)
Posted: Mon May 21, 2012 12:22 pm
Hey! First post here, what a great forum. So much info here that it's kept me busy for the last few weeks trying to take it all in!
But anyway, here's my first go at a cannon with a little build report. I hope the pics aren't too big but if they are just say and I'll resize them.
The design is your average old barrel sealing piston valve built into a t piece pvc cannon with some parts machined out of delrin. I just wanted something simple to get me started. I intend to eventually build a huge piston valve actuated pneumatic ram for one of my battlebots so making something like this is a good introduction i guess!
So onto the build. The construction was limited by what i could get at bunnings (from Aussieland btw), so nothing is particularly ideal. Keep that in mind
Chopping up the bits of pipe on the abrasive saw. It seemed to give the best surface finish of any tool i tried. Super quick too. The pipe is pn18 grade for the fittings, pn9 for the "tank" and pn12 for the barrel. Unfortunately that's the best you can get at most tool shops here, so 100psi max will have to do.
Lathing the ridge out of the reducer so the barrel can slide all the way through it.
Turns out the first one i got was manufactured wrong and wouldn't seat properly into the T piece. You can see it sitting a fair way proud of the surface in the below pic. So that gun assembly is pretty well junk, though it may get turned into a low pressure net gun later 8)
The second one worked flawlessly though. Did i mention the pvc cement and primer is the messiest stuff known to man? well it is for me at least XD! anyone got any tips for stopping the glue going everywhere or is it just a fact of life?
I put the barrel a little less than half way in, in this one, i figured it would flow a bit better then having it half way.
The pipe is 36mm and 50mm respectively IIRC.
Next up is machining the rear bulkhead. I didn't want to use a glued PVC fitting because the piston was going to be fairly heavy at around 200 grams. Having it fire out the ass end wouldn't have been too pleasant lol.
This 80mm piece of delrin i was given should work nicely. Unfortunately though it was basically cut off at the finished length, so i couldn't really hold it in the chuck. Just pressing it against the chuck with the tailstock works great for plastic however.
All machined to size. To install it requires a firm push but nothing excessive. It still needs the o-ring and mounting holes drilled, but we will come back to that later
Next up is the important part. the piston. Made from a longer piece of delrin which was also cut to roughly the finished size. The same technique was used to make it. Here's the finished piece. Its about 55.5mm diameter and 57mm long with a weight of 214 grams.
Some details about the design of it -
There's a small stub on the end that locates the sealing washer, it protrudes about half way into it. The hole in the center is tapped for m6 to keep the end cap attached.
Now the funky part. I noticed a few topics about issues with the barrel sealing. So i tried something a little different. These washers have a ridge on the front of them. So i lathed a cap out of aluminium that, when bolted down, would press on the center of the washer forcing the edges to lift a little, which should help it seal against a not so perfectly flat pvc barrel (hey it happens )
Seems to work nicely
There's always a catch though, and it was the washers being too big in diameter by a fraction of a tenth of a bees d**k Damn! A jig was cooked up to hold the washers in the lathe while i dremeled them down in the lathe. I figured replacements would be needed so having the tool to just crank them out is always handy.
All seated nicely in the T piece. The barrel was lapped with sand paper glued to a conveniently sized piece of aluminium rod. I got glue all over it so that took some time As seen its a small ratio design.
Here's the internals layed out. Theres a 15mm foam chunk between the end cap and the piston which takes up half that space between the two pieces. So the pilot volume is pretty minimal.
Speaking of bumpers here it is being made. It's just a camping sleeping mat i had around. A friend used the same stuff in a 1000psi pneumatic ram as a bumper so this will do nicely. Turns out it lathes pretty good for foam too
In position with the finished end cap. You can't really see it but there's a 1/4" BSP threaded hole in the center of it. Why is white delrin so hard to photograph? Arhrahhghghgh?!
The valve work that will almost certainly be temporary. 1/4" T piece onto nitto style quick disconnect fittings. One goes to the compressor to charge and the other goes to a air duster gun to vent the pilot. (We'll come back to that disaster of an idea soon >.>)
Here she is all ready to go The end cap bolts on with 4x m6 stainless bolts that are done up only very lightly. Don't want to deform or crack the pvc after all.
Time to fire! I slowly cracked open the valve on the compressor, feeling the piston slide foward with a thud and the air equalised around the piston. Standing well away i fired the air gun annnnndddd....
Nothing happened. At all The air just slowly dribbled out the air gun and piston only slid back when there was maybe 5psi left in the thing. It seems way too much leakage around the piston and a completely crap pilot valve was to blame there. After multiple tries air was starting to leak out the barrel too.
Sooooooo it all came apart again.. Little PVC particles on the pistons sealing washer was stopping that sealing (i sanded the inside of the tube it slid in and forgot to clean it out.. whoops) But the leakage around the piston was fairly major even with tight tolerances. On a small ratio valve with a poor pilot it just wasn't going to work. An o ring and equalizer port (1.5mm diameter) was the only way to fix it.
It isn't a floating o ring because i didn't have the right size o ring. They are really hard to come by here. 31mm is way too small and 32mm is way too big. So i just stretched a 31mm one over the piston. Just got some urethane o ring material that you melt together to make your own o rings though so i'll try that later on.
To fix the poor pilot valve i basically just yanked the quick disconnect out of the compressor to dump the air out. Lo and behold - IT WORKED Here it is at 30psi firing a golf ball into some 7mm thick plywood. The air leak sound is the air pissing out the air gun that i had cracked open to help the pilot vent faster.
[youtube][/youtube]
Annnnd that's where I'm up to now Tommorow i need to sort out a much better pilot valve and start winding up the pressure. Sprinkler valves are really expensive here and ball valves are pretty tiny. So i reckon a custom made mini 1/2" Quick dump valve will get the job done. Then i can remotely fire it with a servo if i want.
I'll definitely have some more questions later though regarding some ideas to improve it, like cutting out piston weight etc.
Lastly here's the GGDT data on it
Oh and the parts for a much more serious QDV cannon i hope will run off of full pressure CO2. (its all 6mm walled steel pipe) Which should be pretty awesome.
Cheers for checking out my long winded ramble anyway XD! More to come.
But anyway, here's my first go at a cannon with a little build report. I hope the pics aren't too big but if they are just say and I'll resize them.
The design is your average old barrel sealing piston valve built into a t piece pvc cannon with some parts machined out of delrin. I just wanted something simple to get me started. I intend to eventually build a huge piston valve actuated pneumatic ram for one of my battlebots so making something like this is a good introduction i guess!
So onto the build. The construction was limited by what i could get at bunnings (from Aussieland btw), so nothing is particularly ideal. Keep that in mind
Chopping up the bits of pipe on the abrasive saw. It seemed to give the best surface finish of any tool i tried. Super quick too. The pipe is pn18 grade for the fittings, pn9 for the "tank" and pn12 for the barrel. Unfortunately that's the best you can get at most tool shops here, so 100psi max will have to do.
Lathing the ridge out of the reducer so the barrel can slide all the way through it.
Turns out the first one i got was manufactured wrong and wouldn't seat properly into the T piece. You can see it sitting a fair way proud of the surface in the below pic. So that gun assembly is pretty well junk, though it may get turned into a low pressure net gun later 8)
The second one worked flawlessly though. Did i mention the pvc cement and primer is the messiest stuff known to man? well it is for me at least XD! anyone got any tips for stopping the glue going everywhere or is it just a fact of life?
I put the barrel a little less than half way in, in this one, i figured it would flow a bit better then having it half way.
The pipe is 36mm and 50mm respectively IIRC.
Next up is machining the rear bulkhead. I didn't want to use a glued PVC fitting because the piston was going to be fairly heavy at around 200 grams. Having it fire out the ass end wouldn't have been too pleasant lol.
This 80mm piece of delrin i was given should work nicely. Unfortunately though it was basically cut off at the finished length, so i couldn't really hold it in the chuck. Just pressing it against the chuck with the tailstock works great for plastic however.
All machined to size. To install it requires a firm push but nothing excessive. It still needs the o-ring and mounting holes drilled, but we will come back to that later
Next up is the important part. the piston. Made from a longer piece of delrin which was also cut to roughly the finished size. The same technique was used to make it. Here's the finished piece. Its about 55.5mm diameter and 57mm long with a weight of 214 grams.
Some details about the design of it -
There's a small stub on the end that locates the sealing washer, it protrudes about half way into it. The hole in the center is tapped for m6 to keep the end cap attached.
Now the funky part. I noticed a few topics about issues with the barrel sealing. So i tried something a little different. These washers have a ridge on the front of them. So i lathed a cap out of aluminium that, when bolted down, would press on the center of the washer forcing the edges to lift a little, which should help it seal against a not so perfectly flat pvc barrel (hey it happens )
Seems to work nicely
There's always a catch though, and it was the washers being too big in diameter by a fraction of a tenth of a bees d**k Damn! A jig was cooked up to hold the washers in the lathe while i dremeled them down in the lathe. I figured replacements would be needed so having the tool to just crank them out is always handy.
All seated nicely in the T piece. The barrel was lapped with sand paper glued to a conveniently sized piece of aluminium rod. I got glue all over it so that took some time As seen its a small ratio design.
Here's the internals layed out. Theres a 15mm foam chunk between the end cap and the piston which takes up half that space between the two pieces. So the pilot volume is pretty minimal.
Speaking of bumpers here it is being made. It's just a camping sleeping mat i had around. A friend used the same stuff in a 1000psi pneumatic ram as a bumper so this will do nicely. Turns out it lathes pretty good for foam too
In position with the finished end cap. You can't really see it but there's a 1/4" BSP threaded hole in the center of it. Why is white delrin so hard to photograph? Arhrahhghghgh?!
The valve work that will almost certainly be temporary. 1/4" T piece onto nitto style quick disconnect fittings. One goes to the compressor to charge and the other goes to a air duster gun to vent the pilot. (We'll come back to that disaster of an idea soon >.>)
Here she is all ready to go The end cap bolts on with 4x m6 stainless bolts that are done up only very lightly. Don't want to deform or crack the pvc after all.
Time to fire! I slowly cracked open the valve on the compressor, feeling the piston slide foward with a thud and the air equalised around the piston. Standing well away i fired the air gun annnnndddd....
Nothing happened. At all The air just slowly dribbled out the air gun and piston only slid back when there was maybe 5psi left in the thing. It seems way too much leakage around the piston and a completely crap pilot valve was to blame there. After multiple tries air was starting to leak out the barrel too.
Sooooooo it all came apart again.. Little PVC particles on the pistons sealing washer was stopping that sealing (i sanded the inside of the tube it slid in and forgot to clean it out.. whoops) But the leakage around the piston was fairly major even with tight tolerances. On a small ratio valve with a poor pilot it just wasn't going to work. An o ring and equalizer port (1.5mm diameter) was the only way to fix it.
It isn't a floating o ring because i didn't have the right size o ring. They are really hard to come by here. 31mm is way too small and 32mm is way too big. So i just stretched a 31mm one over the piston. Just got some urethane o ring material that you melt together to make your own o rings though so i'll try that later on.
To fix the poor pilot valve i basically just yanked the quick disconnect out of the compressor to dump the air out. Lo and behold - IT WORKED Here it is at 30psi firing a golf ball into some 7mm thick plywood. The air leak sound is the air pissing out the air gun that i had cracked open to help the pilot vent faster.
[youtube][/youtube]
Annnnd that's where I'm up to now Tommorow i need to sort out a much better pilot valve and start winding up the pressure. Sprinkler valves are really expensive here and ball valves are pretty tiny. So i reckon a custom made mini 1/2" Quick dump valve will get the job done. Then i can remotely fire it with a servo if i want.
I'll definitely have some more questions later though regarding some ideas to improve it, like cutting out piston weight etc.
Lastly here's the GGDT data on it
Oh and the parts for a much more serious QDV cannon i hope will run off of full pressure CO2. (its all 6mm walled steel pipe) Which should be pretty awesome.
Cheers for checking out my long winded ramble anyway XD! More to come.