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Taking some serious inspiration from SB15's TBMA2.0 and Mr.C's Mjollnir, I have been working on a hybrid of my own. I have been using mainly the pictures in SB15's thread to go on, and have learned a lot about hybrids and pistons from the forums here, and I feel like I am ready to tackle the challenge.
So far, I have the gun design made from 2 inch galvanized steel pipe fittings rated at 150 psi, although I have been assured that the yield strength is much higher. A barrel made from 1.5 inch EMT conduit, and am planning to use the 30kV ignition system from ultimatespudgun.com along with a spark plug. Anyway, I just wanted to show you guys what I have so far so you can point out anything I am doing wrong, or to potentially help others who wish to build a similar cannon. Anyway, here are some pics:
First is the barrel, 1.5inch EMT conduit x 48 inches long:
I was unable to get the pipe threaded, but found that a 2inchx1.5inch reducer fit VERY snugly over the pipe, the threads even grabbed it a bit. So I filled the fitting with JB Weld and rammed it onto the end of the pipe essentially. I will be sure to aim far away from anything destructible in case I end up with a flying pipe for the first shot. I plan on actually welding it later. The pipe passes through the cam lock fitting perfectly:
Next, I have started on the piston valve:
Here is the vent port at the back, closed:
Here are some pictures of the 1.5 inch nipple inside the reduction fitting which acts as a 'hard' stop for the piston, to keep the o-rings from slipping out of the 2 inch nipple. I think I will JB weld this in there as well. I now need to figure out what to use as a bumper and where to pit it. I am thinking I will drill holes around the circumference of the 1.5 inch nipple to allow more airflow, and use half a tennis ball wedged in the reducer for a bumper. Any suggestions?:
Here is a picture of how the piston seals inside the 2 inch nipple. I haven't had a chance to pressure test it yet, but from what I can tell, the double o-rings will seal well:
Here is the piston so far. It is a floating O-ring design, and fits inside the 2 inch nipple. I am planning for an actuation of around 1.5 inches, so the piston has full movement from one end of the nipple to the other. I plan to JB Weld the nuts in place to seal the piston once I have the distances fine tuned, and then trim the ends of the rods. I think I will also make a handle for the end of the port to help get the piston seated before filling the pilot with air pressure. Also, a gratuitous amount of JB Weld was used to attach the 5/16" inch rod to the 1/2" coupling nut. I could not find any reducing nuts or bushings ANYWHERE. This seems like an acceptable alternative (correct me if I'm wrong):
Finally, here is the sealing face. It is a 1.5 inch nipple JB Welded to the inside of a 2x1.5 inch bushing. It protrudes into the tee, which the rubber on the piston seals against:
If any of you experienced builders can point out any problems I might have, that would be great. Thanks for looking!
Finally, here is a shot of the layout. I am planning on putting the meter and electronics on the right hand side to facilitate firing from the right hand shoulder. I will also be using my homemade pistol grip as a trigger for the igniter circuit:
I will have more progress in approximately a week when I am done finals and move home and have access to my Dad's shop and the drill press and pipewrenches, taps, etc. I'm a little bit limited in my basement here in the city.
I've tried a tennis ball, didn't work. I think the problem was mainly that it couldn't absorb enough energy, the piston treated it like it wasn't even there. Finding the right bumper is one of the more annoying things about building these cannons because you can damage your piston on the first shot if you don't have a good bumper in place. Both SB15 (IIRC) and I used these plumbing gasket thingys:
Another example here
You may want to find the thickest washer you have (that is the same diameter as the other piston washers) to put on the rear of the piston. That nipple you have in there will probably bend your rear washer and jam your piston when the piston is piloted. Even with an adequate bumper this will probably still happen. This is made worse by the fact that the forces will be concentrated around the edges of the washer due to the 1.5" nipple. In my design, the back of the piston housing is a 50 x 25mm reducer, so the forces are more evenly distributed throughout the whole washer face. What you essentially will have is the problem you can get from piston bounce, but it will happen every time the piston is piloted.
I had some seriously thick washers on both the rear and front of my piston that were completely bent by piston bounce (the sealing face bouncing back in to the valve seat) or just firing with an inadequate bumper.
It could be difficult to use the above bumpers in your design since they may be pushed back in to the 1.5" fitting where they will be rendered useless. I would instead recommend doing what I did. Remove the 1.5" nipple from inside the housing, use a bumper like the one above, grab the thickest washer you can find (maybe something like this), place it on the piston rod 1/2" from the rear o-ring washers and epoxy it in place. I would fill the rod space between the rear o-ring washer and this protective washer with as many fastening nuts that can fit so there's no chance of the protective washer shearing off or being pushed back and damaging the o-ring washer. It is important that the washer is as large in diameter that will fit so there's no chance of a bumper like the one above slipping around the edge of the washer and being dragged along with it.
edit: Good job, by the way! This is the kind of stuff I would like to see more of around here.
I've considered your advice, and have come up with something for now. I have removed the nipple from the reducer at the back of the pilot housing.
Instead, I have another of the large rubber "furniture feet" that I used on the sealing face, and I have put this on the back of the piston. It is placed on the the 5/16" rod. I put 2 washers (smaller diameter than the rubber disk, but larger than the coupling nut) for a little support, then the rubber disk, a smaller washer, and a nut to secure it.
The plan is for the rubber disk to touch the outside of the reducer as the piston moves backward, and purposefully "deform", as you were warning me about the other metal washers. The further the piston moves back, the smaller the diameter of the reducer, and the more force required to deform the rubber disk.
At the small end of the reducer, a 1.5x1 inch bushing is threaded in. The disk eventually dead ends on this, and deformation allows the piston to move slightly further. Close to the end, I have some foam tubing inside the bushing which presses back against the disk as well.
I have not calculated what the opening force on the piston will be, but by grabbing the whole housing and slamming the end of the piston rod into the floor (with considerable force), the piston slams back and the rubber disk absorbs the force nicely, with no obvious signs of damage. As an added bonus, due to the deformation of the disk, the piston gets "stuck" at the back, which I'm hoping will help prevent piston bounce. A good tap on the piston at the back dislodges it so it can be seated for another shot.
If I find the disk is not resilient enough for the task, I have two more, so I could triple the thickness which will help.
Yeah it looks like it might be worth a shot (literally). I spent about a year refining my piston design, making small changes every time I learned something new about ways to improve it. So it's probably not that bad if this design of yours only lasted a few shots because we can quickly work out the problem and fix it, which is probably easier than trying to figure out the perfect design a priori.
Can't recall if I mentioned this earlier but have you thought about manometric metering? It can make fuel mixing a bit easier, you can reduce the size of the metering setup, the calculations are easier, and you can detach the meter and use it on any hybrid cannon.
I have read about manometric. My problem is that I don't have access to a small PSI gauge that I could use to do it accurately, and McMaster-carr will not ship to Canada. I found this out trying to get a reducing rod coupler. Plus, I like the idea of the "air through meter" design of SB15's if it helps overall mixing of the air-fuel mixture...
I am planning to use a "fuelling manifold" that I made to connect propane and air to the meter at the same time, which is detachable. That wall, all that stays on the the gun is the meter pipe, which I don't think is so bad, and makes for a handy barrel support.
That pistol grip looks 3D printed...
It is indeed! I modelled it in Solidworks and printed on my RepRap. It came out quite nice I think, and it's really comfortable.
Great job, I have to say. Puts you a notch above MrC, got to be worth it
One small gripe, could you resize your pictures before posting? This thread is hurting my screen
Duly noted. All my pics should be resized 50% smaller now.
A little more progress. Since the washers I am using to create the gaps for the o-rings on the piston are a way smaller diameter than the o-rings, they fit really loose in the gap ie. they are all over the place. You can see this in my previous pictures of the piston.
So, I modelled up some spacers and printed them out to keep the o-rings in place. They fit over top of the existing washers, which still serve to create the gap, and inside the o-rings perfectly. Now, the o-rings can spin freely and are still floating inside they're gaps, but they don't go so far one way that they can fall right off!
EDIT: Also, I made some bushings for the barrel support in the same fashion:
The thread now loads in less than 27 hours, cheers
Nice to see the 3D printer in action, this is the definition of rapid prototyping looking forward to see this thing fire.
Lots of progress today and yesterday. I assembled the piston housing, chamber, meter, and trigger circuit and have it all attached! In my excitement, I didn't take very many pictures, but can explain a few things.
I fine tuned the spacing on my piston and used JB weld to seal around the nuts and washers to ensure a good seal. Next, I used gas line Teflon to wrap all the threads on the piston housing and tightened it in the vice using a huge pipewrench.
To fill the pilot volume, I used the drill press to drill a hole in the side of the reducer coupling in the back of the housing. I tapped this with a 1/4" NPT tap, and was then able to thread on my pilot valve set-up. I discovered that the 1/4" inch nipple obstructed the rubber washer - bumper I explained earlier, so I ground the nipple with the bench grinder on an angle, and surprisingly, it still threaded into the hole fine, and no longer protruded into the pilot housing.
After all this, I was able to test out my piston by filling the pilot housing with air and it sealed! The O-ring at the end of the piston fits just inside the brass adapter and seals nicely.
After that, I assembled the chamber and meter pipe as you would normally. At this point, I was able to fill the chamber with air to test the sealing face, which also works well. No leaks whatsoever at 100 psi, which is sufficient for the 7X I want to shoot the gun at. The piston works well as an air cannon, I shot a potato around 150 - 200 yards on air alone, around 100 psi in the chamber.
Chamber in the vice:
For the ignition system, I am using the 30kV igniter from ultimatespudgun.com. I put this, along with a battery, and a switch inside a project box, along with an "armed" LED. For connecting to the electrode, I have some banana-binding posts, and in a moment of creativity, I used a molex connector at the other end to connect various trigger devices.
As an electrode, I bought an NGK-CR6E spark plug. This plug has M10x1.0 threads, so I simply drilled a hole in the 2 inch coupling at the end of my chamber, tapped the hole, and threaded the spark plug in with gas line Teflon tape. This plug is a deep reach plug, and reaches into the chamber a little ways, which I think will be beneficial to combustion.
After a moment of confusion regarding fuelling, I refined the procedure and got the gun to fire reasonably reliably at 5X. Boy this thing packs a punch. You'll see in later photos that I had to add a large washer to the end of the piston. This is to catch the pilot o-ring, which the gun likes to eject on some shots. I nearly lost it on the first shot, as it went across the yard. Luckily I was able to find it.
The piston bumper appears the be holding up after 6 or 7 blank shots, and has the desired behavior of getting stuck open, requiring some force to dislodge it and seal the piston again.
Overall, I am quite happy with it so far. I will update this post with pictures and a video tomorrow likely.[/img]
So far I have had around 25 successful shots with this thing. I've zeroed in on a good mixture at 5X. Only had one piston failure, I managed to melt the sealing face. I replaced it, and now it works good as new.
My bumper design appears to be working. The washers that support the rubber disk have bent back significantly, but they don't appear to be bending any further. The bumper stops the piston at the back of the stroke and holds it there until I reset it, which is perfect. No piston bounce! I do know that I will have to replace the rubber disk eventually, because the signs of wear are quite obvious.
This is the semi-final design. I will rework the mounting for the electrical box once I order some new plastic for the my printer, and I may make some supports for the meter pipe out of plastic as well:
The bit on the floor is a manifold that both air and fuel go into. It quick-disconnects onto the meter pipe, and is removed when the gun is fired. I can definitely see why SB15 went with a DCV to fill his gun. I have a fair number of ball valves, and have to vent the pressure after I fill the pilot so I can measure the propane (an unforeseen drawback to my design). It works well though, and is consistent.
I did some shooting today. The gun has no problem with 3/8" plywood, just punches holes straight through. So I decided to take on 3/4" plywood. These are just potatoes:
I have some cool vids from the back of the plywood at 240 fps so you can see the potatoes exit. I am working on putting it up on youtube.
I think this gun is pretty much done now, so I will probably make a showcase post soon.
Thanks for reading! I hope I can help someone else who wants to make a cannon like this one!
Very well done, looking forward to seeing this thing actually fire
I hope you tried something a bit more hardcore than potatoes though, this thing has a lot of power potential and it would be a shame to see it wasted on farm produce.
Since you have a 3D printer, consider making fins for large nails in this vein, for an APFSDS effect that should go through much harder targets.
How are you holding it exactly? The more line is pretty high above the pistol grip...
Just what I needed, love seeing a nicely built piston hybrid!
Kick it up to 10x with some frozen vegetables or fruit, the recoil starts to get excitingly dangerous
Do you hip fire it or rest the chamber elbow against your shoulder?
It's a bit goofy looking, but with the 90 street elbow against my shoulder, the piston housing goes over top of the shoulder, and you hold it like a rifle. It actually feels pretty good, and it is comfortable to shoot.
Well, I got the video up! These are the two shots from the pics in my previous post. I plan on filming some more stuff maybe this weekend and I will make a longer compilation video:
On the list of things to film:
3D assembly visualization
Piston movement during firing
More damage shots of course!
Last edited by sudpuzzer on Thu May 02, 2013 2:43 pm, edited 1 time in total.
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