Mounting the Stock to the Cannon

I mentioned briefly when I started phase 2 that I’ve been considering many different methods of attaching the stock to cannon. There are definitely simpler solutions than this clamping arrangement, but at the time I was making the decision I’d already worked the whole thing out in cad, so might as well do it that way I guess. I went through a LOT of extra trouble to avoid using hose clamps or any other bands with tensioners. The up side is that it should look really nice and be easy to disassemble.

Anyway, enough talk here’s the design:

LBC Stock Mount

The arrangement consists of a 1.5×1.5″ 1/8″ wall U-channel that’s attached to the cannon body with 3 stainless steel bands (red). Each band is attached to an aluminum block with machine screws. The bands will be able to slid around ..for now. If you look carefully there are holes on the bottoms of the blocks as well, bolts will go through those holes in the U-Channel and thread into the blocks. The idea is that when you tighten the bolt it pulls the u channel firmly onto the cannon body, and at the same time tensions the bands so they can’t slide.

Q-Loaders use a similar system to mount onto a paintball marker. I had one and was surprised at how sturdy the connection was, that’s what gave me the idea.

Lets get started.

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I started by drilling the holes in the u and making the blocks. All the pieces are square and easy to fixture so it went really fast.

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I cut the blocks from a 6″ piece of 3/4″ square bar, I then milled off the bandsaw’d area to make them a little square-er and prettier, that wasn’t even really necessary though. After that it was just clamping, drilling and tapping to finish them up.

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Next I started making the bands. I only needed 5ft of the banding, but the least I could buy was 25 so I had plenty to spare. The bands cut nicely with sheet metal sheers. I made the holes with a sheet metal punch.

The hard part about the bands turned out be getting them to just the right length. I’m glad I had extra.

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These are 2 of my rejects. The top one is too short, and the bottom one is too long, it has to be pretty precise. I made the rest of the bands about half way between these 2 and they fit nicely.

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Here’s one of the bands assembled. If you look carefully, this is actually one of the too-short ones.

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Here’s one of the finished ones after I got the band length dialed in.

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I threw the u channel in the mill real quick and milled out some space for the bands.

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Time to check the fit. Looks good. I proceeded to tighten the whole thing down.

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Conclusion: It looks nice and feels very sturdy. I might eventually put some cross braces in the u channel just so the legs wont push apart if you really crank down the bolts, but I’m not too worried about it for now. I’m really happy with how nice this turned out.

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Woah! Look at this.

I wish I had found this before. This guy Damon Williams designed his own version of the Metadyne launcher. And posted full plans!

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Forum: http://www.pbnation.com/showthread.php?t=3297780

Plans: http://www.scenariodreams.com/productpages/launcher.htm

Very interesting and useful. I can’t quite figure out how the poppet valve on the trigger works in his design. ..It has to act like a 3-way valve somehow.

This is incredibly cool.

Phase 2: Building the Stock

Here’s my plan:

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Basically I’m going to use stainless steel bands to clamp a u channel to the bottom of the cannon. Onto this I’ll mount a butt, grip and foregrip. The butt will be milled hdpe.

I’ve been playing with this design in cad for a couple of days. A couple of thoughts:

I considered many methods of attaching the u channel to the cannon tube. In the past I’ve always used hose clamps, bur I wanted to go with something nicer this time. The U-channel doesn’t attach to the pressurized portion of the pipe so welding is an option, but I don’t know how nice it would look with my tig skills being what they are. It would also make  working inside the u-channel a pain.

I’d originally planned on cnc milling the grip and forgrip out of hdpe in addition to the butt, but I think the gun parts will be easier and give it a cool prototype-weapon look.

Phase 1: COMPLETE!! (It works)

Ok, quite a bit has happened since last time.

1. I built the ball check valve.

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The operation’s were pretty simple, just a bunch of drilling on the mill.

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Here it is all buttoned up.

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2. I cut out and attached the bumper.

The cutting out part turned out to be very difficult. The rubber I used is soft, but tough and scissor resistant. I eventually got it cut, but the edge looked awful. It was usable, but I wanted to do better.

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I tried a bandsaw (it just gummed up immediately). I tried lubricating the blade and sandwiching the the rubber with wood; that helped, but still left a nasty looking edge. I ordered and tried a razor saw but that didn’t work very well either. I was getting pretty stuck.

I started asking my friends at work about it. First response: “Aren’t you fixing a trim press right now that’s designed pretty much exactly for that?”  …I probably should have thought of that. Anyway, I was, it it was the perfect tool for this. This is it:

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As it happened, I just got it working.

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I made a quick die out of 3″ pvc pipe.

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The press has a low power height-set mode. Unfortunately even that was too much for my die.

I tried again, this time I put the ring in the press before sanding down the cutting edge. It survived this time.

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The edge quality came out great this time. (I cut out the center hole with the press as well). The disk is now smaller than the pipe ID, but that may be a good thing, since this’ll give it room to deform and soften the impact.

Lastly, I glued the bumper to the back plate with the loctite #380.  I forgot about the smaller ID and put some extra around the edge. I’ll have to sand that off later. No biggie though.
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After applying the glue I clamped the assembly between some boards and left it for 24 hours.

3. ASSEMBLY.  ..After 60 or more hours of machining and designing, I couldn’t wait to test it out.

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It’s very awkward to hold with no stock (that’s phase 2), but I did a couple of test shots.

It’s working pretty awesome. I noticed a few things in testing though. The length of the piston needs tweaking, it’s not moving all the way back, which means I could make it longer (and therefor faster) without any downsides. The other thing it the QEV. It’s VERY sensitive. During testing I accidently set off the cannon a few times by removing the schrader valve too slowly. I added an inline check valve after the schrader (you can see it in the above pic) to solve that problem.

I’m happy with the performance for now. I’ll save any further tweaking for after phase 2, when the cannon is less awkward to take out and shoot.

Some design changes and new parts

In the course of pressure testing the air camber, I discovered that you can’t pressurize a quick exhaust valve from the cylinder side. I was eventually able to get it to seal by holding my thumb over the exhaust port, thus giving air some time to leak around the diaphragm into the “in” side. I was then able to slowly add air to the cylinder without it opening. Obviously that’s not a permanent solution.

I bought some new fittings from McMaster so I can put the fill valve on the in side. So that solves that problem.

Next issue: I think the piston back might seal too well in the pipe, therefore not allowing the air chamber to fill. I haven’t tested this, but as great as the o-ring seals have turned out, I don’t think I can count on any leakage. If too much pressure builds up behind the piston it could potentially bend the piston shaft, so I don’t want to risk it.

I’ll have to add a one way valve across the piston back. I could do it from the outside with banjo fittings, but I’m worried that a tapped hole in the pipe wall might be leaky. Luckily I think I’ll be able to fit a valve directly into piston back (it’s a 3/8″ thick disk).

These looked like exactly what I want, but the quote came back at $40 each. I could swing that, but eh.. Feels like a ripoff for something that could be as simple as a ball and spring.

Enter the ball and spring:

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I think building it will actually be pretty easy. I originally wanted to retain the spring with a hollow set screw, but the fit would be very tight with the plate as thin as it is. Using the little aluminum plate will give me an extra 1/8″ to play with. I added a little spring (#9435K35), a delrin ball (#9614K22), and some #4 screws (#92949A105) to my order. Done.

There’s one last know problem. I’m not sure how to attach the rubber bumper to the cannon back.

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I’m hesitant to use adhesive, because I don’t want the joint to degrade if the cannon sits in storage for years. I’m going for heirloom quality here.

I haven’t been able to think of a better method, so I’m going to give adhesive a shot. I feel like maybe I’ve just been using the wrong adhesives in the past, so I called up Loctite and asked what they recommend for this. I was actually able to talk to a person about it which is nice. The woman I talked to recommended #380 so I’ll try that. Hopefully I’ll get a pretty permanent bond.

I also bought a razor saw for 10 bucks on amazon to hopefully make cutting the rubber bumper easier. I tried with a razor blade previously, and it was just not having it.

Drilling the Radial Holes for the Chamber/Barrel Divider

Here’s what i’ve got to make:

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I’ve been putting this off because I’m not sure exactly how to do it. Specifically, I’m not sure how to hold the divider square in the pipe while I drill the holes. It’s pretty deep in there, so fixturing it from the outside would be difficult.

I laid awake in bed thinking about this for a couple of days.

I eventually decided on this method: I’ll cut wood blocks that I can bolt around the divider to hold is square. I’ll then push that whole assembly into place in the pipe. It doesn’t have to be absolutely square so I think that’ll be accurate enough.

The next problem is how to get the divider to the right depth in the pipe. The radial holes need to go through a pretty narrow band on the divider. I don’t think a tape measure from the end would be accurate enough. To solve this problem, I’ll drill one hole in the pipe and divider before inserting it. Once it’s inserted I can put a screw through this hole and both lock the divider in place, and also be sure it’s at the right position.

Ok.. Lets do this.

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I spend a lot of time squaring saw blades for this job.

I eventually came out with a square block that fit tightly in the pipe. (Square in the sense that the sides were perpendicular)

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I squared the chopsaw and cut the clock into two pieces.

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Then I drilled holes through both and put a threaded rod through them. That completes the wood work.

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Looks good.

Next, I drilled the first hole in the divider.

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I don’t have any pictures, but I punched spots for the holes on the pipe using the same tattered template as last time. I then drilled one of them out.

I’m a jackass and stopped taking pictures for while here.. Basically I jammed the block and divider into the pipe and lined up the holes. Getting the holes to line up turned out to be pretty difficult (i should have anticipated that). Anyway, I eventually got them to overlap, then worked an awl into the hole to force them into alignment. After that I put that first screw in.

From here, the operation was pretty simple. I built a little jig on the mill and drilled each of the radial holes.

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I used one of my trusty wood v-blocks I made before. Again, not incredibly accurate, but it should be plenty good enough.

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Here’s divider after I pulled it out of the pipe. The holes came out very nicely.

The last steps were to tap the divider holes, and drill out the pipe holes for screw clearance.

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Bam, done. (I’ll add a finished picture here sometime hopefully)

Making the Piston Shaft

I spent a relatively short evening making the piston shaft. I cut the threads using the treading mode on the lathe.

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One unanticipated benefit of the lathe method was that I was able to creep up on the right minor diameter and try the fit for the last few passes until it was just perfect.

I was kind of intimidated by this step at first, but it turns out threading on a lathe is pretty easy.

Edit: Here’s the complete piston. Pretty spiffy eh?|
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