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.

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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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