Hose guide and Pressure Guage

I finally decided to take care of that dangling hose. I ordered an aluminum tube from mcmaster, but I didn’t notice the lead time was 2 weeks, so I picked up a piece of chrome plated toilet water line from home depot to hold me over. It definitely looks a lot nicer now. I picket up a pack of grommets as well to get the lines through the lower receiver wall.

I also picket up a mini pressure gauge.

That leaves only 2 more steps that I can think of. Finishing the metal, and mounting the schrader fill-valve. I read around online and I think I’m going to sand the surfaces shiny, then seal them with opticoat. I bought a bulkhead fitting to mount the schrader valve, so that should be pretty easy.

 

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Trigger Guard!

I’ve been shooting the cannon a lot lately, and the lack of a trigger guard is starting to be a little worrisome. While I was working on some other stuff at the shop I bent one out of some aluminum scrap. I did it first with some thin flexible metal, then copied the design with pliers and a thicker more ridged strip.

I tapped bolted onto the trigger mounting block and the stock. I didn’t really measure the hole position on front, I drilled a hole in the trigger guard first, then just flexed the guard until it looked right and then marked the point on the receiver with a sharpie. When I clamped it in the mill, I adjusted the hole to be centered on the receiver, but kept the x position from my mark.

Performance Tweaks 2

Another issue, with the cannon has been the slow fill time. Even when connected straight to the 150psi shop air line, it takes about 15 seconds for air to stop hissing through the check valve in the piston back (you can hear it). The check valve I made simply wasn’t moving enough air.

I made a couple of changes to improve the flow rate. First I drilled smaller hole out until it was only barely smaller than the ball. I also removed the spring, which wasn’t necessary.

At this point I realized the ball would definitely plug the hole in the retainer plate. To Fix this, I replaced the plate with an S shape piece of aluminum tig rod. If this works, I’ll make a nicer version at some point.

And.. it totally does work. I reassembled the cannon and tried filling it. Fill time is now about 2 seconds. the cannon even gets slightly warm from the compression.

Performance Tweaks

The cannons pretty much done now. Normally I’d leave it at that, but this time I’m going to stick it out a little longer and do some finish work.

One thing I’ve noticed is that the piston doesn’t retract all the way during firing. If there’s half an inch left behind the piston after firing that means that half an inch had to be vented unnecessarily, which slows down the piston movement.

The first thing I tried to fix this was to replace the QEV with something with a higher flow rate. I borrowed a sprinkler valve from one of my PVC cannons for this. I modify these valves a lot for speed, so I know for a fact that this valve can move a lot of air.

The results: no change in the firing impulse or piston travel. I guess the QEV is working pretty well after all.

Here’s what’s inside a quick exhaust valve btw. It’s surprisingly simple.

So.. all that’s left to do really is lengthen the piston. This will make the valve open less widely, but it will also make it open faster.

First, I fired the cannon with nothing in it, then disassembled it to check the piston position.

The piston had retracted only .42″ out of 1.28! The piston is way too short. I decided to cut the travel down to .65″, which meant lengthening the piston shaft by .63″.

I had some extra half inch rod lying around in anticipation of this. I spent an hour and a half or so on the lathe and made a new piston shaft .63″ longer than the first. This is the 3rd one I’ve made…

I’ll put a comparison video here soon. Basically, with shorter travel the discharge became much shorter and sharper. The deep, wall-shaking voooom sound was reduced as well unfortunately. The difference with a grapefruit loaded was HUGE. I don’t have a chrono for it yet, but subjectively, cannon went from “pretty powerful” to “scary powerful”. It pretty much vaporizes grapefruits now, leaving only a little mound of rind stuck to the wall at the impact point. The kick is similar to that of a shotgun with a 12ga slug in it. I’ll do more experiments with this once I have a chrono, but I’m very satisfied with the current performance.

Mounting the Trigger

Time to take care of that dangling trigger! I wung this part a little more than I usually do.

Based on the cad model, I know that the the distance from the trigger face to the back of the grip will be too long unless I shorten one of the parts. For reference, here’s a very interesting forum post with grip measurements from several handguns. The optimal distance between the back of the grip and the trigger face seems to be about 2.7 inches.

As luck would have it, due to the design of my trigger valve (a dental air valve), it was possible to mill off 1/8″ or so without hitting any of the internals. I lost the treads on the outlet side, but I don’t need those anyway. The brass machined really nicely.

Next I machined a little block to mount valve through. I would’ve liked to thread the valve directly into the block, but it has some kind of non-standard threads, so I decided to just use the nut instead of ordering a special tap just for this.

Here’s my plan for the block: ..please ignore the embarrassingly bad scaling. In this picture I had already milled the external dimensions of the block.

First, I drilled the giant hole for the valve. The giant final drill is show (i forget the size), but first I hit it with center drill, then a 1/4″.

Next I drilled and taped the two holes for the mounting screws. I drilled down until the holes were just touching the circumference of the large hole. I only had starting taps on hand, which don’t fully finish the first 4 threads, so I had to make the holes deeper than I would have liked to get enough complete threads for the mounting screws.

I lost a few photos here, but next I cleanup this block and drilled corresponding holes in the stock. I’m actually kind of amazed they line up so well. I’ve gotten a lot better throughout this project. Those ugly Philips screws holding the grip on are just temporary.

Here’s the completed trigger. I also mounted the picatinny foregrip. I didn’t take any pictures, but it just bolted on easily with 2 nuts and bolts.

 

 

Attaching the Grip

Attaching the grip will be straightforward, but a bit tedious if I want to do it right. AK grips are designed to bolt onto a mounting boss on the lower receiver. I’ll have to make one for my stock. You can see it in the exploded view below.

Here’s my version. I designed it a bit bulkier than usual, since I’ll be making it out of aluminum, not steel.

I usually just take a few notes on paper from my cad model and bring that to the shop, but this time I made a real drawing.  You can see version 1 there in the back, I’ll get to that in a second.

I won’t get into it, but it’ll suffice to say that all the following steps I had to do twice. I estimated some of the grip dimensions in the cad drawing before I got it, then forgot to update them. I did this whole process once and ended up with a part that didn’t fit. Bummer.

On to version 2..

First I faced a plate down to the correct thickness. I was able to find a suitable piece of stock in the scrap bin, score :).

Next I used the readouts to cut out the square profile based on the coordinates from my drawing.

After that I re-clamped the piece a couple of times and milled off the radius left over from before.

I marked the bottom edge where the point of the triangle should go, then scribed the new edges by hand. There’s a little gap between these surfaces and the grip, so they don’t need to be perfectly accurate.

To cut these faces, I clamped the piece in the vice such that the lines I scribed were parallel to the vice jaw.

Looks good! I accidentally cut the long face a little too deep, but luckily it shouldn’t effect the calculations or functionality.

The radius part isn’t critical or visible, so I decided to just rough it with a file. It was actually easier than I though. It only took a few minutes, and came out nice and accurate.

After that it was just regular drilling and tapping.

It fits! Woot!

Time to drill matching holes in the stock.

Here’s the finished Boss. it could use a little cleaning up but it works great. If I had to guess, I’d say this little piece took me 4 hours. More if you include v1. Machining requires a lot of patience.

Here it is with the grip mounted. I’m really happy with it, it doesn’t wiggle at all.

 

 

 

 

Machining the Butt

Here’s the goal:

My first challenge was finding the material to make it out of. Pretty much any plastic would work. Even so I called around and had a hard time finding a reasonable price on anything less than a large sheet. I originally though mcmaster was too expensive, at $20, it turned out to be my cheapest option. The piece I got was a 12″x6″x1″ block of HDPE.

First (not shown) I used a table saw to cut the stock to the correct height. After that I milled off the depressed area on the top. I’ve never machined HDPE before, it cuts great! Nice clean shavings and no melting!

I decided to tackle the radius for shoulder surface next. This was going to be the hardest part, so I wanted to get it out of the way early,  lest I ruin the part when it’s almost finished. My plan is to attached the work piece to a board and mounted the board such that it can pivot around the center point of the radius. That way I ccan sweep the part under the endmill in an arc.

The pictures a bit blurry, but you can sort of see the center point I drilled on the left side of the board. The radius is 6″.

The drill  bit sticking out of the  board is the pivot point. I locked the end mill 6″ out from that point.

It worked! The cutting was a bit rough, in retrospect I should have done a rough cut with a bandsaw first so I didn’t have to plow through so much material.

The next step is to make the cutout in the center. The vertical and horizontal slots were easy enough.

The diagonal slot was harder. I can’t make a straight cut diagonally on the mill, so I have to somehow fixture the part at an angle. ..I don’t really want to make a jig for this one cut.

I thought about it for a bit and realized that if I cut the corner off now I can use the scrap piece to hold the part at the right angle like so:

The diagonal slot and the tapered front are parallel, so it works out.

To keep the pieces from slipping I put a threaded rod through both pieces. (I quickly viced the piece and drilled the bolt holes first).

And cut. mrrrrrrrrrrrrrr. The quarter inch endmill isn’t long enough to do the whole depth at once so I had to flip the piece after this and make the other half of the cut.

To finish the stock up, I put a 5/8″ endmill on the lathe and used it to counter-bore the bolt holes. Because the centers were already drilled plunge cut went really easy.

Looks awesome! That’s one part down. The right bolts hadn’t come in yet, but I really wanted to get it mounted so I scavenged some bolts from around the shop.

Next up: mounting the ak47 grip.