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.

2014-03-14 13.50.06

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!

2014-03-14 14.36.46

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.

2014-03-14 15.31.37

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″.

2014-03-14 15.34.11

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

2014-03-14 15.40.26

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.

2014-03-14 16.20.30

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:

2014-03-14 17.43.15

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).

2014-03-14 17.40.36

2014-03-14 17.51.39

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.

2014-03-14 19.43.11

2014-03-15 15.40.38

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.


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.

2014-02-21 04.43.08

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.

2014-02-21 04.41.55

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.

2014-02-21 17.35.33

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.

2014-02-21 17.23.45

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.

2014-02-21 17.19.30

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

2014-02-21 18.47.18

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

2014-02-21 21.06.05

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

2014-02-21 18.49.11

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

2014-02-21 21.19.42

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.

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!




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:


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.